344 Encyclopedia of the Solar System
FIGURE 20 Images of rocks from the Columbia Hills in Gusev
crater. (a) Pancam image of Wooly Patch, after several RAT
grinds showing darker interior and natural dusty surface. (b) MI
of Wooly Patch RAT hole, showing clastic texture. (c) Pancam
image of Tetl, which exhibits fine layering. (d) MI of Tetl,
illustrating coherent layers separated by finer grained material.
Husband Hill, a promontory within Gusev crater (one of the
Columbia Hills in Fig. 10). The Hills outcrops are distinct
from the plains basalts. Some are massive, others are lami-
nated, and most are altered and deeply weathered (Fig. 20).
Pancam, Mini-TES, and MB spectra suggest highly varying
mineralogy. Some rocks contain combinations of olivine, py-
roxene, feldspar, and iron oxides (as on the plains), whereas
others contain large amounts of glass, sulfate, ilmenite, and
phosphate. APXS analyses have been used to divide the
rocks into several different classes according to their chem-
istry, but the mineralogy can vary considerably even within a
class. Some rocks appear to be relatively unaltered, but most
show very high contents of sulfur, phosphorus, and chlorine,
suggesting a high degree of alteration. The chemical compo-
sitions of these rocks are not illustrated in Fig. 13 because
that classification is only applicable to unaltered igneous
rocks. The textures of Hills rocks, as revealed by the MI,
are also highly variable but commonly indicate alteration
of rocks composed of angular particles and clasts (Fig. 20).
RAT grinding indicates that these rocks are much softer
than the plains basalts. They have been interpreted as mix-
tures of materials formed by impacts or explosive volcanic
eruptions, and subsequently altered by fluids. Two classes
of rocks on the northwest flank of Husband Hill have what
appear to be roughly concordant dips to the northwest sug-
gesting a stratigraphy (Fig. 15). The lower rock has layered
materials and angular to rounded clasts in a matrix that com-
pares favorably to impact ejecta that has been altered by
water to various extents. The upper rock is a finely layered
sedimentary rock that has been cemented by sulfate, ex-
cept the aqueous alteration did not affect the basaltic char-
acter of the sediment. A few distinctive rock types found
as loose stones (geologists call these “float”) in the Hills
include Backstay, Irvine, and Wishstone, which are dark,
fine-grained basaltic rocks with compositions distinct from
the plains basalts (Fig. 13), and only limited signs of al-
teration by water. These rocks appear to have formed by
removal of crystals from magmas similar in composition to
plains basalts.
OnceSpiritgained the crest of Husband Hill, it traveled
down the south face, encountering olivine-rich rocks of the
Algonquin class (Fig. 13). Upon reaching the bottom,Spirit
traversed an area containing highly vesicular rocks (scoria)
to Home Plate, tentatively interpreted as a small volcanic
edifice formed of ash. The compositions of all the relatively
unaltered igneous rocks in Gusev crater are rich in alkalis
and low in silica (Fig. 13), allowing their classification as
alkaline rocks. These are the first alkaline rocks recognized
on Mars.
Rocks at theOpportunitylanding site are mostly ex-
posed in the walls of impact craters and where the sand
is thin. Outcrops in Eagle crater were studied extensively
after landing (Fig. 11), and a thicker stratigraphic section
in Endurance crater was analyzed later in the mission. Pan-
cam and MI images (Fig. 21) show that the rocks are finely
laminated, sometimes exhibiting cross-bedding (Fig. 22),
and RAT grinds indicate that they are very soft. At the
microscopic scale, they consist of sand grains bound to-
gether by fine-grained cement. Small gray spherules, called
blueberries (Figs. 21 and 22), are embedded within the
rock (the spherules are actually gray, but appear bluish in
many false color images). Some parts of the outcrop also ex-
hibit tabular voids (Fig. 21). APXS analyses of these rocks
indicate very high concentrations of sulfur, chlorine, and
bromine (highly water soluble elements), demonstrating
that the cement and sand (partially) consists of sulfate and
halide salts. MB spectra reveal the presence of iron sul-
fate, and Mini-TES spectra suggest magnesium and cal-
cium sulfates also occur. The spherules are at least half
hematite, the mineral seen from orbital TES spectra of
the Meridiani region. The rocks are interpreted as sand-
stones composed of dirty evaporites of basaltic and sulfate
composition formed by the evaporation of brines. Their
textures suggest repeated cycles of flooding, exposure, and
desiccation. Exposure and desiccation allowed some of the
sediments to be mobilized into sand dunes (Fig. 23). Af-
ter deposition, the rocks underwent a number of different
phases of diagenesis by groundwater of varying composi-
tion that circulated through the rocks, mobilizing and re-
precipitating iron in the form of hematite spherules (con-
cretions) and dissolving highly soluble minerals to leave the
voids.