[meteorite-list] The Tricky Business of Identifying Rocks on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:51 2004
Message-ID: <200205231735.KAA00354_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/May02/MarsTES.html

The Tricky Business of Identifying Rocks on Mars
Planetary Science Research Discoveries
May 22, 2002

     --- A new analysis of thermal emission spectra suggests a new [Mars]
     interpretation for the composition of the Martian surface.

Written by G. Jeffrey Taylor
Hawaii Institute of Geophysics and Planetology

The Mars Global Surveyor mission carried a remote-sensing gizmo called the
Thermal Emission Spectrometer (TES). TES detected heat waves flowing from
the surface of the Red Planet. The TES team, led by Phil Christensen
(Arizona State University), identified two large regions on Mars that have
distinctive spectral properties. Using mathematical mixing calculations
based on the thermal emission spectra of numerous materials, the TES team
reported in papers led by Josh Bandfield and Victoria Hamilton that the two
regions had mineral abundances similar to basalt (Surface Type 1) and
andesite (Surface Type 2), two common volcanic rock types on Earth. Andesite
has more silicon than does basalt, giving rise to a distinctive mineralogy.

Scientists had mixed reactions to the possibility of andesite on Mars,
greeting the news with fascination, consternation, or skepticism. One
question raised is how uniquely the spectra of Surface Type 2 matches
andesite. Michael Wyatt and Harry Y. McSween (University of Tennessee) have
taken another look at the TES spectra by using a larger collection of
aqueous alteration (weathering) products in the spectral mixing
calculations. They show that weathered basalt also matches the spectral
properties of Surface Type 2. Wyatt and McSween also note that Type 2
regions are generally confined to a large, low region that is the site of a
purported Martian ocean that sloshed around billions of years ago. They
suggest that basalts like those in Surface Type 1 were altered in the
ancient Martian sea. Independent data are needed to test the andesite vs.
altered-basalt hypotheses. For now, we may have to be satisfied with at
least two working hypotheses and a lively debate.

     References:

     Wyatt, M. and McSween Jr., H. Y. (2002) Spectral evidence for weathered
     basalt as an alternative to andesite in the northern lowlands of Mars.
     Nature, vol.417, p. 263-266.

     Bandfield, J. L., Hamilton, V. E., and Christensen, P.R. (2000) A
     global view of martian surface compositions from MGS-TES. Science, vol.
     287, p. 1626-1630.

     Hamilton, V. E., Wyatt, M. B., McSween Jr., H. Y., and Christensen, P.
     R. (2001) Analysis of terrestrial and martian volcanic compositions
     using thermal emission spectroscopy: II. Application to martian surface
     spectra from the Mars Global Surveyor Thermal Emission Spectrometer, J.
     Geophys. Res., vol. 106, p. 14,733-14,746.

             --------------------------------------------------

Thermal Eyes[EM specs]

There is a wealth of information in the heat waves emitted from the surface
of a planet. TES measured the intensity of the heat radiated in the
wavelength range from 6 to 50 micrometers, well beyond what we humans can
see. The intensity at different wavelengths (called spectra) allow experts
like Phil Christensen and his team to deduce some physical properties of the
surface, such as the abundance of boulders versus dust.

Thermal spectra also allow us to infer what minerals are present on the
surface and in what proportions. This is possible because most minerals have
unique spectra--a spectral fingerprint. The trouble is that a planet's
surface has more than one mineral, so all the fingerprints are on top of one
another. The TES team had to separate each fingerprint, a technique called
spectral deconvolution. This unmixing requires correcting for the effects of
the gases and dust in the Martian atmosphere, calibrating the response of
the instrument at each wavelength, and making other corrections. The whole
effort has been done as well as the best forensic laboratories do in
identifying the culprits of crimes from smudgy fingerprints at a crime
scene.

The thermal spectra of numerous minerals have been measured in the
laboratory. Christensen and his colleagues have assembled all the
measurements into a spectral library--a database of spectral fingerprints
like that maintained by the FBI for human fingerprints. This allows them to
mathematically combine the spectra of several minerals into a theoretical
composite spectrum for comparison with the Martian surface. If there is a
good match between calculated and theoretical spectra it suggests that the
minerals used in the calculation are present in the proportions that
produced the good match. The trouble is that there is not necessarily a
unique combination of minerals that match the measured Martian spectra. It
depends on which minerals go into the theoretical mix, the chemical
compositions of the minerals, and how distinctive each mineral's spectrum
is. It's a tricky business.

             --------------------------------------------------

Distinctive Surface Types

Much of the surface of Mars is covered with reddish dust. The dust gives
Mars its dramatic red color, but obscures the materials beneath it.
Fortunately, there seem to be areas relatively free of dust. Josh Bandfield
and other TES team members interpreted the spectra from these areas as
falling into two categories. One (Surface Type 1) is similar to spectra from
basalt. This was not too surprising as basalt is the most common rock type
on Earth and occurs on Venus, the Moon, and even some asteroids.

The other category (Surface Type 2) was a surprise. Banfield and coworkers
(reaffirmed by Hamilton and others) interpreted the spectra of Surface Type
2 as indicating a volcanic rock called andesite. This interpretation is
supported by analyses of rocks by an instrument on the Mars Pathfinder
rover. Andesite contains more SiO2 than does basalt (52-63 wt% in andesite
vs. < 52 wt% in basalt). On Earth, andesite forms in two ways. Most andesite
forms where oceanic crust descends at converging margins of tectonic plates.
Water released from the wet oceanic crust rises to promote melting in the
wedge of mantle above it. This happens in the Andes Mountains, from which
andesite gets its name. There is no evidence, such as the presence of
arc-shaped mountain ranges, that large plates dove beneath other plates on
Mars.

      [surface type 1]
      Spectral mixing calculations of Surface Type 1 (above) and
      Surface Type 2 (below) indicate that they have different mineral
      abundances. Detailed analysis and conversion of the mineralogy
      to chemical composition suggests that Type 1 is basalt and Type
      2 is andesite.
      [surface type 1]

The other way of making andesite is by removing crystals as they form in
basalt magma, a process called fractional crystallization. Removal of
minerals that contain less SiO2 than does the magma causes SiO2 to increase,
eventually reaching the andesite range. (Some geologist call such magma
icelandite rather than andesite, to distinguish the two ways it can form.
For simplicity, I'll stick with andesite.) McSween and members of the Mars
Pathfinder team have argued that this process could not produce the large
volume of andesite observed (if it is really andesite). They point out that
the amount of SiO2 observed in andesite is not reached until 90% of the
original basalt magma has crystallized. In other words, there ought to be
much more basalt than andesite.

It is possible to produce andesite when only about half of a basalt has
crystallized if the basalt magma contains dissolved water. Michelle Minitti
and Mac Rutherford (Brown University) showed that fractional crystallization
of a magma with the composition of shergottites (a type of Martian
meteorite) will reach 58 wt% SiO2 after only 60% crystallization.
Nevertheless, Wyatt and McSween still think that there ought to be lots of
basalt associated with the andesite--about equal amounts of each. This is
consistent with Banfield's observations: If Surface Type 2 is andesite, maps
of the distribution of basalt and andesite suggest that equal amounts of
basalt and andesite exist on Mars (you can estimate the abundances of
Surface Types 1 and 2 in the maps below). On the other hand, if andesite
formed by fractional crystallization, it ought to be associated more
intimately with the basalt, rather than concentrated in different places.
This drove Wyatt and McSween to look at other ways to interpret the TES
data. They tested the idea that Surface Type 2 is not andesite at all. They
suggest it could be basalt altered by interaction with water. One clue to
this possibility is that the andesite-like surface is confined mostly to a
region suspected to have been the site of an ancient Martian sea [See PSRD
article: Outflow Channels May Make a Case for a Bygone Ocean on Mars].

                [locations of surface types 1 and 2]
                The basalt and andesite identified by Josh
                Bandfield and the TES team occur in
                different places on Mars (shown on the
                lefthand maps of the northern and southern
                hemispheres.) The surface interpreted as
                andesite (red, Surface Type 2) is
                concentrated in the northern hemisphere in
                a large, low region previously interpreted
                as an ancient ocean on Mars, as seen in the
                topographic map on the right. The white
                line outlines the location of the purported
                shoreline. Click on the image for a larger
                version.

             --------------------------------------------------

Back to the Spectral Library for Some Weathering Products

To test this hypothesis, Wyatt and McSween used a different set of minerals
from the Arizona State spectral library. If the Type 2 area lies at the
bottom of a former ocean, they reasoned, maybe all that water chemically
altered basalt. Water certainly affects the mineralogy of rocks on Earth,
forming an assortment of water-bearing (hydrous) minerals. Which minerals
form depends on numerous factors, including the temperature, the acidity,
and the amount of oxygen and other elements dissolved in the water.
Nevertheless, the basic idea of altered basalt can be tested by a greater
number of hydrous minerals in their mixing calculation compared to the set
used by Bandfield and Hamilton. Most important, Wyatt's calculations
differed from Bandfield's and Hamilton's by not including a component of
glass rich in SiO2. Such glass is a logical component of andesitic rock, but
is not abundant in basalt.

The new calculations produce good matches to the observed spectra. For
Surface Type 1, Wyatt's calculation agrees with Banfield's and Hamilton's.
It looks like relatively unweathered basalt: lots of pyroxene and
plagioclase and not much weathering products (clay minerals, sulfates,
carbonates). For Surface Type 2, however, the modeled mineral abundances are
quite different. Bandfield and Hamilton concluded that there were high
abundances of plagioclase feldspar and high-silica glass, and low abundances
of pyroxene and weathering products. In contrast, Wyatt finds high
abundances of hydrous minerals and other weathering products. He finds no
high-silica glass, of course, because he did not use it in the calculation.
Quantitative assessments of the quality of the match between measured and
calculated spectra indicate that all matches are very good--another example
of how tricky it is to determine mineralogy from thermal emission data.

       [surface type 1]
       Mike Wyatt's calculation suggests that it is possible that
       Surface Type 2 (below) is weathered basalt rather than
       andesite. His calculated spectrum contains more clay minerals
       than do the spectra calculated by Bandfield and Hamilton.
       [surface type 1]

Wyatt's calculation differs from the others in that he substituted hydrous
minerals for high-silica glass. It turns out that the clays used in Wyatt's
calculation have very different spectral properties between 18 and 20
microns. Unfortunately, the Martian atmosphere (mostly carbon dioxide) is
opaque in this wavelength range, so a definitive test cannot be made. Some
independent measurements will be needed, perhaps made by instruments
directly on the surface.

             --------------------------------------------------

Fresh and Weathered Natural Basalt

Like silverware, rocks tarnish. They rust. They rot. Take a good look at any
natural rock surface on Earth. Look at its color. Then smash off some of the
surface with a rock hammer. The fresh surface will look different, usually
shinier and a different shade of color than the weathered surface.

Wyatt and McSween decided to test their idea by obtaining spectra of
weathered and fresh basalt from the Columbia River plateau in Washington and
Oregon, USA. These are extensive flows of basalt and are fairly typical of
terrestrial basalts. Fresh surfaces of Columbia River basalts are somewhat
browner than weathered surfaces, which are darker in color.

                              This road-side photograph by Thor
                              Thordarson shows lobes of pahoehoe lava
      [Columbia River basalts]from the Columbia River basalts.
                              Unweathered (grey-brown) pahoehoe lobes
                              overlie strongly weathered, darker
                              basalt pahoehoe lobes.

Wyatt measured the thermal spectra of fresh and weathered basalt and
compared the results to Surface Types 1 and 2. He found that the fresh
basalt is somewhat similar to Surface Type 1, the one interpreted by
everyone to be basalt. On the other hand, the weathered surface of the
basalt was not a bad match for Surface Type 2, the one interpreted
previously by Bandfield and Hamilton to be andesite. One problem with using
the Columbia River basalts as terrestrial analogs is that they were not
weathered under the same conditions as Wyatt and McSween hypothesize for
Surface Type 2 on Mars. The Columbia River basalts were weathered by rain
and the atmosphere. They were not sitting at the bottom of a Martian ocean.

               [basalt spectra]
               Fresh and weathered terrestrial basalt spectra
               match Surface Type 1 and 2, respectively. This
               is consistent with Wyatt and McSween's
               suggestion that Surface Type 2 is weathered
               basalt, not andesite.

Wyatt also did spectral deconvolution calculations on the Columbia River
basalt spectra. He found that the fresh basalt contains mostly pyroxene and
plagioclase, which matches detailed study of the Columbia River basalts. In
contrast, the weathered basalt should contain lots of clay minerals (about
30%), which it does. Using a smaller number of clay minerals and adding
glass, as Bandfield and Hamilton did, produces a calculated mineralogy that
it should contain lots of glass (25%), which it does not. Nevertheless, the
Bandfield technique still yields a good fit to the data. The difference,
Wyatt and McSween suggest, may be that the silica glass mimics
noncrystalline weathering products that could be present in the weathered
rock surfaces, rather than volcanic glass.

             --------------------------------------------------

So what is Surface Type 2?

Surface Type 2 might be andesite. It might be basalt. It might be something
else nobody has considered. Finding out what it is will probably require
other types of data for Mars. It is an important issue because it makes a
huge difference in how we picture the igneous history of Mars and even the
nature of processes operating inside the plumbing systems of Martian
volcanoes. The Gamma Ray Spectrometer (GRS) carried onboard the Mars Odyssey
spacecraft, currently in orbit around Mars, may help settle the argument.
The GRS can measure Si quite readily, so should be able to determine if
Surface Types 1 and 2 differ in Si, as they would if they were composed of
basalt and andesite, respectively. If the Si concentrations are similar, it
would favor the hypothesis that Surface Type 2 is made of weathered basalt.
We should know this answer in a year or two as the GRS onboard Odyssey
methodically determines the composition of the Martian surface.

[ADDITIONAL RESOURCES]

     Bandfield, J. L., Hamilton, V. E., and Christensen, P.R. (2000) A
     global view of martian surface compositions from MGS-TES. Science, vol.
     287, p. 1626-1630.

     Hamilton, V. E., Wyatt, M. B., McSween Jr., H. Y., and Christensen, P.
     R. (2001) Analysis of terrestrial and martian volcanic compositions
     using thermal emission spectroscopy: II. Application to martian surface
     spectra from the Mars Global Surveyor Thermal Emission Spectrometer, J.
     Geophys. Res., vol. 106, p. 14,733-14,746.

     Mars Odyssey homepage.

     Mars Odyssey Gamma Ray Spectrometer homepage.

     Minitti, M.E. and Rutherford, M.J. (2000) Genesis of the Mars
     Pathfinder "sulfur-free" rock from SNC parental liquids. Geochim.
     Cosmo. Acta, vol. 64, p. 2535-2547.

     Wyatt, M. and McSween Jr., H. Y. (2002) Spectral evidence for weathered
     basalt as an alternative to andesite in the northern lowlands of Mars.
     Nature, vol.417, p. 263-266.

----------------------------------------------------------------------------
Received on Thu 23 May 2002 01:35:21 PM PDT