[meteorite-list] Show Me the Carbonates

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:28:36 2004
Message-ID: <200310282155.NAA16027_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/Oct03/carbonatesMars.html

Planetary Science Research Discoveries
October 28, 2003

Show Me the Carbonates
Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology

--- Carbonate minerals intermingle with silicates in the Martian surface dust.

The Martian surface dust is 2 to 5 weight % carbonate minerals. Joshua
Bandfield, Timothy Glotch, and Philip Christensen (Arizona State University)
reported the result after examining Mars Global Surveyor Thermal Emission
Spectrometer (TES) data from 21 high-albedo, dusty surfaces on Mars located
between 30oS and 15oN. Trace amounts of carbonates are widely distributed in
the silicate-rich dust, but no evidence has been found in the TES data for
widespread deposits of exposed carbonate rock. The small amount of detected
carbonate is more consistent with the idea that Mars has long been cold and
mostly dry rather than a place formerly warm and wet with a thick carbon
dioxide atmosphere, and especially favorable for life.

Reference:

     Bandfield, J. L., Glotch, T. D., and Christensen, P. R. (2003)
     Spectroscopic identification of carbonate minerals in the Martian dust.
     Science, v. 301, p. 1084-1087.

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

Why Look for Carbonates on Mars?

[Artist Veenenbos' rendering of ancient water on Mars]

The motivation to search for carbonates on Mars is the mineral's
relationship to water. Carbonates form when carbon dioxide gas
dissolves in water releasing negatively charged carbonate
ions (anions, CO32-) that bind to a variety of positively charged
ions (cations) such as calcium or magnesium. This means that carbonate
minerals precipitate out of carbon dioxide-rich solutions; they form
readily in the presence of water and a carbon dioxide atmosphere. If the
hypotheses for a ancient thick carbon dioxide atmosphere and water on Mars
are true, including an ancient northern ocean, widespread smaller standing
bodies of water, and outflow channels (as depicted in the graphic on the
right), then one line of evidence would be the presence of carbonate rocks.

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

Carbonates in Martian Meteorites

Cosmochemical studies of Martian meteorites give us a direct look at
water-precipitated minerals including carbonates, salts, sulfates, and clays
of extraterrestrial origin (e.g. James Gooding, NASA Johnson Space Center
and colleagues). This cosmochemical evidence indicates that water was
chemically active on Mars for at least the time span represented by the
radiometric ages of the meteorites, that is the past 200-1300 million years.
But the Martian meteorites are not severely weathered. The alteration
products found in these rocks suggest only intermittent contact with water
on Mars [see PSRD article "Liquid Water on Mars: The Story from
Meteorites."]

Martian carbonate minerals stirred up a commotion in 1996 when a group of
scientists from Johnson Space Center, Lockheed Martin, and three
universities published a paper in Science called "Search for Past Life on
Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001." ALH
84001 was a slowly-cooled igneous rock in
the Martian crust before it was excavated
by an impact, altered by fluids, sent to Earth 13,000 years ago by another
impact, and finally collected off the Antarctic ice in 1984. The researchers
suggested that the meteorite contained evidence of potential fossils within
the carbonate globules that were found in crushed zones and cracks in the
rock. Fluids rich in carbon dioxide presumably flowed through cracks in the
Martian rock depositing globules, plates, and veins of carbonate minerals.
Whether these carbonate minerals formed by biologic origin or not and the
temperature at which they formed are still issues of debate. [For
background, see PSRD articles "Shocked Carbonates may Spell N-o L-i-f-e in
Martian Meteorite ALH84001," "Low-temperature Origin of Carbonates
Consistent with Life in ALH84001," and "Life on Mars?"] Simply and plainly,
the existence of carbonates in Martian meteorites confirms the presence of
the minerals on Mars.

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

Finding Carbonates on Mars Remotely

Carbonate minerals have unique absorptions in the near-infrared and thermal
infrared spectral regions because of vibrations due to stretching and
bending of the carbon-oxygen bonds. Examples of thermal infrared emission
spectra of carbonate minerals collected in the laboratory are shown below
next to a photo of the instrument used by the researchers.

 [spectra of carbonates][spectrometer at Arizona State Univ.]

     LEFT: Spectra of laboratory samples. The absorption
     band between 1350 and 1580 cm-1 is diagnostic of
     carbonate minerals. Spectra are offset for easier
     comparison. RIGHT: Spectrometer.

Laboratory spectra serve as the standards against which TES data of Mars are
compared. Since these comparisons began, no areas of carbonate rocks have
been found anywhere on Mars within the resolution limits of the instrument
(e.g. areas a few tens of square kilometers.) But this year's new work on
the TES spectra by Bandfield and colleagues resulted in the detection of
carbonate minerals in the Martian surface dust.

Josh Bandfield and Michael Smith (NASA Goddard Space Flight Center)
developed a way to mathematically separate effects of the atmosphere from
the surface in the TES data. Their work led to the first detailed spectrum
of the dusty surface isolated from the interfering effects of atmospheric
dust, water ice aerosols, carbon dioxide, and water vapor.

Then Bandfield, Glotch, and Christensen examined 21 TES sequences from a
variety of dusty regions between 30oS and 15oN. Though the 21 sites were in
different places there were no detectable variations in the shapes of the
spectra, probably because the Martian wind is so effective in mixing and
moving the dust over the entire globe.

 [comparison of 2 spectra]

The spectrum of the Martian surface dust is shown
in the upper curve on the graph shown on the
left. To determine the compositions of the
materials that can produce such a spectrum,
Bandfield and coauthors mixed pure labradorite
(calcium sodium aluminum silicate) dust
separately with several different carbonate
minerals: calcite, dolomite, siderite, and
magnesite. Other non-carbonates were also mixed
with the labradorite for comparison, including
hematite, gypsum, and high silica glass.
Bandfield and colleagues chose labradorite as the
silicate mineral in their model because of its
general similarity to the Martian high-albedo
surface spectrum, its purity, and its easy
availability. Of all the mineral mixtures tested,
only the addition of carbonate minerals and
gypsum had any effect on the spectrum of the
fine-grained labradorite. Furthermore, the
spectral shape between 1350 and 1580 cm-1 is
unique to materials that contain carbonate
minerals, and no other mixture matched the
Martian spectrum at these wavelengths. Only the
addition of magnesite to the labradorite produced
a match with the Martian surface dust spectrum in
both magnitude and shape at wavenumbers greater
than 1300 cm-1.

The researchers tested the effects of particle
sizes of the laboratory samples on the resulting
spectrum. They separated labradorite and
magnesite into particle size fractions of 0-5,
0-10, 10-20, 20-30, and 30-41 microns. The
magnesite dust was added to the labradorite dust
in 0.5 weight % increments until the resulting
spectrum matched the Martian surface dust
spectrum at wavenumbers greater than 1300 cm-1.
The 0 to 10 micron particle sizes were the most
sensitive to the addition of small amounts of
carbonate. They found that the Martian spectrum
could be reproduced with at least 2 to 3 weight %
magnesite. A tentative upper limit on the
abundance of carbonates in the Martian surface
dust is set at 5 weight % as suggested by other
remote sensing research in the near- and
mid-infrared spectral regions. In the final
analysis, magnesite may not be a unique answer if
future work shows that other carbonate mineral
mixtures also match the Martian spectrum.
Significantly, these new TES results demonstrate
that where there is dust on Mars, there are
carbonates in low concentrations.

 ABOVE: The TES spectrum of the Martian dust (top) matches well with the
 spectrum of a laboratory sample mixture (bottom) of 98 weight % labradorite
 silicate and 2 weight % magnesite carbonate (MgCO3) of particle sizes
 between 0-10 microns. The spectra have been offset for comparison.

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

What a Trace of Carbonates Might Mean

The work by Bandfield and colleagues found that carbonate minerals at
concentrations of <5 weight % are common in the Martian surface dust (see
the image below for a typical dusty surface). Previous work also recognized
carbonates (1 to 3% volume) in the airborne dust using thermal emission
spectra of Mars from telescopes (e.g. the Kuiper Airborne Observatory data
used by James Pollack and colleagues in 1990). Bits of carbonate minerals
have been studied in Martian meteorites. The task is to figure out how these
findings of carbonate minerals fit into the history of water and climate on
Mars.

Martian dust appears to have ubiquitous low levels
of carbonate minerals yet the planet appears to lack carbonate cliffs or
outcrops. Does this mean there were no oceans, lakes, or running water on
the Red Planet? No, it's not that simple. Carbonates are simply expected as
a natural product of weathering in a wet Martian environment, and there are
good reasons for thinking that Mars was wet. A variety of surface features
are attributed to ancient surface water (e.g. channels, outflow channels)
and recent groundwater seepage or snow (gullies) even though water is not
stable on the surface of Mars today. Subsurface water ice was detected last
year by the Mars Odyssey suite of instruments (see PSRD article Dirty Ice on
Mars). It is possible that the small amounts of carbonate minerals in the
dust are erosional remnants of ancient carbonate source rocks whose
formation served as a buffer for atmospheric CO2. These carbonate rock
layers may simply be hidden now beneath layers of the silicate-rich dust.
It's also possible that the carbonates were washed underground and are now
too deep to be detected by TES.

On the other hand, there are reasons favoring a colder, dryer Mars where
extensive carbonate rock layers never formed. It is most probable, say
Bandfield and coauthors, that the small amounts of carbonate minerals
detected in the surface dust were not derived from carbonate outcrops but
formed through the ages by simple reactions between the dust and moisture in
the thin Martian atmosphere. Additionally, spectroscopic evidence for clays,
which are known indicators of aqueous weathering, remains inconclusive.

The questions of how carbonate minerals formed on Mars and what that means
about the environment and climate in which they formed are still being
answered. Ultimately, they'll lead us to answers about the duration of
surface and near-surface water in Mars' past. In August, 2003 in reference
to finding traces of carbonates and what that may mean for water on Mars,
Phil Christensen, principal investigator for the TES instrument said, "Maybe
instead of calling them oceans, we should call them glaciers. A frozen ocean
will not form carbonate. I believe Mars has a lot of water, but it is cold
and frozen most of the time. That is consistent with what we have seen."

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

ADDITIONAL RESOURCES

     Archived news release, August 22, 2003, from Arizona State University.

     Bandfield, J. L., Glotch, T. D., and Christensen, P. R. (2003)
     Spectroscopic identification of carbonate minerals in the Martian dust.
     Science, v. 301, p. 1084-1087.

     Bandfield, J. L. and Smith, M. D. (2003) Multiple emission angle
     surface-atmosphere separations of thermal emission spectrometer data.
     Icarus, v. 161, p. 47.

     Gooding, J. L. (1992) Soil mineralogy and chemistry on Mars: Possible
     clues from salts and clays in SNC meteorites. Icarus, v. 99, p. 28-41.

     Lane, M. D. and Christensen, P. R. (1997) Thermal infrared emission
     spectroscopy of anhydrous carbonates, Journal of Geophysical Research,
     v. 102, p. 25581-25592.

     Mars Meteorite Compendium-2003

     McKay, D.S., Gibson, Jr. E.K., Thomas-Keprta, K.L., Vali, H., Romanek,
     C.S., Clemett, S.J., Chillier, X.D.F., Maechling, C.R. and Zare, R.N.
     (1996) Search for Past Life on Mars: Possible Relic Biogenic Activity
     in Martian Meteorite ALH84001, Science, v. 273, p. 924-930.

     Pollack, J. B, Roush, T., Witteborn, F., Bregman, J., Wooden, D.,
     Stoker, C., Toon, O. B., Rank, D., Dalton, B., and Freedman, R. (1990)
     Thermal emission spectra of Mars (5.4-10.5 microns): evidence for
     sulfates, carbonates, and hydrates, Journal of Geophysical Research, v.
     95 (B9), p. 14595-14627.

     Scott, E. R. D. (1997) Shocked Carbonates may Spell N-o L-i-f-e in
     Martian Meteorite ALH84001. Planetary Science Research Discoveries.
     http://www.psrd.hawaii.edu/May97/ShockedCarb.html.

     Taylor, G. J., (2002) Dirty Ice on Mars. Planetary Science Research
     Discoveries. http://www.psrd.hawaii.edu/June02/MarsGRSice.html.

     Taylor, G. J. (2000) Liquid Water on Mars: The Story from Meteorites.
     Planetary Science Research Discoveries.
     http://www.psrd.hawaii.edu/May00/wetMars.html.

     Taylor, G. J. (1997) Low-temperature Origin of Carbonates Consistent
     with Life in ALH84001. Planetary Science Research Discoveries.
     http://www.psrd.hawaii.edu/May97/LowTempCarb.html.

     Taylor, G. J. (1996) Life on Mars? Planetary Science Research
     Discoveries. http://www.psrd.hawaii.edu/Oct96/LifeonMars.html.

     Thermal Emission Spectrometer (TES) on Mars Global Surveyor (MGS).
Received on Tue 28 Oct 2003 04:55:01 PM PST