[meteorite-list] Meteorites Reveal Extended Deep-Freeze on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jul 21 20:14:10 2005
Message-ID: <200507220013.j6M0DLv16983_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn7709

Meteorites reveal extended deep-freeze on Mars
David L Chandler
New Scientist
21 July 2005

Mars has never been much warmer than it is now, reveals the first
detailed analysis of the planet's long-term thermal history - the
current temperature on the equator is a bitter -58?C on average. The
study suggests liquid water could not have survived for long periods on
the Red Planet's surface, lowering the chances that life could have
taken hold on the frigid world.

Previous observations of channels and rocks altered by liquid water have
pointed to periods when the surface temperature must have climbed above
freezing. But this study adds to a growing body of evidence suggesting
Mars has been piercingly cold during most of its 4.6-billion-year history.

David Shuster of the California Institute of Technology in Pasadena and
Ben P Weiss of the Massachusetts Institute of Technology in Cambridge,
both in the US, arrived at this conclusion after studying data from
eight meteorites from Mars.

They used previously published data on the ratio of two isotopes in the
meteorites - potassium 40 and argon 40 - to compute the maximum
temperatures reached over the rocks' lifetimes. And that is a very long
time in the case of meteorite ALH84001, which in 1996 gained notoriety
when some scientists announced it contained signs of past microbial
life. The new analysis reveals the rock could not have risen above 0?C
for more than a million years since it formed, 3.5 billion years ago.
          
Gas leak

The method works because radioactive potassium 40, a solid, decays into
the gas argon 40, which then diffuses very slowly out of the rock. But
it does so in a temperature-dependent way so that, at higher
temperatures it, leaks out faster.

The method is based on a very precise rock-dating technique. So by
dividing each rock's age by the amount of argon it lost, the team could
calculate the average annual rates of escape.

The new study suggests average temperatures were too low to keep bodies
of liquid water on the surface for long periods of time. But it does
allow for liquid water to exist on the surface over short periods. That
is because the meteorites came from somewhere below the surface of Mars
where they were not warmed by sunlight. So liquid water might have been
able to flow on the planet's surface during the warmest part of the day.

Life without water?

The study also shows that these eight meteorites - which represent about
20% of all known Mars meteorites - were never subjected to temperatures
high enough to sterilise them, even during the shock from the impact
that ejected them from Mars. That proves that any living cells they
carried could have survived the journey to Earth, Weiss told New Scientist.

"Billions of tonnes of material have come from Mars to Earth over
geological time," he adds, which "would really seem to suggest the
planets have not been biologically isolated".

But the study also casts doubt on whether there would be any life on the
frigid world to make the trip. "Mars may have just cooled off too
quickly [for life to evolve]," says Weiss.

Journal reference: Science (vol 309, p 594)
Received on Thu 21 Jul 2005 08:13:21 PM PDT