328 Encyclopedia of the Solar System
FIGURE 10 View from theSpirit Roverin the Columbia Hills. The level plains of Gusev are in the background. The hills in the
distance are part of a delta-like deposit at the mouth of a large channel that enters the crater from the south. The rocks in the
foreground have been aqueously altered to varying degrees. The origin of the hills is unknown, but they may have been uplifted by an
impact event that postdated the formation of Gusev itself. (Mars Exploration Rover.)ranging from almost unaltered olivine basalts like those on
the plains to almost completely altered, soft rocks enriched
throughout with mobile elements such as S, Cl, and Br.
Primary basalt minerals are almost absent having been re-
placed by secondary minerals such iron oxides and oxyhy-
droxides that have high Fe^3 +/Fe^2 +ratios compared with
the unaltered rocks. A sulfate cement in some rocks sug-
gests evaporation of sulfate-rich waters. On some of the
rocks, there is a surface rind that is harder than the in-
terior, so the rocks have been hollowed out by the wind.
Layered rock is common, and a coarse stratification ap-
pears to follow the contours of the hills. The origin of the
Columbia Hills rocks is still being debated. Some may have
formed by aqueous alteration of newly deposited impact
or volcanic debris. Some may have been hydrothermally
altered long after deposition. For others, waters from the
postulated Gusev lake may have been implicated. What-
ever the cause was, aqueous processes were involved. (See
Fig. 10.)
Opportunitylanded in Meridiani Planum on a thick stack
of layered rocks that had been observed from orbit. The site
was chosen because a particular form of the iron mineral
hematite that forms in aqueous environments had been de-
tected there. The number of impact craters superimposed
on the layered rocks suggests that they formed at the end
of the heavy bombardment period around 3.8 billion years
ago. The rovers demonstrated unequivocally that the lo-
cal rocks are reworked evaporitic sandstones with roughly
equal proportions of basaltic debris and evaporitic minerals
such as Mg, Ca, Fe, and Na sulfates and chlorides. Although
most of the rocks were deposited by the wind, there had to
be a nearby source for the evaporites, which form by evap-
oration of bodies of water. The source had to be substantial
because the layered sequence on which the rover landed
extends for several hundred kilometers. A small fraction
of the rocks have depositional textures that indicate that
they were deposited in standing water. The environment in
which the Meridiani sequence accumulated is thus thought
to be one in which there were wind-blown dunes with in-
terdune ponds.Opportunityspent much of its time exam-
ining the local rock section in a crater called Endurance.
(See Figs. 11 and 12.) The relations in the crater indicated
that there had been almost no aqueous activity since the
crater formed. Thus, the two rovers, although landing on
very different geologic materials, are telling a somewhat
similar story. The oldest rocks, those that formed during
heavy bombardment, have abundant evidence for aqueous
processes, but the evidence for such processes after the end
of heavy bombardment is sparse or absent.10. Summary
Mars is a geologically variegated planet on which have op-
erated many of the geologic processes familiar to us here on
Earth. It has been volcanically active throughout its history;
the crust has experienced extensive deformation, largely as
a result of massive surface loads; and the surface has been
eroded by wind, water, and ice. Despite these similarities,
the evolutions of Mars and Earth have been very differ-
ent. The lack of plate tectonics on Mars has prevented the
formation of linear mountain chains and cycling of crustal