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  • How does hydrogen associate with sulfur on Mars across latitudes, longitudes, and compositional extremes?

The hydration of sulfates in the midlatitudes of Mars may control water activity, brine pH, and even atmospheric relative humidity cycles. These factors collectively influence 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 bio-signatures. The Mars Odyssey Gamma Ray Spectrometer with hundreds of km 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 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, H2O concentration may affect the degree of sulfate hydration more than S. 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 hemispheric differences in the progression of hydration with latitude. Our observations support chemical association of H2O with S in the southern hemisphere, including the possibility of Fe-sulfates as a key mineral group. This encourages further laboratory modeling of what roles Fe-sulfates may play in the H2O cycle of Mars involving the atmosphere and bulk soil.

Hydration state plotted against latitude to identify latitudinal variations in average hydration. Blue error bars signify one standard error and the black dot indicates the average, both accounting for heteroscedasticity [Karunatillake et al., 2011, url:]. Mid-point of each 10̊ -wide latitudinal band is shown on the x-axis. Note nearly monotonic increase in hydration state to a maximum of ~3.5 toward northerly latitudes. Hydration initially decreases towards southerly latitudes to a minimum of ~2.8, then increases.

  • Is there correlation between Hydrogen, Chlorine, and Sulfur on Mars?

Using PCA, I have found that there are prominent, consistent correlations between chlorine, sulfur, and hydrogen. Currently, I am working to constrain what minerals can explain the correlations and molar ratios shown in the data.

A map of hydrogen concentration projected onto MOLA shaded relief. Polar regions are excluded due to incomplete data. Pixelated appearance due to resolution of GRS data.

  • What causes cave skylights on Mars?

In preliminary investigation, I have found that there are inconsistencies between the suggested opening mode of Martian pit-craters and the cave skylights seen amongst the crater chains. Thus far, these skylights have been associated with pit-crater chains in very limited locales on Mars, which may suggest that cave skylights may form by more complex processes than currently viewed.

Pit crater chain on the eastern flank of Arcia Mons with potential cave skylight in the middle. Hirise Image ESP_011756_1735_RED

  • Can the distinguishing features between bombsags and dropstones be used to classify a clast definitively as one or the other?  How can this classification be applied to clasts on Mars?

Bombsags and dropstones are features of their respective genetic environments, volcanic for the former and glaciolacustrine for the latter. In terrestrial settings, these settings are distinguished easily since the geologic environment is known from contextual evidence. However, on Mars, glaciolacustrine and volcanic environments are not always discernable from remote sensing or in situ observations and may overprint on the same terrain. Nevertheless, in situ mission images of clasts embedded within sediment layers hold the promise of distinguishing glaciolacustrine environments from volcanic ones. At Home Plate in the Columbia Hills at Gusev Crater, Spirit discovered the only identified clast on Mars [Squyres et al., 2007]. This discovered clast motivates developing a robust method, based on terrestrial image analogs, to distinguish bombsags from dropstones in order to ultimately distinguish a volcanic environment from a glaciolacustrine environment. Accordingly, we developed an assessment method to definitively classify a clast as a bombsag or dropstone with limited sedimentological data. The robustness of our method was tested using sample dropstones from the Mount Rogers Formation, Virginia. The Home Plate clast was also re-assessed, which confirmed its origin as a bombsag, accordingly eliminating any uncertainty from the priori analysis by Squyres et al. [2007].

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