Hydration state of the Martian surface as seen by Mars Express OMEGA: 2. H₂O content of the surface

Abstract

[1] Visible-near infrared reflectance spectra acquired by the Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces et l'Activité (OMEGA) spectrometer are used to estimate the absolute water content within the uppermost fraction of the Martian regolith. This upper surface layer represents the boundary between the regolith and atmosphere; thus the amount of water stored in these two reservoirs and at this boundary is expected to vary spatially and temporally with changing equilibrium conditions. We have applied models derived from laboratory experiments relating the strength of the 3 mm hydration feature to absolute water content to OMEGA spectra acquired during the first $1200 orbits of the Mars Express mission to estimate the H 2 O content of the Martian surface. Three methods were used to examine the strength of the 3 mm absorption: integrated band depth, apparent absorbance, and the effective single-particle absorptionthickness (ESPAT) parameter. Integrated band depth and apparent absorbance values are correlated to albedo when derived from reflectance spectra, implying that bright regions are more hydrated than dark regions. The ESPAT parameter, however, relies on single scattering albedo instead of reflectance and is capable of estimating absolute water content within ±1 wt.% H 2 O for a wide range of albedo values, compositions, and particle sizes. Applying this model to the OMEGA data reveals that bright and dark regions commonly have similar water contents in equatorial regions and the largest spatial variations in H 2 O occur as a function of latitude. Equatorial regions exhibit water contents in the range of $2-5 wt.%, whereas latitudes higher than $45°N are characterized by a continuous increase in H 2 O with latitude from $5-15 wt.%. Phyllosilicate and sulfate bearing terrains are more hydrated than average bright and dark regions and their locations are in widely separated areas of Noachian-aged material, suggesting chemical alteration by water-rock interaction may have been spatially extensive in the early history of Mars.

Astronomical Observations of Volatiles on Asteroids

“…Increased laboratory capabilities, including the ability to drive off terrestrial water and take spectra without breaking vacuum, have led to a much better sense of the spectral properties of hydroxyl in carbonaceous chondrites (Section 2.2 and below), and mapped out OH band centers with mineralogy. Work by Milliken et al (2007) in the context of Martian studies provides a potential way to remotely measure water in meteorites through calculation of spectral parameters such as band depth and integrated band depth, integrated band area, mean and normalized optical path length, and effective single-particle absorption thickness, though to this point these have not been applied to estimating hydroxyl amounts, nor been used on carbonaceous chondrites in the published literature.…”

“…Increased laboratory capabilities, including the ability to drive off terrestrial water and take spectra without breaking vacuum, have led to a much better sense of the spectral properties of hydroxyl in carbonaceous chondrites (Section 2.2 and below), and mapped out OH band centers with mineralogy. Work by Milliken et al (2007) in the context of Martian studies provides a potential way to remotely measure water in meteorites through calculation of spectral parameters such as band depth and integrated band depth, integrated band area, mean and normalized optical path length, and effective single-particle absorption thickness, though to this point these have not been applied to estimating hydroxyl amounts, nor been used on carbonaceous chondrites in the published literature.CM (Meghei-like) and CI (Ivuna-like) carbonaceous chondrite meteorites are widely thought to be the possible analogs for dark and primitive asteroids (e.g., C-, D-, G-, K-, F-, and B-types in the Tholen taxonomy, C-complex in the Bus taxonomy: e.g., Burbine et al, 2002). These meteorites experienced varying degrees of fluid-assisted alteration on their parent bodies (McSween, 1979).…”

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

“…Thermodynamic models of ice stability [e.g., (7)] and equilibration models of various hydrous minerals [e.g., (8,9)] have ruled out some simple explanations for the origin of this water reservoir. Although hydrated sulfates and clay minerals have been detected from orbit, they cover only a small fraction of the entire surface (10)(11)(12) and, to the extent of what can be observed by remote sensing, are limited to bedrock outcrops (13,14), whereas the hydration of the topmost microns of the regolith probed by the Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) aboard the European Space Agency's Mars Express orbiter is global and usually unrelated to bedrock exposures (6,15). Therefore, the nature and origin of this global hydration remain largely unknown.…”

“…From orbital observations (neutron, gammaray, and near-infrared spectroscopy), the martian surface is known to hold 2 to~10 weight percent (wt %) of water-equivalent hydrogen at mid-and low latitudes (4)(5)(6). Thermodynamic models of ice stability [e.g., (7)] and equilibration models of various hydrous minerals [e.g., (8,9)] have ruled out some simple explanations for the origin of this water reservoir.…”

火星の海・湖・アウトフローチャネルの比較惑星地質学

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