Mars Odyssey Gamma Ray Spectrometer Research Articles

Atmospheric injection of sulfur from the Medusae Fossae forming events

A plethora of in-situ bulk chemical and mineralogical analyses, remote sensing data, and geochemical models suggest that the Martian crust is sulfur(S)-rich, exceeding the S content of terrestrial basalts by almost an order of magnitude. The main source of crustal S on Mars is the volcanic exhalation of SO2 and H2S gases. Volcanic ash, expelled along with gases, may be a sink for up to 30% of the exhaled S gas species. We analyze the elemental composition of the Martian shallow sub-surface using the Mars Odyssey Gamma Ray Spectrometer to find areas that are simultaneously enriched in constituents of major volcanic gases such as S, Cl, and H2O. A large sedimentary deposit of likely pyroclastic origin called the Medusae Fossae Formation (MFF) is located within this region of possibly extensive alteration by volcanic gases. Based on reasonable terrestrial analog estimates that the MFF scavenged, at maximum, 30% of the exhaled S gas species, we find that a significant amount of S (>1017 kg) would have been delivered to the atmosphere over the time it took for the MFF to be deposited on Mars. The mass of the S emitted from the MFF-forming event(s) is up to 8 orders of magnitude higher than the mass of S emitted from the largest Quaternary volcanic eruption on Earth. Thus, volcanic degassing from the MFF forming events could have significantly affected surface geological processes, climate evolution, and habitability of Mars.

Assessing the geologic evolution of Greater Thaumasia, Mars

AbstractThe Greater Thaumasia region consists of three chemical provinces that include Syria, Solis, and Thaumasia Planae, the Corprates Rise, part of the Thaumasia Highlands, and the transition zone northwest of the Argyre basin. Chemical signatures obtained from the Mars Odyssey Gamma Ray Spectrometer suggest low abundances of K and Th to the west, with low H abundances and high Si abundances to the east, relative to the bulk Martian crust at midlatitudes. These observations are confirmed and quantified with a modified box and whisker analysis that simultaneously captures the degree of deviation and significance of the regionally anomalous chemistry. Motivated by regionally unique chemistry, as well as its diverse geological history, we characterize Greater Thaumasia in terms of chemistry, mineralogy, and mapped geology to determine how such complementary data record the evolution of this region. Our observations are inconsistent with a proposed salt‐lubricated landslide origin, particularly given the lack of chemical or mineralogical signatures to support near‐surface salt deposits that should arise over geological timescales. Our observations instead support magmatic processes, such as mantle evolution over geological time, which may impart the Si‐enriched signature of the eastern portion of Greater Thaumasia as well as the K and Th depletion of the southeastern flank of Syria Planum. While the observed trend of decreasing K and Th from Noachian to Hesperian lavas is inconsistent with previous models of Martian mantle evolution, we see an increase in Ca content at the Noachian‐Hesperian boundary, consistent with predictions from thermodynamic modeling.

The association of hydrogen with sulfur on Mars across latitudes, longitudes, and compositional extremes

AbstractMidlatitudinal hydrated sulfates on Mars may influence brine pH, atmospheric humidity, and collectively water activity. These factors affect the habitability of the planetary subsurface and the preservation of relict biomolecules. Regolith at grain sizes smaller than gravel, constituting the bulk of the Martian subsurface at regional scales, may be a primary repository of chemical alteration, mechanical alteration, and biosignatures. The Mars Odyssey Gamma Ray Spectrometer with hundreds of kilometers of lateral resolution and compositional sensitivity to decimeter depth provides unique insight into this component of the regolith, which we call soil. Advancing the globally compelling association between H2O and S established by our previous work, we characterize latitudinal variations in the association between H and S, as well as in the hydration state of soil. Represented by H2O:S molar ratios, the hydration state of candidate sulfates increases with latitude in the northern hemisphere. In contrast, hydration states generally decrease with latitude in the south. Furthermore, we observe that H2O concentration may affect the degree of sulfate hydration more than S concentration. Limited H2O availability in soil‐atmosphere exchange and in subsurface recharge could explain such control exerted by H2O on salt hydration. Differences in soil thickness, ground ice table depths, atmospheric circulation, and insolation may contribute to hemispheric differences in the progression of hydration with latitude. Our observations support chemical association of H2O with S in the southern hemisphere as suggested by Karunatillake et al. (2014), including the possibility of Fe sulfates as a key mineral group.

Sulfates hydrating bulk soil in the Martian low and middle latitudes

AbstractThe evidence for sulfate‐bearing strata, across Late‐Noachian to Amazonian eons, suggests a central role for sulfates in acidity and salinity of Martian paleofluids and the planet's habitability. However, details remain unclear owing to shallow sampling and the limited ability of visible/near‐infrared spectroscopy to distinguish among some sulfates. Using chemical data from the Mars Odyssey gamma ray spectrometer, including the sulfur map of Mars, we confirm the possibility of hydrous sulfates acting as key hydrates throughout the southern midlatitudinal soil at decimeter depths. An H2O:S molar ratio between 2.4 and 4.0 for 80% of the midlatitudes is also consistent with hydrous sulfate phases, including the many Fe sulfates hydrated in this range or mixtures of Ca and Mg sulfates. Nevertheless, hydrous Fe sulfates could explain our observations in a simpler manner relative to Ca/Mg mixtures. Furthermore, phyllosilicates, zeolites, amorphous phases, and H2O(s) do not seem to be strong candidates to explain the H‐S variations. Consequently, we speculate that sulfates, as the primary contributor of H2O in bulk soil, may influence modern aqueous processes including warm‐season slope lineae in the southern hemisphere.

Experimental study of radon production and transport in an analogue for the Martian regolith

The suggestion that radon could be used as a radioactive tracer of regolith-atmosphere exchanges and as a proxy for subsurface water on Mars, as well as its indirect detection in the Martian atmosphere by the rover Opportunity, have raised the need for a better characterization of its production process and transport efficiency in the Martian soil. More specifically, a proper estimation of radon exhalation rate on Mars requires its emanation factor and diffusion length to be determined. The dependence of the emanation factor as a function of pore water content (at 267 and 293 K) and the dependence of the adsorption coefficient on temperature, specific surface area and nature of the carrier gas (He, He + CO 2) have been measured on a Martian soil analogue (Hawaiian palagonitized volcanic ash, JSC Mars-1), whose radiometric analysis has been performed. An estimation of radon diffusion lengths on Mars is provided and is used to derive a global average emanation factor (2–6.5%) that accounts for the exhalation rate inferred from the 210Po surface concentration detected on Martian dust and from the 214Bi signal measured by the Mars Odyssey Gamma Ray Spectrometer. It is found to be much larger than emanation factors characterizing lunar samples, but lower than the emanation factor of the palagonite samples obtained under dry conditions. This result probably reflects different degrees of aqueous alteration and could indicate that the emanation factor is also affected by the current presence of pore water in the Martian soil. The rationale of the “radon method” as a technique to probe subsurface water on Mars, and its sensitivity to soil parameters are discussed. These experimental data are useful to perform more detailed studies of radon transport in the Martian atmosphere using Global Climate Models and to interpret neutron and gamma data from Mars Odyssey Gamma Ray Spectrometer.

Recipes for Spatial Statistics with Global Datasets: A Martian Case Study

The Mars Odyssey Gamma Ray Spectrometer has yielded planetary data of global extent. Such remote-sensing missions usually assign the value of a continuous-valued geospatial attribute to a uniform latitude-longitude grid of bins. Typical attributes include elemental-mass fraction, areal fraction of a mineral type, areal fraction of rocks, thermal inertia, etc. The fineness of the grid is chosen according to the spatial resolution of the orbiter and concomitant data processing. We describe methods to maximize the information extracted from both bin and regional data. Rigorous use of statistical parameters and related methods for inter- and intra- regional comparisons are also discussed. While we discuss results from the Mars Odyssey mission, the techniques we describe are applicable whenever continuous-valued attributes of a planet's surface are characterized with bins and regions. Our goal is to distill the simplest statistical methods for regional comparisons that would be intuitively accessible to planetary scientists.

K and Cl concentrations on the Martian surface determined by the Mars Odyssey Gamma Ray Spectrometer: Implications for bulk halogen abundances in Mars

Orbital gamma ray spectrometry shows that the Martian surface has a mean Cl/K ratio of 1.3 ± 0.2, indistinguishable from the ratio in CI chondrites (1.28). Although Cl and K fractionate by magma degassing and aqueous processing, during igneous partial melting both elements are highly incompatible. Thus, if the surface Cl/K reflects the bulk crustal value, then the mantle, hence primitive silicate Mars, also has a roughly CI ratio. Martian meteorite data indicate that Cl/Br is also approximately chondritic, suggesting that elements that condensed in the nebula between ∼1000 K (K and Cl) to ∼500 K (Br) are uniformly depleted in Mars at about 0.6 × CI chondrite concentrations. Mars clearly does not contain 0.6 × CI levels of H2O, which would be ∼6 wt%, indicating that Mars was constructed by planetesimals rich in volatile elements, but not in water.

Mapping Mars geochemically

Using multivariate cluster analysis, we identify six distinct geochemical provinces on Mars from the concentrations of K, Th, Fe, Si, Ca, Cl, and H 2 O determined by the Mars Odyssey gamma ray spectrometer. The results show that the Martian surface is dominated by basaltic rocks that vary in their abundance of incompatible (K, Th) and major elements (Fe, Si, and Ca). These chemically distinct geochemical provinces are in large, contiguous regions comprising a mixture of geologic units. The K/Th ratios are uniform among the geochemical provinces. To prevent measurable fractionation of K from Th, aqueous events must have been brief and/or the total throughput of water small. The muted weathering effects led to deposition of younger sedimentary deposits with the same compositions as older igneous units, explaining why a geochemical province may contain mapped units with the same composition but a range of ages.

Chemically striking regions on Mars and Stealth revisited

The Mars Odyssey Gamma Ray Spectrometer Suite has yielded global chemical information for Mars. In this work, we establish regions of unusual chemical composition relative to average Mars primarily on the basis of Ca, Cl, Fe, H, K, Si, and Th. Using data from Mars Odyssey; the Mars Exploration Rovers; the Mars Reconnaissance Orbiter Imaging; and 3.5 cm and 1.35 cm radar observations from Earth, we examine a chemically striking ≈2.E6 km2 region and find it to overlap significantly with a radar Stealth region on Mars. It is remarkably enriched in Cl and depleted in Fe and Si (along with minor variations in H, K, and Th) relative to average Mars. Surface dust observed at the two rover sites mixed with and indurated by Ca/Mg‐bearing sulfate salts would be a reasonable chemical and physical analog to meter‐scale depths. We describe potential scenarios that may have contributed to the unique properties of this region. The bulk dust component may be an air fall deposit of compositionally uniform dust as observed in situ. Hydrothermal acid fog reactions on the flanks of nearby volcanoes may have generated sulfates with subsequent deflation and transport. Alternatively, sulfates may have been produced by low‐temperature, regional‐scale activity of ground ice–driven brine and/or regional‐scale deposition of acidified H2O snowfall.

Modeling sublimation of ice exposed by new impacts in the martian mid-latitudes

New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the “Dodo-Goldilocks” trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible.

The very early thermal state of Terra Cimmeria: Implications for magnetic carriers in the crust of Mars

Large areas of strongly magnetized crust are located in Terra Cimmeria and Terra Sirenum, in the southern highlands of Mars, the older terrains on the planet. The typical depth to the base of the magnetized layer, deduced from the magnetic spectrum of Mars and modeling of isolated magnetic anomalies, would ∼40–60 km. In this paper the thermal state of Terra Cimmeria is revisited by calculating heat flows and crustal geotherms consistent with a previous estimation of the effective elastic thickness of the lithosphere, and with crustal abundances of heat-producing elements based on surface measurements performed by the Mars Odyssey Gamma Ray Spectrometer. The results suggest that, at the time of formation of the magnetized terrains surface (which have a minimum age of 4.1 Ga), Curie depths for magnetite and hematite were shallower and similar, respectively, than the typical depth to the base of the magnetized layer. This implies that hematite is an important (if not dominant) contributor to the magnetization of the martian crust. However, an early warm climate would enhance the chance of magnetite to be a significant, even dominant, magnetization carrier in the lower portion of the magnetic layer.

The effect of ground ice on the Martian seasonal CO 2 cycle

The mostly carbon dioxide (CO 2) atmosphere of Mars condenses and sublimes in the polar regions, giving rise to the familiar waxing and waning of its polar caps. The signature of this seasonal CO 2 cycle has been detected in surface pressure measurements from the Viking and Pathfinder landers. The amount of CO 2 that condenses during fall and winter is controlled by the net polar energy loss, which is dominated by emitted infrared radiation from the cap itself. However, models of the CO 2 cycle match the surface pressure data only if the emitted radiation is artificially suppressed suggesting that they are missing a heat source. Here we show that the missing heat source is the conducted energy coming from soil that contains water ice very close to the surface. The presence of ice significantly increases the thermal conductivity of the ground such that more of the solar energy absorbed at the surface during summer is conducted downward into the ground where it is stored and released back to the surface during fall and winter thereby retarding the CO 2 condensation rate. The reduction in the condensation rate is very sensitive to the depth of the soil/ice interface, which our models suggest is about 8 cm in the Northern Hemisphere and 11 cm in the Southern Hemisphere. This is consistent with the detection of significant amounts of polar ground ice by the Mars Odyssey Gamma Ray Spectrometer and provides an independent means for assessing how close to the surface the ice must be. Our results also provide an accurate determination of the global annual mean size of the atmosphere and cap CO 2 reservoirs, which are, respectively, 6.1 and 0.9 hPa. They also indicate that general circulation models will need to account for the effect of ground ice in their simulations of the seasonal CO 2 cycle.

Chemical compositions at Mars landing sites subject to Mars Odyssey Gamma Ray Spectrometer constraints

The Mars Odyssey Gamma Ray Spectrometer (GRS) is the first instrument suite to return elemental abundances throughout the midlatitudes of Mars. Concentrations of Cl, Fe, H, K, Si, and Th have been determined to tens of centimeter depths as mass fractions with reasonable confidence. Comparing such data with, or normalizing them to, in situ compositional data is difficult due to issues such as dramatic differences in spatial resolution; difficulties in convolving densities, abundances, and compositions of different regolith components; and a limited number of elements observed in common. We address these concerns in the context of the GRS, using Si at Pathfinder to normalize remote data. In addition, we determine representative in situ compositions for Spirit (both with and without Columbia Hills rocks), Opportunity, and Viking 1 landing sites using GRS‐derived H content to hydrate the soil component. Our estimate of the Si mass fraction at Pathfinder, with 13% areal fraction of rocks, is 21%. The composition of major elements, such as Si and Fe, is similar across the four landing sites, while minor elements show significant variability. Areal dominance of soil at all four landing sites causes representative compositions to be driven by the soil component, while proportionally large uncertainties of bulk densities dominate the net uncertainties. GRS compositional determinations compare favorably with the in situ estimates for Cl and K, and for Si by virtue of the normalization. However, the GRS‐determined Fe content at each landing site is consistently higher than the in situ value.

Geochemistry of Martian soil and bedrock in mantled and less mantled terrains with gamma ray data from Mars Odyssey

Surficial materials, including soil and dust, are abundant in the upper tens of centimeters of the Martian surface sensed by the Mars Odyssey Gamma Ray Spectrometer (GRS). Seven large areas (14% of the Martian surface) that represent possible compositional end‐members were selected, including three regions heavily mantled with surficial materials. The selection process included mapping the ratio of exposed rocky terrain to surficial materials using high‐resolution imagery. GRS data for H, Cl, Fe, Si, K, and Th were obtained for each area. The areas are chemically homogeneous within each area, given the spatial resolution and analytical uncertainty of the GRS data. However, substantial chemical differences exist among the areas, including the different mantled terrains, contrary to earlier assumptions that surficial materials are globally homogeneous due to aeolian mixing. The observed chemical differences among the areas may be due to variations in the protolith compositions, extent of alteration of the protolith regions, or post soil formation processes. The abundances of Cl, K, and Th in rockier (but still soil‐rich) areas such as Syrtis Major Planum can be explained by mixing between a soil with higher concentrations of Cl, K, and Th, similar to the abundances in the mantled terrains (and some of the landing sites), and crustal rocks containing lower abundances of these elements, similar to Martian meteorites.

Effects of soil heterogeneity on martian ground-ice stability and orbital estimates of ice table depth

Data from the Mars Odyssey Gamma-Ray Spectrometer (GRS) instrument suite and results from numerical simulations of subsurface ground-ice stability have been used to estimate the depth of martian ground-ice. Geographic correlation between these estimates is remarkable; the relative ice table depth distributions also agree well. However, GRS-based estimates of ice table depth are generally deeper than predictions based on ground-ice stability simulations. This discrepancy may be related to heterogeneities in the martian surface such as rocks, dust, and albedo variations. We develop a multi-dimensional numerical model of ground-ice stability in a heterogeneous martian subsurface and use it to place the first quantitative constraints on the response of the ice table to meter-scale heterogeneities. We find that heterogeneities produce significant undulations/topography in the ice table at horizontal length scales of a few meters. Decimeter scale rocks create localized areas of deep ice, producing a vertical depression of 10–60 cm in the ice table over a horizontal range of 1–2 rock radii. Decimeter scale dust lenses produce locally shallow ice; however the magnitude of the vertical deflection of the ice table is small (1–4 cm). The effects of decimeter scale albedo variations on the ice table are nearly negligible, although albedo very weakly enhances the effects of dark rocks and bright dust on the ice table. Additionally, we investigate the role played by rocks in estimates of ice table depth based on orbital data. Surface rocks can account for more than half of the discrepancy between ice table depths inferred from GRS data and those predicted by theoretical ice-stability simulations that utilize thermophysical observations. Our results have considerable relevance to the up-coming Mars Scout Mission, Phoenix, because they indicate that the uncertainty in the ice table depth of a given region is greater than differences between current depth estimates. Likewise, small-scale depth variability due to heterogeneities at the eventual landing site is potentially greater than differences between current depth estimates.

Equatorial and midlatitude distribution of chlorine measured by Mars Odyssey GRS

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.

Theoretical fluxes of gamma rays from the Martian surface

Theoretical fluxes of gamma rays escaping the surface of Mars were calculated. These and other calculated fluxes are needed to model the counting rates in the Mars Odyssey gamma ray spectrometer that are used to determine elemental compositions and other results using these measurements. Cross sections for the formation of gamma rays by both thermal and fast neutrons were compiled and evaluated. These evaluated cross sections were used with neutron fluxes calculated with the Monte Carlo N Particle Extended (MCNPX) code to get gamma ray production rates as a function of depth in the Martian surface. The fluxes of these gamma rays as a function of angle at the Martian surface were then calculated using gamma ray attenuation coefficients.

Analysis of gamma ray spectra measured by Mars Odyssey

Gamma ray spectra measured by the Mars Odyssey gamma ray spectrometer while in orbit around Mars were analyzed to identify the sources of 334 observed peaks and features. Most peaks were of a standard Gaussian shape with a low‐energy tail. However, Doppler‐broadened and sawtooth‐shaped peaks were also observed in the spectra. The sources of most peaks were identified. Many peaks come from the gamma ray detector material or materials around the detector, particularly Ti and Mg. Identifications were often confirmed by analyzing other spectra, such as those measured during the cruise to Mars, before and after the boom deployment, at the end of solar particle events, and over thick frozen CO2 polar caps during winters.

Composition of northern low‐albedo regions of Mars: Insights from the Mars Odyssey Gamma Ray Spectrometer

Analysis of data acquired by the Mars Global Surveyor Thermal Emission Spectrometer (TES) instrument led to the identification of two distinct surface types.“Surface type 1,” lying mostly within southern low‐albedo terrain, is likely to be composed of basalt. However, there are several competing mineralogical models for “surface type 2,” which is found primarily in northern low‐albedo terrain. We attempt to identify which models better agree with element concentrations determined through the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) instrument suite aboard the Mars Odyssey spacecraft. We have binned GS data over threshold regions derived from maps of surface types 1 and 2 and examined the spatial correlation of element concentrations with surface type 2. The elements we have examined are Cl, Fe, H, K, Si, and Th. Our results show that K concentration, Th concentration, and the areal abundance of surface type 2 are strongly correlated, with significant enrichment of both K and Th in regions representative of surface type 2. In addition, Si does not appear to be significantly enriched in surface type 2. These results are more consistent with surface type 2 originating from a compositionally distinct mantle source than the aqueous alteration of basalts.

Seasonal polar carbon dioxide frost on Mars: CO2 mass and columnar thickness distribution

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.