Martian Soil and Rock Analysis
In this diagram, preliminary
Pathfinder APXS analyses of soils (yellow dots) extend the range of Viking soil analyses.
The analysis of Yogi appears to be contaminated by dust adhering to the rock's surface.
The rock composition can be estimated by subtracting a portion of dust; the resulting Yogi
composition is very similar to that of Barnacle Bill (we assumed 50% dust having the
composition of drift analysis A-5 and used a linear mixing model to subtract the dust
which is only strictly valid if the dust, where present, is thicker than the APXS
penetration depth). Barnacle Bill is also contaminated by dust, but to a lesser extent.
APXS analyses of Martian soils
are compared with Viking soil analyses. Each element is normalized to silicon in this
diagram. The yellow boxes representing Viking data include all analyses and their
analytical uncertainties reported by B.C. Clark and others (1982) Journal of Geophysical
Research, vol. 87, p. 10,064. Although the first APXS soil analysis (A-2) was reported to
be almost identical to Viking soils, ssubsequent analyses demonstrate some variability and
a few significant differences from Viking analyses. Specifically, soils at the Pathfinder
site generally have higher aluminum and magnesium, and lower iron, chlorine, and sulfur.
Scooby Doo, which appears to be a sedimentary rock composed primarily of compacted soil,
also exhibits a few chemical differences form the surrounding soils. Analysis A-5
represents a deposit of windblown dust (called drift), whereas the other soil analyses may
be cemented materials.
This diagram (preliminary X-ray data) illustrates chemical differences
between terrestrial rocks and meteorites inferred to have been derived from Mars. The
Martian meteorites (as well as Viking soil analyses) all plot to the left of the fields
for Earth rocks. Pathfinder APXS analyses of rocks (stars) and soils (yellow dots) appear
to plot in the gap between these previously defined fields, although they are similar to
at least one basaltic meteorite. The other two stars represent the compositions of
Barnacle Bill and Yogi. The analysis of Yogi appears to be contaminated by dust adhering
to the rock's surface. The rock composition can be estimated by subtracting a portion of
dust; the resulting Yogi composition is very similar to that of Barnacle Bill (we have
assumed 50% dust having the composition of drift analysis A-5 and used a linear mixing
model to subtract the dust which is only strictly valid if the dust, where present, is
thicker than the APXS penetration depth). Barnacle Bill is also contaminated by dust, but
to a lesser extent.
The Pathfinder APXS chemical analyses of
Barnacle Bill and Yogi (corrected for adhering dust) have been recast into plausible
minerals using the CIPW norm calculation. If they are fully crystalline igneous rocks,
both possibly consist of orthopyroxene (magnesium-iron silicate), feldspars (aluminum
silicates of potassium, sodium, and calcium), quartz (silicon dioxide), and other minerals
that include magnetite, ilmenite, iron sulfide, and calcium phosphate.
This commonly used chemical classification
for lavas show that Barnacle Bill and Yogi (corrected for adhering dust) are distinct from
basaltic Martian meteorites (shown as red squares). The Pathfinder APXS analyses have been
corrected for the presence of a small amount of salt, and sulfur is assumed to be present
as sulfide. These rocks plot in or near the field of andesites, a type of lava common at
continental margins on the Earth. The preliminary data for alkalis are likely to represent
upper limits, so refinement of these analyses could shift them to slightly lower Na2O +
K2O and higher SiO2. We do not presently know whether these are igneous (crystallized from
a melt), sedimentary (grains/fragments deposited by wind or water or precipitates), or
metamorphic rocks (deformed).