[meteorite-list] Wet and Mild: Caltech Researchers Take the Temperature of Mars' Past (ALH84001)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 13 Oct 2011 09:49:45 -0700 (PDT)
Message-ID: <201110131649.p9DGnjE8000432_at_zagami.jpl.nasa.gov>

http://news.caltech.edu/press_releases/13462

Wet and Mild: Caltech Researchers Take the Temperature of Mars' Past
California Institute of Technology
October 12, 2011

PASADENA, Calif. - Researchers at the California Institute of Technology
(Caltech) have directly determined the surface temperature of early Mars
for the first time, providing evidence that's consistent with a warmer
and wetter Martian past.

By analyzing carbonate minerals in a four-billion-year-old meteorite
that originated near the surface of Mars, the scientists determined that
the minerals formed at about 18 degrees Celsius (64 degrees Fahrenheit).
"The thing that's really cool is that 18 degrees is not particularly
cold nor particularly hot," says Woody Fischer, assistant professor of
geobiology and coauthor of the paper, published online in the
Proceedings of the National Academy of Sciences (PNAS) on October 3.
"It's kind of a remarkable result."

Knowing the temperature of Mars is crucial to understanding the planet's
history - its past climate and whether it once had liquid water. The Mars
rovers and orbiting spacecraft have found ancient deltas, rivers,
lakebeds, and mineral deposits, suggesting that water did indeed flow.
Because Mars now has an average temperature of -63 degrees Celsius, the
existence of liquid water in the past means that the climate was much
warmer then. But what's been lacking is data that directly points to
such a history. "There are all these ideas that have been developed
about a warmer, wetter early Mars," Fischer says. "But there's precious
little data that actually bears on it." That is, until now.

The finding is just one data point - but it's the first and only one to
date. "It's proof that early in the history of Mars, at least one place
on the planet was capable of keeping an Earthlike climate for at least a
few hours to a few days," says John Eiler, the Robert P. Sharp Professor
of Geology and professor of geochemistry, and a coauthor of the paper.
The first author is Itay Halevy, a former postdoctoral scholar who's now
at the Weizmann Institute of Science in Israel.

To make their measurement, the researchers analyzed one of the oldest
known rocks in the world: ALH84001, a Martian meteorite discovered in
1984 in the Allan Hills of Antarctica. The meteorite likely started out
tens of meters below the Martian surface and was blown off when another
meteorite struck the area, blasting the piece of Mars toward Earth. The
potato-shaped rock made headlines in 1996 when scientists discovered
tiny globules in it that looked like fossilized bacteria. But the claim
that it was extraterrestrial life didn't hold up upon closer scrutiny.
The origin of the globules, which contain carbonate minerals, remained a
mystery.

"It's been devilishly difficult to work out the process that generated
the carbonate minerals in the first place," Eiler says. But there have
been countless hypotheses, he adds, and they all depend on the
temperature in which the carbonates formed. Some scientists say the
minerals formed when carbonate-rich magma cooled and crystallized.
Others have suggested that the carbonates grew from chemical reactions
in hydrothermal processes. Another idea is that the carbonates
precipitated out of saline solutions. The temperatures required for all
these processes range from above 700 degrees Celsius in the first case
to below freezing in the last. "All of these ideas have merit," Eiler says.

Finding the temperature through independent means would therefore help
narrow down just how the carbonate might have been formed. The
researchers turned to clumped-isotope thermometry, a technique developed
by Eiler and his colleagues that has been used for a variety of
applications, including measuring the body temperatures of dinosaurs and
determining Earth's climate history.

In this case, the team measured concentrations of the rare isotopes
oxygen-18 and carbon-13 contained in the carbonate samples. Carbonate is
made out of carbon and oxygen, and as it forms, the two rare isotopes
may bond to each other???clumping together, as Eiler calls it. The lower
the temperature, the more the isotopes tend to clump. As a result,
determining the amount of clumping allows for a direct measurement of
temperature.

The temperature the researchers measured - 18 ? 4 degrees Celsius - rules
out many carbonate-formation hypotheses. "A lot of ideas that were out
there are gone," Eiler says. For one, the mild temperature means that
the carbonate must have formed in liquid water. "You can't grow
carbonate minerals at 18 degrees other than from an aqueous solution,"
he explains. The new data also suggests a scenario in which the minerals
formed from water that filled the tiny cracks and pores inside rock just
below the surface. As the water evaporated, the rock outgassed carbon
dioxide, and the solutes in the water became more concentrated. The
minerals then combined with dissolved carbonate ions to produce
carbonate minerals, which were left behind as the water continued to
evaporate.

Could this wet and warm environment have been a habitat for life? Most
likely not, the researchers say. These conditions wouldn't have existed
long enough for life to grow or evolve - it would have taken only hours to
days for the water to dry up. Still, these results are proof that an
Earthlike environment once existed in at least one particular spot on
Mars for a short time, the researchers say. What that implies for the
global geology of Mars - whether this rock is representative of Martian
history or is just an isolated artifact - is an open question.

The research described in the PNAS paper, "Carbonates in the Martian
meteorite Allan Hills 84001 formed at 18 ? 4 ??C in a near-surface
aqueous environment," was supported by a Texaco Postdoctoral Fellowship,
NASA, and the National Science Foundation.

Written by Marcus Woo

Deborah Williams-Hedges
626-395-3227
debwms at caltech.edu <mailto:debwms at caltech.edu>
Received on Thu 13 Oct 2011 12:49:45 PM PDT