Encyclopedia of the Solar System 2nd ed

302 Encyclopedia of the Solar System

the northern circumpolar dunes to layered deposits in the
equatorial regions. The extent and timing of the presence of
liquid water are central to the question of whether microbial
life ever arose and evolved on Mars.
Atmospheric volatiles are substances that tend to form
gases or vapors at the temperature of a planet’s surface.
Consequently, such volatiles can influence climate. Here
we review the current understanding of volatile reservoirs,
the sources and sinks of volatiles, the current climate, and
the evidence for different climates in the past. We focus
on the hypothesis that there have been one or more ex-
tended warm and wet climate regimes in the past, the prob-
lems with that hypothesis, and the alternative possibility that
Mars has had a cold, dry climate similar to the present cli-
mate over nearly all of its history, while still allowing for
some fluid flow features to occur on the surface. Mars un-
dergoes very large orbital variations (Milankovitch cycles),
and the possible relevance of these differences to climate
history will be discussed. Whether or not extended periods
of warm, wet climates have occurred in the past, wind is
certainly an active agent of surface modification at present
and has probably been even more important in the past. We
discuss the evidence for modification of the surface by wind
erosion, burial, and exhumation and the resulting compli-
cations for interpreting Mars’ surface history. We conclude
with a brief overview of open questions.

2. Volatile Inventories and their History

2.1 Volatile Abundances

Mars’ thin atmosphere is dominated by carbon dioxide
(Table 1). In addition to the major gaseous components
listed, the atmosphere contains a variable amount of wa-
ter vapor (H 2 O) up to 0.1%, minor concentrations of pho-
todissociation products of carbon dioxide (CO 2 ) and water
vapor (e.g., CO, O 2 ,H 2 O 2 , and O 3 ), and trace amounts
of noble gases neon (Ne), argon (Ar), krypton (Kr), and
xenon (Xe). Recently, trace amounts of methane (CH 4 )
have also been identified, averaging∼10 parts per billion

by volume, although currently a wide range of methane

values have been reported, and these differences have yet

to be reconciled. The differences may represent measure-

ment errors or variability in the source of methane and its

transport.

Volatiles that can play important roles in climate are

stored in the regolith and near-surface sediments. Crude es-

timates of some of these are given in Table 2. Water is stored

in the permanent north polar cap, north polar cap layered

terrains, and layered terrains surrounding the South Pole,

and as ice, hydrated salts, or adsorbed water in the regolith.

The regolith is a geologic unit that includes fine dust, sand,

and rocky fragments made up of the Martian soil together

with loose rocks, but excluding bedrock. Although the sur-

face of the residual northern polar cap is water ice, the

∼5 km deep cap itself consists of a mixture of ice and fine

soil with an unknown proportion of each. Layered south po-

lar terrains may also contain an amount of water ice equiv-

alent to a global ocean 20 m deep. Measurements of the

energy of neutrons emanating from Mars into space have

provided evidence for abundant water ice, adsorbed water,

and/or hydrated minerals in the upper 1–2 m of regolith

at high latitudes and in some low-latitude regions (Fig. 1).

Cosmic rays enter the surface of Mars and cause neutrons

to be ejected with a variety of energies depending on the el-

ements in the subsurface and their distribution. Abundant

hydrogen serves as a proxy for water and/or hydrated min-

erals. If water ice extends deep into the regolith, it could

correspond to tens of meters of equivalent global ocean. It

is also possible that Mars has liquid water aquifers beyond

the depth where the temperature exceeds the freezing point

(the so-called melting isotherm), but direct evidence is cur-

rently lacking.

Carbonate weathering of dust has occurred over billions

of years in the prevailing cold dry climate, and as a conse-

quence some CO 2 appears to have been irreversibly trans-

ferred from the atmosphere to carbonate weathered dust

particles. The total amount depends on the global average

depth of dust. Some CO 2 is likely to be adsorbed in the

soil also, but the amount is limited by competition forad-

sorptionsites with water. Despite an extensive search from

TABLE 1 Basic Properties of the Present Atmosphere

Average surface pressure ∼ 6 .1millibars (mbar), varying seasonally

by∼30%

Surface temperature Average 215 K, range: 140–310 K

Major gases CO 2 95.3%,^14 N 2 2.7%,^40 Ar 1.6%

Significant atmospheric isotopic ratios

relative to the terrestrial values

D/H= 5

(^15) N/ (^14) N=1.7
(^38) Ar/ (^36) Ar=1.3
(^13) C/ (^12) C=1.07