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The Lunar Exploration Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter (LRO) detects a widespread suppression of the epithermal neutron leakage flux that is coincident with the pole-facing slopes (PFS) of the Moon’s southern hemisphere. Suppression of the epithermal neutron flux is consistent with an interpretation of enhanced concentrations of hydrogen-bearing volatiles within the upper meter of the regolith. Localized flux suppression in PFS suggests that the reduced solar irradiation and lowered temperature on PFS constrains volatility to a greater extent than in surrounding regions. Epithermal neutron flux mapped with LEND’s Collimated Sensor for Epithermal Neutrons (CSETN) was analyzed as a function of slope geomorphology derived from the Lunar Orbiting Laser Altimeter (LOLA) and the results compared to co-registered maps of diurnally averaged temperature from the Diviner Lunar Radiometer Experiment and an averaged illumination map derived from LOLA. The suppression in the average south polar epithermal neutron flux on equator-facing slopes (EFS) and PFS (85–90°S) is 3.3 ± 0.04% and 4.3 ± 0.05% respectively (one-sigma-uncertainties), relative to the average count-rate in the latitude band 45–90°S. The discrepancy of 1.0 ± 0.06% between EFS and PFS neutron flux corresponds to an average of ∼23 parts-per-million-by-weight (ppmw) more hydrogen on PFS than on EFS. Results show that the detection of hydrogen concentrations on PFS is dependent on their spatial scale. Epithermal flux suppression on large scale PFS was found to be enhanced to 5.2 ± 0.13%, a discrepancy of ∼45 ppmw hydrogen relative to equivalent EFS. Enhanced poleward hydration of PFS begins between 50°S and 60°S latitude. Polar regolith temperature contrasts do not explain the suppression of epithermal neutrons on pole-facing slopes. The Supplemental on-line materials include supporting results derived from the uncollimated Lunar Prospector Neutron Spectrometer and the LEND Sensor for Epithermal Neutrons.

The requirements-driven OSIRIS-REx Camera Suite (OCAMS) acquires images essential to collecting a sample from the surface of Bennu. During proximity operations, these images document the presence of satellites and plumes, record spin state, enable an accurate digital terrain model of the shape of the asteroid and identify any surface hazards. They confirm the presence of sampleable regolith on the surface, observe the sampling event itself, and image the sample head in order to verify its readiness to be stowed. They document the history of Bennu as an example of early solar system material, as a microgravity body with a planetesimal size-scale, and as a carbonaceous object. OCAMS is fitted with three cameras. The MapCam records point-source color images on approach to the asteroid in order to connect ground-based point-source observations of Bennu to later higher-resolution surface spectral imaging. The SamCam documents the sample site before, during, and after it is disturbed by the sample mechanism. The PolyCam, using its focus mechanism, observes the sample site at sub-centimeter resolutions, revealing surface texture and morphology. While their imaging requirements divide naturally between the three cameras, they preserve a strong degree of functional overlap. OCAMS and the other spacecraft instruments allow the OSIRIS-REx mission to collect a sample from a microgravity body on the same visit during which it was first optically acquired from long range, a useful capability as humanity explores near-Earth, Main-Belt and Jupiter Trojan asteroids.

Data collected by Phoenix Lander’s Thermal and Evolved Gas Analyzer (Phoenix-TEGA) indicate carbonate thermal decomposition at both low and high temperatures. The high-temperature thermal decomposition is consistent with calcite, dolomite, or ankerite, (3–6 wt.%) or any combination of these phase or, presumably, solid solutions of these phases having intermediate composition. The low-temperature thermal decomposition is consistent with the presence of magnesite or siderite, their solid solutions, or any combination of magnesite and siderite, and possibly other carbon-bearing phases (e.g., organics). The carbonate concentration for the low temperature release, assuming magnesite–siderite, is ∼1.0 wt.%. This revised interpretation of the Phoenix-TEGA data resulted from new laboratory measurements of carbonate decomposition at a Phoenix-like 12 mbar atmospheric pressure. Phoenix carbonate was inherited in ejecta from the Vastitas Borealis and Scandia regions, inherited from material deposited by aeolian processes, and/or formed in situ at the Phoenix Landing site (pedogenesis). Inherited carbonate implies multiple formation pathways may be represented by carbonates at the Phoenix Landing site. Soil carbonates and associated moderate alkalinity indicate that the soil pH is favorable for microbial activity at the Phoenix Landing site and presumably throughout the martian northern plains.

During its three flybys of Mercury, the MESSENGER spacecraft made the first detection of gamma-ray emission from the planet's surface. With a closest approach distance of ∼200 km, the flybys provided an opportunity to measure elemental abundances of Mercury's near-equatorial regions, which will not be visited at low altitude during MESSENGER's orbital mission phase. Despite being limited by low planetary photon flux, sufficient counts were accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong gamma-ray spectral peaks, including Si, Fe, Ti, K, and Th. Only for Si is the standard deviation σ sufficiently small to conclude that this element was detected with 99% confidence. Iron and potassium are detected at the 2− σ (95% confidence) level, whereas only upper bounds on Ti and Th can be determined. Relative to a Si abundance assumed to be 18 weight percent (wt%), 2− σ upper bounds have been estimated as 9.7 wt% for Fe, 7.0 wt% for Ti, 0.087 wt% for K, and 2.2 ppm for Th. The relatively low upper bound on K rules out some previously suggested models for surface composition for the regions sampled. Upper bounds on Fe/Si and Ti/Si ratios are generally consistent with Ti and Fe abundances estimated from the analysis of measurements by the MESSENGER Neutron Spectrometer during the flybys but are also permissive of much lower concentrations.

Phoenix Ascending The Phoenix mission landed on Mars in March 2008 with the goal of studying the ice-rich soil of the planet's northern arctic region. Phoenix included a robotic arm, with a camera attached to it, with the capacity to excavate through the soil to the ice layer beneath it, scoop up soil and water ice samples, and deliver them to a combination of other instruments—including a wet chemistry lab and a high-temperature oven combined with a mass spectrometer—for chemical and geological analysis. Using this setup, Smith et al. (p. 58 ) found a layer of ice at depths of 5 to 15 centimeters, Boynton et al. (p. 61 ) found evidence for the presence of calcium carbonate in the soil, and Hecht et al. (p. 64 ) found that most of the soluble chlorine at the surface is in the form of perchlorate. Together these results suggest that the soil at the Phoenix landing site must have suffered alteration through the action of liquid water in geologically the recent past. The analysis revealed an alkaline environment, in contrast to that found by the Mars Exploration Rovers, indicating that many different environments have existed on Mars. Phoenix also carried a lidar, an instrument that sends laser light upward into the atmosphere and detects the light scattered back by clouds and dust. An analysis of the data by Whiteway et al. (p. 68 ) showed that clouds of ice crystals that precipitated back to the surface formed on a daily basis, providing a mechanism to place ice at the surface.

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H2O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.

We present the results of five-year observations of the southern seasonal cap of Mars based on neutron spectroscopy of the surface fulfilled by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. The numerical modeling of the observational data allowed us to reconstruct the curves of the variations of the total mass of the southern seasonal cap of Mars for different years (three Martian years) and to find the year-to-year variations of the seasonal cycle.

The Mars Odyssey spacecraft carries two experiments which are capable of detecting cosmic gamma-ray bursts and soft gamma repeaters. Since April 2001 they have detected over 275 bursts and, in conjunction with the other spacecraft of the interplanetary network, localized many of them rapidly and precisely enough to allow sensitive multi-wavelength counterpart searches. We present the Mars Odyssey mission and describe the burst capabilities of the two experiments in detail. We explain how the spacecraft timing and ephemeris have been verified in-flight using bursts from objects whose precise positions are known by other means. Finally, we show several examples of localizations and discuss future plans for the Odyssey mission and the network as a whole., 22 pages, 10 figures. To be submitted to the Astrophysical Journal Supplement Series

In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water–ice in subsurface layers and about peculiarities of the CO2 seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water–ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10–20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO2 deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3 × 10 15 kg , which is larger than the total mass of the seasonal deposition at the north by 40–50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO2 occurred 10–15 degrees of L s later than is predicted by the GCM.

In this paper, we have analyzed neutron spectroscopy data gathered by the high energy neutron detector instrument onboard Mars Odyssey in order to study the behavior of CO 2 condensed mass at the polar regions of Mars. It is known that observation of the neutron albedo of Mars provides not only information on distribution of water ice/bounded water through Martian subsurface but is also sensitive to the condensation/sublimation of atmospheric CO 2 at the Martian poles. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the creation of multidimensional model of CO 2 deposition describing behavior of CO 2 condensed mass as a function of longitude, latitude and time. Based on this model, we have estimated the total masses of north seasonal cap ((3.8 ± 0.4) × 10 15 kg), south seasonal cap ((6.3 ± 0.6) × 10 15 kg) and compared our estimations with predictions of General Circulation Model. Furthermore, using the data gathered during second Martian year on the orbit, we have conducted preliminary comparison between seasonal cycles of two successive Martian years.

We present the results of 20 months of observations of Mars by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. We show that there are two extended subpolar regions with a soil water content of several tens of percent in the northern and southern hemispheres of Mars. The southern subpolar region is well described by a two-layer model, according to which a soil with a water content of up to 55% by mass lies under a relatively dry soil with a water mass fraction of 2% and a thickness of 15–20 g/cm2. The distribution of water in Martian regolith northern subpolar region is in good agreement with the homogeneous model and does not require invoking the more complex two-layer soil model. The water-ice content in the subsurface layer of the northern subpolar region reaches 53 % by mass. We show that there are two regions with a relatively high water content near the Martian equator. These are Arabia Terra and the Medusae Fossae formation region southwest of Olympus Mons. In these regions, a lower layer with 9–10% of water by mass may underlie the upper layer of relatively dry material ∼30 g/cm2 in thickness. The “moistest” spot near the equator is at about 30° E and 10° N. Its lower-layer soil may contain more than 16% of water by mass.

We present the results of eighteen months of observations of the seasonal caps of Mars based on data from the neutron spectroscopy of the surface by the Russian HEND Instrument mounted aboard the NASA 2001 Mars Odyssey spacecraft. A four-dimensional model of the Martian seasonal caps was developed on the basis of these observation data. The model shows how the thickness of the frozen carbon dioxide changes in different surface regions. Using the results of the model, we estimated the total mass of the seasonal caps for the period of maximal accumulation of seasonal depositions and the rates of condensation and sublimation of the seasonal cover.

Observations of seasonal variations of neutron flux from the high-energy neutron detector (HEND) on Mars Odyssey combined with direct measurements of the thickness of condensed carbon dioxide by the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor show a latitudinal dependence of northern winter deposition of carbon dioxide. The observations are also consistent with a shallow substrate consisting of a layer with water ice overlain by a layer of drier soil. The lower ice-rich layer contains between 50 and 75 weight % water, indicating that the shallow subsurface at northern polar latitudes on Mars is even more water rich than that in the south.

We present the first results of the global neutron mapping of Mars by the Russian High-Energy Neutron Detector (HEND) onboard the US 2001 Mars Odyssey spacecraft. Global neutron maps of Mars in various spectral ranges allow the content of water ice and adsorbed and bound water in a near-surface layer of the planet 1 to 2 m in thickness to be estimated. Huge regions of permafrost with a high (several tens of percent by weight) content of water ice are shown to be present in the north and the south of Mars. The continuous observations of Mars for 12 months, from February 18, 2002, through February 8, 2003, are indicative of significant seasonal variations on Mars where the transition from northern winter to northern summer occurred.

Using the Gamma-Ray Spectrometer on the Mars Odyssey, we have identified two regions near the poles that are enriched in hydrogen. The data indicate the presence of a subsurface layer enriched in hydrogen overlain by a hydrogen-poor layer. The thickness of the upper layer decreases with decreasing distance to the pole, ranging from a column density of about 150 grams per square centimeter at –42° latitude to about 40 grams per square centimeter at –77°. The hydrogen-rich regions correlate with regions of predicted ice stability. We suggest that the host of the hydrogen in the subsurface layer is ice, which constitutes 35 ± 15% of the layer by weight.

After 55 days of mapping by the High Energy Neutron Detector onboard Mars Odyssey, we found deficits of high-energy neutrons in the southern highlands and northern lowlands of Mars. These deficits indicate that hydrogen is concentrated in the subsurface. Modeling suggests that water ice–rich layers that are tens of centimeters in thickness provide one possible fit to the data.

The MESSENGER Gamma-Ray Spectrometer measured the average surface abundances of the radioactive elements potassium (K, 1150 ± 220 parts per million), thorium (Th, 220 ± 60 parts per billion), and uranium (U, 90 ± 20 parts per billion) in Mercury's northern hemisphere. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursor materials, and supports formation from volatile-containing material comparable to chondritic meteorites. Abundances of K, Th, and U indicate that internal heat production has declined substantially since Mercury's formation, consistent with widespread volcanism shortly after the end of late heavy bombardment 3.8 billion years ago and limited, isolated volcanic activity since.

[1] A new technique has been developed for characterizing gamma-ray emission from a planetary surface in the presence of large background signals generated in a spacecraft. This technique is applied to the analysis of Al gamma rays measured by the MESSENGER Gamma-Ray Spectrometer to determine the abundance of Al on the surface of Mercury. The result (Al/Si = 0.29� 0.13 +0.05 ) is consistent with Al/Si ratios derived from the MESSENGER X-Ray Spectrometer and confirms the finding of low Al abundances. The measured abundance rules out a global, lunar-like feldspar-rich crust and is consistent with previously suggested analogs for surface material on Mercury, including terrestrial komatiites, low-iron basalts, partial melts of CB chondrites, and partial melts of enstatite chondrites. Additional applications of this technique include the measurement of other elements on Mercury’s surface as well as the analysis of data from other planetary gamma-ray spectrometer experiments.

[1] Orbital gamma-ray measurements obtained by the MESSENGER spacecraft have been analyzed to determine the abundances of the major elements Al, Ca, S, Fe, and Na on the surface of Mercury. The Si abundance was determined and used to normalize those of the other reported elements. The Na analysis provides the first abundance estimate of 2.9 ± 0.1 wt% for this element on Mercury's surface. The other elemental results (S/Si = 0.092 ± 0.015, Ca/Si = 0.24 ± 0.05, and Fe/Si = 0.077 ± 0.013) are consistent with those previously obtained by the MESSENGER X-Ray Spectrometer, including the high sulfur and low iron abundances. Because of different sampling depths for the two techniques, this agreement indicates that Mercury's regolith is, on average, homogenous to a depth of tens of centimeters. The elemental results from gamma-ray and X-ray spectrometry are most consistent with petrologic models suggesting that Mercury's surface is dominated by Mg-rich silicates. We also compare the results with those obtained during the MESSENGER flybys and with ground-based observations of Mercury's surface and exosphere.

[1] More than 3 Mars' years (MY) of atmospheric argon (Ar) measurements are used to study annual and seasonal variations in atmospheric transport and mixing. Data are obtained over the period 20 May 2002 to 4 May 2008 by the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) on the Mars Odyssey spacecraft in orbit around Mars. Here we augment previous studies of Mars' Ar in which strong seasonal variations were observed and horizontal meridional mixing coefficients for the southern hemisphere were computed. Comparison of year-to-year seasonal abundance shows strong similarity but also some short-period (∼15°–30° Ls) and interannual variations. Evidence for short periods of strong eddy transport is exhibited during autumn and winter. The seasonal change in Ar concentration for southern latitudes is relatively gradual and well defined, but seasonal changes at high northern latitudes are chaotic and indicate that atmospheric disturbance is ubiquitous. Major topographic landforms (Elysium, Tharsis, Noachis Terra, Hellas) apparently have little control over seasonal Ar concentration at the spatial resolution of the GRS data set. Some indication of local enhanced Ar concentration is present from 30°N to 60°N for the Hellas and Tharsis sectors in late winter and early spring. The data show some significant (3σ) differences between MY 26 and MY 27 in geographical sectors that are likely produced by local weather. The GS data do not show seasonal variation of Ar at equatorial and low-latitude zones, in contrast to those from the Alpha Particle X-ray Spectrometer (APXS) measurements from the Mars Exploration Rovers.

X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets. Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust. The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles. Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance.

[1] We report maps of the concentrations of H, Si, Cl, K, Fe, and Th as determined by the Gamma Ray Spectrometer (GRS) on board the 2001 Mars Odyssey Mission for ±∼45° latitudes. The procedures by which the spectra are processed to yield quantitative concentrations are described in detail. The concentrations of elements determined over the locations of the various Mars landers generally agree well with the lander values except for Fe, although the mean of the GRS Fe data agrees well with that of Martian meteorites. The water-equivalent concentration of hydrogen by mass varies from about 1.5% to 7.5% (by mass) with the most enriched areas being near Apollinaris Patera and Arabia Terra. Cl shows a distribution similar to H over the surface except that the Cl content over Medusae Fossae is much greater than elsewhere. The map of Fe shows enrichment in the northern lowlands versus the southern highlands. Silicon shows only very modest variation over the surface with mass fractions ranging from 19% to 22% over most of the planet, though a significant depletion in Si is noted in a region west of Tharsis Montes and Olympus Mons where the Si content is as low as 18%. K and Th show a very similar pattern with depletions associated with young volcanic deposits and enrichments associated with the TES Surface Type-2 material. It is noted that there appears to be no evidence of significant globally distributed thick dust deposits of uniform composition.

[1] To elucidate the nature of permafrost in the shallow subsurface of Mars, we analyze jointly neutron albedo from the High Energy Neutron Detector (HEND) on the Mars Odyssey spacecraft and near-IR (1064-nm) surface reflectance from the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor. The first dataset measures the content of hydrogen (in the form of water or hydroxyl) in the soil, and the second yields the flux of absorbed solar energy by the surface. We identify a statistically–significant negative cross-correlation between these data at latitudes poleward of 40° latitude in the northern hemisphere and in the latitude band 40°–60° in the southern hemisphere, which we interpret as evidence for the presence of stable water ice under a dry equilibrium top layer (ETL). We deduce an empirical relation between near-IR reflectance and thickness of this ETL, which allows the burial depth of the water ice table to be estimated with km-scale spatial resolution. We observe no correlation between neutron and near-IR albedo within the southern hemisphere poleward of 60° latitude. While it is known from previously analyzed neutron and gamma-ray data that subsurface water ice is present within this region and is covered by a layer of dry regolith, the absence of a correlation indicates that the thickness of this layer is not controlled by an equilibrium condition between the ice table and atmosphere.

[1] In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument on board Mars Odyssey for two Martian years of orbital mapping. We focused on the observations of the Martian southern seasonal cap for this period of time. Nuclear methods based on numerical modeling allow us to derive an estimation of the distribution of CO2 frost column density and mass from the temporal variations of neutron flux between summer and winter seasons. We have estimated the seasonal behavior of both the column density and mass of CO2 frost. Comparison between two Martian years revealed minor (

[1] Quantifying secular variations in the chemical composition of the Martian crust provides unique insights into the processes that have guided the evolution of the Martian crust-mantle system. Using global abundances for a suite of elements determined by the Gamma Ray Spectrometer (GRS) on board the Mars Odyssey spacecraft and global mapping of apparent surface age adapted from existing geologic maps in the USGS Martian Geologic Investigation series, we report the average abundance of K, Th, Fe, Cl, H, and Si for the major Martian geologic epochs (Noachian, Hesperian, and Amazonian). Average GRS-determined K and Th abundances generally decrease by 9% and 7%, respectively, between the Hesperian and the Amazonian, possibly implying evolving magma chemistry throughout major resurfacing events (although the effects of surficial alteration processes cannot be entirely discounted). GRS-determined Fe and Cl averages increase by 12% and 19%, respectively, with younger apparent relative surface age, suggesting the possible mobilization and transport of these elements through aqueous processes (although an igneous origin for the variation in Fe also cannot be excluded). While H abundance does vary with surface age, the relationship is likely not governed by geologic processes. No statistically reliable apparent surface age relation was found for Si.

[1] K/Th determined by the Mars Odyssey Gamma Ray Spectrometer varies by a factor of 3 on Mars (3000 to 9000), but over 95% of the surface area has K/Th between 4000 and 7000. K/Th is distinctly lower than average in some areas, including west of Olympus Mons in the Amazonis Planitia, the region around Memnonia Fossae, Chryse Planitia, southeastern Arabia Terra, Syrtis Major Planum, and northwest of Apollinaris Patera. On the other hand, K/Th is distinctly higher than average in other areas, including the central part of Valles Marineris and the surrounding highlands, and in the northern part of Hellas. The generally modest variation in K/Th may be explained by inherent variations in igneous rocks and by variations in the extent of aqueous alteration.

[1] The 2001 Mars Odyssey Gamma Ray Spectrometer (GRS) has made the first measurement of the equatorial and midlatitude distribution of Cl at the near-surface of Mars. A mean concentration value of 0.49 wt% Cl has been determined from a grand sum of GRS spectra collected over the planet excluding high-latitude regions. Cl is significantly enriched within the upper few tens of centimeters of the surface relative to the Martian meteorites and estimates for the bulk composition of the planet. However, Cl is not homogeneously distributed and varies by a factor of ∼4 even after smoothing of data with a 10°-arc-radius filter. Several contiguous, geographically large (>20°) regions of high and low Cl concentrations are present. In particular, a region centered over the Medusae Fossae Formation west of Tharsis shows significantly elevated Cl. A large region north of Syrtis Major extending into Utopia Planitia in the northern hemisphere shows the lowest Cl concentrations. On the basis of hierarchical multivariate correlations, Cl is positively associated with H while negatively associated with Si and thermal inertia. We discuss four possible geologic mechanisms (aeolian, volcanic, aqueous, and hydrothermal) that may have affected the Cl distribution seen by GRS. While some of the distribution may be due to Cl-rich dust deposits transported by aeolian processes, this mechanism does not appear to account for all of the observed variability. We propose that reactions with volcanic exhalations may have been important for enriching Cl in Medusae Fossae Formation material.

[1] Conclusions are drawn about the column density (g/cm2), spatial extent, and mass of the seasonal carbon dioxide frost on the poles of Mars as a function of time utilizing data from the 2001 Mars Odyssey Gamma Ray Spectrometer (GRS). Quantification of these CO2 values is achieved by observing attenuation effects of the surface-emitted hydrogen gamma ray flux as the frost condenses and sublimates in a seasonal exchange of CO2 between the ground and the atmosphere. Columnar thickness and mass results are discussed and plotted for latitudes including ±60° and poleward. GRS observations are compared to predictions from the NASA Ames Research Center General Circulation Model and to similar experimental results from the Mars Odyssey High Energy Neutron Detector and Neutron Spectrometer. Models for north and south polar atmosphere and regolith distributions are incorporated, and our results indicate that the assumption of a 100% H2O-ice residual cap underlying the seasonal frost in the north is accurate. The GRS CO2 frost observations are in good agreement with the other studies mentioned, in particular for the timing of the beginning of frost deposition to the complete sublimation of surface CO2 back into the atmosphere. The total amount of condensed carbon dioxide mass seen by the GRS is on the order of 6.0 × 1015 kg and verifies previous reports that nearly 25% of the Martian CO2 reservoir participates in the ground-atmosphere exchange cycle.

[1] The thickness of seasonal CO2 frost that covers the north pole of Mars during the winter and spring seasons is studied using gamma ray and neutron data measured using the gamma ray spectrometer suite of instruments aboard Mars Odyssey. The resultant seasonal variation of CO2 ice/frost thickness for latitudes greater than +85° is in close agreement (within 9% at maximum) with those predicted using all three major Mars general circulation models presently in use. These observed CO2 ice results are significantly different from those inferred from the Mars Obiter Laser Altimeter experiment aboard Mars Global Surveyor (MGS). A possible explanation is that the density of the CO2 ice/frost cap is considerably lower than that inferred from observed variations of the MGS orbit. Other significant results are that the thermal neutron counting rates provide evidence for variations in the abundance of noncondensable components of the polar atmosphere of Mars and that the basement terrain below the seasonal cover of CO2 frost within about 10° of the pole is nearly 100% water ice.

The Mars Odyssey Gamma-Ray Spectrometer is a suite of three different instruments, a gamma subsystem (GS), a neutron spectrometer, and a high-energy neutron detector, working together to collect data that will permit the mapping of elemental concentrations on the surface of Mars. The instruments are complimentary in that the neutron instruments have greater sensitivity to low amounts of hydrogen, but their signals saturate as the hydrogen content gets high. The hydrogen signal in the GS, on the other hand, does not saturate at high hydrogen contents and is sensitive to small differences in hydrogen content even when hydrogen is very abundant. The hydrogen signal in the neutron instruments and the GS have a different dependence on depth, and thus by combining both data sets we can infer not only the amount of hydrogen, but constrain its distribution with depth. In addition to hydrogen, the GS determines the abundances of several other elements. The instruments, the basis of the technique, and the data processing requirements are described as are some expected applications of the data to scientific problems.

The Thermal and Evolved Gas Analyzer (TEGA) on the Mars Polar Lander spacecraft is composed of two separate components which are closely coupled: a Differential Scanning Calorimeter (DSC) and an Evolved Gas Analyzer (EGA). TEGA has the capability of performing differential scanning calorimetry on eight small (0.038 mL) soil samples selected in the vicinity of the lander. The samples will be heated in ovens to temperatures up to 950°C, and the volatile compounds water and carbon dioxide, which are released during the heating, will be analyzed in the EGA. The power required by the sample oven is continuously monitored during the heating and compared to that required to heat simultaneously a similar, but empty, oven. The power difference is the output of the DSC. Both endothermic and exothermic phase transitions can be detected, and the data can be used in the identification of the phases present. By correlating the gas release with the calorimetry, the abundance of the volatile compounds associated with the different phases can be determined. The EGA may also be able to detect the release of oxygen associated with any superoxide that may be on the surface of the soil grains. The instrument can detect the melting of ice in the DSC down to abundances on the order of 0.2% of the sample, and it can detect the decomposition of calcite, CaCO3, down to abundances of 0.5%. Using the EGA, TEGA can detect small amounts of water, down to 8 ppm in the sample, and it can detect the associated release of CO2 down to the equivalent abundances of 0.03%. The EGA also has the ability to determine the 13C/12C ratio in the evolved CO2, but it is not clear if the accuracy of this ratio will be sufficient to address the scientific issues.