Encyclopedia of the Solar System 2nd ed

Mars Atmosphere: History and Surface Interactions 303

TABLE 2 Volatile Reservoirs

Water (H 2 O) Reservoir Equivalent Global Ocean Depth

Atmosphere 10 −^5 m

Polar caps and layered terrains 5–30 m

Ice, adsorbed water, and/or hydrated salts

stored in the regolith

0.1–100 m

Deep aquifers Unknown

Carbon Dioxide (CO 2 ) Reservoir Equivalent Surface Pressure

Atmosphere ∼6 mbar

Carbonate in weathered dust ∼200 mbar per 100 m global average layer of

weathered dust

Adsorbed in regolith <200 mbar

Carbonate sedimentary rock ∼0 (at surface)

Sulfur Dioxide (SO 2 ) Reservoir Equivalent Global Layer Depth

Atmosphere 0

Sulfate in weathered dust ∼8 m per 100 m global average layer of

weathered dust

Sulfate sedimentary rock reservoirs Extensive, but not yet quantifiable

orbit, no carbonate sedimentary rock outcrops have been
identified down to a spatial resolution of about 100 m.
Table 2 also lists sulfates. Although there are no de-
tectable sulfur-containing gases in the atmosphere at
present, sulfur is an important volatile for climate because
it may have briefly resided in the atmosphere in the past.
Measurements by NASA’sMars PathfinderandVikinglan-
ders showed that sulfur is a substantial component of soil
dust (∼7–8% by mass) and surface rocks. Hydrated sul-
fate salt deposits have also been recently identified in nu-
merous deposits in the Martian tropics from near-infrared
spectral data on the European Space Agency’sMars Ex-

pressspacecraft. Observed sulfate minerals include gyp-

sum (CaSO 4 ·2H 2 O) and kieserite (MgSO 4 ·H 2 O), while

jarosite has been found by theOpportunityrover. [Jarosite

is XFe 3 (SO 4 ) 2 (OH) 6 , where X is a singly charged species

such as Na+,K+, or hydronium (H 3 O+).] Anhydrous sul-

fates, such as anhydrite (CaSO 4 ), are also likely to be present

but would give no signature in the spectral region studied

byMars Express.

Evidence of volatile abundances also comes from analy-

sis of a certain class of meteorites, the Shergotite, Nahkla,

and Chassigny or SNC meteorites. [SeeMeteorites.]

These meteorites are known to be of Martian origin from

FIGURE 1 Water-equivalent

hydrogen content of subsurface

water-bearing soils derived from

theMars Odysseyneutron

spectrometer. (From Feldman

et al., 2004,J. Geophys. Res.

109 , E09006,

doi:10.1029/2003JE002160.)