APXS Composition Results
Detailed information on the APXS instrument
Volcanic Rock Classification
Igneous rocks are usually classified according
to the minerals they contain. In the absence of
mineralogic data, volcanic rocks can be
classified using their chemical compositions.
Shown here is a scheme
which classifies volcanic rocks on the basis of their
alkalis (Na2O and K2O) and silica (SiO2) contents.
This commonly used chemical classification for lavas shows 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).
If they are igneous rocks,
this classification would indicate that they has been
remelted and reprocessed, implying that Mars may
have had a more active thermal history than
previously thought. However, this measurement could
also be a result of a physical mixture of
particles of rocks such as granite and basalt.
spot reflectance spectra of Barnacle Bill
shows that if the rock is made of a mixture, the
particles are smaller than about 1 cm.
Magnesium/Silicon versus Aluminum/Silicon
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 (estimated based on
a correction 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
Sodium/Silicon versus Iron/Silicon
Shown here are the analyses of Yogi (A-7)
and Barnacle Bill (A-3) on a plot of Na/Si vs.
Fe/Mn. Na/Si is not a good indicator of
different planetary bodies (and the APXS
analyses of Na have a large error), but the
Fe/Mn ratio is a diagnostic feature that
separates Martian rocks from all other rocks.
As can be seen, Yogi and Barnacle Bill are
Calcium/Silicon versus Iron/Silicon
These three elements are especially well suited
for APXS analysis. The compositions of SNC
meteorites, as well as Viking soils and Mars
Pathfinder soils, have
higher iron/silicon ratios than terrestrial rocks.
Barnacle Bill's composition (A-3) plots to the
left, because of its high silicon content.
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.
Martian Soil Composition
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)
JGR, vol. 87, p. 10,064. Although the first APXS soil analysis (A-2)
was reported to be almost identical to Viking soils, subsequent 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.
Table of elemental compositions -
Preliminary results for rocks and soils in wt %
Detailed information on the APXS instrument
APXS Mars surface composition results
- Press Release 04 December 1997
Mars Fact Sheet
Mars Pathfinder home page at NSSDC
Questions and comments about this page should be addressed to:
Dr. David R. Williams, firstname.lastname@example.org, (301) 286-1258
NSSDC, Mail Code 633, NASA/Goddard Space Flight Center, Greenbelt, MD 20771
NASA Official: J. H. King, email@example.com
Last Updated: 18 December 2001, DRW