Spectral and geological study of the sulfate-rich region of West Candor Chasma, Mars

Research areas:
Sulfates have been discovered by the OMEGA spectrometer in different
locations of the planet Mars. They are strongly correlated to light
toned layered deposits in the equatorial regions. West Candor Chasma is
the canyon with the thickest stack of layers and one with the largest
area covered by sulfates. A detailed study coupling mineralogy derived
from OMEGA spectral data and geology derived from HRSC imager and other
datasets leads to some straightforward issues. The monohydrated sulfate
kieserite is found mainly over heavily eroded scarps of light toned
material. It likely corresponds to a mineral present in the initial rock
formed either during formation and diagenesis of sediments, or during
hydrothermal alteration at depth, because it is typically found on
outcrops that are eroded and steep. Polyhydrated sulfates, that match
any Ca-, Na-, Fe-, or Mg-sulfates with more than one water molecule, are
preferentially present on less eroded and darker outcrops than outcrops
of kieserite. These variations can be the result of a diversity in the
composition and/or of the rehydration of kieserite on surfaces with
longer exposure. The latter possibility of rehydration in the current,
or recent, atmosphere suggests the low surface temperatures preserve
sulfates from desiccation, and, also can rehydrate part of them. Strong
signatures of iron oxides are present on sulfate-rich scarps and at the
base of layered deposits scarps. They are correlated with TES gray
hematite signature and might correspond to iron oxides present in the
rock as sand-size grains, or possibly larger concretions, that are
eroded and transported down by gravity at the base of the scarp.
Pyroxenes are present mainly on sand dunes in the low lying terrains.
Pyroxene is strongly depleted or absent in the layered deposits. When
mixed with kieserite, local observations favor a spatial mixing with
dunes over layered deposits. Sulfates such as those detected in the
studied area require the presence of liquid water to form by
precipitation, either in an intermittent lacustrine environment or by
hydrothermal fluid circulation. Both possibilities require the presence
of sulfur-rich groundwater to explain fluid circulation. The elevation
of the uppermost sulfate signatures suggests the presence of aquifers up
to 2.5 km above datum, only 1 km below the plateau surface. (C) 2007
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