Current Science Data
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Shown here is a comparison of the APXS analysis
of Yogi (A-7) and Barnacle Bill (A-3). Also shown are the Martian meteorites,
Viking soils, and terrestrial ultramafic rocks and basalts. The two lines
on the graph are Martian and terrestrial fractionation trends, respectively.
Recent images suggest that the analysis of Yogi may have a significant soil
component, and this rock analysis must be considered to be very preliminary.
Yogi and Barnacle Bill are more earthlike than
we would have expected from consideration of the Martian meteorites. Yogi
is a more primitive rock than Barnacle Bill, having not gone through as
much "cooking" as Barnacle Bill. |
Shown here is a scheme which classifies volcanic rocks
on the basis of their alkalis (Na2O and K2O) and silica (SiO2) contents.
The possibility that these rocks are not volcanic, but rather the result
of an impact event, cannot be ignored. If Yogi (A-7) and Barnacle Bill (A-3)
are volcanic, then Yogi is a basalt and Barnacle Bill is an andesite. Basalts
are the most common rocks in our solar system, covering most of Mercury,
Venus, Earth, our Moon, and Mars. Basalts are important rocks for deciphering
the history of rocky bodies in the solar system, and the Pathfinder Martian
basalts will undoubtedly lead to major revisions in our understanding of
the red planet. |
Shown here are the three soil analyses labeled A-2,
A-4, and A-5 compared to the average of soils analyzed by the Viking landers
in 1976. The APXS (Alpha Proton Xray Spectrometer) analyses are preliminary,
and the least amount of error is induced by normalizing to the major element
silicon. As can be seen, the three Sojourner analyses are all quite similar
to the Viking soils, even though the Viking landers were over a 1000 kilometers
away from the Pathfinder landing site. This implies that there is a globally
homogeneous soil layer on Mars, probably the result of many, many years
of mixing by the winds of Mars. As a result, the Sojourner rover will mainly
concentrate on analyzing rocks for the rest of the mission. |
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| 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 quite Martian. |
Shown here are three pie charts showing the normative
mineralogy of Yogi and Barnacle Bill. A norm is a way of taking a chemical
analysis and converting it into mineral phases. The two pie charts on the
left are conventional CIPW normative plots of Yogi and Barnacle Bill. The
main difference between these two rocks is in the normative quartz contents.
Yogi has less normative quartz than Barnacle Bill, indicative of the primitive
nature of Yogi. Yogi has not undergone the extensive "cooking"
that Barnacle Bill has experienced.
On the right is the "Mars Norm" for Yogi.
The Mars Norm is a calculation made by Dr. Amitabha Ghosh (University of
Tennessee) which attempts to calculate the actual mineralogy of a Martian
basalt. The Mars Norm has been tested and calibrated with the Martian meteorites,
and may be a better indicator of the actual mineralogy of Yogi. Interestingly,
the Mars Norm of Yogi is quite similar to the CIPW norm of Yogi (top left),
pointing towards its more earth-like character. |
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The Mars Pathfinder Magnetic Properties
Experiment primarily involves an array of permanent magnets on the lander.
The magnets are assembled to produce a bullseye pattern of attracted dust
using an outer annular ring magnet, 18mm in diameter, surrounding a central
cylindrical magnet. In each of two magnet arrays, five such magnets of progressively
increasing strength, are mounted in magnesium blocks.
These magnets are intended to attract
any magnetic particles in the windborne dust. The picture shows a magnet
array on Sol 6 and on Sol 13. Dust has clearly accumulated on the two strongest
magnets. As more dust is attracted with time, we expect the patterns on
the magnets to become clearer.
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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. Using this system, Martian rock Barnacle Bill
(labelled here as APXS site A-3) is classified as an andesite. The term
"andesite" derives from the Andes Mountains in South America,
where this type of lava is particularly abundant. Barnacle Bill's andesitic
composition could indicate that it is a volcanic rock (a true andesite)
or a physical mixture of particles of rocks such as granite and basalt.
Such a mixture could have been formed as the particles were transported
and deposited as a sediment, or could have been produced during a large
impact that pulverized and mixed the target rocks in andesitic proportions. |
Magnesium/Silicon versus Aluminum/Silicon
SNC meteorites are thought to be Martian samples, based primarily on the
fact that they contain trapped gases that have the composition of the Martian
atmosphere. All the known SNC meteorites, as well as Viking soils, lie well
to the left of the trend for Earth rocks in this diagram. Weight ratios
of elements tend to be more precise than absolute element abundances, so
ratios are used to plot these preliminary APXS data. Martian rock Barnacle
Bill (labelled as APXS site A-3), plots near the line defined by SNCs, perhaps
providing support for the idea that SNC meteorites are actually derived
from Mars. |
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, have higher iron/silicon ratios
than terrestrial rocks. The APXS soil analysis A-2 plots directly on top
of Viking soils in this diagram. However, Barnacle Bill's composition (A-3)
plots to the left, because of its high silicon content. |
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One of the first "multispectral spots"
obtained by the IMP camera was of the Stripe Rock on Sol 4. A multispectral
spot measurement obtains small images of a region of interest in all geology
filters with no image compression. Stripe rock is of interest to Mars Pathfinder
scientists because of a bright vertical stripe that appears on the center
of the rock face. It was thought that this stripe might be an intruded vein
of material of different composition than the surrounding rock.
The color image of this rock shows that the stripe
is of similar color to the surrounding soils (see arrow). A detailed examination
of the rock was conducted to extract preliminary reflectance spectra (that
is, the variation of brightness with color) from nearby bright and dark
soils, the stripe, and the surrounding rock. Although these data require
further calibration (e.g., the lower reflectance at 965 nm is not reliable
at this time), they do show that the general spectral characteristic of
the stripe is quite similar to the nearby dark soil. This suggests that
the "stripe" is actually an accumulation of soil deposited in
a crack in the rock face.
Mars Pathfinder Mission
Mineralogy and Geochemistry Science Operations Group |
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Barnacle Bill Rock
Hypothesis: APXS data show composition of rock is consistent with
volcanic andesite, but rough texture of surface suggests it may be a "breccia."
Could it be composed of many different rock fragments that combine to
give a similar overall composition?
Method: Target Barnacle Bill with "multispectral spot" (all
geology filters at full spatial resolution of about 1-2 cm per picture element)
Goal: Determine variability of reflectance spectra (mineralogy) across
the face of the rock
If all spectra are similar: rock is "homogeneous" (composed
of the same material)
If spectra vary: rock may be "heterogeneous" (such as an impact
melt breccia or sedimentary conglomerate)
Result: Spectra taken from many different locations show only two
basic kinds of spectra:
- Soil-like deposits
- Dark rock face
Implication: At spatial resolution of 1-2 cm, rock composition is
homogeneous. However, rock may be composed of fine-grained materials (<
1-2 cm) that cannot be seen with this method. |
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| This image shows the location of Barnacle Bill rock
(left of the Sojourner rover) and the approximate location of the full-resolution
"multispectral spot" acquired on Barnacle Bill. Lossless (no compression)
images were taken in all geology filters using the IMP camera to study in
detail the variation of brightness in each filter, which provides information
regarding the mineralogy of the material sampled. Spectra were extracted
from several study regions (shown to the right of the high resolution view).
The green area represents soil found behind the rock. Red patches represent
brighter areas on the rock that are interpreted as accumulations of wind-blown
dust found in small holes, or vesicles, on the rock. Blue patches represent
darker rock faces not contaminated by a soil deposit. The spectra of these
materials are shown in the accompanying figure. |
Preliminary data acquired from the "multispectral
spot" image sequence for Barnacle Bill rock. Images were acquired with
no compression in all geology filters. Reflectance spectra (that is, the
variation of brightness with wavelength, or color) are shown for background
soil (green), soil-like deposits found on and within small holes in the
rock (red), and dark portions of the rock face (blue). Comparison of the
spectra of these three types of materials demonstrates that the rock has
relatively homogeneous composition at the spatial resolution of the patches
sampled (about 1-3 cm). That is, all soil-like deposit and rock face spectra
cluster in both their overall brightness (reflectance) and shape of their
reflectance curves. A more heterogeneous rock would show variable spectral
characteristics across its face. Note that the spectra of the soil-like
deposit is intermediate to that of the background soil and rock face spectra.
This is consistent with the interpretation that the soil-like deposit is
a relatively thin layer in which portions of the rock are also sampled within
the patches selected.
Also shown are laboratory spectra of oxidized and
unoxidized volcanic rocks from Earth. Scientists will compare spectra of
terrestrial materials such as these to help determine the composition of
the rocks observed at the landing site in combination with data returned
by other instruments such as the APXS. |
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