Variations in the association of H2O with sulfur on Mars

Abstract:

Sulfates may constrain the salinity and acidity of martian paleo-fluids, with consequent impact on the planet’s habitability. However, information about sulfate mineralogy, subsurface abundance, and hydration remains unclear owing to shallow sampling and limited ability of infrared spectroscopy to distinguish among some sulfates. We address this with H₂O and S chemical maps, derived from the Mars Odyssey Gamma Ray Spectrometer (GRS) spectra, sampling at regional scales and decimeter depths[1]. Resulting chemical constraints apply primarily to bulk soil, which dominates at such spatial scales.

We assess trends in the association of H₂O and S, with bivariate correlations(r) across 10° wide latitudinal bands, corresponding molar ratios H₂O:S denoted as hydration state, and hydration states across compositional extremes. We characterize the hydration state distribution with kernel density estimated frequency histograms (KDE). Twenty seven sulfate minerals confirmed or suggested on Mars, from in situ, orbital, and analog observations, provide 13 reference hydration states.

Bivariate correlations across latitudinal bands show consistency with hemispheric results. The strengths of correlations are similar at extreme north and south latitudes, with r values of 0.2 at ±55° centered latitudinal bands. However, most southern latitudinal bands manifest r values no less than 0.60, while northern bands remain below 0.60. Accordingly, at the coarsest hemispheric scale and the finest latitudinal scale, H₂O may associate with S in bulk soil of the southern hemisphere. Regions of compositional extremes show that H₂O concentration, not S, affects the degree of sulfate hydration. Variable soil thickness, ground ice table depths[2], atmospheric processes[3], and insolation may contribute hemispheric differences in the progression of hydration with latitude. KDE results suggest Fe-sulfates as a possibly key hydrous mineral group[4]. This encourages further modeling of Fe-sulfate distribution and deliquescence, potentially derived from local hydrothermalism[5], at regional scales.

References.[1]Boynton et al, JGR 112, E12S99 (2007) [2]Jakosky et al., Icarus 175, 58–67 (2005) [3]Beck et al., JGR 115, E10011 (2010) [4]Lane et al., Am. Mineral. 93, 728–739 (2008) [5]Yen et al., JGR 113, E06S10 (2008).