The SCITEAS experiment: Optical characterizations of sublimating icy planetary analogues

Planetary and Space Science 109-110 (2015) 106–122

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The SCITEAS experiment: Optical characterizations of sublimating icy planetary analogues A. Pommerol a,n, B. Jost a, O. Poch b, M.R. El-Maarry a, B. Vuitel a,c, N. Thomas a a

Physikalisches Institut, Universität Bern, Bern, Switzerland Centre for Space and Habitability, Universität Bern, Bern, Switzerland c Swiss Space Systems Holding SA, Zone Industrielle la Palaz A3, 1530 Payerne, Switzerland b

art ic l e i nf o

a b s t r a c t

Article history: Received 21 November 2014 Received in revised form 14 January 2015 Accepted 4 February 2015 Available online 18 February 2015

We have designed and built a laboratory facility to investigate the spectro-photometric and morphologic properties of different types of ice-bearing planetary surface analogs and follow their evolution upon exposure to a low pressure and low temperature environment. The results obtained with this experiment are used to verify and improve our interpretations of current optical remote-sensing datasets. They also provide valuable information for the development and operation of future optical instruments. The Simulation Chamber for Imaging the Temporal Evolution of Analogue Samples (SCITEAS) is a small thermal vacuum chamber equipped with a variety of ports and feedthroughs that permit both in-situ and remote characterizations as well as interacting with the sample. A large quartz window located directly above the sample is used to observe its surface from outside with a set of visible and near-infrared cameras. The sample holder can be easily and quickly inserted and removed from the chamber and is compatible with the other measurement facilities of the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern. We report here on the results of two of the first experiments performed in the SCITEAS chamber. In the first experiment, fine-grained water ice mixed with dark organic and mineral matter was left to sublime in vacuum and at low temperature, simulating the evolution of the surface of a comet nucleus approaching the Sun. We observed and characterized the formation and evolution of a crust of refractory organic and mineral matter at the surface of the sample and linked the evolution of its structure and texture to its spectro-photometric properties. In the second experiment, a frozen soil was prepared by freezing a mixture of smectite mineral and water. The sample was then left to sublime for 6 h to simulate the loss of volatiles from icy soil at high latitudes on Mars. Colour images were produced using the definitions of the filters foreseen for the CaSSIS imager of the Exomars/TGO mission in order to prepare future science operations. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Photometry Spectroscopy Physical properties Water ice Analogues Comets Mars Icy satellites

1. Introduction The visible/near-infrared reflectance of planetary objects and small bodies is one of the prime sources of information on the physical and chemical processes that occur at their surface. Relating the observed spectro-photometric properties of planetary surfaces to quantitative properties of the surface layer is however a challenging task because of the complexity of both the natural materials that compose these surfaces and the physical process of light scattering in particulate media. Numerical experiments with parameterized particulate layers and laboratory experiments with n Correspondence to: Physikalisches Institut, Universität Bern, Sidlerstrasse, 5, CH-3012 Bern, Switzerland. Tel.: þ 44 31 631 39 98. E-mail address: [email protected] (A. Pommerol).

http://dx.doi.org/10.1016/j.pss.2015.02.004 0032-0633/& 2015 Elsevier Ltd. All rights reserved.

natural or synthetic analog materials are both necessary to understand the potentials and limitations of the models that are currently in use in the remote-sensing community. In addition, laboratory investigations provide catalogues of data that can be used for direct comparison with remote-sensing datasets. Water ice is a particularly interesting and challenging material to work with for such investigations. Because of its widespread occurrence in the Solar System, both pure and mixed with mineral and/or organic material, it is crucial to have a comprehensive and accurate knowledge of its photometric properties. However, the diversity of shapes that water ice can adopt depending on environmental conditions, the difficulty to produce and maintain the extreme conditions relevant to the surface of cold outer Solar System bodies in the laboratory and to operate accurate

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instrumentation in these conditions make it a particularly difficult material to investigate. The Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern has been designed and built to overcome these difficulties and broaden our knowledge of the spectrophotometric properties of icy planetary analogs. Pommerol et al. (2011) have described in detail the rationale for the construction of this laboratory (previously referred to as the “Planetary Ice Laboratory”) and presented extensively the PHIRE-2 (PHysikalisches Institut Radiometry Experiment) radio-goniometer. This instrument has been used to measure the reflectance of ice/minerals mixtures with implications for the surface of comets or Mars polar caps (Pommerol et al., 2011, 2013b), various types of meteorites (Beck et al., 2012a), and micrometre-sized ice particles with implications for the surface of the icy satellites of Jupiter and Saturn (Jost et al., 2013). In this article, we present in detail the design of the second main facility of the LOSSy laboratory, a simulation chamber designed for evolving large and thick (4cm-sized) icy samples at low pressure and temperature while continuously characterizing some of their most important spectro-photometric properties (Fig. 1). We refer to this chamber as SCITEAS, “Simulation Chamber for Imaging the Temporal Evolution of Analog Samples”. Our initial objective with the development of this chamber was to be able to expose the samples characterized by the PHIRE-2 instrument to conditions of low temperature and pressure found at the surface of icy Solar System objects such as comets, icy satellites of the giant planets or the polar caps of Mars. The effects of textural and compositional changes of the sample (sublimation, sintering, formation on an upper desiccated layer…) on its reflectance properties would be characterized by regularly moving the sample between the PHIRE-2 radio-goniometer and the SCITEAS chamber. Therefore, two main conditions had to be met in the design of the chamber, and the sample holder: (1) it should be easy and quick to insert and extract, and (2) it should be compatible with the ones used by the radio-goniometer (160 mm diameter and 155 mm high cylinder). As commercial solutions could not provide the required flexibility and adaptability to existing hardware, we have opted for an entirely custom design and construction with special optimizations to fulfil our objectives of ease of use, compatibility and versatility.

Fig. 1. Picture of the SCITEAS facility. The chamber is in the centre of the image, with its window clearly visible. The two cameras are positioned above the window, in the standard measurement position. The pumps are visible in the lower right part of the picture and the dewars of liquid nitrogen behind the chamber, on the right. On the left side of the picture is the shelf containing all the electronics and light source optics for both the SCITEAS and PHIRE-2 facilities. Table 1 (see text) identifies all elements labelled with numbers in the picture.

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Many other facilities have been built and used in the past to expose various types of icy analog materials to simulated space or planetary surface conditions. The design and foreseen capabilities of the SCITEAS facility have been largely influenced by the scientific results, technical solutions and limitations of some of these setups, which are listed below. The most comprehensive effort to date to simulate the evolution of cometary matter in the laboratory is the KOSI (KOmeten SImulation) program, undertaken by the DLR (Deutsche Zentrum für Luft- und Raumfahrt) between 1986 and 1993 (see for example Grün et al., 1991; Seidensticker and Kochan, 1992; Benkhoff et al., 1995; Sears et al., 1999 provide a good review). Large samples, made out of mixtures of water ice and mineral were exposed to vacuum and low temperature conditions in the large space simulator of DLR Köln. A total of 11 experimental runs were conducted over 7 years, considerably enhancing our understanding of the sublimation of cometary material. New protocols to produce cometary analogs in large amount have been developed and many different properties of the samples as well as the evolved gases have been studied in detail. Despite the broad scope of this simulation program in terms of characterization techniques, the photometric and spectroscopic properties of the simulated nucleus material were not investigated in-situ. Reflectance spectra of the initial samples before exposure to vacuum and of the residue after the experiments have been measured for three of the KOSI experiments, KOSI-3, -4 and -6 by Oehler and Neukum (1991) but the spectro-photometric properties could not be characterized in-situ during the sublimation. The authors note in addition that the measurements of the residual samples might not be reliable, due to a possible contamination of the surface of the sample by frost, condensed during transfer from the space simulator to the spectrometer. During the course of the sublimation experiments, the surface of the sample was monitored using television cameras. The images were used in particular to characterize the trajectories and speed of grains ejected from the sample. Unfortunately, the images extracted from the movies and reprinted in the literature are generally of poor quality and cannot be used for quantitative analyses of the surface. Another research program particularly relevant for our own investigations is the series of experiments with thick ice samples conducted by Bar-Nun and co-workers at the University of Tel-Aviv (Bar-Nun and Laufer, 2003; Bar-Nun et al., 2007; Pat-El et al., 2009; Laufer et al., 2013). Here too, large icy samples were exposed to space conditions and the effects of sublimation investigated by a few different techniques, including visible photography of the surface. These results highlight the importance of gases trapped inside amorphous ice during condensation for the evolution of the ice during sublimation. Experiments with large and thick samples have also regularly been performed at the Institut für Weltraumfoschung (IWF) in Graz. Kossacki et al. (1997) largely followed the experimental protocols used for the KOSI experiments and show interesting schematic vertical cross-sections of their samples after several hours of evolution into vacuum and exposure to simulated solar radiation. All evolved samples show strong physical and chemical stratification. Using a similar setup, Kaufmann et al. (2006) concentrated their efforts on characterizing the solid-state greenhouse effect in icy samples. They exposed solid ice to simulated solar radiation under vacuum and observed changes in the transparency of the samples due to the appearance of thermal cracks. The thermal structure inside the sample of compact ice was compared to simulations using glass beads as an analog for a porous icy regolith. Gundlach et al. (2011a) built a dedicated simulation setup to study quantitatively the sublimation of ice into vacuum and in particular the effect of mineral dust on the sublimation, complementing previous qualitative results from the KOSI program. They

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found that the sublimation coefficient of water ice, defined as the ratio between the observed and the maximal sublimation rate of water ice can be as low as 0.1 for temperatures between 194 K and 227 K, in agreement with the conclusions of Kossacki et al. (1999). This can result in much higher near-surface temperatures than generally predicted with important implications for cometary activity. Laboratory simulations with icy analogs placed under simulated Martian surface conditions have also been performed by a few groups in the past. Chevrier et al. (2007, 2008) and Bryson et al. (2008) exposed water ice mixed with various amounts of different types of mineral analogs to Mars surface pressure and temperature. The observed diffusion rates of water vapour in the regolith have implications for calculating the survival times of shallow subsurface ice on Mars. This important question was investigated as well by Hudson et al. (2007) and Hudson and Aharonson (2008) with a wide range of analogs for Mars surface material. The diffusive emplacement of ground ice was observed and studied experimentally by Hudson et al., (2009). In the Mars community, many groups have concentrated on measurements of infrared reflectance spectra of minerals, pure and mixed, as they are crucial in interpreting many remotesensing datasets. Spectrometers are sometimes associated with small environmental chambers in which the pressure, temperature and humidity can be controlled (e.g. Bishop and Pieters, 1995; Cloutis et al., 2008; Pommerol et al., 2009; Beck et al., 2010). Ices and frost have received much less attention, although it is recognized that additional spectral characterizations of ices would aid in the understanding of polar seasonal processes (e.g. Pommerol et al., 2013a). Carbon dioxide in particular has not been fully characterized in the laboratory and new facilities are currently under development to address this specific issue (Grisolle et al., 2011; Kaufmann et al., 2013; Merrison et al., 2013). Finally, laboratory characterization of analogs of the surfaces of outer Solar System objects are highly challenging, as the temperature conditions relevant for their surfaces are difficult to maintain in the laboratory with large samples, especially if the surface must be observed through a window. For this reason, most studies so far have been restricted to thin films of condensed ice (e.g. Warren, 1986; Cruikshank et al., 1991; Quirico and Schmitt, 1997; Bernstein et al., 2006; Moore et al., 2007). From our review of past experimental studies of the sublimation of icy samples, we have identified one particular field in which considerable progress has been achieved recently, which could justify undertaking new studies and develop new facilities: digital imaging. In addition to its role as a complement to the PHIRE-2 radio-goniometer, we have thus decided to focus the design of the SCITEAS chamber on the use of digital imaging to characterize the evolution of icy analog samples under low temperature and pressure conditions. Using this technique, our main goal for this new facility is to complement past studies by offering detailed photometric and spectrometric characterization of the evolution of large (4cm-size) analog samples during sublimation, an aspect that has been mostly neglected in previous studies despite the high importance of these properties for the interpretation of many passive optical remote-sensing datasets. The PHIRE-2 and SCITEAS facilities are complementary to each other by the fact that the SCITEAS imaging system allows measurements of reflectance spectra of the samples over the spectral range 0.4–2.5 mm, whereas the PHIRE-2 instrument is restricted to reflectance measurements inside a few discrete bandpasses in the 0.4–1.1 mm range. However, the PHIRE-2 radio-goniometer provides very accurate measurements of the reflectance over a very wide range of measurement geometries, whereas reflectance measurements in SCITEAS are restricted to low phase angles and their accuracy is limited by the presence of the window between

the sample and the detectors. Interchangeable sample holders allow us to combine efficiently these different capabilities and characterize our samples in detail. Being involved in the development, construction and operation of optical remote-sensing systems for various planetary exploration missions, one of our additional objectives in developing the SCITEAS Imaging System was to be able to simulate in the laboratory, on small spatial scales, the data that would be returned by current and future space-borne instruments. In that way, we can optimize the choice and definition of the spectral ranges to improve the discrimination between different chemical and mineralogical compositions. Section 2 provides a complete technical description of the different subsystems of the SCITEAS facility. Our goal with this section is to provide enough detail on the design and construction so that particular technical solutions implemented here can be reused by other teams in the development of future experimental systems. In Section 3, we present the results and discussions of the first two experiments conducted with the SCITEAS facility. The first experiment is dedicated to the sublimation of a mixture of mm-sized H2O ice, mineral and organic matter with implications for the evolution of the surface of cometary nuclei. The second experiment is dedicated to the sublimation of a frozen soil made of clay mineral with direct implications for remote-sensing studies of the surface of Mars. Finally, Section 4 contains some conclusive remarks and prospective ideas for future use and improvement of the system.

2. Design and construction of the SCITEAS experiment 2.1. General design/structure The SCITEAS is a small thermal-vacuum chamber designed to accommodate at its centre one of the sample holders used with the PHIRE-2 radio-goniometer. These holders are cylinders with a diameter of 160 mm and a height of 155 mm (Fig. 2, Table 1). The main driver for the design of this chamber was the necessity to easily and quickly open and close the chamber, insert or extract a sample holder, and quickly (within a couple of minutes) lower the temperature and pressure as soon as the sample is installed. Like the sample holder, the chamber is shaped as a vertical cylinder with a diameter of 300 mm and a height of 270 mm. It is closed on its top by a thick stainless steel plate holding a 135 mm diameter (clear aperture) window, making it possible to image the entire surface of the sample from the outside. This lid is fixed at its side on a spring-loaded horizontal rotation mechanism that allows full opening of the chamber and thus guarantees free access to the sample holder in a few seconds and conversely, rapid closing of the chamber, cooling and pumping down shortly after the sample has been installed. The sample holder can be grabbed with a specially built handle to easily insert and extract it. Located next to the upper rim of the sample holder when in place in the chamber, a 32-pin electric connector permits the easy and fast connection of sensors placed inside the sample holder to an electric feedthrough port connected on the outside to a data logger. Inside the chamber, the base of the sample holder fits into a metal disk, which guarantees its correct positioning and centring. This disk is fixed to the tip of a linear mechanical feedthrough, which allows translation of the sample vertically over a distance of 60 mm once it is installed in the chamber. The linear mechanical feedthrough is installed in the centre of the large flange that constitutes the bottom side of the chamber. Around are installed other feedthroughs, electrical and optical, the connection to the pumping station and the input and output feedthroughs for liquid nitrogen. This liquid nitrogen circulates in

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Table 1 List of the important parts of the SCITEAS facility, shown and labelled on Figs. 1 and 2.

Fig. 2. CAD model views of the SCITEAS facility. Top: vertical cross-section of the chamber revealing its internal structure. Bottom: 3D-view of the chamber, without its external insulation layer, and connection to the pumping station. Table 1 (see text) identifies all elements labelled with numbers in the figures.

a cylindrical shroud, which laterally surrounds the sample holder at a distance of 20 mm. Connectors on the sides of the chamber are used for pressure sensors and for the gas input. Additional details on the different sub-systems of the SCITEAS facility are provided in Sections 2.2–2.5. 2.2. Vacuum/atmosphere control The SCITEAS chamber has been designed for use with either a good secondary vacuum (down to 10–7 mbar) or with a lowpressure controlled atmosphere. The different flanges, joints, connectors, feedthroughs and valves are all metallic, except the large top cover of the chamber for which the use of metallic joints was precluded by the necessity of closing the chamber very quickly once the sample is installed. Here we have used two concentric O-rings with continuous primary pumping in between to reconcile good vacuum performance and fast closing/opening capabilities. The intermediate pumping between the two O-rings is used to mitigate the permeation of gas through the O-rings, which results in leak rates lower by orders of magnitude than those possibly achieved with single O-rings. The chamber is evacuated using two membrane primary pumps and one turbo-molecular pump. The turbo-molecular pump is connected to the chamber on its low-pressure side and is evacuated on its high-pressure side by the membrane pumps.

Number Element

Visible in figure

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

1, 2a, 2b 1, 2a, 2b 1, 2a, 2b 2a 2a 2a, 2b 2a 2a, 2b 2a 2a 2a 2a 2a 1, 2a, 2b 2a 1 1, 2b 1, 2b 1, 2b 1, 2b 1, 2b 1 1, 2b 1 1 1 1 1 1 1 1 1 1

Upper lid Quartz window Lid rotation mechanism Radiation shield Standard sample holder Bottom flange Optical fibre feedthrough Linear mechanical feedthrough DN 63 connection to the pumping station 32-Pin electrical feedthrough Liquid nitrogen feedthrough Main cylindrical structure Cold shroud Pressure gauge Void for pumping between the concentric O-rings Thermal isolation foam Valve, DN 63 Leak valve Gate valve, DN 63 Turbomolecular pump Pumping station with membrane pump Primary pump Liquid nitrogen run-off Liquid nitrogen Dewar Liquid nitrogen Dewar Visible CCD camera Near-infrared MCT camera Cameras holding structure Illumination optical fibre Data loggers for temperature and pressure Light source radiometric power supply Ethernet hub Monochromator exit slit and fibre bundle

The most powerful of the two membrane pumps is also connected directly to the volume in between the two concentric O-rings of the top cover to limit the diffusion of air through the innermost joint. From the turbo-molecular pump, there are two connections to the chamber. The “direct” connection through a large gate valve is used to efficiently evacuate the chamber and achieve good secondary vacuum (10  7 mbar). A small diameter pipe, connected to an all-metal leak-valve, shortcuts the large gate valve of the direct connection. This configuration is used when an actively controlled atmosphere is required. In this case, a motorized computer-controlled leak-valve, connected to the gas input port on the side of the chamber, is used to control the input gas rate and actively regulate the pressure inside the chamber. The pressure in the chamber can be measured by two sensors, a cold cathode Pirani hybrid sensor (model PKR 251, Pfeiffer vacuum) to obtain approximate (gas-dependent) readings of the pressure from the lowest pressure achievable inside the chamber and up to about 100 mbar, and an absolute temperature-regulated capacitive sensor (model CMR 373, Pfeiffer vacuum) operating in the pressure range from 10  2 to 10 mbar. The latter sensor is used in combination with the motorized leak valve to keep the pressure inside the chamber constant at values representative of those of the Martian surface. One of the two pressure sensors (depending on the desired pressure range) is connected to the motorized leak valve controller, which is itself connected to the main computer via a RS232 interface. The value of pressure inside the chamber can be recorded, either regularly with a frequency defined by the user, or synchronized with other measurements (images, temperature).

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2.3. Cooling system and temperature measurements The simulation chamber is cooled by the continuous circulation of liquid nitrogen (LN2) in the cylindrical cold shroud surrounding the sample. We use two pressurized dewar vessels to store the LN2 and keep it flowing through the shroud. They are connected to each side of the shroud by flexible pipes. For one of the dewars, the venting valve is kept slightly open to the atmosphere to release the pressure produced by the evaporation. The venting valve remains closed on the other Dewar. By carefully adjusting the outgoing gas flow through the opened venting valve, we maintain a small difference of pressure between the dewars in order to keep the shroud full of LN2 while avoiding displacing large amounts of LN2 from one Dewar to the other. When the correct configuration is found, the system can remain stable for several days, evaporating a few litres of nitrogen per hour. The system is completely symmetric and reversible so that LN2 can flow either direction between the two dewars. In order to optimize the heat exchange between the cold shroud and the sample holder, they have both been painted with space-grade high-emissivity paint. On the other hand, in order to minimize the exchange of heat between the structure of the chamber and the cold shroud, a low-emissivity shield made out of polished steel is installed between the cold shroud and the inner vertical walls of the chamber. The temperatures inside the chamber can be monitored at multiple locations using up to 16 Pt100 temperature sensors, connected through a 32-pin electric feedthrough to an external data logger (Ethernet based data acquisitions system, model Keithley 2701 equipped with a 32-channel high-speed differential multiplexer, model 7703). Two of these sensors are permanently installed on the top of the cold shroud and their temperature is constantly monitored to verify that they stay at low temperature and thus ensure that the shroud remains full of LN2. All other sensors can be installed at any location, inside or outside the sample holder, fixed either mechanically, with Kapton and aluminium tape or with special thermal glue. They can also be inserted inside the sample during its preparation. Sensors placed inside the sample holders are quickly and easily connected onto the multipins connector located next to the top of the sample holder after it is introduced in the simulation chamber. The data logger is connected through Ethernet to the main computer, which controls the entire experiment and temperature values can be recorded at chosen intervals of time or synchronized with other measurements. We have used the COMSOL Multiphysicss (Comsol Inc.) finiteelement software to model the temperature fields and heat fluxes inside the chamber, both in perfect vacuum and under lowpressure conditions. The mesh for the finite element model was created from the actual CAD model of the chamber and simplified to keep 10,000 and 200,000 nodes, for the vacuum and lowpressure atmosphere cases, respectively. The surface emissivity values of the different materials that compose the chamber, which haven’t been measured, were left as free parameters, as well as the thermal conductivity of some of the insulation materials. The steady-state models were then correlated with the actual temperature measurements in the chamber by optimizing the values of these parameters. We reused here the optimization technique based on the Adaptive Particle Swarm Optimization (APSO) implemented by Bieler et al. (2011) and adapted to the problem of thermal correlation by Beck et al. (2012b). This correlation process converged toward plausible values for all parameters, both in the vacuum and low-pressure cases, with final average differences between model and measurements below 3 K. Examples of such calculations for an early configuration of the chamber used during the development and testing are represented in Fig. 3.

There were a couple objectives for this modelling study. First, we used the correlated model to optimize the thermal performance of the chamber by identifying the most critical parts and the most influential material properties. This has led us to improve our initial design by applying modifications to the setup that enhanced its performance in terms of cooling time, equilibrium temperature and LN2 consumption. Second, the correlated model provides us with useful information about the temperature field and heat fluxes inside the sample holder. This helps us assessing the amplitude of temperature gradients inside the sample. In particular, setting the sample thermal conductivity as a free parameter and correlating the model with values of temperature measured at different positions inside the sample can be used to accurately determine the bulk thermal properties of the sample. 2.4. Optical characterization of the samples Optical characterization of the samples to study their photometric properties and surface morphology is the prime objective of the SCITEAS facility. Most of the optical characterization is performed through the large window (135 mm clear aperture) located at the centre of the upper lid, between 20 and 70 mm above the surface of the sample holder, depending on its vertical positioning. The 15 mm-thick window is made of uncoated HOQ 310 fused quartz (Heraeus), which shows a nearly perfect transmission from 0.27 to 2.50 mm. For most applications, the sample is both illuminated and observed through this window. On the bottom flange of the chamber, an optics fibre feedthrough with SMA connector can be used to transmit light to the sample holder from below. The SCITEAS Imaging System subsystem has been designed for imaging the surface of the sample through the window. It consists of a monochromatic light source, which can illuminate the entire surface of the sample, and two cameras one covering the visible spectral range and the other the near-infrared spectral range. The light source consists of a 100 W Quartz Tungsten Halogen lamp illuminating the entrance slit of an Oriel MS257 monochromator equipped with three different gratings and five different bandpass filters to fully cover the wavelength range from 0.4 to 2.5 mm. The Full Width Half Maximum of the transmitted bandpasses is about 0.015 mm for the first two gratings (used from 0.4 to 1.3 mm) and about 0.03 mm for the third grating (used from 1.3 to 2.5 mm). The monochromatic light transmitted by the monochromator through its exit slit is focused on the extremity of a custom-made fibre bundle with a good transmission up to 2.3 mm. Its attenuation at longer wavelengths is currently limiting our ability to acquire high SNR images. The light beam emitted from the extremity of the fibre bundle has a divergence half-angle of 12.71. The entire surface of the sample can thus be illuminated by placing the extremity of the fibre bundle at a distance of about 300 mm from the sample (230 to 280 mm from the window). As the sample is illuminated by a divergent beam, the light source incidence and azimuth angles are slightly variable from pixel to pixel. The average incidence and azimuth angles can be changed by placing the fibre at different positions above the window. The light flux incident at the surface of the sample is heterogeneous, clearly showing spatial patterns influenced by the structure of the fibre bundle. A flat field correction is thus necessary to produce good quality images. The light scattered by the surface of the sample and transmitted through the window is collected by a near-infrared and a visible camera, placed 200 and 300 mm above the window of the chamber, respectively, and looking at the sample with an angle of about 131 from the nadir direction. The visible camera is a 1.4-Megapixel scientific-grade camera (model TSI 1500M-GE from Thorlabs) equipped with a 1392  1040 CCD sensor. It can be used

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Fig. 3. Steady-state temperature fields inside the SCITEAS chamber, modelled by a finite-element software and correlated to measurements performed with a first version of the facility, at sixteen different positions inside and outside the chamber. Shown here are cases modelled for a perfect vacuum (left) and a 4-mbar atmosphere (right).

to image the sample over the wavelength range 0.38–1.08 mm with a high sensitivity and a low noise level. The near-infrared camera (model Xeva-2.5-300 from Xenics) is equipped with a 320  256 pixel HgCdTe (MCT) detector cooled by a four-stage Peltier element, which has a high sensitivity over the 0.85–2.5 mm wavelength range. Both cameras are mounted on a holding structure, rolling on rails above the chamber so that they can be shifted away from the top of the chamber and re-positioned above the window with good precision. This was done to ensure a quick and easy access to the chamber at the beginning and the end of the measurement sequence. In addition, this mounting structure also allows us to tilt the cameras away from their central orientation by a maximum angle of 301 on each side. This can be used to acquire pairs of images of the sample with different emission angles and reconstruct the topography of the surface from the stereo images. The objectives of both cameras are interchangeable. For all experiments described in this paper, optics were chosen to allow imaging of the entire surface of the samples with both cameras. It is however also possible, for future experiments, to have one camera looking at the entire sample while the other camera is targeted on a particular and smaller subset of the sample. The two cameras, the light source and the monochromator are connected to a computer located in the laboratory and accessible remotely through the University network. The pressure controller and the data logger used for reading all temperature sensors are connected to this computer as well. Hyperspectral cubes of the sample’s surface are measured by shifting the wavelength of the incident light transmitted by the monochromator and synchronizing the acquisition of images at each wavelength by the visible and/or near-infrared camera. The typical time needed to acquire a hyperspectral cube is about 20 min but is highly dependent on the parameters chosen, such as spectral range and spectral sampling. We also generally synchronize the temperature and pressure measurements with the image acquisitions. Images recorded by the cameras, 12- and 14-bit for the VIS and NIR camera, respectively, are stored as 16-bit PNG images with lossless compression on a file server. Our standard calibration procedure includes: – Subtraction of an average dark image acquired while the internal shutter in the monochromator is closed. 20 dark images for both cameras are systematically acquired at the beginning and at the end of each hyperspectral cube measurement and averaged. The variability of the dark signal can be used for the estimation of uncertainties on the final spectral data.

– Division of the entire hyperspectral cube of the sample by a hyperspectral cube acquired in the same conditions but with a large plate of Spectralon™ (Labsphere) instead of the sample. We refer to this part of the calibration procedure as the “flatfield” correction. It efficiently corrects the effects of the inhomogeneous illumination of the sample and results in images calibrated in units of reflectance factor (I/F.cos i). The flat field measurements are normally performed each time the system is turned on. It should be noted that this correction is only efficient if the surface of the sample is flat and smooth. – Normalization of the entire image by the value measured over an internal reference (small disk of Spectralon) inside the frame, for each wavelength. This is done to mitigate the temporal variability of the sensitivity of the cameras, which can be significant if the laboratory conditions are changing.

An accurate evaluation of the uncertainties on the values of the spectral parameters determined with SCITEAS is a complicated task. In particular, various parameters of the spectra (absolute level of reflectance in the continuum, general slope, band depth…) show different sensitivities to the measurement noise, sample preparation, and instrumental bias. By design and construction, the hyperspectral system of SCITEAS cannot reach the high level accuracy permitted by photo-goniometer such as PHIRE-2 (relative errors of 1 to 2%). The main reasons are the effect of the window placed between the sample and the instruments, which causes non-linearity due to multiple reflection (Pommerol et al., 2009), and the use of cameras for which noise, bias, dark level and sensitivity variations are much higher than for mono detectors used on high accuracy radiometers. In addition, by design, the directions of illumination and observation vary slightly across the field of view, which causes spatial heterogeneity in the images acquired. We use a combination of different techniques to assess the uncertainties in our measurements. The dark level of the cameras is carefully characterized by acquiring 20 dark images before and after the hyperspectral cube. The effect of the dark level is negligible for the visible camera but significant for the infrared camera. In addition, the dark level seems to vary with time, possibly as a result of room temperature variations. Systematic characterization of the dark level for each observation is thus crucial and resulting uncertainties are variable and depending as well on the absolute level of the signal, which is varying from one sample to another and with wavelength. Measurements performed with spatially homogeneous samples are used to estimate the spatial variability of the reflectance across the field of view,

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which is of the order of 10% if the flat field correction could be done properly. Finally, it is possible to get some hints a-posteriori into the uncertainty of the measured reflectance by analysing its variability inside a region of interest defined over a homogenous part of the sample. Although the errors vary from one measurement to another, a typical value for the absolute uncertainty on the calibrated reflectance factor is 0.05. 2.5. Sample irradiation During operation in vacuum at low temperature, the surface of the sample is constantly heated by infrared radiation produced by the upper window. The only way to modulate this heating is by moving the sample vertically inside the chamber to modify the distance between the window and the surface of the sample. The temperature of the inner surface of the window is stable around 240 K during operation at low temperature, resulting in an irradiance of about 50 W m  2 onto the sample when placed at its standard vertical position, 7 cm below the window. In order to illuminate the sample with high fluxes of visible light, we use a small Sun simulator equipped with a high pressure 300 W Xenon short arc bulb and a filter designed to simulate the spectrum of extra-terrestrial Solar radiation up to 1 solar constant. The intensity of the flux can be reduced by attenuators to the desired value of irradiation and day/night cycles can be programmed using an integrated electronic shutter.

3. First results of the SCITEAS experiment We report in this section on the first two series of laboratory measurements performed with the SCITEAS facility, which reflect two of our three main scientific interests relative to the design and construction of the SCITEAS facility: the surface of comet nuclei and icy soils on Mars. Our third main interest, the icy satellites, will be addressed in future work. 3.1. Comet simulation (comet 1) The “comet 1” experimental run is the first of our comet simulations for which valuable measurements could be obtained. It was designed as a reference run, with which results obtained in subsequent and more complex simulations can systematically be compared. 3.1.1. Sample nature and preparation We have used a rectangular 120  60 mm sample holder with a thickness of 20 mm, entirely filled with the sample. This sample holder shape is carved in the centre of a black anodized aluminium disk, inserted at the top of one of our standard cylindrical sample holder. A heavy piece of steel, cooled to LN2 temperature, was placed in the void space of the cylinder below the sample holder to increase its thermal inertia. Producing a plausible simulant for cometary nucleus material is challenging for two reasons. First, the production, storage and handling of the individual components (ices, mineral matter, organic matter) can be challenging. Second, the way these components are mixed as well as their exact nature and physical state is poorly constrained, leaving us with a large range of compositions and textures to explore. The “comet 1” run being designed as a reference for subsequent studies, a simple “baseline” composition and texture was defined. Water is the only volatile used in the mixture. Carbon black (1 wt%) is used to represent the dark organic matter and fine volcanic ash (5 wt%) is used as the silicate component.

The SPIPA (Setup for the Production of Icy Planetary analogs) setup developed in the LOSSy laboratory (Jost et al., submitted) with strong heritage from a setup built at the University of Braunschweig (Gundlach et al., 2011b) was used to produce the water ice. Micrometre-sized spherical particles of ice are produced by freezing a nebula of water droplets at low temperature. Scanning electron microscope pictures of the produced sample were used to characterize the particle shape, spherical, and size distribution: mean diameter¼4.5 mm and standard deviation ¼2.5 mm (number distribution). Carbon black was purchased from Alfa-Aesar (product 39724). It is a solid residue obtained after controlled combustion of acetylene and consists of amorphous carbon shaped in spherical particles with an average size of 42 nm. We chose to include carbon black in the mixture of our reference comet experiment because amorphous carbon inherited from interstellar dust is estimated to represent up to 10% of cometary materials, and contributes to the low albedo of cometary nuclei (Wooden, 2008). We used the JSC1-AF lunar regolith simulant (Owens, 2006) as the silicate component of the mixture. It is a fine airfall volcanic ash of basaltic composition. The main advantage of this sample is that it is well characterized, in terms of composition (chemistry and mineralogy) and texture (particle size distribution). In addition, we have already characterized the photometry of this sample on the PHIRE-2 instrument in the past (Pommerol et al., 2011). The mafic minerals of basalt, pyroxene and olivine, are both expected in comets. The feldspar minerals, however, are not present in cometary material but feldspar is completely transparent and spectrally neutral over the spectral range investigated and will thus have a limited influence on the spectral and photometric properties of the sample. After experimenting with different procedures to mix the three components, we have decided to use mixing into liquid nitrogen for this experiment, as it is the method that gives the most reproducible results and is thus appropriate for a reference run. The steel bowl containing 15.3 g of mm-sized water ice is first filled with liquid nitrogen. The carbon black (0.15 g) and the volcanic ash (0.77 g) are then deposited, in known proportions, in this bowl. With a steel spatula, we continuously mix the cold slurry, which becomes increasingly viscous as the nitrogen evaporates. While the mixture is still liquid enough to flow, we deposit a fraction of it in the aluminium sample holder, which has also been previously cooled to liquid nitrogen temperature. As the LN2 completely evaporates, the deposited layer shrinks and cracks. Some holes appear, caused by the formation of nitrogen bubbles at the contact between the sample and the sample holder. For this reason, the sample is deposited in successive thin layers in order to fill entirely the holder. Whereas one can obtain very smooth and homogeneous samples by always depositing mixtures very rich in LN2 in the sample holder, we have voluntarily decided to deposit more viscous layers at the end of the procedure to produce some topography in the sample. The entire preparation procedure took place at the bottom of a running chest freezer where large volumes of liquid nitrogen were evaporated. This kept the atmosphere in the freezer very cold and dry during the entire preparation process and no frost deposition was observed on the black anodized aluminium during that time. For most of the materials that we have used, this method for mixing solids generally results in very homogeneous mixtures. The carbon black, however, shows a peculiar behaviour. While a fraction of it is homogeneously dispersed into the mixture, another fraction builds up large agglomerates that are clearly visible on all images. Four temperature sensors were used in this experiment to monitor the temperature inside the sample. Sensor 1, was fixed to the bottom of the sample holder. Sensor 2, was positioned close to

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the bottom of the sample holder but not held in contact with the metal, floating a few millimetres above. Sensors 3 and 4 were positioned in the middle of the sample, i.e. at a height of about 10 mm. Sensor 4 was a few millimetres higher than sensor 3. Because they were simply attached by their wires, the sensors slightly moved when the sample was inserted inside the holder so that their vertical position cannot be known very accurately.

3.1.2. Experimental conditions and dataset description Prior to the beginning of the experiment, the entire sample holder was constantly kept at liquid nitrogen temperature. The sample holder was then quickly moved into the simulation chamber, at room temperature. After inserting the sample into the chamber, linking the four temperature sensors to their connectors took less than a minute. The acquisition of visible pictures as well as pressure and temperature measurements (one per minute) was automatically triggered when the last temperature sensor was connected. The chamber was then hermetically closed and we immediately started to cool it down by circulating liquid nitrogen into the shroud. After about 8 min, we started the first primary pump to slowly lower the pressure into the chamber. The pressure dropped to 20 mbar in about 30 min, at which point the second primary pump was started as well as the turbo-molecular pump, at 20% of its nominal velocity. The pressure then quickly dropped to 10  4 mbar in about 10 min. The velocity of the turbomolecular pump was finally increased to 100% about 30 min later, quickly lowering the pressure down to about 10  5 mbar and was left at this value of rotation speed for the rest of the experiment. This entire procedure was performed with the ambient light of the laboratory turned on, in order to be able to inspect the sample visually. Visible images were acquired, which are useful for documenting the evolution of its surface, but hardly usable for accurate photometric studies. The light was turned off six times during the 5 h in order to acquire hyperspectral reflectance cubes of the samples, which can be used for spectro-photometric studies. We refer to these first five hours of measurements as “phase 1” of the experiment. Later, the ambient light in the laboratory was turned off, and hyperspectral cubes were regularly acquired, once per hour. Monochromatic images at 0.60 mm were also acquired every minute. These monochromatic images are calibrated and used to follow quantitatively the photometric evolution of the sample surface. This second part of the experiment is referred to as “phase 2”. The profiles of pressure and sample temperature through the entire experiment are presented on Fig. 4. Over 30 h, the pressure slowly decreases over one order of magnitude, from about 2.10  5 mbar at the beginning of the experiment to about 2.10  6 mbar at the end. Superposed over this general trend, one can see various additional lower amplitude fluctuations. Particularly obvious is the bump in the pressure curve between 5 and 8 h (labelled “A” on Fig. 4). A second one, broader and with a lower amplitude, shows a maximum between 10 and 11 h (labelled “B” on Fig. 4). Starting 8.5 h after the beginning of the experiment, the pressure curves also display numerous examples of pressure spikes, with typical durations of a few minutes only. The most prominent of these spikes are labelled with numbers, from 1 to 6. The origin of the bumps and spikes will be discussed in more detail in Section 3.1.3, when analysing the images taken simultaneously. The temperature profiles inside the sample holder show a continuous and logarithmic increase of temperature from about 165 K, 1 h after the beginning of the experiment to about 205 K just before the cooling is stopped. The temperatures inside the sample show a non-continuous behaviour during the first hour of the experiment, with a first phase characterized by a quick rise of

Fig. 4. Measured experimental conditions during the comet 1 experiment. Top: value of pressure inside the chamber. Bottom: values of temperature at four different vertical positions inside the sample, sensor 1 to sensor 4, from bottom to top, respectively. Sensor 1 is fixed onto the aluminium at the bottom of the sample holder. Sensor 2 is located a few millimetres above the surface of the holder. Sensor 3 and 4 are both located in the middle of the sample, about 10 mm from the bottom, Sensor 4 being high than Sensor 3 by a few millimetres. The labels, letters and numbers, identify some features on the curves that are described and discussed in the text.

the temperature, a second phase characterized by a quick decrease of temperatures of a few K, and finally a slower increase of temperature during the third phase. We note that this initial bump in the temperature curve, labelled C on the temperature curve in Fig. 4, is most pronounced for the sensor close to the surface of the sample (sensor 4) and barely visible for the one fixed at the bottom of the sample (sensor 1). The reason for this behaviour is possibly the way the sample has been cooled down and the air evacuated after introduction in the chamber. Indeed, the fast increase of temperature during the first phase corresponds to the time when a significant amount of gas was still present in the chamber. The presence of gas in the chamber increases the exchange of heat between the surface of the sample and the hot top cover of the chamber by conduction and convection. This air is evacuated after the turbo-molecular pump is turned on, and the heat exchange between the two surfaces is then limited to radiation. Simultaneously, the increase in sublimation rate when the pressure is lowered results in an increase of latent heat absorption by the sample, which probably explains the quick decrease of temperature before a new equilibrium is reached. Once the sample is in a good vacuum, it absorbs heat by its upper surface, radiated from the upper window (internal temperature around 240 K). Simultaneously, it loses energy by radiating heat toward the cold shroud from its sides. The temperatures

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Fig. 5. Selection of eight snapshots from the comet 1 video—colour (SOM 2), separated in time by 2 to 5 h, showing the evolution of the surface structure and albedo markings. All images are converted to apparent reflectance factor (I/F.cos i) using both the flat field and the internal calibration target visible on the center-right in every image. Snapshots on the top, (a)–(d), have histograms stretched between 0 and 0.7 whereas snaphsots on the bottom, (e)–(h), have histograms stretched between 0 and 0.3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

inside the sample, initially very low, evolve toward a new equilibrium situation with a higher average temperature and a different temperature field structure. The heavy piece of steel placed below the sample holder, cooled by LN2 prior to the beginning of this experiment, slows down this rise of temperature. The excavation of the temperature sensor from the ice is clearly visible on the temperature curve for the most surficial sensor, 4, around label B on Fig. 5. It is shortly followed by a similar feature for sensor 3. A change of slope on the temperature curve for sensor 2 is barely distinguishable, between t ¼16 and t¼ 20 h. No such feature is visible for sensor 1, the most deeply buried temperature sensor.

3.1.3. Evolution of the surface morphology and albedo pattern Pictures acquired every minute, simultaneously with pressure and temperature readings, have been assembled to create a video depicting the evolution of the sample (SOM 1). The obvious differences between the first 235 frames and the rest of the video are due to the different illumination conditions during phase1 (ambient laboratory light) and phase 2 (monochromatic illumination and flat-field correction) of the experiment (see Section 3.1.2). Only the second phase of the video (frame 236 to frame 1678) can be used to quantitatively analyse the photometric evolution of the surface. A similar video was produced using RGB colour composites extracted from the 35 hyperspectral cubes (SOM 2). Eight

frames extracted from this video and showing some of the most interesting features in the evolution of the surface are reproduced in Fig. 5. The overall strong decrease of reflectance seen in the video and on Fig. 5 will be analyzed quantitatively and in details in Section 3.1.4. Beside this strong general trend, many other interesting features are worth mentioning and discussing here. Our observations are listed below and the most interesting of them will be discussed in the rest of this section. 1. Rapid disappearance of the initial surface topography to produce a surface that is smooth at the cm-scale but rough at the mm-scale. 2. Bright mm-sized agglomerates of clean water ice fade away in 3 to 4 h. 3. New dark particles of all sizes (agglomerates) previously embedded into the ice are continuously exposed at the surface as the ice sublimates. 4. The small (unresolved) dark particles of basalt and carbon black accumulate and are responsible for the general and homogeneous darkening of the surface. 5. Occasional displacement of the bigger dark particles (agglomerates of carbon black) are observed, over distances of a few millimetres. 6. When moving, the large particles often expose bright ice at their initial location.

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7. Three of the four temperature sensors are observed to successively emerge from the subliming ice (4, 3, and 2 at t¼ 12, 14, and 16 h, approximately). The fourth one (1) fixed to the bottom of the holder remains covered by the dark mantle until the end of the sequence but its wires become visible after t ¼25 h. The first sensor to be excavated, sensor 4, is doing so very abruptly, while sensors 3 and 2 emerge more progressively, explaining the differences of behaviour noted on the temperature curves in Fig. 4. 8. For the last 15 to 20 h of the experiment, the surface evolves by sublimation of the ice in the form of regularly growing circular depressions with bright ice exposed on their periphery. 9. As a result of the continuous expansion of these circular depressions and their interactions, the bright markings appear to be continuously moving at the surface of the sample. 10. Water frost is clearly visible on the sides of the sample holder for the first 8 h of the experiment, then quickly disappears.

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Three Regions Of Interest (ROI) have been defined and are represented on both the VIS and NIR colour composites in Fig. 6. For the VIS hyperspectral cube, the definition of the ROIs is strictly the same as for the monochromatic images and the corresponding video (SOM 1). Because of the difference of field of view, resolution and exact orientation between the cameras, it is difficult to define ROIs on the NIR cube exactly as they are defined on the VIS cube. ROI 1 and 2 are 20  20 pixel large on the VIS images and 6  6 pixel large on the NIR images. ROI 3 is 220  170 pixel large on the VIS images and 49  65 pixel large on the NIR images. This temporal series of visible and near-infrared hyperspectral data is a complex four-dimensional dataset from which a variety of information can be extracted. As an illustration, Fig. 7 shows a selection of reflectance spectra averaged over the Region Of Interest “ROI1” represented on Figs. 5 and 6. Not surprisingly, the recorded spectra resemble those of pure water ice, responsible for the strong absorption bands at 1.5 and 2 mm. Both the continuum level and the level of reflectance at the bottom of the H2O absorption bands are strongly influenced by the amount of dark dust at the surface. Whereas the pure carbon black is spectrally neutral, displaying a constant reflectance factor of about 0.02 in the entire spectral range investigated, the basalt powder displays a non-negligible visible red slope that is noticeable in spectra measured after a few hours of sample sublimation. The temporal evolutions of the red slope and the H2O spectral signature, as well as the reflectance at different wavelengths in the continuum of the spectra, are represented in Fig. 8 for ROI 3 and in Fig. 9 for ROI 1. The red slope is calculated between 0.4 and 1.0 mm according to (Eq. (1)) and the depth of the 1.5 mm absorption band of H2O according to (Eq. (2)).

This last observation offers a plausible explanation for the interpretation of the pressure bump labelled A on Fig. 4 and described in Section 3.1.2. The thin layer of frost deposited on the external surface of the sample holder sublimes as the temperature of the top of the sample holder must reach the free sublimation temperature of water ice, 200 K. The pressure bump labelled A, from t¼5 h to t ¼8 h corresponds to the sublimation of the frost observed at the surface. The second pressure bump, labelled B, might be caused by sublimation of frost located at another location of the sample holder, out of the field of view of the cameras. At least some of the shorter spikes identified on the pressure curve in Fig. 4 can be explained by grain movements visible in the video. The most obvious example is the spike number 5 at t¼ 20.5 h, which is caused by the movement of a 8 mm long dark particle on the lower right part of the sample holder (between frames 861 and 862, t ¼20.52 to t ¼20.54) suddenly exposing a large surface of bright ice-rich material at the surface.

Band Depth ¼

3.1.4. Spectro-photometry of the surface The spectro-photometric characteristics of the surface of the sample are studied as a function of time through the detailed analysis of the hyperspectral cubes. Figs. 6 and 7 present a subset of the hyperspectral data acquired with the imaging spectrometer. The hyperspectral cubes acquired at t¼ 1.6 h with both cameras are presented in Fig. 6. We show here two RGB colour composites, each produced from three different monochrome images, acquired with the visible CCD (left) and near-infrared MCT (right) cameras. For the VIS colour composite, monochrome images acquired at 0.40, 0.52, and 0.60 mm are used, for the blue, green, and red channel, respectively. For the NIR colour composite, monochrome images acquired at 1.1, 1.8, and 2.0 mm are used, for the blue, green, and red channel, respectively. The 1.1 and 1.8 mm wavelengths (blue and green channels, respectively) are located in the continuum of the spectra whereas the 2.0 mm wavelength (red channel) corresponds to the maximum of absorption by H2O-ice in the studied spectral range. As a consequence, the NIR colour composite shows a distinctive bluish colour for areas rich in water ice. In addition to the sample itself, this also includes the sides of the aluminium sample holder. For this series of measurements, both cameras had been oriented and equipped with adequate optics to image the entire sample holder. Differences between the images, arising from the difference of observation direction between the cameras, are clearly visible. The difference of sensor resolution is also obvious, the visible images having 16 (4  4) times more pixels than the infrared images.

where REFFλ is the reflectance factor at the wavelength λ and CONTλ is the continuum above the absorption band, defined as a straight line between 1.38 and 1.82 mm. The obvious differences of behaviours between these two regions clearly illustrate the influence of the spatial resolution at which the surface photometry is studied. ROI 3 is large enough (40  30 mm), so that the evolution of the mm-scale bright and dark markings is averaged and a continuous decrease of the reflectance is observed, although the rate of this decrease varies strongly between the beginning and the end of the experiment. The reflectance first shows a fast decrease, dropping by a factor of four during the first five hours of sublimation. Then, the rate of decrease becomes much smaller, as the reflectance drops by a factor of three in about 25 h. After t¼30 h, all the ice has sublimated and the reflectance remains constant. The depth of the 1.5-mm absorption band of H2O ice displays a nearly linear decrease during the course of the experiment. Slightly negative values of this spectral parameter after the ice has entirely sublimated are probably caused by the low signal-tonoise ratio for spectral measurements of very low albedo material after t¼30 h and the way the continuum is defined. The red slope displays a different temporal evolution than the other spectro-photometric criteria described here. It shows no evolution between the first two measurements, a strong increase between t¼1 h and t¼5 h, a slower and almost linear increase between t¼5 and t ¼27 h, and a strong final increase during the last few hours of the ice sublimation (t¼27 h to t¼31 h).

Red slope ¼

ððREFF1:0 mm  REFF0:4 mm Þ=ð1 0:4ÞÞ ð0:5  ðREFF1:0 mm þ REFF0:4 mm ÞÞ 1  REFF1:5 mm CONT0:5 mm

ð1Þ

ð2Þ

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Fig. 6. Examples of VIS (left) and NIR (right) colour composites obtained from the hyperspectral cube acquired at t¼ 1.6 h. The visible colour composite on the left is assembled using the images at 0.60, 0.52, and 0.40 mm for the R, G, and B channels, respectively. The near-infrared colour composite on the right is assembled using the images at 2.0, 1.8, and 1.1 mm for the R, G, and B channels, respectively. Three different Regions Of Interest (ROI) are represented on these images, as well as a “reference” ROI located on the internal calibration target and use for reflectance calibration. Because the images used for each of these colour composites are acquired by two different cameras with different orientation and optics, geometric differences between the two images are obvious. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0.7

0.8 0.6

Band Depth Red slope Reflectance

t=0.33h

0.5 t=1.01h

0.6

0.3

t=1.93h t=2.66h t=3.91h

0.2

0.4

−

Reflectance

−

0.4

t=5.17h

0.2

0.1

0.0 0.0

0.5

1.0

1.5

2.0

Wavelength [µm]

0

10

20

30

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Fig. 7. Temporal series of reflectance spectra averaged over the ROI 1 (see definition in Fig. 6). These spectra are obtained by combining information collected by the CCD camera (at wavelengths shorter than 0.9 mm) and the MCT camera (at wavelengths longer than 0.9 mm). The overall level of reflectance in these spectra decreases with time, as well as the strength of the prominent features of water ice centred at 1.5 and 2.0 mm. Timing information is provided for the first 6 spectra, which were acquired at irregular intervals of time. Other spectra were acquired regularly, once per hour.

Fig. 8. Temporal evolution of the reflectance at four different wavelengths, the strength of the H2O ice band at 1.5 mm and the visible red slope, during the 37 h of the comet 1 experiment. See text for the definition of these two spectral criteria. All values are spatially averaged over the ROI 3 (see definition in Fig. 6). In the case of the reflectance at 0.60 mm, two types of measurements, hyperspectral cube every hour and monochromatic image every minute (labelled “1 min” in the legend), are compared. In order to correctly display the evolution of the red slope on the same plot as the other criteria, red slope values were divided by 2.

The temporal evolutions of the same spectral criteria for the much smaller ROI 1 (5  5 mm) show the same general trends as for ROI 3 but show in addition a strong increase of the spectral signature of H2O ice between t¼10 and t¼17 h, accompanied by a more subtle increase of reflectance at all wavelengths and a simultaneous decrease of the red slope. Studying the hyperspectral cubes and monochromatic images acquired at this time shows that ROI 1 is then partly filled by a bright ridge, causing the observed spectro-photometric variations. ROI 2 (not shown) displays a behaviour similar to ROI 1 but the increase of the spectral signature of H2O ice is shifted to later times, around t¼ 24 h. As for ROI 1, the evolution of the relative fraction of the ROI filled by bright features is responsible for the observed evolution of the reflectance and spectral criteria.

3.1.5. Discussion For obvious reasons, developing on Earth a perfect comet simulator is an impossible task. In particular, the difference of gravity between a km-long comet nucleus and a laboratory on Earth will always remain a major discrepancy between laboratory simulations and reality. The limited size of the simulation chamber is another factor that limits our ability to simulate some of the processes occurring at the surface of comet nuclei. Last but not least, the exact composition and physical properties of cometary matter is not yet accurately known and the production of any cometary analog is thus based on strong assumptions and hypotheses. Rather than trying to imitate in every detail the situation of a comet nucleus in space, it is much more feasible and instructive to concentrate on a few processes and observables for which the key

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0.6

−

Band Depth Red slope Reflectance

0.8

0.4

− 0.2

0.0

0

10

20

30

Time [hours]

Fig. 9. Same as Fig. 8 for the ROI 1.

physical parameters can be accurately constrained and, if possible, controlled. A prerequisite for any application of the laboratory results to real cometary cases is thus a perfect understanding of all processes occurring in the simulation chamber. As any other simulation of comet nucleus evolution undertaken in a laboratory, the comet 1 simulation offers a combination of plausible and unrealistic parameters and environmental conditions. This particular experiment is seen as a starting point and reference run for the use of the SCITEAS facility to simulate cometary processes. Future experiments can now be planned to assess the importance of each of these parameters. Two particular observations made during the comet 1 experiments seem particularly interesting to us and should be the subject of future investigations. The first one is the appearance and development of circular features after about 10 h of evolution. The second one is the persistence of the H2O spectral signatures as the ice sublimes and as a desiccated mantle develops. Observation of the morphology and photometry of the sample surface during sublimation clearly shows a transition between two different behaviours, which occurs between t¼5 h and t¼ 10 h. This transition is characterized by the appearance of circular features and a strong decrease of the rate of darkening of the surface. During the first five hours, the ice is subliming fast from the entire surface. The dark dust accumulates at the surface, resulting in a fast and homogeneous darkening. Heterogeneities are only observed at the mm scale of the individual agglomerates of water ice particles, which appear white, and carbon particles, which appear black. Between t¼5 and 10 h, the upper desiccated mantle must reach a sufficient thickness to provide good thermal insulation and be optically thick. Horizontal areas covered by this mantle reach the low reflectance of the desiccated basalt/carbon mixture whereas bright elongated features caused by exposure of water ice on slopes appear, increase in length and acquire circular shapes, delimiting circular depression which then steadily increase in diameter until the total sublimation of the ice. The desiccated mantle slows down the sublimation of the water ice below and a transition of behaviour is observed between a vertical and a horizontal sublimation regime. Although the environmental conditions are drastically different, the literature on snowfields and glaciers on Earth is useful to understand some of the physical principles at play in the appearance and expansion of these circular depressions. Movements of dark dust grains at the surface of ice have been studied to interpret various types of geomorphologic features observed at the surface of dirty ice, dirt cones and sun cups in particular. Betterton (2001) analyses analytically the dynamics of dust particles at the surface

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of a snowfield during ablation. The consequence of particles following trajectories normal to the ablating surface of ice is a concentration of these particles on peaks (stable equilibrium) and valleys (unstable equilibrium). The unstable equilibrium of the dust in the valley results in a deepening and expansion of the depressions. Accumulation of thick layers of dust can result in the appearance of dirt cones corresponding to an inversion of the topography, where initially low areas having accumulated dust are later protected from sublimation by the insulating effect of the dust mantle. Numerical modelling of these processes under different conditions of pressure and temperature should now be undertaken to verify to which extent the physical processes described on terrestrial snowfields can be extrapolated to the surface of a comet and overcome some limitations of the experimental setup such as the limited size of the sample and the effect of the much lower gravity. The organization of the surface into circular depressions with exposed ice on slopes explains why the spectral signature of water ice remains detectable for the entire experiment, although an optically thick layer of dust covers most of the surface. The strong near-infrared absorption bands of water seem much more sensitive to the presence of small amounts of water ice than the visible albedo of the sample. The visible red slope of the sample however, shows a strong evolution during the sublimation due to the enhanced contrast of reflectance between water ice and dark contaminants in the blue region of the visible spectrum. Visible red slope and infrared absorption bands show a high correlation and can both be used to assess the presence of water ice at the surface of a mostly dust-covered surface from remote-sensing. 3.2. Smectite/H2O-ice mixture Of a much shorter duration than the comet 1 experiment, the smectite-ice mixture experiment, referred to as the Mars 1 experiment thereafter, was focused on the use of the hyperspectral imaging system of SCITEAS to study the evolution of an icy Martian soil analogue. Our interest for laboratory experiments in this field is related to our studies of glacial and periglacial features on Mars using colour imaging and VIS–NIR hyperspectral imaging (Pommerol et al., 2011, 2013a). The study performed here also ideally complements results that we have recently published on the photometric characterization of icy Martian soils analogs with the PHIRE-2 instrument (Pommerol et al., 2013b) and demonstrates the synergy between the two main facilities of the LOSSy laboratory. In addition, this experiment also constitutes a preliminary step for a planed series of experiments aimed at studying the formation of desiccation cracks under simulated Martian conditions and the spectral evolution of desiccating chloridebearing materials, which were presented in detail elsewhere (ElMaarry et al., submitted). Finally, another reason to use clay minerals mixed with ice for this experiment is the strong interest for a clear distinction between the presence of ice in the ground and the occurrence of hydrated minerals in a high-latitude terrains, from near-infrared reflectance spectroscopy (e.g. Horgan et al., 2009). 3.2.1. Sample nature and preparation The SWy-2 smectite, a Na-rich montmorillonite from the Source Clay Repository (Costanzo and Guggenheim, 2001, and companion papers), was selected for this experiment. Although the chemical composition of this particular mineral is relatively different from clay minerals detected on Mars, this samples has other advantages that make it a good choice for this experiment. First, it shows extreme water uptake and swelling capabilities, which is particularly important for the study of desiccation

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Fig. 10. Selection of three snapshots from the Mars 1 video (SOM 2), separated in time by 3 to 4 h, showing the evolution of the surface structure and albedo markings. All images were acquired under the ambient light of the laboratory. The images shown here are converted to apparent reflectance factor (I/F.cos i) using both a flat field image and an internal calibration target. All snapshots have histograms stretched between 0 and 1.1. Three different Regions Of Interest (ROI) are represented on these images: ROI3, ROI1 and ROI2, from left to right, respectively. See also Fig. 11.

Fig. 11. Examples of VIS (left) and NIR (right) colour composites obtained from the hyperspectral cube acquired at t¼ 7 h. The visible colour composite on the left is assembled using the images at 0.60, 0.52, and 0.40 mm for the R, G, and B channels, respectively. The near-infrared colour composite on the right is assembled using the images at 2.0, 1.8, and 1.1 mm for the R, G, and B channels, respectively. Three different Regions Of Interest (ROI) are represented on these images, as well as a “reference” ROI located on the internal calibration target and use for reflectance calibration. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

cracking. Second, the NIR spectroscopy of this particular sample has already been extensively studied in the past (Pommerol and Schmitt, 2008; Pommerol et al., 2009). The sample was prepared by mixing SWy-2 with enough distilled water to create a slurry (  700% water content by weight for SWy-2). The slurry was mixed in a planetary mixer for 30 min and then left to settle for 1 day in a fridge to remove any excess water. The sample (at an initial weight of 85.6 g) was poured into a standard glass petri dish that had a temperature sensor attached to its base from the inside. An additional sensor was inserted into the slurry to have two different temperature measurements. Finally, the sample was left inside an initially switched-off deep freezer to cool down to a temperature of 30 1C (243 K). Upon cooling, the sample exhibited small (mmsized) fractures on its surface (probably due to thermal contraction cracking) and fibrous-looking foliations (Fig. 10), which were probably the result of pore-water crystallization. We did not observe evidence for cryoturbation effects during freezing.

3.2.2. Experimental conditions and dataset description Prior to its introduction inside the chamber, the sample holder inside its petri dish was kept at a constant temperature of 243 K inside a laboratory freezer for several hours. After the petri dish was installed in the chamber, the two temperature sensors were quickly connected, which triggered the data acquisition sequence. The chamber was then immediately closed and liquid nitrogen flown in the shroud to cool it down as fast as possible. A few minutes later, the pumps were turned on to evacuate the air from the chamber. The turbo-molecular pump was set at a low and constant rotation speed (20% nominal speed), for the duration of the entire experiment. The temperature and pressure quickly equilibrated to values of about 210 K and 10  4 mbar, respectively, and remained at these values for about 7 h, until the end of the experiment. Although this value of total pressure is much lower than the Martian total pressure, we wanted to avoid accumulating water vapour at high level in the chamber and keep it very dry to accelerate the sublimation of ice from the sample. This experiment

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is not a good thermodynamic simulation of the Martian environment but focuses on the acquisition of useful spectro-photometric data. Most of the measurements were performed with the ambient light of the laboratory turned on. It has only been turned off 4 times to acquire hyperspectral cubes. The rest of the time, one picture of the sample illuminated by ambient light was acquired every minute. The image acquisition was synchronized with readings of the temperature and pressure values.

3.2.3. Spectro-photometric results and discussion Although they cannot be used to provide accurate quantitative estimates of the absolute reflectance of the sample, the pictures acquired every minute with ambient light (video SOM 3 and Fig. 10) are useful to analyse the relative temporal variations of the sample reflectance in addition to geomorphological changes. The shrinking and brightening of dark ice-rich regions over time can be observed in many places, in particular over the ROI 1 (see Fig. 11 for definition). Very few other changes are visible. Some minor changes can be seen in the foliation features, as thin platelets of dry clay are slightly moving. No cracking of the sample was observed. Because only four were recorded, the VIS–NIR hyperspectral cubes (Fig. 11) are of limited interest to analyse the temporal changes occurring in the sample. On the other hand, the spectral dimension permits a detailed analysis of the surface composition of the sample. The first (t¼1.25 h) and last (t¼ 7 h) average reflectance spectra calculated for each of the three ROIs shown in Fig. 11 are displayed on Fig. 12. Comparison between the initial and final measurements shows significant evolutions of the spectra in the near-infrared, especially around 1.2, 1.5 and 2.0 mm where the decrease in the strength of the water ice absorption bands reveals the shape of the smectite hydration bands. In the case of ROI 2 and ROI 3, the smectite is clearly dominating the final spectra. In the case of ROI 1, the final spectrum still shows a combination of the spectral signatures of the hydrated clay mineral and the water ice. Colour composite images based on the near-infrared hyperspectral cube, as shown in Fig. 11, reveal immediately the areas that contain water ice.

Fig. 12. Average VIS–NIR reflectance spectra of the three ROI defined in Fig. 11. The dash dotted lines show the spectra extracted from the first measured hyperspectral cube, at t¼ 1.25 h and the full lines represent the spectra extracted from the last hyperspectral cube, measured at t¼ 7 h. The four colour filters of the CaSSIS camera are represented on the figure with transparent colours over the 0.4–1.1 mm range. We refer to these filters as BLU, PAN, RED, and NIR, from short to long wavelength, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 13. Example of simulated colour CaSSIS images, calculated from the VIS hyperspectral cube measured at t ¼7 h. The four RGB colour composites are generated from the different combinations of the BLU (0.40–0.57 mm), PAN (0.55– 0.80 mm), RED (0.78–0.88 mm) and NIR (0.90–1.10 mm) colour channels, simulated using the spectral sensitivity curves for the CaSSIS camera of the Exomars TGO orbiter. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Although an imaging spectrometer cannot compete with accurate mono-detector laboratory spectrometers in terms of absolute accuracy and signal/noise ratio, it offers other advantages that are particularly significant, as visible and near-infrared hyperspectral imaging has been established as one of the most frequently used technique to study planetary surfaces. Spectral mapping methods developed to reduce and analyse the large amount of data contained in hyperspectral cubes can be directly tested on similar cubes acquired in the laboratory at a reduced scale. Colour images can also be generated, which can directly be compared to the ones acquired by instruments in orbit around Mars or currently under construction, as illustrated in Section 3.2.4.

3.2.4. Simulation of CaSSIS colour images The CaSSIS, Colour and Stereo Science Imaging System, of the Exomars Trace Gas Orbiter, will return stereo images of the Martian surface, at a spatial resolution of about 4 m/pixel and in four colours. It will be assembled and calibrated at the University of Bern in 2015 and launch toward Mars on-board EM-TGO in 2016 (Thomas et al., 2014). The four broadband filters providing four colour channels with high signal to noise ratio over the 0.4–1.1 mm spectral range will be particularly useful for analysing the surface composition. Contrary to other colour imaging systems for which all colour channels are systematically acquired, CaSSIS has a significant operational flexibility as the number of colour channels acquired, the width of the images and their binning, can be freely chosen for each observation. This means that trade-offs will regularly have to be discussed and decided to optimize the scientific output of each observation, as the available bandwidth to transmit images to Earth will always limit the total size of the observations. In this context, we have developed a simulation of CaSSIS images based on the laboratory hyperspectral images of Martian analog samples acquired in the SCITEAS facility and characterizations of the various optical components of the instrument (mirrors, colour filters, detector). The model allows us to convert a visible hyperspectral cube measured on SCITEAS with the CCD camera into a synthetic colour CaSSIS image that can be used to

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assess the usefulness of the different colour channels for analysing the surface composition. Fig. 13 compiles four different RGB colour composites generated from the last hyperspectral cube acquired during the Mars 1 experiment, at t ¼7 h. To generate these colour composites, the images acquired through the four different colour filters, BLU, PAN, RED and NIR were first calculated using the actual CaSSIS filters spectral sensitivity curves then combined three by three to produced RGB colour images. The comparison of these colour images clearly shows that the BLU filter (0.40–0.57 mm) is crucial to provide a colour contrast between areas which contain water ice and areas which are completely desiccated. The colour composite which doesn’t include the BLU filters shows a clear contrast of reflectance between icy and desiccated areas but very limited difference of colours whereas the three colour composite which include the BLU filters display the icy areas with a blue colour and the dry smectite clay with a distinctive yellow tone. This exercise can be repeated for other cameras than CaSSIS. In particular, we are interested in simulating HiRISE colour images with icy samples to better interpret the peculiar colours observed over the high-latitude terrains covered by ice during the Martian spring (Pommerol et al., 2013a).

4. Conclusion and perspectives For more than four years, we have been continuously developing at the University of Bern the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) in order to study the reflectance properties of icy planetary analogs in the laboratory. The experiments and characterizations performed in this laboratory are essential to deepen our understanding of past, current and future optical remote-sensing datasets acquired on a wide range of icy planetary bodies. Following the construction of the PHIRE-2 radiogoniometer in 2011, the completion of the SCITEAS facility now considerably broadens our range of possible investigations. We have presented in detail in this article the design of the instrument, showing some of the technical solutions found to overcome experimental challenges. These might be of use for other groups developing laboratory instrumentation. In particular, we have succeeded in building a simulation chamber that is easy to operate and in which samples of various sizes can be introduced and extracted very simply and quickly. We have also managed to reach a high degree of automation. Once the samples are inserted and the chamber has reached its nominal pressure and temperature, all control operations and measurement sequences can execute fully automatically and the facility can run for days without human assistance, the data being stored on a file server accessible through the University network. Possibly as important as the overall performance of the chamber and its instrumentation, the ease of use and automation of the facility will guarantee its usefulness in the future. The two series of measurements presented in this paper, comet 1 and mars 1, illustrate the capabilities of the system. The comet 1 experiment was designed as a reference run with which the results of future more complex comet simulation experiments will be compared. These first results perfectly demonstrate the complementarity between some thermodynamics characterizations of the evolution of the system (temperature inside the sample, pressure inside the chamber) the analysis of the surface texture and spectro-photometry; for example the observation of movements of grains at the surface that cause spikes in the pressure measured inside the chamber or the persistence of spectral signatures of water ice as fresh ice is continuously exposed on the slope of expanding circular depressions.

Our future comet simulation experiments will differ from the baseline comet 1 experiment in the following ways: – Various types of organics suspected to be present in comets will be introduced in the ice/mineral mixture. In addition to amorphous carbon, comets are composed of a mixture of organic molecules and macromolecules made of different combinations of C, H, N and O atoms (Fomenkova et al., 1994; Mumma and Charnley, 2011). We will focus in particular on carbon–oxygen and carbon–nitrogen polymers, which have been proposed as extended sources of gases in the coma, and exhibit peculiar absorptions in the visible and near-infrared (Mumma and Charnley, 2011). – The basalt powder will be replaced by a mixture of pure mafic minerals, olivine and pyroxene, of known chemistry and in a known amount. – Different initial textures of sample (particle size, surface roughness, bulk density) will be prepared and tested. – The heat flux reaching the surface of the sample will be increased and controlled by illuminating the surface of the sample or a fraction of the surface with a Sun simulator through the window. – Small amounts of CO2 ice can be introduced in the samples either mixed homogeneously with the other components or as vertical stratification. – We will produce thicker samples to run longer experiments. In the longer term, we are planning to upgrade the experimental system with different instruments to allow for additional and complementary studies: a high-speed camera to capture the rapid processes associated with grain movement and ejection, a mass spectrometer to study the composition of the gases produced by the sublimation and a profiling microscope to characterize quantitatively the surface texture of the samples. In the case of Mars, the association between a simulation chamber and a VIS–NIR hyperspectral imager offers interesting possibilities for interpreting the colour images and infrared hyperspectral cubes acquired by many instruments from orbit as well as on the ground. Beyond the measurement of average reflectance spectra of analogs, which is possible in a number of laboratories, the measurement of hyperspectral cubes on cm-sized samples offers unique opportunities to test efficiently some of the spectral mapping methods applied to orbital remote-sensing datasets. An alternative way of using the laboratory data to help analysing orbital or in-situ colour images is to simulate the colours that would be measured by instruments on board spacecraft from the data measured in the laboratory. We are planning to use this approach to understand the origins of some colour variations seen on the central colour swath of HiRISE images in polar as well as low latitude regions. In addition, we have shown in this article an example of simulation of a CaSSIS colour image, a camera still in construction and to be launched toward Mars on the Exomars TGO orbiter (Thomas et al., 2014, Mars 8 abstract). Because of the foreseen high flexibility in the operation of the camera and necessary trade-offs between the number of colours, the number of images to be acquired per orbit, the necessity of acquiring a stereo pair or the swath of the images, simulated images constructed from laboratory data will be a useful help for decision making and will support the optimization of the scientific output of the instrument operation. Although comets and Mars are currently on top of our list or priorities because of missions currently in operation and others in preparation, many other measurements related to other planetary objects are planned and will be performed in the near future. We are particularly interested in the spectro-photometric properties

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of the water ice regolith that cover the surfaces of the Jovian and Saturnian satellites and more particularly the surfaces of Europa and Enceladus. We have recently studied the bidirectional reflectance of such fine-grained ice and noticed the strong influence of the sintering of the ice particles on the shape of the phase function (Jost et al., 2013). We now plan to use SCITEAS and its hyperspectral imaging system to complement these observations by studying the sintering process through near-infrared reflectance spectroscopy. While we will be studying the VIS–NIR reflectance of abiotic organics potentially present on comets and icy planetary satellites, we are also interested in characterizing the spectro-photometric signatures of biotic organic matter in the form of biomolecules or colonies of bacteria mixed with ice, soils and in different environmental conditions. We want to assess the potential of optical remote-sensing methods to detect biosignatures at the surface of Solar System objects. Our ability to prepare and study large samples of controlled composition consisting of ice, minerals and organics mixed in different ways, would allow us to propose spectral signatures of interest for landing site selection, prior to in-situ analyses, for future missions toward icy satellites.

Acknowledgements The construction of the facility was funded by the University of Bern and by the Swiss National Science Foundation, in particular through the R’equip grant no. 206021_133827. We are grateful to all the engineers and technicians of the WP department at the University of Bern who participated in this project. Careful reading of this paper by three anonymous reviewers resulted in significant improvements of its quality and clarity.

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