- Email: [email protected]
Digging deep for ice in Isidis Planitia—New constraints on subsurface volatile concentrations from thermal modeling
ARTICLE IN PRESS
Planetary and Space Science 54 (2006) 331–336 www.elsevier.com/locate/pss
Digging deep for ice in Isidis Planitia—New constraints on subsurface volatile concentrations from thermal modeling J. Helbert, J. Benkhoff1 Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany Received 18 August 2004; received in revised form 16 December 2005; accepted 20 December 2005 Available online 17 February 2006
Abstract The Isidis Planitia region on Mars usually is regarded as a comparably attractive site for landing missions based on engineering constraints such as elevation and smooth regional topography. The Mars Express landed element Beagle 2 was deployed to this area, and the southern margin of the basin was selected as one of the backup landing sites for the NASA Mars Exploration Rovers. Especially in the context of the Beagle 2 mission, Isidis Planitia has been discussed as a place which might have experienced a volatilerich history with associated potential for biological activity [e.g. Bridges et al., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108(E1), 5001, doi: 10.1029/2001JE001820]. However the measurements of by the GRS instrument on Mars Odyssey indicate a maximum inferred water abundance of only 3 wt% in the upper few meters of the surface [Feldman et al., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi: 10.1029/2003JE002160]. Based on these measurements this area seems to be one of the driest spots in the equatorial region of Mars. To support future landing site selections we took a more detailed look at the minimum burial depth of stable ice deposits in this area, focusing as an example on the planned Beagle 2 landing site. We are especially interested in the likelihood of ground ice deposits within the range of proposed subsurface sampling tools as drills or ‘mole’-like devices [Richter et al., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903–913] given reasonable physical constraints for the surface and near surface material. For a mission like ExoMars [Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15–19 March 2004, League City, TX] with a focus on finding traces of fossil life the area might be of potential interest, because these traces would be better conserved in the dry soil. Modeling and measurement indicate that Isidis Planitia is indeed a dry place and any hypothetical ground ice deposits in this region are out of range of currently proposed sampling devices. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mars; Water; Ice deposits; Beagle 2; ExoMars; Exobiology
1. Introduction Isidis Planitia is interpreted to be an ancient impact basin and is situated at the highland–lowland boundary, possibly having been filled by sediments from drainage leading into this area. It has been suggested that this
Corresponding author. Tel.: +49 30 67055319; fax: +49 30 67055303.
E-mail address: [email protected] (J. Helbert). Currently. Research and Scientific Support Department, ESTEC, Keplerlaan1, 2201AZ Noordwijk ZH, The Netherlands. 1
0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.12.017
sedimentary material may still be volatile-rich (Bridges et al., 2001). For this reason it is worthwhile to study this region in greater detail. In this study, we have focused on the likelihood of near-surface ice deposits, which might be important for the exobiological potential of this area in terms of favorable conditions for the preservation of organic molecules and biomarkers from putative early Martian life (e.g. McDonald et al., 1998). We present here estimates for the minimum burial depth of ground ice deposits, which are stable over long periods of time (410 ka) for the Beagle 2 landing site. We have used the Berlin Mars near Surface Thermal model
ARTICLE IN PRESS J. Helbert, J. Benkhoff / Planetary and Space Science 54 (2006) 331–336
Dust cover
Pores icefree Base material C
n io n at io at lim ns de on
b Su
The BMST model is characterized by a high vertical resolution—down to the centimeter range—, a realistic treatment of the thermal properties of ice–rock mixtures, a detailed treatment of the gas flux within the surface and into the atmosphere and a variable temporal resolution which allows studying diurnal as well as seasonal variations. While most thermal models for the near-surface layer of Mars in use today assume constant physical properties with depth, this model is based on a layered structure of the subsurface material in which each layer can have different physical and thermo-physical properties. The model includes a detailed treatment of the energy transfer into and out of the surface, including energy transported by gas flux and energy required for sublimation and energy released by recondensation of volatiles within the regolith. The model simultaneously solves the time-dependent mass and energy equations for the different volatiles. Solar energy input varies due to orbital and rotational motion of the planet. Heat is transferred into the interior of the regolith by solid-state heat conduction in the
Energy Transport
2. The BMST model
dust–rock–ice mixture (matrix) and by vapor flowing through the porous matrix. The gas flow from the sublimation fronts is driven by vapor pressure gradients. The numerical details of the model are discussed in Helbert and Benkhoff (2003). The simplified sketch in Fig. 1 shows the working principle of the BMST, the so-called ‘dirty ice approach’. The modeling is started assuming a porous layered soil in which the pores to a certain percentage are filled with ice. For this study only H2O ice has been included, but CO2 can be added as well. We have also neglected adsorbed phases for this study, as they have little influence on the effective depth of the ice table (Schorghofer and Ahardson, 2005). During the simulation, the thermal behavior, the energy transport and the gas flux within the soil are calculated over a number of Mars years. The thermal conductivity of each soil layer is in each step recalculated based on the ice content within the pores. In the version used here the BMST approximates the atmosphere interaction by assuming a constant water content of the atmosphere as derived from the data by Smith (2002). The simulation continues until the model has reached a dynamic steady state. Dynamic steady state is defined by an annual repetition of the temperature gradients within the subsurface and by a stabilization of the predicted ground ice table. Typically, steady state is reached after 1000 or more Mars years for the region studied here. From the distribution of ice with depth obtained upon convergence of the model, a predicted minimum depth of the ice table can be derived. Ice deposits stable over several annual cycles are possible only for depths in which the model predicts an ice content of the pores. For all other depths any existing ice would sublime over time. The version of the model used here does not consider diurnal frost deposits, which might occur at or very close to the surface. From the data set of the Gamma-Ray Spectrometer (GRS) on Mars Odyssey we have a limited knowledge of
Diffusion
(BMST) (Helbert and Benkhoff, 2003) to derive a map for the minimum burial depth of possible ice deposits. Beagle 2, the lander of the ESA Mars Express mission, was scheduled to land in the Isidis Planitia basin in December 2003. It would have repeatedly probed the nearsurface regolith to depths of 2 m with a mechanical ‘mole’ for sampling and thermal measurements (Richter et al., 2002). Unfortunately, no signal was received from the lander and it was declared lost. Therefore we cannot compare predictions from our model with in-situ results. However, we have compared the modeled surface temperatures with measurements from the Planetary Fourier Spectrometer on the Mars Express orbiter (Helbert et al., 2005). As a result of our study it should be noted that from a scientific point of view, especially focusing on volatilerelated issues and extant life, Isidis Planitia is a less favorable location. Based on our results ice can only be stable at depths of 4.5 m and below. If the focus is however on traces of extinct life, Isidis Planitia might indeed be an interesting landing site. The dry soil which has been most likely free of ice for most of the history of Mars might have conserved traces of life from the very early phase of Mars history. This would apply especially to ExoMars, the first mission in the ESA Aurora program (Kminek and Vago, 2005). The rover is supposed to search for traces of past and present life and will most likely be equipped with a drilling device, giving access to the first meters below the surface. The results on the ice stability are in agreement with findings by the GRS instrument on Mars Odyssey, which suggest a maximum inferred water abundance of 3 wt% for this area (Feldman et al., 2004) while noting that measurement depth of this instrument is no more than 1 m.
Heat Flux
332
Pores partially filled with ice
Icetable Stable ice deposits Start conditions
Simmer under Martian conditions for several hundred or thousand years
Dynamic steady state
Fig. 1. The ‘dirty ice’ approach used in the BMST.
ARTICLE IN PRESS J. Helbert, J. Benkhoff / Planetary and Space Science 54 (2006) 331–336
inferred water abundance in the uppermost layer of Mars (Feldman et al., 2004). Due to the method used, this knowledge is limited to a depth of 1 m at most. Furthermore, the spatial resolution of the GRS data is extremely coarse with a footprint of about 600 km. The data provided by the GRS instrument and the modeling using the BMST can be considered complimentary. GRS can provide the greater context and gives the limit on water abundance in the top layer of the surface, while the BMST can model local variations and can model vertical gradients to a larger depth than is accessible from GRS.
333
map of the thermal conductivity of surface materials surrounding the Beagle 2 landing site as shown in Fig. 2. The targeted landing site and the lateral extension of the landing ellipse are marked on the map. For the landing site area a density r ¼ 1500 kg m3 and a heat capacity c ¼ 795 J kg1 K1 have been assumed for dry near-surface regolith. These are typical values for fine sand assuming a basaltic composition. A classification of the thermo-physical properties of the general area yields 3 distinct units. Unit A exhibits a low thermal conductivity and is found mainly in the west of the studied area, while most of the area is classified as unit B with an intermediate thermal conductivity of 0.1 W m1 K1. In the south and east of the studied terrain a higher thermal conductivity is observed. This has been classified as unit C. To verify that the difference between the units is due to a difference in the physical properties and not only in the morphology, Fig. 3 shows the thermo-physical classification superimposed on a cropped MOC wide angle image of the region. No obvious correlation between morphology and thermo-physical classification is apparent with the exception of the crater in the southeastern edge which
3. Thermo-physical properties of the studied area For this study, a simplified subsurface structure has been assumed which consists of a 0.5 m thick layer with thermo-physical properties as remotely observed for the surface, and a subjacent base layer with a typical value of 0.5 Wm1 K1 for sedimentary material (Mellon et al., 2000). This follows a model proposed by Bridges et al. (2003). For the top layer data from the MGS thermal inertia measurements (Putzig et al., 2005; Mellon et al., 2000; Jakosky et al., 2005) have been used to construct a
Thermal Conductivity [10-3 Wm-1 K-1]
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Fig. 2. Inferred thermal conductivity of surface regolith for Beagle 2 landing area in 103 W m1 K1—based on MGS-TES thermal inertia retrievals; three distinct regions are identifiable (see text).
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Longitude [°E] Fig. 3. Classification from Fig. 2 superimposed on MOC wide-angle image of the area.
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ARTICLE IN PRESS J. Helbert, J. Benkhoff / Planetary and Space Science 54 (2006) 331–336
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coincides with the highest values of the thermal conductivity which may be indicative of a widespread, blocky ejecta field. A more detailed study of the correlation between morphology and thermo-physical properties by Murphy et al. (2004) reports similar findings. The parameters derived from the general classification are shown in Table 1. The albedo values have been obtained from an albedo map similar to the map shown in Fig. 2, also based on MGS-TES measurements (Mellon et al., 2000). The thermal conductivity listed in Table 1 is for the dry matrix. As described above the BMST calculates the actual thermal conductivity in each soil layer at each time step based on the ice content of the pores. The porosity is at the lower limit of the plausible range. We have assumed this low value to get an estimate for the minimal burial depth of ice. A low porosity limits the vapor diffusion and would tend to bring the ice table closer to the surface. This might
Table 1 Thermo-physical properties of the three identified surface units
Albedo (%) k surface (W m1 K1) k base layer (W m1 K1) Porosity
Unit A
Unit B
Unit C
23 0.07 0.5 0.1
22 0.1 0.5 0.1
23 0.12 0.5 0.1
Temperature profile Region A
for example reflect a cementation of the soil due to high amounts of salts. In order to map the minimum burial depth of ice for the region with a one-dimensional model as the BMST, a number of sampling points defining subareas within the three main thermophysical units have been modeled. In all, 37 different locations have been modeled within the area studied. This approach is based on the assumption that lateral heat transport within the subsurface is negligible. For each subarea the parameters have been set according to the above-derived classification. For the boundaries between regions the parameters have been interpolated linearly. 4. Results Each of the 37 model runs yields, among other parameters, the temperature at the surface, the temperature gradient within the subsurface for each time step of the model run and—from the distribution of ice at the end of the modeling run—a limit for the burial depth of ice. For each of the three thermophysical units, Fig. 4 shows an example of the typical temperature gradient in the subsurface. Displayed are the temperatures down to a depth of 0.4 m for a time interval from northern spring to northern summer for a local time of 13:00LST. As has been noted before (Helbert and Benkhoff, 2003), the steepest temperature gradient is observed within the first centimeters below
Temperature profile Region B
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Temperature profile Region C
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Fig. 4. Variation of the temperature for 13:00LST with depth and season for the first 0.4 m below the surface. Each panel shows a typical result for one of the three thermophysical units as defined in the classification (see Table 1),
ARTICLE IN PRESS J. Helbert, J. Benkhoff / Planetary and Space Science 54 (2006) 331–336
0
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Fig. 5. Example for the pore filling with ice versus depth for a model run in unit A.
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From these results, ice deposits anywhere in this area cannot be expected within 4.5 m of the surface. This bounding value is reached in unit A (compare Fig. 5) which has a low thermal conductivity of the top layer and which thus favors stability of near-surface ice deposits. The difference in predicted minimum burial depth in this area is approx. 0.5 m between the higher thermal conductivity areas in unit C and the lower thermal conductivity areas in unit A. This translates to a variation of approx. 10% with respect to the absolute burial depth. This is interesting because it shows the limited influence of the assumed surface layer, the thermal conductivity of which varies by a factor of 2 between units A and C. However, whether or not ice deposits do actually exist beyond the predicted minimum depth is of course unclear and depends on the extent of past emplacement of water in this area of Mars. 5. Conclusions Using the BMST and MGS-TES-derived surface thermal inertia, we have derived a map of the predicted minimum depth to any stable ice tables for the Beagle 2 landing region in Isidis Planitia, suggesting stable conditions only beyond 4.5 m depth. This study also is an example of the ability to derive useful information on the subsurface of Mars by a combination of remote sensing and modeling. Beagle 2 would have allowed to confirm or disprove the existence of any subsurface ice within the uppermost 2 m of the regolith by sampling and analysis for volatiles, as well as to perform depth-resolved temperature measurements in the regolith (Richter et al., 2003) to investigate the accuracy of the BMST soil thermal model. Unfortunately, due to the loss of the lander mission, both of these types of in-situ measurements could not be realized. However, an indirect verification of the thermal predictions of the BMST was performed by comparison with surface temperature data from the Planetary Fourier Spectrometer on the Mars Express orbiter and showed a very good agreement (Helbert et al., 2005).
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the surface, in agreement with results from other workers (e.g. Tokano, 2003). At greater depth, the seasonal variations are within a range of less than 5 K. One of the key capabilities of the BMST is the detailed treatment of water vapor transport within the pores. To derive a stability limit for ice deposits it was assumed that the pore space is homogenously filled with ice at the start of the model run. The model allows tracing the distribution of ice within the subsurface over time. Fig. 5 shows the filling state of the pores at the end of a model run for unit A. As can be seen, the pores down to a depth of 4.4 m are completely desiccated. From this, a minimum depth of the ice table of 4.5 m can be derived. All ice within the pores at shallower depth is removed over time. As pointed out above, this model does not consider transient ice, like diurnal frost deposits on or close to the surface. Based on the results for the depth of the ice table for each model run, a map of minimum burial depth of ice can be derived for the whole area. The result of this mapping is shown in Fig. 6.
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Longitude [°E] Fig. 6. Predicted minimum burial depths (in meters) of stable ground ice deposits around the Beagle 2 landing area in Isidis Planitia—ticks on the contour lines point toward shallower depths.
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The results of our modeling indicate a very small likelihood to find ground ice deposits close to the surface in Isidis Planitia which is in agreement with other investigators (e.g. Clifford and Hillel, 1983; Tokano, 2003). Our reported ice table of 4.5 m is a best case assumption because we used properties at the limit of the reasonable parameter space. Therefore we can clearly state that any ground ice deposits within 5 m below the surface in this region are very unlikely. This diminishes the scientific importance of this area for any future lander missions targeted towards present day volatile-related processes, especially the sampling of possible ice containing soil. However, it might be an interesting landing site for missions with a focus on finding fossil traces of life. An ideal candidate would be ExoMars, the first mission within the ESA AURORA program. This rover will be equipped with instrumentation addressing the potential of past and present life on Mars and will have a drilling device to access the first few meters below the surface of Mars (Kminek and Vago, 2005). The findings presented here are in good agreement with the water abundance as inferred from Mars Odyssey GRS measurements and they complement that data by providing a much higher spatial resolution. The GRS instrument reports a ‘dry spot’ in the area studied here with a maximum inferred water abundance of not more than 3 wt%. This would exclude any significant amounts of ice within the first 1 m below the surface. Acknowledgements This work has been funded by the German Research Council (DFG) under Grant no. BE 1630/2. We would like to thank the two reviewers for their comments which helped to greatly improve this manuscript. References Bridges, J.C., Seabrook, A.M., Rothery, D.A., Kim, J.R., Pillinger, C.T., Sims, M.R., Golombek, M.P., Duxbury, T., Head, J.W., Haldemann, A.F.C., Mitchell, K.L., Muller, J.-P., Lewis, S.R., Moncrieff, C., Wright, I.P., Grady, M.M., Morley, J.G., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108 (E1), 5001, doi:10.1029/2001JE001820. Clifford, S.M., Hillel, D., 1983. The stability of ground ice in the equatorial Region of Mars. J. Geophys. Res. 88 (B3), 2,456–2,474.
Feldman, W.C., Prettyman, T.H., Maurice, S., Plaut, J.J., Bish, D.L., Vaniman, D.T., Mellon, M.T., Metzger, A.E., Squyres, S.W., Karunatillake, S., Boynton, W.V., Elphic, R.C., Funsten, H.O., Lawrence, D.J., Tokar, R.L., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi:10.1029/ 2003JE002160. Helbert, J., Benkhoff, J., 2003. A new approach to assessing the burial depth of ground ice deposits and its application to proposed MER landing sites in Isidis Planitia. J. Geophys. Res. 108 (E12), ROV28-1. Helbert, J., Arnold, G., Benkhoff, J., Hirsch, H., Maturilli, A., Formisano, V., Giuranna, M., 2005. Comparison of surface temperatures measured by the Planetary Fourier Spectrometer (PFS) on Mars Express with predictions from the Berlin Mars near Surface Thermal model (BMST) for the BEAGLE 2 landing site in Isidis Planitia. Adv. Space Res. doi:10.1016/j.asr.2005.05.007. Jakosky, B.M., Hynek, B.M., Pelkey, S.M., Mellon, M.T., Martı´ nezAlonso, S., Putzig, N.E., Murphy, N., Christensen, P.R., 2005. Thermophysical properties of the MER and Beagle II landing site regions on Mars. J. Geophys. Res., submitted for publication. Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15–19 March 2004, League City, TX. McDonald, G.D., de Vanssay, E., Buckley, J.R., 1998. Oxidation of organic macromolecules by hydrogen peroxide: implications for stability of biomarkers on Mars. Icarus 132, 170–175. Mellon, Michael T., Jakosky, Bruce M., Kieffer, Hugh H., Christensen, P.R., 2000. High-resolution thermal inertia mapping from the Mars Global Surveyor Thermal Emission Spectrometer. Icarus 148 (2), 437–455. Murphy, N., Hynek, B.M., Jakosky, B.M., Martı´ nez-Alonzo, S., Putzig, N.E., Mellon, M.T., Pelkey, S., 2004. Thermophysical properties of Isidis Basin, Mars. In: 35th Lunar and Planetary Science Conference, pp. 1797. Putzig, N.E., Mellon, M.T., Kretke, K.A., Arvidson, R.E., 2005. Global thermal inertia and surface properties of Mars from the MGS Mapping Mission. Icarus 173 (2), 325–341. Richter, L., Coste, P., Gromov, V.V., Kochan, H., Nadalini, R., Ng, T.C., Pinna, S., Richter, H.-E., Yung, K.L., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903–913. Richter, L., Gromov, V.V., Kochan, H., Kosacki, K., Tokano, T., 2003. Scientific objectives and operational scheme of the Planetary Underground Tool (PLUTO) experiment on the Beagle 2 Mars Lander. In: XXVIIIth General Assembly of the European Geophysical Society. Schorghofer, N., Aharonson, O., 2005. Stability and exchange of subsurface ice on Mars. J. Geophys. Res. 110 (E5) CiteID E05003. Smith, M.D., 2002. The annual cycle of water vapor on Mars as observed by the Thermal Emission Spectrometer. J. Geophys. Res. 107 (E11), 25-1. Tokano, T., 2003. Spatial inhomogeneity of the Martian subsurface water distribution: implication from a Global Water Cycle Model. Icarus 164, 50–78.
Planetary and Space Science 54 (2006) 331–336 www.elsevier.com/locate/pss
Digging deep for ice in Isidis Planitia—New constraints on subsurface volatile concentrations from thermal modeling J. Helbert, J. Benkhoff1 Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany Received 18 August 2004; received in revised form 16 December 2005; accepted 20 December 2005 Available online 17 February 2006
Abstract The Isidis Planitia region on Mars usually is regarded as a comparably attractive site for landing missions based on engineering constraints such as elevation and smooth regional topography. The Mars Express landed element Beagle 2 was deployed to this area, and the southern margin of the basin was selected as one of the backup landing sites for the NASA Mars Exploration Rovers. Especially in the context of the Beagle 2 mission, Isidis Planitia has been discussed as a place which might have experienced a volatilerich history with associated potential for biological activity [e.g. Bridges et al., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108(E1), 5001, doi: 10.1029/2001JE001820]. However the measurements of by the GRS instrument on Mars Odyssey indicate a maximum inferred water abundance of only 3 wt% in the upper few meters of the surface [Feldman et al., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi: 10.1029/2003JE002160]. Based on these measurements this area seems to be one of the driest spots in the equatorial region of Mars. To support future landing site selections we took a more detailed look at the minimum burial depth of stable ice deposits in this area, focusing as an example on the planned Beagle 2 landing site. We are especially interested in the likelihood of ground ice deposits within the range of proposed subsurface sampling tools as drills or ‘mole’-like devices [Richter et al., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903–913] given reasonable physical constraints for the surface and near surface material. For a mission like ExoMars [Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15–19 March 2004, League City, TX] with a focus on finding traces of fossil life the area might be of potential interest, because these traces would be better conserved in the dry soil. Modeling and measurement indicate that Isidis Planitia is indeed a dry place and any hypothetical ground ice deposits in this region are out of range of currently proposed sampling devices. r 2006 Elsevier Ltd. All rights reserved. Keywords: Mars; Water; Ice deposits; Beagle 2; ExoMars; Exobiology
1. Introduction Isidis Planitia is interpreted to be an ancient impact basin and is situated at the highland–lowland boundary, possibly having been filled by sediments from drainage leading into this area. It has been suggested that this
Corresponding author. Tel.: +49 30 67055319; fax: +49 30 67055303.
E-mail address: [email protected] (J. Helbert). Currently. Research and Scientific Support Department, ESTEC, Keplerlaan1, 2201AZ Noordwijk ZH, The Netherlands. 1
0032-0633/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.12.017
sedimentary material may still be volatile-rich (Bridges et al., 2001). For this reason it is worthwhile to study this region in greater detail. In this study, we have focused on the likelihood of near-surface ice deposits, which might be important for the exobiological potential of this area in terms of favorable conditions for the preservation of organic molecules and biomarkers from putative early Martian life (e.g. McDonald et al., 1998). We present here estimates for the minimum burial depth of ground ice deposits, which are stable over long periods of time (410 ka) for the Beagle 2 landing site. We have used the Berlin Mars near Surface Thermal model
ARTICLE IN PRESS J. Helbert, J. Benkhoff / Planetary and Space Science 54 (2006) 331–336
Dust cover
Pores icefree Base material C
n io n at io at lim ns de on
b Su
The BMST model is characterized by a high vertical resolution—down to the centimeter range—, a realistic treatment of the thermal properties of ice–rock mixtures, a detailed treatment of the gas flux within the surface and into the atmosphere and a variable temporal resolution which allows studying diurnal as well as seasonal variations. While most thermal models for the near-surface layer of Mars in use today assume constant physical properties with depth, this model is based on a layered structure of the subsurface material in which each layer can have different physical and thermo-physical properties. The model includes a detailed treatment of the energy transfer into and out of the surface, including energy transported by gas flux and energy required for sublimation and energy released by recondensation of volatiles within the regolith. The model simultaneously solves the time-dependent mass and energy equations for the different volatiles. Solar energy input varies due to orbital and rotational motion of the planet. Heat is transferred into the interior of the regolith by solid-state heat conduction in the
Energy Transport
2. The BMST model
dust–rock–ice mixture (matrix) and by vapor flowing through the porous matrix. The gas flow from the sublimation fronts is driven by vapor pressure gradients. The numerical details of the model are discussed in Helbert and Benkhoff (2003). The simplified sketch in Fig. 1 shows the working principle of the BMST, the so-called ‘dirty ice approach’. The modeling is started assuming a porous layered soil in which the pores to a certain percentage are filled with ice. For this study only H2O ice has been included, but CO2 can be added as well. We have also neglected adsorbed phases for this study, as they have little influence on the effective depth of the ice table (Schorghofer and Ahardson, 2005). During the simulation, the thermal behavior, the energy transport and the gas flux within the soil are calculated over a number of Mars years. The thermal conductivity of each soil layer is in each step recalculated based on the ice content within the pores. In the version used here the BMST approximates the atmosphere interaction by assuming a constant water content of the atmosphere as derived from the data by Smith (2002). The simulation continues until the model has reached a dynamic steady state. Dynamic steady state is defined by an annual repetition of the temperature gradients within the subsurface and by a stabilization of the predicted ground ice table. Typically, steady state is reached after 1000 or more Mars years for the region studied here. From the distribution of ice with depth obtained upon convergence of the model, a predicted minimum depth of the ice table can be derived. Ice deposits stable over several annual cycles are possible only for depths in which the model predicts an ice content of the pores. For all other depths any existing ice would sublime over time. The version of the model used here does not consider diurnal frost deposits, which might occur at or very close to the surface. From the data set of the Gamma-Ray Spectrometer (GRS) on Mars Odyssey we have a limited knowledge of
Diffusion
(BMST) (Helbert and Benkhoff, 2003) to derive a map for the minimum burial depth of possible ice deposits. Beagle 2, the lander of the ESA Mars Express mission, was scheduled to land in the Isidis Planitia basin in December 2003. It would have repeatedly probed the nearsurface regolith to depths of 2 m with a mechanical ‘mole’ for sampling and thermal measurements (Richter et al., 2002). Unfortunately, no signal was received from the lander and it was declared lost. Therefore we cannot compare predictions from our model with in-situ results. However, we have compared the modeled surface temperatures with measurements from the Planetary Fourier Spectrometer on the Mars Express orbiter (Helbert et al., 2005). As a result of our study it should be noted that from a scientific point of view, especially focusing on volatilerelated issues and extant life, Isidis Planitia is a less favorable location. Based on our results ice can only be stable at depths of 4.5 m and below. If the focus is however on traces of extinct life, Isidis Planitia might indeed be an interesting landing site. The dry soil which has been most likely free of ice for most of the history of Mars might have conserved traces of life from the very early phase of Mars history. This would apply especially to ExoMars, the first mission in the ESA Aurora program (Kminek and Vago, 2005). The rover is supposed to search for traces of past and present life and will most likely be equipped with a drilling device, giving access to the first meters below the surface. The results on the ice stability are in agreement with findings by the GRS instrument on Mars Odyssey, which suggest a maximum inferred water abundance of 3 wt% for this area (Feldman et al., 2004) while noting that measurement depth of this instrument is no more than 1 m.
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Pores partially filled with ice
Icetable Stable ice deposits Start conditions
Simmer under Martian conditions for several hundred or thousand years
Dynamic steady state
Fig. 1. The ‘dirty ice’ approach used in the BMST.
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inferred water abundance in the uppermost layer of Mars (Feldman et al., 2004). Due to the method used, this knowledge is limited to a depth of 1 m at most. Furthermore, the spatial resolution of the GRS data is extremely coarse with a footprint of about 600 km. The data provided by the GRS instrument and the modeling using the BMST can be considered complimentary. GRS can provide the greater context and gives the limit on water abundance in the top layer of the surface, while the BMST can model local variations and can model vertical gradients to a larger depth than is accessible from GRS.
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map of the thermal conductivity of surface materials surrounding the Beagle 2 landing site as shown in Fig. 2. The targeted landing site and the lateral extension of the landing ellipse are marked on the map. For the landing site area a density r ¼ 1500 kg m3 and a heat capacity c ¼ 795 J kg1 K1 have been assumed for dry near-surface regolith. These are typical values for fine sand assuming a basaltic composition. A classification of the thermo-physical properties of the general area yields 3 distinct units. Unit A exhibits a low thermal conductivity and is found mainly in the west of the studied area, while most of the area is classified as unit B with an intermediate thermal conductivity of 0.1 W m1 K1. In the south and east of the studied terrain a higher thermal conductivity is observed. This has been classified as unit C. To verify that the difference between the units is due to a difference in the physical properties and not only in the morphology, Fig. 3 shows the thermo-physical classification superimposed on a cropped MOC wide angle image of the region. No obvious correlation between morphology and thermo-physical classification is apparent with the exception of the crater in the southeastern edge which
3. Thermo-physical properties of the studied area For this study, a simplified subsurface structure has been assumed which consists of a 0.5 m thick layer with thermo-physical properties as remotely observed for the surface, and a subjacent base layer with a typical value of 0.5 Wm1 K1 for sedimentary material (Mellon et al., 2000). This follows a model proposed by Bridges et al. (2003). For the top layer data from the MGS thermal inertia measurements (Putzig et al., 2005; Mellon et al., 2000; Jakosky et al., 2005) have been used to construct a
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Fig. 2. Inferred thermal conductivity of surface regolith for Beagle 2 landing area in 103 W m1 K1—based on MGS-TES thermal inertia retrievals; three distinct regions are identifiable (see text).
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coincides with the highest values of the thermal conductivity which may be indicative of a widespread, blocky ejecta field. A more detailed study of the correlation between morphology and thermo-physical properties by Murphy et al. (2004) reports similar findings. The parameters derived from the general classification are shown in Table 1. The albedo values have been obtained from an albedo map similar to the map shown in Fig. 2, also based on MGS-TES measurements (Mellon et al., 2000). The thermal conductivity listed in Table 1 is for the dry matrix. As described above the BMST calculates the actual thermal conductivity in each soil layer at each time step based on the ice content of the pores. The porosity is at the lower limit of the plausible range. We have assumed this low value to get an estimate for the minimal burial depth of ice. A low porosity limits the vapor diffusion and would tend to bring the ice table closer to the surface. This might
Table 1 Thermo-physical properties of the three identified surface units
Albedo (%) k surface (W m1 K1) k base layer (W m1 K1) Porosity
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Unit C
23 0.07 0.5 0.1
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23 0.12 0.5 0.1
Temperature profile Region A
for example reflect a cementation of the soil due to high amounts of salts. In order to map the minimum burial depth of ice for the region with a one-dimensional model as the BMST, a number of sampling points defining subareas within the three main thermophysical units have been modeled. In all, 37 different locations have been modeled within the area studied. This approach is based on the assumption that lateral heat transport within the subsurface is negligible. For each subarea the parameters have been set according to the above-derived classification. For the boundaries between regions the parameters have been interpolated linearly. 4. Results Each of the 37 model runs yields, among other parameters, the temperature at the surface, the temperature gradient within the subsurface for each time step of the model run and—from the distribution of ice at the end of the modeling run—a limit for the burial depth of ice. For each of the three thermophysical units, Fig. 4 shows an example of the typical temperature gradient in the subsurface. Displayed are the temperatures down to a depth of 0.4 m for a time interval from northern spring to northern summer for a local time of 13:00LST. As has been noted before (Helbert and Benkhoff, 2003), the steepest temperature gradient is observed within the first centimeters below
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Fig. 4. Variation of the temperature for 13:00LST with depth and season for the first 0.4 m below the surface. Each panel shows a typical result for one of the three thermophysical units as defined in the classification (see Table 1),
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From these results, ice deposits anywhere in this area cannot be expected within 4.5 m of the surface. This bounding value is reached in unit A (compare Fig. 5) which has a low thermal conductivity of the top layer and which thus favors stability of near-surface ice deposits. The difference in predicted minimum burial depth in this area is approx. 0.5 m between the higher thermal conductivity areas in unit C and the lower thermal conductivity areas in unit A. This translates to a variation of approx. 10% with respect to the absolute burial depth. This is interesting because it shows the limited influence of the assumed surface layer, the thermal conductivity of which varies by a factor of 2 between units A and C. However, whether or not ice deposits do actually exist beyond the predicted minimum depth is of course unclear and depends on the extent of past emplacement of water in this area of Mars. 5. Conclusions Using the BMST and MGS-TES-derived surface thermal inertia, we have derived a map of the predicted minimum depth to any stable ice tables for the Beagle 2 landing region in Isidis Planitia, suggesting stable conditions only beyond 4.5 m depth. This study also is an example of the ability to derive useful information on the subsurface of Mars by a combination of remote sensing and modeling. Beagle 2 would have allowed to confirm or disprove the existence of any subsurface ice within the uppermost 2 m of the regolith by sampling and analysis for volatiles, as well as to perform depth-resolved temperature measurements in the regolith (Richter et al., 2003) to investigate the accuracy of the BMST soil thermal model. Unfortunately, due to the loss of the lander mission, both of these types of in-situ measurements could not be realized. However, an indirect verification of the thermal predictions of the BMST was performed by comparison with surface temperature data from the Planetary Fourier Spectrometer on the Mars Express orbiter and showed a very good agreement (Helbert et al., 2005).
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the surface, in agreement with results from other workers (e.g. Tokano, 2003). At greater depth, the seasonal variations are within a range of less than 5 K. One of the key capabilities of the BMST is the detailed treatment of water vapor transport within the pores. To derive a stability limit for ice deposits it was assumed that the pore space is homogenously filled with ice at the start of the model run. The model allows tracing the distribution of ice within the subsurface over time. Fig. 5 shows the filling state of the pores at the end of a model run for unit A. As can be seen, the pores down to a depth of 4.4 m are completely desiccated. From this, a minimum depth of the ice table of 4.5 m can be derived. All ice within the pores at shallower depth is removed over time. As pointed out above, this model does not consider transient ice, like diurnal frost deposits on or close to the surface. Based on the results for the depth of the ice table for each model run, a map of minimum burial depth of ice can be derived for the whole area. The result of this mapping is shown in Fig. 6.
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Longitude [°E] Fig. 6. Predicted minimum burial depths (in meters) of stable ground ice deposits around the Beagle 2 landing area in Isidis Planitia—ticks on the contour lines point toward shallower depths.
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The results of our modeling indicate a very small likelihood to find ground ice deposits close to the surface in Isidis Planitia which is in agreement with other investigators (e.g. Clifford and Hillel, 1983; Tokano, 2003). Our reported ice table of 4.5 m is a best case assumption because we used properties at the limit of the reasonable parameter space. Therefore we can clearly state that any ground ice deposits within 5 m below the surface in this region are very unlikely. This diminishes the scientific importance of this area for any future lander missions targeted towards present day volatile-related processes, especially the sampling of possible ice containing soil. However, it might be an interesting landing site for missions with a focus on finding fossil traces of life. An ideal candidate would be ExoMars, the first mission within the ESA AURORA program. This rover will be equipped with instrumentation addressing the potential of past and present life on Mars and will have a drilling device to access the first few meters below the surface of Mars (Kminek and Vago, 2005). The findings presented here are in good agreement with the water abundance as inferred from Mars Odyssey GRS measurements and they complement that data by providing a much higher spatial resolution. The GRS instrument reports a ‘dry spot’ in the area studied here with a maximum inferred water abundance of not more than 3 wt%. This would exclude any significant amounts of ice within the first 1 m below the surface. Acknowledgements This work has been funded by the German Research Council (DFG) under Grant no. BE 1630/2. We would like to thank the two reviewers for their comments which helped to greatly improve this manuscript. References Bridges, J.C., Seabrook, A.M., Rothery, D.A., Kim, J.R., Pillinger, C.T., Sims, M.R., Golombek, M.P., Duxbury, T., Head, J.W., Haldemann, A.F.C., Mitchell, K.L., Muller, J.-P., Lewis, S.R., Moncrieff, C., Wright, I.P., Grady, M.M., Morley, J.G., 2003. Selection of the landing site in Isidis Planitia of Mars Probe Beagle 2. J. Geophys. Res. 108 (E1), 5001, doi:10.1029/2001JE001820. Clifford, S.M., Hillel, D., 1983. The stability of ground ice in the equatorial Region of Mars. J. Geophys. Res. 88 (B3), 2,456–2,474.
Feldman, W.C., Prettyman, T.H., Maurice, S., Plaut, J.J., Bish, D.L., Vaniman, D.T., Mellon, M.T., Metzger, A.E., Squyres, S.W., Karunatillake, S., Boynton, W.V., Elphic, R.C., Funsten, H.O., Lawrence, D.J., Tokar, R.L., 2004. Global distribution of near-surface hydrogen on Mars. J. Geophys. Res. 109, E09006, doi:10.1029/ 2003JE002160. Helbert, J., Benkhoff, J., 2003. A new approach to assessing the burial depth of ground ice deposits and its application to proposed MER landing sites in Isidis Planitia. J. Geophys. Res. 108 (E12), ROV28-1. Helbert, J., Arnold, G., Benkhoff, J., Hirsch, H., Maturilli, A., Formisano, V., Giuranna, M., 2005. Comparison of surface temperatures measured by the Planetary Fourier Spectrometer (PFS) on Mars Express with predictions from the Berlin Mars near Surface Thermal model (BMST) for the BEAGLE 2 landing site in Isidis Planitia. Adv. Space Res. doi:10.1016/j.asr.2005.05.007. Jakosky, B.M., Hynek, B.M., Pelkey, S.M., Mellon, M.T., Martı´ nezAlonso, S., Putzig, N.E., Murphy, N., Christensen, P.R., 2005. Thermophysical properties of the MER and Beagle II landing site regions on Mars. J. Geophys. Res., submitted for publication. Kminek, G., Vago, J.L., 2005. The Aurora Exploration Program—The ExoMars Mission. In: Proceedings of the 35th Lunar and Planetary Science Conference, abstract no. 1111, 15–19 March 2004, League City, TX. McDonald, G.D., de Vanssay, E., Buckley, J.R., 1998. Oxidation of organic macromolecules by hydrogen peroxide: implications for stability of biomarkers on Mars. Icarus 132, 170–175. Mellon, Michael T., Jakosky, Bruce M., Kieffer, Hugh H., Christensen, P.R., 2000. High-resolution thermal inertia mapping from the Mars Global Surveyor Thermal Emission Spectrometer. Icarus 148 (2), 437–455. Murphy, N., Hynek, B.M., Jakosky, B.M., Martı´ nez-Alonzo, S., Putzig, N.E., Mellon, M.T., Pelkey, S., 2004. Thermophysical properties of Isidis Basin, Mars. In: 35th Lunar and Planetary Science Conference, pp. 1797. Putzig, N.E., Mellon, M.T., Kretke, K.A., Arvidson, R.E., 2005. Global thermal inertia and surface properties of Mars from the MGS Mapping Mission. Icarus 173 (2), 325–341. Richter, L., Coste, P., Gromov, V.V., Kochan, H., Nadalini, R., Ng, T.C., Pinna, S., Richter, H.-E., Yung, K.L., 2002. Development and testing of subsurface sampling devices for the Beagle 2 Lander. Planet. Space Sci. 50, 903–913. Richter, L., Gromov, V.V., Kochan, H., Kosacki, K., Tokano, T., 2003. Scientific objectives and operational scheme of the Planetary Underground Tool (PLUTO) experiment on the Beagle 2 Mars Lander. In: XXVIIIth General Assembly of the European Geophysical Society. Schorghofer, N., Aharonson, O., 2005. Stability and exchange of subsurface ice on Mars. J. Geophys. Res. 110 (E5) CiteID E05003. Smith, M.D., 2002. The annual cycle of water vapor on Mars as observed by the Thermal Emission Spectrometer. J. Geophys. Res. 107 (E11), 25-1. Tokano, T., 2003. Spatial inhomogeneity of the Martian subsurface water distribution: implication from a Global Water Cycle Model. Icarus 164, 50–78.