346 Encyclopedia of the Solar System
of reddish dust, as seen by the color change when it is dis-
turbed in rover tracks (Fig. 17) or airbag bounces. Most
measurements of soil mineralogy or chemistry represent a
mixture of soil and dust, sometimes with an admixture of
small particles of the local rocks.
At all these sites, the soils have broadly similar compo-
sitions, consisting of basaltic sands mixed with fine-grained
dust and salts. Pancam, Mini-TES, and MB spectra of
bright dust are dominated by nanophase ferric oxides, es-
pecially hematite. MB spectra of dark soils indicate abun-
dant olivine, pyroxene, and magnetite at the MER landing
sites. The degree of alteration appears to be limited, but
fractionation of chlorine and bromine in some soils sug-
gest some mobilization by water. APXS chemical analyses
show that plagioclase is also an important component of
soils, and that their compositions resemble basalts with ex-
tra sulfur, chlorine, and bromine. At thePathfindersite, lo-
cal andesitic rock fragments are present in varying amounts,
and at theOpportunitysite hematite spherules occur abun-
dantly at the surface as a lag of granules. Trenches dug by
theSpiritandOpportunityrovers reveal clods, suggesting
greater proportions of salts that precipitated in the sub-
surface have bound sand into weakly cohesive clumps, and
APXS analyses of some subsurface soils show high concen-
trations of magnesium sulfate salt. Soils also contain sig-
nificant amounts of nickel, which may reflect admixture of
meteorite material into the regolith. Dust appears similar in
composition to the soil (basaltic). Analysis of dust adhered
to magnets on the rovers indicates it contains olivine, mag-
netite, and a nanophase iron oxide (likely hematite) that
suggests the dust is an oxidation or alteration product of
fine-grained basalt. The presence of olivine in the dust sug-
gests liquid water was not heavily involved in its formation
because olivine would readily alter to other minerals (espe-
cially serpentine) in the presence of water.
6. Implications for the Evolution of Mars
6.1 Origin of Igneous Rocks
Igneous rocks form by partial melting of the planet’s
deep interior. The significance of the olivine-rich basaltic
compositions found bySpiriton the Gusev plains is that
they appear to represent “primitive” magmas formed by
melting in the mantle. Most magmas partly crystallize as
they ascend toward the surface, losing the crystals in the
process, so that the liquid progressively changes composi-
tion. Primitive magmas retain their original compositions
and thus reveal the nature of their mantle source regions.
It is unlikely that rocks with andesitic composition at the
Mars Pathfinderlanding site formed by partial melting of
the mantle, unless the mantle contains large quantities of
water-bearing minerals. More likely, andesite lavas would
form by partial melting of previously formed basaltic crustal
rocks (the crust forms an outer layer above the mantle). An
alternative, previously mentioned, is that these rocks are not
really andesites at all, but instead are basalts with silica-rich
weathering rinds. The latter idea seems especially plausible
considering that Surface Type 2 (andesitic) rocks are found
primarily in places (like the northern lowlands) where sur-
face waters would collect and the sediments they carried
would be deposited. If this is correct, the orbital data and
the samples of rocks at the five landing sites strongly argue
that Mars is a basalt-covered world. Basalts, sediments de-
rived from basalts, and dust derived from mildly weathered
basalts are confirmed or suspected at all the landing sites.
Adding the thermal emission spectra of Type 1 and Type 2
materials as basalt and weathered basalt would suggest that
most of Mars is made of this primitive volcanic rock.
The gamma ray spectrometer (GRS) on the Mars
Odysseyorbiter has provided direct chemical measure-
ments of large areas of the martian surface. These measure-
ments are of the top meter or so of material, rather than the
topmost few hundred micrometers of the surface analyzed
by TES spectra. The measured silica contents of Surface
Types 1 and 2 terrains are not significantly different, but
the potassium content of Surface Type 2 is higher. These
conflicting results do not clearly support either proposed
origin.6.2 Chemical Evolution and Surface Water
The minerals that form outcrops of evaporites at theOppor-
tunitylanding site could only have precipitated from highly
acidic water (“acidic” means low pH, or hydrogen ion con-
centration). Any sea at Meridiani was more like battery acid
than drinking water. Given the abundance of basaltic lavas
on the martian surface, it is surprising that these waters
would be so acidic. Reactions between water and basalt on
Earth tend to produce neutral to basic solutions. On Mars,
huge volumes of sulfur and chlorine emitted from volca-
noes must have combined with water to make sulfuric and
hydrochloric acids. Only a few locations on Earth—mostly
areas devastated by acidic waters released by weathering
of sulfides that drained from mines—mimic this kind of
fluid. Acidic water dissolves and precipitates different min-
erals than the waters we are more familiar with on Earth.
Carbonates are not precipitated, and iron sulfates are more
common in acidic solutions.
If carbonates could not precipitate in an acid water en-
vironment, interesting additional constraints can be placed
on the evolution of the atmosphere on Mars. If the early
environment of Mars was wetter and warmer and liquid wa-
ter was in equilibrium, then the atmospheric pressure and
temperature both had to be higher. Higher atmospheric
pressure requires much more carbon in the atmosphere
(mostly composed of carbon dioxide). As the atmosphere
thinned, substantial deposits of carbonate would normally
be deposited in the crust (as occurs on Earth). In an acid
environment, carbonate could not be deposited in the crust,
which would then require that the carbon in the atmosphere