[meteorite-list] Antarctic Guide to Martian Weathering

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 14 11:03:31 2005
Message-ID: <200504141503.j3EF33j25882_at_zagami.jpl.nasa.gov>

http://www.psrd.hawaii.edu/April05/DryValleysSoils.html

Antarctic Guide to Martian Weathering
Planetary Science Research
April 13, 2005

--- Soils in the Antarctic Dry Valleys contain traces of silicate
alteration products and secondary salts much like those found in Martian
meteorites.

Written by Linda M. V. Martel
Hawai'i Institute of Geophysics and Planetology

Field sites in the Antarctic Dry Valleys serve as useful (and relatively
nearby) analogs to the surface of Mars for several reasons that have
been known since the Viking Lander experiments. The environmental
similarities include: low mean temperatures; strong, desiccating winds;
lack of rain; sparse snowfall; sublimation;
diurnal freeze-thaw cycles; low humidity; high solar radiation; and the
presence of salts in the soils. A team of planetary scientists have
recently reexamined earlier work on
cold Antarctic desert soils to better understand the Martian
near-surface environment and weathering processes. The ongoing studies
by Susan Wentworth (ERC/ESCG at Johnson Space Center), Everett Gibson
and David McKay (Johnson Space Center), and Michael Velbel (Michigan
State University) include Martian meteorites, most of which contain
traces of aqueous weathering products. They report that these meteorite
weathering features, which are of Martian origin, are remarkably similar
in composition, nature, and abundance to those found in the Antarctic
Dry Valleys soils, suggesting the weathering processes are also similar.

Reference:

    * S. J. Wentworth, Gibson, E. K., Velbel, M. A., and McKay, D. S.
      (2005) Antarctic Dry Valleys and indigenous weathering in Mars
      meteorites: implications for water and life on Mars. Icarus, v.
      174, p. 382-395.

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

Insights from Polar Desert Soils

Workers in Wright Valley, Antarctica The photograph on the left is a
general scene of the environment of Wright Valley, Antarctica. The
ice-free landscapes of the Dry Valleys seem out of place on this
continent otherwise known for its permanent ice coverage. During the
austral summer field season, temperatures in the Dry Valleys fluctuate
around freezing. Mean temperatures are -23.7 oC to 0.7 oC. The
photograph below is the specific soil pit in the Prospect Mesa of Wright
Valley where soil samples were collected and studied by Wentworth and
colleagues. For scale, the length of the metal tube on the pit wall is
78 centimeters.

photo of soil pit

In the original 1983 study Gibson and colleagues examined soil samples
from a 1-meter-deep pit in Wright Valley, Antarctica (see photos above)
to determine chemistry and micro-scale textures. The samples were
collected from several different layers in the soil column including a
coarse lag deposit at the surface, a salt-rich layer just below it, an
underlying active zone (unfrozen during the austral summer), and a
permanently frozen soil below 40-centimeters depth. They found evidence
of slow weathering processes as shown by the movement of water and salts
through the soil. The water content decreases from the bottom to the top
of the pit (even within the permanently frozen zone) whereas
concentrations of water-soluble ions (salts) increase upwards (see
graphs below). They interpreted these opposing trends of water and salts
as effects of an upward migration of brines and the progressive
evaporation and precipitation of salts.

water and salt trends in soil column
Graphs showing the water content (left) and water-soluble ions (right)
measured in soil samples from the Wright Valley soil pit. Below about 40
centimeters (indicated by the dashed line) the soil is permanently frozen.

Wentworth and colleagues compare their results to those of several more
recent Dry Valleys field studies by other research teams (e.g.,
Dickinson and Rosen, 2003). A characteristic common to Dry Valley soils
in general is the presence of one or more salt-rich layers of typically
the same kinds of salts: calcium carbonate, calcium sulfate, and
magnesium sulfate. Soils from different locations in the Dry Valleys
have different characteristics. Differences are found in the depths
(beneath the surface) of the salt layers, in the number of salt layers
in each soil, and also in the spatial distribution of the types of
salts. It is clear that water and salt migration patterns are markedly
different in different locations in the Dry Valleys, and one
interpretation is that the differences are largely due to the amount of
precipitation and the availability of surface or subsurface water at the
sampling sites. Some soils, especially those at elevations higher than
the Prospect Mesa soil pit, have occasional snow accumulations, while
some locations in the valleys are affected significantly by summer
meltwater. If only minute amounts water become available, however, only
the most soluble salts will be dissolved and carried away; these salts
would later be deposited elsewhere, resulting in the spatial segregation
of different salt species. Where relatively abundant moisture is
present, salts and water migrate downward from the surface and become
concentrated at one specific horizon; in this case some recent
researchers suggest that subsurface ice acts like a cold trap and
results in the net gain of both salts and water within the soils.

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

Comparing Wright Valley Soils with Martian Meteorites

Wentworth and colleagues have teased out the compositional and textural
details of the terrestrial soils using scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). They find that
silicate grains have been altered by water throughout the Antarctic soil
column, even in the permanently frozen zone. The images, below, show
similarities between silicate alteration features in the Wright Valley
soils and in the Martian meteorite Shergotty, which was collected in
August of 1865 in India.

silicate dissolution features
When silicate minerals dissolve they leave behind pits with sharp
"sawtooth" margins or honeycomb patterns. Figures (a) and (c) show
effects of aqueous dissolution on amphibole
found in the permanently frozen soil collected in Wright Valley,
Antarctica. Figures (b) and (d) show dissolution features on pyroxene
in Shergotty.

Trace amounts of salts, notably calcium carbonate, calcium sulfate, and
magnesium sulfate, occur in the Wright Valley soils and also in Martian
meteorites Chassigny and Nakhla, as shown in the images below. Chassigny
was collected in October of 1815 in France and Nakhla was collected in
June of 1911 in Egypt.

secondary salts
Images (a) of calcite rhombs on quartz and (c) calcium sulfate, labeled
"gyp" for gypsum, are both from the Wright Valley soil. Image (b) shows
calcium carbonate grains (labeled carb) around a central calcium sulfate
grain (labeled Ca-sulf) in Martian meteorite Chassigny. Image (d) of the
Martian meteorite Nakhla shows calcium sulfate on pyroxene (labeled px).

You might wonder if the alteration products and salts in these
meteorites formed after the rocks landed on Earth and are a result of
terrestrial weathering. Though it is true that some meteorites in the
world's collections have been altered by terrestrial weathering, this is
not the case with the Martian meteorites considered in this study by
Wentworth and colleagues. First, Chassigny, Shergotty, and Nakhla were
observed falls and were picked up soon after they landed, so there was
little time for weathering in a terrestrial soil. Second, even if they
had been weathered after landing, the meteorites were not exposed to an
Antarctic-like environment, as they fell in France, India, and Egypt,
respectively. Third and most important, the Martian origin of the
alteration products and salts in Chassigny, Shergotty, and Nakhla have
been proven unambiguously. For instance, the researchers see alteration
features that have been heated or truncated by the fusion crust
(the glassy coating remaining after
the rock blazed through the Earth's atmosphere.)

The mineral assemblages in Martian meteorites, so similar in composition
and abundance to those found in the Dry Valleys soils, tell a story
about the interactions between fluids and rocks in the Martian crust.
Planetary geologists and meteoriticists are reading these long and
complicated weathering histories. It seems likely that many of the salts
in Mars meteorites formed through evaporation of briny Martian water by
processes similar to that proposed by John Bridges and Monica Grady
(Natural History Museum, London), for example, for the salt minerals in
Nakhla.

------------------------------------------------------------------------
What it Could Mean for Mars

Click for high resolution and details
<http://www.jpl.nasa.gov/missions/mer/images.cfm?id=1302>
Rocks in the "Columbia Hills" show evidence of past alteration by water
as determined by the Mars Exploration Rover Spirit. [Click image for
high resolution version and additional details.]

There has been a tremendous upsurge in rover and orbital missions to
Mars as well as additional identification of Martian meteorites since
Wentworth and colleagues' first Antarctic soil studies in 1983. The data
taken together--the water and salt trends in the Wright Valley soil pit
and the evidence for aqueous alteration in the Martian meteorites--are
consistent with soil volatile enrichments and salts found by Mars
landing missions (Viking, Pathfinder, Mars Exploration Rovers). For
instance, the Mars Exploration Rover Opportunity science team, headed by
Stephen Squyres (Cornell University) has reported finely laminated
sedimentary rocks rich in sulfur and sulfate salts at Meridiani Planum.
They say the rocks record a history of episodic inundation by shallow
surface water followed by evaporation, exposure, and desiccation. The
OMEGA hyperspectral imager on ESA's current Mars Express orbital mission
has also identified sulfates and water-being minerals near the north
pole and associated with layered terrains in several locations.

Since aqueous processes are active even in the permanently frozen zones
of Dry Valleys soils, Wentworth and colleagues postulate that similar
processes are probably also occurring on Mars today, especially at the
mid-latitudes or in the polar regions during their respective Martian
summers. They say that if the analogy to Antarctica holds true, then
local areas on the Martian surface will certainly have distinct
weathering histories. In the Dry Valleys soils, weathering products and
life are distributed heterogeneously. Hence, Wentworth and coauthors say
that such variations should be taken into account in future studies of
Martian soils and also in the search for possible life on Mars.

The valuable studies by Wentworth and colleagues on terrestrial analog
soils and Martian meteorites are providing insights into how to best
characterize the past and present weathering environments on Mars and
how to search for life on the planet. Their work demonstrates the need
for many samples of rock and soil from different climate regimes, from
different locations within each regime, and from different depths
beneath the surface of Mars.

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

ADDITIONAL RESOURCES

    * Bibring, J.-P., Langevin, Y., Gendrin, A., Gondet, B., Poulet, F.,
      Berth?, M., Soufflot, A., Arvidson, R., Mangold, N., Mustard, J.,
      Drossart, P., and the OMEGA team (2005) Mars surface diversity as
      revealed by the OMEGA/Mars Express observations. Science,, v. 307,
      p. 1576-1581.


    * Bridges, J. C., Catling, D. C., Saxton, J. M., Swindle, T. D.,
      Lyon, I. C., and Grady, M. M. (2001) Alteration assemblages in
      martian meteorites: Implications for near-surface processes. In
      Chronology and Evolution of Mars: Space Science Reviews 96 (R.
      Kallenbach, J. Geiss, and W. K. Hartmann, Eds.), pp. 365-392.
      Kluwer Academic Publishers, Dortrecht.


    * Dickinson, W.W. and Rosen, M.R. (2003) Antarctic permafrost: An
      analogue for water and diagenetic minerals on Mars. Geology, v.
      31, p.199-202.


    * Gibson, E. K., Wentworth, S. J., McKay, D. S. (1983) Chemical
      weathering and diagenesis of a cold desert soil from Wright
      Valley, Antarctica: an analog of martian weathering processes.
      Proceedings of the Lunar and Planetary Science Conference 13,
      A912-A928.


    * Mars Meteorites <http://www2.jpl.nasa.gov/snc/> web site by Ron
      Baalke at the Jet Propulsion Laboratory.


    * Squyres, S. W. and 49 coauthors, (2004) The Opportunity Rover's
      Athena Science investigation at Meridiani Planum, Mars. Science,,
      v. 306, p. 1698-1703.


    * Wentworth, S. J., Gibson, E. K., Velbel, M. A., and McKay, D. S.
      (2005) Antarctic Dry Valleys and indigenous weathering in Mars
      meteorites: implications for water and life on Mars. Icarus, v.
      174, p. 382-395.
Received on Thu 14 Apr 2005 11:03:01 AM PDT


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