H on Mars during northern summer

Icarus 330 (2019) 204–216

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IRTF/CSHELL mapping of atmospheric HDO, H2O and D/H on Mars during northern summer

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Alain S.J. Khayata,b, , Geronimo L. Villanuevaa, Michael D. Smitha, Scott D. Guzewicha a

NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States Center for Research and Exploration in Space Science & Technology (CRESST II), Department of Astronomy, University of Maryland, College Park, MD 20742, United States b

A B S T R A C T

Mapping the D/H isotopic ratio across Mars provides unique insights into the evolution and climatology of its atmosphere, and may help to identify the sources and sinks of atmospheric water vapor on the planet. We present new spatially-resolved measurements of atmospheric H2O, HDO and D/H on Mars during its northern summer at Ls = 126°, on March 21, 2016. High-resolution spectra were acquired at ν/Δν~40,000 using CSHELL, the Cryogenic Near-IR Facility Spectrograph at the 3 m NASA Infrared Telescope Facility (IRTF) on top of Maunakea, Hawaii. We targeted the 2ν2 spectral band of H2O around 2990 cm−1 (3.3 μm), and its deuterated form HDO at its ν1 fundamental band around 2720 cm−1 (3.7 μm). The water vapor and HDO show increased mixing ratios in the northern hemisphere, reaching peak values of 400 ppmv for H2O, 170 ppbv–450 ppbv for HDO, as compared to the southern hemisphere where depleted values of < 20 ppmv for H2O and < 10 ppbv for HDO were observed. The resulting D/H measurements indicate an enrichment over the terrestrial value, exhibiting a strong variation with latitude, longitude and local times. We report a strong dependence of D/H on local time, with high HDO abundances towards local noon. We observed higher D/H enrichment above basins (Utopia), lower enrichment above high-altitude Mons (Elysium Mons), and low D/H variations over “flat” regions on the planet.

1. Introduction Tracing isotopic ratios in planetary atmospheres provides unique insights into the evolution of such atmospheres. In the case of Mars, the preferential escape of the lighter hydrogen over deuterium in its atmosphere is responsible for the strong enrichment of deuterium in water as compared to the terrestrial value. Hence, measuring current abundance ratios of deuterium to hydrogen (hereafter D/H) can help constrain the water loss in the Martian atmosphere. The D/H value of earlier epochs on Mars can be obtained from ancient water trapped in Martian SNC meteorites that are collected on Earth. For instance, Greenwood et al. (2008) reported measurements of high D/H value of 4 relative to the Vienna Standard Mean Ocean Water (VSMOW = [HDO] / [H2O] = 3.1 × 10−4) in the primordial atmosphere of Mars between 4.5 and 3.9 Ga in the ALH84001 meteorite, and a D/H of 5.6 VSMOW in the young shergottites, dating as early as 0.17 Ga. The increase in the D/H value indicates a continuous hydrogen escape from the atmosphere of Mars. The first in-situ measurements of the isotopic ratios of water in the Martian atmosphere were conducted at Gale Crater by the Curiosity rover that landed on Mars in August 2012. Measurements using the rover's Sample Analysis at Mars (SAM)'s tunable laser spectrometer (TLS) show an enrichment in deuterium by a factor of 6 ± 1 VSMOW (Webster et al., 2013). From the Earth, using the Fourier transform spectrometer at the ⁎

Canada-France-Hawaii 3.6-meter telescope, Owen et al. (1988) measured in the infrared the total integrated line-of-sight column abundances of HDO and H2O and derived a first present day D/H for Mars. The ratio found is enhanced by a factor of 6 ± 3 over the Earth's standard mean ocean water, indicating a great loss of water from Mars throughout its history. Bjoraker et al. (1989) later reported measurements of HDO and H2O using the Fourier Transform Spectrometer on NASA's Kuiper Airborne Observatory around 2.65 μm, indicating a D/H ratio of 5.2 ± 0.2 relative to the standard mean ocean water (SMOW). Several global measurements of D/H followed, including infrared observations using the Kitt Peak National Observatory (Krasnopolsky et al., 1997), and millimeter observations using heterodyne spectroscopy with the IRAM 30 m antenna (Encrenaz et al., 1991; Encrenaz et al., 2001) that derived D/H values of 5.5 ± 0.2 and 6 (+6, −3), respectively. In order to track the geographic variations of D/H, the first spatially resolved retrievals were presented by Mumma et al. (2003) who conducted nearly simultaneous observations of H2O (years 2001–2003) and HDO (years 1997–2003) using CSHELL, the Cryogenic Near-IR Facility Spectrograph (Tokunaga et al., 1990; Greene et al., 1993) at the 3 m NASA Infrared Telescope Facility (IRTF) on top of Maunakea, Hawaii. Novak et al. (2011) presented a latitudinal map of D/H at the central meridian of Mars during the 2008 mid-northern spring of Mars, and noticed that the ratio peaks around the sub-solar point, before

Corresponding author at: Solar System Exploration Division, Mailstop 693, NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States. E-mail address: [email protected] (A.S.J. Khayat).

https://doi.org/10.1016/j.icarus.2019.04.007 Received 26 January 2019; Received in revised form 4 April 2019; Accepted 5 April 2019 Available online 17 April 2019 0019-1035/ © 2019 Elsevier Inc. All rights reserved.

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Fig. 1. Images of the spectrum of Mars as observed on CSHELL's detector at the H2O (left) and HDO (right) wavelength settings. The beam from Mars (10.63 arcsec) covers ~50 pixels in the vertical direction, and the section that observed the sky around Mars (“sky region”) covers ~100 pixels. The remaining “dead” section (in pitch black) covers the upper 100 pixels on the detector and does not receive any flux from Mars or the sky. The left panel shows telluric methane and water vapor signatures as indicated by the arrows. They are shown in emission (sky radiance) in the “sky region”, and in absorption when observed against Mars (“beam”). The right panel shows telluric HDO lines. In the middle panel, Mars' image is inverted between CSHELL's imager and the spectrometer, and therefore inverting the northern hemisphere of Mars (pointed South) on CSHELL's detector. The Mars image is rendered using the TES albedo map (Christensen et al., 2001). Mars alternates between the A and B beams positions on the detector.

latitudinal maps of H2O, HDO, and the resulting D/H, and finally summarize our findings in Section 5.

decreasing to the north and south directions. In contrast, the D/H ratio was rather constant at several latitudes over four seasons on Mars between 2007 and 2014 (Krasnopolsky, 2015), with a low global mean D/ H ratio of 4.6 ± 0.7 that of the Earth's ratio. Global D/H maps across the Martian globe were obtained in the 2008–2014 period at four seasons between late northern winter and spring seasons (Villanueva et al., 2015). Strong water isotopic anomalies were observed across the disk, characterized by low D/H values at high-altitude regions (1–3 VSMOW), high values at orographic depressions (above 8 VSMOW), and a progressive increase in D/H over the northern hemisphere during northern spring. Following maps of H2O, HDO and D/H were recorded on April, 2014, around 7.2 μm using the Echelle Cross Echelle Spectrograph (EXES) instrument aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) (Encrenaz et al., 2016). A disk-integrated D/H ratio of 4.4 (+1.0, −0.6) is obtained around northern summer, showing an enhancement from 3.5 to 6 VSMOW towards northern latitudes. The molecular abundances were derived from the ratio of their spectral line depths, and the retrieved disk-integrated value of D/H is lower than most of the previous diskintegrated results (Owen et al., 1988; Encrenaz et al., 1991; Krasnopolsky et al., 1997; Encrenaz et al., 2001). Independently from the limited 3 arcseconds (arcsec) spatial resolution of SOFIA, the EXES maps show less variability in the D/H ratio across the disk of Mars when compared to the maps shown in Villanueva et al. (2015). Follow up observations by EXES on March 2016 (northern summer) and January 2017 (northern winter) provided disk-integrated D/H values of 4.0 (+0.8, −0.6) and 4.5 (+0.7, −0.6), respectively (Encrenaz et al., 2018). These observations did not reveal any strong geographical and seasonal variations in the D/H ratio across the disk of Mars between northern summer and winter, but presented relatively lower D/H values over the planet. Ground-based infrared observations offer the advantage of highresolution spectroscopy when compared to low-resolution instruments aboard the Mars orbiters, and the ability to instantaneously probe the latitudinal (seasonal) variations in the abundances of the atmospheric molecular species. Mapping the D/H across Mars gives important clues on the sources and sinks of water on Mars, and permits to separate fast/ localized climatological processes (e.g., Rayleigh and cloud formation) from slow evolutionary processes (D and H differential escape). In Section 2, we present the observation strategy used in this work. In Section 3, we provide details on the data processing methodology, explain the morphology of CSHELL spectra and discuss the resulting uncertainties. In Section 4, we present the retrieval results and the

2. Observations We observed Mars using CSHELL, the Cryogenic Near-IR Facility Spectrograph (Tokunaga et al., 1990; Greene et al., 1993) at the 3 m NASA Infrared Telescope Facility (IRTF) on top of Maunakea, Hawaii. The disk of Mars was observed during its northern summer on March 21, 2016, corresponding to Mars Year (MY) 33 at Ls = 126°, between UT 12:45 and UT 16:20. MY represents the Mars year number, where MY1 begins at the northern spring equinox on April 11, 1955 (Clancy et al., 2000). The angular diameter of Mars in the sky was 10.63 arcsec, with a topocentric Doppler velocity of −15.6 km/s. The sub-Earth and sub-solar latitudes were 7°N and 20°N, respectively. Ground-based observations of Mars through the Earth's atmosphere introduce telluric contamination to the Martian spectrum. Earth's atmospheric water vapor introduces spectral signatures that are more than two orders of magnitude stronger than the ones belonging to Mars, and therefore high Doppler velocities of Mars are needed in order to distinguish between the different water vapor signatures. We targeted the 2ν2 spectral band of H2O around 2990 cm−1 (3.3 μm), and its deuterated form HDO at its ν1 fundamental band around 2720 cm−1 (3.7 μm). We took advantage of CSHELL's entrance slit at its narrowest width at 0.5 arcsec to achieve the maximum spectral resolving power of ν/Δν~40,000, corresponding to spectral resolutions of Δν = 0.0747 cm−1 (at 2990 cm−1), Δν = 0.0692 cm−1 (at 2720 cm−1), and a Doppler velocity resolution of Δv = 7.5 km/s. CSHELL's InSb detector is an array of 256 × 150 pixels, with 256 pixels covering the spectral range at 2.5 pixels per resolution element, and 150 pixels spanning the spatial direction, with each pixel covering 0.2 arcsec on the sky. Fig. 1 shows the image of the Mars spectra on CSHELL's detector at the H2O and HDO settings. Strong telluric features of methane (CH4) and water vapor are observed at the H2O setting, whereas weak telluric HDO features are observed at the HDO setting. The spectra were later aligned in the spatial and spectral directions during the data processing phase. As shown, the spectrometer's 30 arcsec long slit covered the full diameter of Mars and part of the sky at each observation. The purpose of the observations was to map the disk of Mars as the planet rotates around the sub-Earth point, and therefore to obtain simultaneous spectra of the planet at the H2O and HDO wavelength settings. We executed our observational strategy by positioning 205

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Fig. 2. The left panel shows the CSHELL's mapping configuration on Mars on March 21, 2016 when Mars' angular size was 10.63 arcsec, and at 15:30 UT. The two parallel lines in each position represent the proposed entrance slit that is shown to scale, with a 0.5 arcsec width. 12 slit positions on Mars were taken. The directions of the celestial North and East are represented by the perpendicular arrows on the top left of each panel. The directions of the north pole of Mars were oriented 36° east of the celestial north. The longitudes on Mars are positive to the West, and the map of Mars is rendered using Mars24 (Allison, 1997; Allison and McEwen, 2000). The middle panel shows the compiled image of Mars on March 22, 2016, as observed by the Mars Color Imager (MARCI), image credit: NASA/JPL-Caltech/MSSS, (Malin et al., 2016). Water ice clouds are shown above Elysium Mons and Syrtis Major. The right panel shows the position of CSHELL's slit over Mars at the A and B beam positions. CSHELL's imager shows a truncated image of Mars at the bottom, but the spectrometer's detector can sample the full diameter of Mars at the 2 beam positions A and B (see Fig. 1). Such images are used to help monitor to first order CSHELL's footprint at each spectral observation of Mars, and later to help determine the appropriate column abundances of water and HDO above the sampled regions.

τ225GHz = 0.05, where τ225GHz is a metric for the water vapor abundance in Earth's atmosphere, and the telluric water was 1.33 precipitable mm.

CSHELL's slit at the North-South central meridian of Mars at the labeled “A” position along the slit, took an image, and then removed the slit and took another image of the whole disk of the planet. We nodded the telescope 14.4 arcsec along the slit at the “B” position and took the slit/ no slit images of Mars (see Fig. 1). We then switched from the imaging to the spectral mode and took a series of 4 exposures in the ABBA sequence at the HDO setting. The (A1-B1-B2 + A2) cancels the telescope and telluric emissions to first order via pixel-to-pixel subtraction. The same steps were repeated for the H2O setting at the same slit position on Mars, and we alternated between the two settings at each slit position on the planet. The on-source integration time is 30 s for each of the H2O and HDO settings. The mapping took a total of 3 h and 35 min, by positioning the slit at the eastern limb of Mars, and repeating the same ABBA steps for the H2O and HDO, and then by offsetting the slit 0.8 arcsec across the disk of Mars, in a total of 12 slit positions from the eastern to the western limbs of the planet (see Fig. 2). Several factors dictate the number of slit positions on Mars to achieve a map, including the rotation rate of the planet in the sky (15° in longitude every hour), the planet's airmass limit set to observe it (< 1.6), the atmospheric seeing, the time needed to execute the Mars exposures at the HDO and H2O settings, and the overhead time spent in guiding, offsetting and refocusing the telescope. The Mars images at each position were used to make sure that the spectra were sampling the appropriate regions on the planet (Fig. 2, right panel). As part of the data processing, we corrected for guiding inaccuracies by comparing between the measured emergent flux from Mars across the detector and the modeled one, at each slit position across the planet. The atmospheric seeing above Maunakea was measured using the full-width-at-half-maximum (FWHM) of the point spread function of a standard star, and it varied around 0.63 arcsec (400 km on Mars) on March 21, 2016. The observations were conducted in dry weather conditions. The average telluric optical depth at 225 GHz was

3. Data processing We followed our standard data processing methodology as described in more detail in Novak et al. (2011), Villanueva et al. (2015), and Khayat et al. (2017). At the end of the observing night, we took “flat” and “dark” frames at the H2O and HDO grating settings, in order to remove the contamination originating from the dark current in the detector, and correct for the pixel-to-pixel response variations across the detector. After combining all the flats (12 for each setting), we achieved a signal-to-noise ratio per pixel of 430 and 340 at the HDO and H2O settings, respectively. The raw spectra from Mars are subjected to a two-dimensional distortion that is caused by the anarmophicity of the optics. In order to correct for such distortion, also known as the spectral straightening, we observed a narrow and a spectrally featureless source in the sky, the early-type standard star V* alf CrB. By applying a second-degree polynomial fitting to the stars' continuum at the different grating settings, we were able spatially straighten the Mars' spectra across the detector at the A and B beam positions to a milli-pixel accuracy. The row-by-row frequency calibration across the detector was conducted by synthesizing the Earth's atmospheric radiance throughout the entire frequency range using the Line-By-Line Radiative Transfer Model (LBLRTM) (Clough et al., 2005). By stepping CSHELL's slit across the disk of Mars, we obtained two dimensional spectra related to the relevant molecular species (H2O and HDO). Guiding inaccuracies affect the mapping scheme, and therefore the intended topographic regions that are sampled on Mars. We retrieved the exact footprint of CSHELL's slit on Mars in two stages. During the first one, we positioned CSHELL's slit on a selected region on 206

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well as the solar flux contribution (hereafter solar factor) to the total intensity from Mars. We retrieved the solar contribution in the total continuum intensity by measuring the diminishing equivalent width of the solar Fraunhofer lines from the solar radiation that transited Mars twice before reaching the Earth (Novak et al., 2011). The retrieved solar contribution factor was later used to retrieve the molecular abundance of HDO. Fig. 4 shows the adjusted slit positions at the H2O and HDO wavelength settings, after making a comparison between the estimated (left panels) and measured (right panels) continuum intensities from the planet. CSHELL's detector displays the measured continuum intensity in number-of-counts (ADU) units. We normalized the number of counts to their maximum value across each slit position, then multiplied this normalized intensity by the maximum estimated continuum intensity at the same slit position at the time of the observation, which allows the comparison between the measured and the estimated continuum intensities with the same flux units (W/sr/m2/μm), and therefore allowing the proper alignment of the slit positions on Mars. The gaps in covering the full disk of Mars are a result of the selected slit-stepping cadence, which emphasizes spectral integrity between H2O and HDO (two CSHELL settings per location). Suspended dust and water ice particles in the atmosphere of Mars introduce absorption and scattering effects that influence the gas absorption depths. Excluding such extinction (scattering and absorption) effects underestimates the true value of the molecular gas abundances (Villanueva et al., 2015). We therefore accounted for the presence of aerosols during our retrieval process. We used the same extinction model that was implemented by Smith et al. (2000) to interpret the data

Mars and took four images in the ABBA series. In the second stage, we used a general circulation model, the Mars Climate Database (MCD, Millour et al., 2008) to estimate the surface temperature of Mars at the time of the observation, and model the continuum intensity from the planet at several slit positions. The exact position of the slit is retrieved by first relying on the slit images above Mars as seen on CSHELL's detector, and then by adjusting the slit position East or West, North or South on the disk of Mars until a good agreement between the measured flux from stage 1 and the modeled flux from stage 2 is retrieved. At near infrared wavelengths over the H2O grating setting at 3.3 μm, Mars is seen through solar reflected radiation, and the thermal component from Mars is negligible. At the HDO grating setting at 3.7 μm, the thermal emission from Mars increases rapidly with temperature and wavelength (Khayat et al., 2015), following the Planck function. Retrieving the molecular abundance of HDO is therefore related to the fraction of contributions from the thermal emission of Mars (one-way extinction), and the scattered sunlight (two-way extinction). We calculated the solar intensity by considering Mars as a Lambertian surface, and used the solar incidence angle on each element on Mars at the time of the observation as well as the surface albedo from the Mars Global Surveyor's Thermal Emission Spectrometer (TES, Christensen et al., 2001). The thermal emission from Mars was calculated using the emissivity of the surface as well as the surface temperature provided by MCD. The solar contribution is the ratio of the scattered sunlight to the continuum intensity from Mars (solar + thermal). Fig. 3 shows the disk of Mars on March 21, 2016 as seen from Earth, including the surface temperature of the planet as calculated from the MCD, the calculated solar reflected intensity, thermal emission, the continuum intensity as

Fig. 3. The panels show the configuration of the disk of Mars at 15:30 UT on March 21, 2016. a) The surface temperature across the disk of Mars as modeled by MCD, a general circulation model (Millour et al., 2008). b) Our modeled thermal emission from Mars at the time of the observation at Ls = 126° using the TES emissivity map and the surface temperature. c) Our modeled solar reflected intensity from Mars, with the appropriate incidence angles on Mars at the observing configuration. d) The synthetic continuum intensity (thermal + solar reflected) of Mars. e) Contribution of the solar reflected flux to the total continuum intensity, also known as the solar contribution factor in this work. f) The expected surface pressure on Mars as modeled by the MCD. 207

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Fig. 4. The top left panel represents the estimated continuum intensity (W/sr/m2/μm) at the H2O setting at each slit position on Mars as computed by our model using the surface temperature across the disk of Mars as modeled by MCD, a general circulation model (Millour et al., 2008), at the time of each observation on 21 March 2016. In comparison, the top right panel represents the measured continuum intensity at CSHELL's detector. The bottom left panel represents the estimated continuum intensity at the HDO setting, shown against the measured intensity at CSHELL's detector as presented in the bottom right panel.

spectral features of telluric methane, water and ethane (C2H6). We synthesized Earth's atmospheric transmittance at the two frequency settings using the LBLRTM, before removing it from the CSHELL spectra, therefore revealing the residual spectra originating from Mars, and containing HDO, water and solar lines. The retrieved quantities in this work are the spectral resolving power, the abundances of the molecular species in Earth's atmosphere (CH4, CH2, H2O…), the solar contribution to the total continuum intensity from Mars, and the molecular abundances of atmospheric water vapor and HDO on Mars across each latitudinal range covered by CSHELL's slit at the different positions across the Martian disk.

as returned by MGS/TES. In particular, we used the values for dust and ice optical depths from the TES climatological database for MY 26, since it typically describes an average Martian seasonal cycle. We employed the water ice and dust optical constants by Wolff et al. (2009) as derived from observations returned by the Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer (MRO/CRISM).

3.1. Morphology of a CSHELL spectrum The CSHELL spectra include telluric signatures, solar Fraunhofer lines, and Martian atmospheric lines. The spectral range covered by CSHELL at the H2O setting extended between 2993.5 cm−1 and 3000.8 cm−1, and at the HDO setting between 2719.4 cm−1 and 2726.5 cm−1. Figs. 5 and 6 represent two typical Mars spectra as returned by CSHELL on March 21, 2016. Since the spatial resolution on Mars is limited by the seeing (0.63 arcsec), we binned the spectra over 3 pixels along the slit. In Fig. 5 at the HDO setting around 2720 cm−1, the telluric and Doppler-shifted HDO lines from Mars as well as the solar Fraunhofer lines are shown. Fig. 6 at the H2O setting around 2990 cm−1 show the Earth's atmospheric transmittance at the top, containing

3.2. Uncertainties This work shares similar data processing and retrieval assumptions with Novak et al. (2011), Villanueva et al. (2011, 2015)), and Khayat et al. (2017), therefore it is subjected to similar sources of systematic uncertainties. Major sources of such uncertainties include the errors in the spectral parameters of the molecular transitions such as their frequency positions and line broadening. This impacts synthesizing the Earth's atmospheric transmittance, the residual Martian spectrum and 208

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Fig. 5. The Martian residual spectra showing HDO spectral lines along the slit when sampling the north-south central meridian of Mars region on 21 March, 2016, are shown here. The spectra were taken in the observer's reference frame, when Mars is at a topocentric Doppler velocity of −15.6 km/s. Trace ‘a’ in red shows the observed spectrum of Mars, affected by the Earth's atmospheric transmittance and the solar spectrum. Trace ‘b’ shows the synthetic Earth's atmospheric transmittance. Trace ‘c’ in green shows the solar spectrum (multiplied by 10) containing two major Fraunhofer lines. Trace ‘d’ shows the residual spectra of Mars (multiplied by 7) across CSHELL's slit, after removing traces ‘b’ and ‘c’ from trace ‘a’ at each indicated latitude on Mars. The depth of the lines indicates the difference in airmasses and abundances across the N-S central meridian of Mars. Trace ‘d’ in blue represents the synthetic Martian model of HDO at each latitude, blue-shifted by 0.141 cm−1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.1. H2O results from using its 2ν2 spectral band around 2990 cm−1

more importantly the frequency calibration. For instance, a displacement 1 pixel (0.027 cm−1) in the frequency direction at the HDO setting will drop the signal-to-noise ratio (SNR) from 325 to 120 in the residual spectra. Following our methodology of nodding the telescope on the planet in the ABBA series, the secondary sources of uncertainties originating from the telescope, the sky background and other emissions, are removed to a first order in the Taylor series expansion. Weaker sources of systematic uncertainties arise from the improper straightening of the spectrum onto CSHELL's detector, but they were corrected to the milli-pixel accuracy. In addition, the spectral fringing and incoming scattered light were also removed (see Supporting material for Mumma et al. (2009)). After binning the spectra across 3 vertical pixels on CSHELL's detector, the maximum achieved SNR at the HDO frequency setting in the residual spectra at 2720 cm−1 is ~450, and the 1sigma uncertainty in the mixing ratio of HDO is 12 ppbv. With the same binning procedure across the detector, at the H2O setting around 2990 cm−1, the maximum achieved SNR is ~300 and the 1-sigma uncertainty in the mixing ratio H2O of is 15 ppmv.

Even though it is difficult to reliably obtain water vapor abundances on Mars when confronted with the Earth's atmosphere as compared to observations from orbiters around Mars, the dry weather conditions on Maunakea (low τ225GHz opacity = 0.05; 1.33 precipitable mm), combined with the high Doppler velocity of the planet, allowed water vapor abundance retrievals at local times on Mars at each slit position, as shown in Fig. 7. Each plot represents the latitudinal variation of the water vapor's retrieved mixing ratio (ppmv). The meridional position of each slit is indicated on top of each panel by the median value of the longitudes along the slit, with the vertical arrow indicating the latitude of the sub-solar point, and the longitude of the sub-solar point is noted on the upper left part of each plot in Fig. 7. The latitudinal variation of water vapor abundance reveals a general trend of increasing abundance between the southern and the northern hemisphere, with depleted abundances in the south reaching values < 20 ppmv, and higher values reaching 400 ppmv in the northern hemisphere, owing it to the increasing intensity of the solar insolation during northern summer. Due to the observing geometry of Mars, the sub-Earth point is located in the northern hemisphere at latitude 7°N, therefore our observations cover a larger fraction of the northern hemisphere and latitudes in the southern hemisphere northward of 40°S. The overall water vapor abundance peaks at regions between 25°N and 60°N, north of the sub-solar latitude. Atmospheric abundances of water vapor depend on the local temperatures on Mars, which respond to insolation and vary according to surface albedo, leading to local variations in the water vapor abundances by following

4. Results and discussion We have conducted 185 successful retrievals for water and HDO at each wavelength setting along 9 of CSHELL's slit positions across the Martian disk, after binning the spectra from 3 pixels in the spatial direction within the atmospheric seeing. Of the 370 retrievals, we reject results from observations taken at an airmass higher than 6 around the Martian limb. 209

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Fig. 6. The Martian residual spectra showing H2O spectral lines along the slit when sampling the north-south central meridian of Mars region on 21 March, 2016, are shown here. Same as in Fig. 5, the spectra were taken in the observer's reference frame, when Mars was at a topocentric Doppler velocity of −15.6 km/s. Trace ‘a’ in red shows the observed spectrum of Mars, affected by the Earth's atmospheric transmittance and the solar spectrum. Trace ‘b’ shows the synthetic Earth's atmospheric transmittance. Trace ‘c’ in green shows the solar spectrum (multiplied by 30) containing Fraunhofer lines. Trace ‘d’ shows the residual spectra of Mars (multiplied by 5) across CSHELL's slit, after removing traces ‘b’ and ‘c’ from trace ‘a’ at each indicated latitude on Mars. The depth of the lines indicates the difference in airmasses and abundances across the N-S central meridian of Mars. Trace ‘d’ in blue represents the synthetic Martian model of H2O at each latitude, blue-shifted by 0.156 cm−1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

taken on 22 March 2016 (see Fig. 2, middle panel), show that the Martian atmosphere is clear of water ice clouds above the regions sampled by CSHELL in slit positions 6 and 7. However, a water ice cloud is observed around Elysium Mons, which could explain the absence of the local peak in water vapor abundance in set 3. The primary differences between the CSHELL and TES water vapor results could be attributed to the difference in the spatial resolution, the Mars years when the observations were conducted, the operating wavelengths of the spectrometers, the assumptions in the radiative transfer retrieval algorithms, as well as the regions and the local times sampled by the two spectrometers. The advantage in using semi-simultaneous observations of H2O and HDO with CSHELL is to remove the systematics arising from the above-mentioned differences between TES and CSHELL.

the general behavior of increasing water vapor between the two hemispheres. All the observations were taken between one and 4 h (Local True Solar Time; LTST: 13:10 and 15:40) from local noon on Mars when the planet is moving around the sub-Earth point, showing little variations in the peak abundance (290 ppm–400 ppm). The abundance of water vapor retrieved from CSHELL observations presented in Fig. 7 can be directly compared against observations from Mars Global Surveyor (MGS/TES) Thermal Emission Spectrometer (Smith, 2004). TES is mounted on-board MGS, and it operated in the thermal infrared regime (6–50 μm; 200–1600 cm−1). MGS orbits Mars in a near-polar and sun-synchronous orbit, observing the planet near 0200 (nighttime) and 1400 (daytime) local times. The TES water results shown here are nearby averages of near-nadir geometry daytime observations for 3 Martian years, between MY 24 (1999) and MY 26 (2003). No major inter-annual variations in the TES water vapor abundances are observed away from the sub-solar latitude. However, at latitudes within ± 10° of the sub-solar one, moderate year-to-year differences ( ± 20%) between the different peaks in the TES water vapor abundances are observed. Overall, the latitudinal variations of water vapor from CSHELL and TES retrievals show similar trend. Some differences occur in retrievals around the sub-solar latitude. While TES results retrieval a local peak in water vapor abundance around such latitude, CSHELL does not. These two independent CSHELL retrievals of water vapor occur at the same longitude (245°W), and show no local peaks in the water vapor abundance around the latitude of the sub-solar point. Similarly, there is no local peak in water vapor abundance in CSHELL's slit position 3 at longitude 210°W and latitude 20°N. Careful examination MARCI images

4.2. HDO results from using its ν1 fundamental band around 2720 cm−1 Even-though telluric water vapor abundance above Maunakea could exceed that of Mars by two orders of magnitude, the HDO/H2O abundance ratio above Maunakea is depleted in comparison to the Martian atmosphere, making it easier to detect atmospheric HDO and measure its abundance on Mars using ground-based observations (Krasnopolsky, 2015). Fig. 8 represents the latitudinal variation of the HDO's mixing ratio at each of CSHELL slit position across Mars. Just like water vapor, its deuterated form shows increased abundance values in the northern hemisphere as compared to the southern one, where depleted values (< 10 ppbv) are observed southward in the polar direction. High 210

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Fig. 7. Each panel represents the latitudinal dependence of the water vapor's mixing ratio (ppmv) as retrieved by CSHELL, at each slit position (indicated by the set number) across the disk of Mars on March 21, 2016. The median longitude on the planet (°W) that is covered by the spectrometer's slit is indicated at the top of each panel. The set number, sub-solar longitude and the Local True Solar Time (LTST) on Mars at the time of the observation are shown at the upper left part inside the panel. The vertical arrow points out the sub-solar latitude on Mars. The 1-gima error bars are shown. In comparison against the CSHELL retrievals, the water vapor mixing ratio as returned by TES using nearby averages for the same season and longitude during MY 24, 25 and 26 is indicated in red, blue and green, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

same region on Mars. The results in Fig. 8 show strong variations of HDO with longitude and local times, which has direct implications on the zonal change in the D/H ratio. In comparison, by positioning CSHELL's slit along the north-south central meridian of Mars on 26 March 2008, Novak et al. (2011) obtained a maximum HDO abundance of 330 ppbv (8.2 × 1021 molecules/m2) at Ls = 50.1° around latitude 30°N at the morning side at LTST 09:40. Higher abundances of HDO are expected at mid-northern summer and in the afternoon on Mars when our observations were made. In order to divide the HDO by the H2O abundances and retrieve the D/H ratio as a function of latitude and longitude on Mars away from the sub-solar point, we selected slit positions where the HDO and H2O observations are perfectly aligned, and plotted the measured Mars radiances on CSHELL's detector against each other at both wavelength settings. Fig. 9 shows the latitudinal variation of the measured radiance (in units of counts) across CSHELL's detector (pixel numbers) on March 21, 2016. The radiances were normalized to that of HDO in order to allow comparison between the two curves. The ratio of the two abundances with respect to VSMOW is shown in Fig. 10, indicating significant enrichment of D/H compared to that on Earth. There is a strong variation with latitude, longitude and local

insolation in the northern latitudes during northern summer drives the surface and atmospheric heating, therefore leading to high abundances of HDO in the northern hemisphere. At each CSHELL's slit longitudinal position, the HDO mixing ratio reaches its peak abundance north of the sub-solar latitude, followed by a decrease towards the pole, except for slit position 2 at longitude 191°W, where no decline in the HDO abundance is observed. Unlike for water vapor, there are large variations in the peak abundance of HDO (170 ppbv–450 ppbv), reaching a factor of 2.6 between the maximum and the minimum of the peaks. With the exception of the region that is covered by CSHELL's slit position 0 at longitude 222°W, a prominent increase in HDO's peak abundance is observed to start at ~16:00 LTST (185°W) towards ~14:00 LTST (237°W) where it reaches a maximum of 450 ppbv, before gradually decreasing towards local noon. This could be attributed to the surface temperature on Mars, reaching its maximum at about ~14:00 LTST, with the two-hour delay from the maximum insolation occurring at local noon. Within the same atmospheric seeing, the geographic region around 235°W was sampled twice in positions 5 and 6 around LTST ~13:50, and the same sharp increase in the HDO abundance is observed in both positions, reaching about ~450 ppbv above 50°N. These high values of the HDO mixing ratio are reproduced when applying similar observation techniques and retrieval assumptions to the 211

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Fig. 8. Each panel represents the latitudinal dependence of the HDO's mixing ratio (ppbv) as retrieved by CSHELL, at each slit position (indicated by the set number) across the disk of Mars on March 21, 2016. The median longitude on the planet (°W) that is covered by the spectrometer's slit is indicated at the top of each panel. The set number, sub-solar longitude and the Local True Solar Time (LTST) on Mars at the time of the observation are shown at the upper left part inside the panel. The vertical arrow points out the sub-solar latitude on Mars.

Interestingly, Aoki et al. (2015) investigated the longitudinal distribution of D/H between 220° W and 360° W, and at local times between 10:00 LTST and 17:00 LTST, and found no strong dependence of D/H on local time.

times, owing it to the different sublimation/condensation temperatures of HDO and H2O in the Martian atmosphere. As compared to the northern hemisphere, generally lower values of D/H are observed in the south. In the northern hemisphere, a similar trend is observed at all slit positions, indicated by an increase in D/H, reaching a local peak north of the sub-solar latitude, then decreasing in the direction of the north polar cap, except for the regions around longitude 236°W. Slit positions at longitudes 200°W and 222°W cover Elysium Mons (latitudes 20 to 40°N), and show lower enrichment of D/H above Elysium as compared to low-altitude regions in the northern hemisphere. On the other hand, the slit position at longitude 253° W samples Utopia basin (latitude 30 to 55°N), where enriched D/H values are observed. The slit position at longitude 236° W reveals a D/H that is leveling off above the low-lands on Mars, where there are no major basins or Mons at such longitude. A similar behavior of D/H above basins, high-altitude and “flat” regions was also observed by Villanueva et al. (2015) during northern spring (see D/H map at Ls = 80°, Fig. 2 in Villanueva et al. (2015)). With the exception of the observation taken at longitude 222° W, the D/H values in the northern hemisphere show an increase with longitude between late and early afternoon towards the sub-solar longitude where the maximum solar insolation is reached. We therefore report a strong dependence of D/H on local time, mostly driven by the high HDO abundances towards local noon as observed in Fig. 8.

4.3. Comparison with previous results The strong variation of D/H with season, local time, and topography, show a D/H dependence on the climatology. In the same respect, the so-called representative D/H values are biased towards the regions that are sampled at each study. Our atmospheric water vapor and HDO maps taken during northern summer at Ls = 126° on March 21, 2016, show very low values of ~1 VSMOW at low latitudes and at the morning/afternoon terminators, with a significant increase to 6.1 ± 0.4 over the 40°N–60°N latitude range. Villanueva et al. (2015) retrieved a D/H value of 7 for the northern latitudes in which both istopologues were thought to be volatile. Novak et al. (2011) retrieved a D/H value of 6.9 ± 0.2 VSMOW near the sub-solar region, with a decrease to 3.8 ± 0.3 towards the polar regions where temperatures are low due to the preferential condensation of HDO. Large latitudinal variations in D/H were observed in Novak et al. (2011) as well, showing deviations by a factor of 1.32 from the D/H ratio as predicted by GCM models (Montmessin et al., 2005). It should be noted that Novak et al. (2011), Villanueva et al. (2015), and Khayat et al. (2017) share similar 212

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Fig. 9. Each panel represents the latitudinal variation of the measured radiance (in units of counts) from Mars at the HDO and H2O wavelength settings on March 21, 2016. The curve in black indicates the measured HDO radiance across CSHELL's detector (pixel numbers), and the curve in red indicates the measured H2O radiance, after normalizing it to its maximum value and multiplying it by the maximum value of the HDO radiance, to allow comparison between the two curves. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

an excess in water vapor abundances across Mars, reaching 600 ppmv in northern latitudes, therefore presenting a more enriched Mars atmosphere in water vapor, in comparison with the one observed by CSHELL and TES. The maximum water vapor abundances of 400 ppmv were here observed, in agreement with TES results from 3 Martian years. The same happens for HDO, where EXES is observing maximum HDO abundances over the northern latitudes that reached 800 ppbv, as compared to our CSHELL measurements of HDO that reached maximum values around 450 ppbv. The representative D/H indicated using EXES is 4.0 + 0.8, −0.6. The D/H values presented here are consistent with the ones reported by Encrenaz et al. (2018, 2016), Aoki et al. (2015) and Krasnopolsky (2015), but are lower than the ones reported by Novak et al. (2011) and Villanueva et al. (2015). Table 1 lists the most notable D/H results, showing a great range of variability within observations, local times, and seasons on Mars.

data processing and radiative transfer modeling techniques with the current study. Krasnopolsky (2015) also used IRTF/CSHELL to measure the H2O and HDO abundances along the central meridian of Mars and presented single longitude D/H results for 6 seasons on Mars. When having difficulties in measuring water vapor abundances on Mars at the time where telluric water exceeded 2 pr mm above Maunakea, the water vapor abundances were incorporated from averaged TES results. The D/ H ratios were rather latitudinally constant in most observations, with a mean value of 4.6 ± 0.7, and D/H values reaching 6 and 7 around the north polar cap. Using high-dispersion echelle spectroscopy, Aoki et al. (2015) conducted observations of Mars with the Infrared Camera and Spectrograph (IRCS) on the 8.2-m Subaru telescope, and reported a latitudinal mean D/H ratio of 4.1 ± 1.4 during mid-northern spring (Ls = 52°), and 4.4 ± 1.0 during early northern summer (Ls = 96°). Large seasonal variations at high latitudes were observed, with a significant increase from 2.4 ± 0.6 to 5.5 ± 1.1 over the 70°N – 80°N latitude range between Ls = 52° and Ls = 96°. In contrast to our retrievals, no significant variation in the D/H over different local times and longitudes was reported. More water vapor and HDO measurements were conducted in the thermal infrared range at 1383–1391 cm−1 (7.2 μm) by EXES aboard SOFIA (Encrenaz et al., 2018) on 24 March 2016 (Ls = 127°), around the same time the CSHELL observations in this study were made. Even though no strong local variations in the D/H ratio on Mars were found within the large field-of-view of EXES (3 arcsec), significant deviations between CSHELL and EXES are observed. EXES water vapor maps show

5. Summary The spatially-resolved D/H ratio on Mars is an important metric that helps monitor the sources and sinks of water vapor on the planet, and track the history of water loss from the planet. In that respect, we observed the 10.63 arcsec disk of Mars during its northern summer on March 21, 2016, at MY 33 at Ls = 126° using CSHELL at the IRTF on top of Maunakea, Hawaii. We targeted the 2ν2 spectral band of H2O around 2990 cm−1 (3.3 μm), its deuterated form HDO at its ν1 fundamental band around 2720 cm−1 (3.7 μm), and took advantage of CSHELL's entrance slit at its narrowest width to achieve the maximum spectral resolving power of ~40,000. 213

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Fig. 10. Each panel represents the latitudinal dependence of the D/H ratio with respect to VSMOW at different longitudes on Mars away from the sub-solar point. The black dots indicate the D/H using simultaneous observations of HDO and H2O from CSHELL. The vertical arrow indicates the sub-solar latitude. The average Local True Solar Time (LTST) on Mars at the time of the HDO and H2O observations from which the D/H plot is derived are shown at the upper left part inside the panel.

Table 1 This table represents the most notable D/H results as obtained from ground-based and airborne observatories, as well as in-situ. The different wavelength ranges, field of views, seasons when H2O and HDO were targeted, and the notes on the variations of D/H, are presented here. The global D/H results track the evolution of D/H, whereas the spatially-resolved ones track the climatology of the D/H. Authors

Method/wavelength range/FOV

Season/Ls

D/H

Owen et al. (1988)

Late southern summer/Ls = 316°

Global; 6 ± 3 VSMOW

Mid southern spring/Ls = 246°

Global; 5.2 ± 0.2 SMOW

Early northern summer/Ls = 100°

Global; calculated D/H ~6 (+6, −3) from HDO and assumed H2O abundance Localized; 6 ± 1 VSMOW

Villanueva et al. (2015)

Ground based, HDO ~ 2722 cm−1, H2O ~ 9090 cm−1, FOV = 5 arcsec Airborne observatory, HDO and H2O simultaneously ~2.65 μm Ground based, HDO at 226 GHz, FOV = 10.5 arcsec In-situ, Mars Science Laboratory, HDO and H2O ~ 3594 cm−1 Ground based, HDO ~ 2720 cm−1 H2O ~ 2994 cm−1, FOV ~ 0.6 arcsec

Khayat et al. (this work)

Ground based, HDO ~ 2720 cm−1, H2O ~ 2994 cm−1, FOV ~ 0.65 arcsec

Mid northern summer/Ls = 126°

Krasnopolsky (2015)

Ground based, HDO ~ 2722 cm−1, H2O ~ 2994 cm−1 and from TES, FOV ~ 1 arcsec Ground based, HDO ~ 2672 cm−1, H2O ~ 3035 and 3216 cm−1, FOV ~ 0.8 and 0.5 arcsec Airborne observatory, thermal 1383–1390 cm−1, FOV = 3 arcsec Airborne observatory, thermal 1383–1390 cm−1, FOV = 3 arcsec

Ls = 110°, 60°, 42°, 145°, 70°, 20°

Bjoraker et al. (1989) Encrenaz et al. (1991, 2001) Webster et al. (2013)

Aoki et al. (2015)

Encrenaz et al. (2016) Encrenaz et al. (2018)

Average sol 28–106/Sept–Nov 2012 Northern winter and spring/Ls = 335°, 50°, 83°, 80°

Mid and late northern spring/Ls = 52°, 96° Early northern summer/Ls = 113° Northern summer and winter/Ls = 127°,304°

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Strong variation with topography and season. D/H ~1–3 VSMOW in the southern hemisphere, representative D/H ~ 7 in the northern hemisphere where both isotopologues are fully volatile Very low values of ~1 VSMOW at low latitudes and at the morning/afternoon terminators, with a significant increase to 6.1 ± 0.4 over the 40°N–60°N latitude range Mean D/H of 6 seasons: 4.6 ± 0.7, strong variations with latitude and season D/H ratio significantly increases from 2.4 ± 0.6 VSMOW at Ls = 52° to 5.5 ± 1.1 VSMOW at Ls = 96° over the 70°N–80°N latitude range. D/H enhancement from southern to northern latitudes, ranging from 3.5 to 6 VSMOW Slight increase in D/H towards northern latitudes, ranging between 2 and 6 VSMOW/stable between 4 and 5 VSMOW

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The latitudinal variation of water vapor abundance shows a general increase from the southern to the northern hemisphere, with depleted abundances reaching < 20 ppmv in the south, and higher values reaching 400 ppmv in the north. At each meridional observation, the water vapor abundance peaks north of the sub-solar latitude between 25°N and 60°N, and in some occasions it decreases poleward. When compared against MGS/TES retrievals averaged over 3 Mars years between MY 24 (1999) and MY 26 (2003), our observations show a similar trend, but minor differences occur when some TES retrievals reveal the presence of a local peak in water vapor abundance around the sub-solar latitude at some longitudes, a peak that is not found in the CSHELL observations. Just like water vapor, HDO shows increased abundances in the northern hemisphere as compared to the southern one, where depleted values (< 10 ppbv) are observed. The HDO mixing ratio reaches its peak abundance north of the sub-solar latitude in every longitudinal observation, followed by a decrease towards the pole. Unlike for water vapor, there are large variations in the peak abundances of HDO (170 ppbv–450 ppbv), reaching a maximum of 2.6 times between two peaks. A prominent increase in HDO's peak abundance is observed to begin at ~16:00 LTST towards ~14:00 LTST, before gradually decreasing towards local noon. The resulting D/H measurements indicate a significant enrichment over the terrestrial value, showing a strong variation with latitude, longitude and local times. The northern hemisphere is exhibiting an increase in D/H, reaching a local peak north of the sub-solar latitude, before decreasing in the direction of the north polar cap, except for regions around longitude 236°W. The observations show lower enrichment of D/H above Elysium Mons compared to low-altitude regions in the northern hemisphere. On the other hand, the observations indicate enriched D/H values above Utopia basin. D/H shows low variation over “flat” regions in the low-lands of the northern hemisphere of Mars where no major basins or Mons are present. Our Mars atmospheric water vapor and HDO maps taken during northern summer indicate very low values of ~1 VSMOW at low latitudes and at the morning/ afternoon terminators, with a significant increase to 6.1 ± 0.4 over the 40°N–60°N latitude range. More comprehensive D/H measurements for Mars at different seasons and locations including the polar regions are underway with the NOMAD instrument on-board the Trace Gas Orbiter (e.g., Korablev et al., 2014; Neefs et al., 2015; Vandaele et al., 2015; Patel et al., 2017). These observations take advantage of the unprecedented spectral resolving power of an instrument on a Mars orbiter. They also present a major advantage over ground-based observations that are weather sensitive and suffer from the Earth's telluric contamination.

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