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 degrees latitude to about 40 grams per square centimeter at -77 degrees. 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.

/pdf/distribution-of-hydrogen-in-the-near-surface-of-mars-zbal35h4tw.pdf

Distribution of Hydrogen in the Near-Surface of Mars: Evidence for Subsurface Ice Deposits

The presence and stability of ground ice in the southern hemisphere of Mars

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 Mars Odyssey Gamma-Ray Spectrometer Instrument Suite

[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.

Concentration of H, Si, Cl, K, Fe, and Th in the low- and mid-latitude regions of Mars

[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.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JE002679

Equatorial and midlatitude distribution of chlorine measured by Mars Odyssey GRS

The Mars Observer gamma ray spectrometer will return data related to the elemental composition of Mars. The instrument has both a gamma ray spectrometer and several neutron detectors. The gamma ray spectrometer will return a spectrum nominally every 20 s from Mars permitting a map of the elemental abundances to be made. The gamma rays are emitted from nuclei involved in radioactive decay, from nuclei formed by capture of a thermal neutron, and from nuclei put in an excited state by a fast-neutron interaction. The gamma rays come from an average depth of the order of a few tens of centimeters. The spectrum will show sharp emission lines whose intensity determines the concentration of the element and whose energy identifies the element. The neutron detectors, using the fact that the orbital velocity of the Mars Observer spacecraft is similar to the velocity of thermal neutrons, determine both the thermal and epithermal neutron flux. By combining the results from both techniques it is possible to map the depth dependence of hydrogen in the upper meter as well. These data permit a variety of Martian geoscience problems to be addressed including the crust and mantle composition, weathering processes, volcanism, and the volatile reservoirs and processes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/92JE00538%4010.1002/%28ISSN%292169-9100.MAROB1

F. C. McCloskey

Papers

Science applications of the Mars Observer gamma ray spectrometer

Gamma Ray Spectra from the Mars Observer Gamma Ray Spectrometer: Cruise Data Analysis