Mars Astrobiology: Recent Status and Progress - Planetary Exploration and Science: Recent Results and Advances - page 153

component of terrestrial soils), the evaporate minerals gypsum and kieserite,
opaline silica, and phyllosilicates (also called clay minerals), such as kaolinite and
montmorillonite. All of these minerals have been detected on Mars.
One direct effect of chemical weathering is to consume water and other reactive
chemical species, taking them from mobile reservoirs like the atmosphere and
hydrosphere and sequestering them in rocks and minerals. The amount of water
in the Martian crust stored in hydrated minerals is currently unknown but may be
quite large. For example, mineralogical models of the rock outcroppings examined
by instruments on the Opportunity rover at Meridiani Planum suggest that the sulfate
deposits there could contain up to 22 % water by weight.
On Earth, all chemical weathering reactions involve water to some degree. Thus,
many secondary minerals do not actually incorporate water but still require water to
form. Some examples of anhydrous secondary minerals include many carbonates,
some sulfates (e.g., anhydrite), and metallic oxides such as the iron oxide mineral
hematite. On Mars, a few of these weathering products may theoretically form
without water or with scant amounts present as ice or in thin molecular-scale films
(monolayers). The extent to which such exotic weathering processes operate on
Mars is still uncertain. Minerals that incorporate water or form in the presence of
water are generally termed “aqueous minerals.”
Aqueous minerals are sensitive indicators of the type of environment that existed
when the minerals formed. The ease at which aqueous reactions occur (Gibbs free
energy) depends on the pressure, temperature, and concentrations of the gaseous
and soluble species involved.
Two important properties are pH and oxidation-reduction potential (Eh). For
example, the sulfate mineral jarosite forms only in low pH (highly acidic) water.
Phyllosilicates usually form in water of neutral to high pH (alkaline). Eh is a
measure of the oxidation state of an aqueous system. Together, Eh and pH indicate
the types of minerals that are thermodynamically most likely to form from a given
set of aqueous components. Thus, past environmental conditions on Mars, including
those conducive to life, can be inferred from the types of minerals present in the
rocks.

8.3.1.2 Hydrothermal Alteration

Aqueous minerals can also form in the subsurface by hydrothermal fluids migrating
through pores and fissures. The heat source driving a hydrothermal system may
be nearby magma bodies or residual heat from large impacts. One important type
of hydrothermal alteration in the Earth's oceanic crust is serpentinization, which
occurs when seawater migrates through ultramafic and basaltic rocks. The water-
rock reactions result in the oxidation of ferrous iron in olivine and pyroxene to
produce ferric iron (as the mineral magnetite) yielding molecular hydrogen (H 2 )asa
by-product. The process creates a highly alkaline and reduces (low) Eh environment
favoring the formation of certain phyllosilicates (serpentine minerals) and various
carbonate minerals, which together form a rock called serpentinite. The hydrogen