Vida en Marte, el debate continua

[arteaga@...>]

	Hace poco se anuncio que los resultados de un nuevo analisis
hecho al famoso meteorito apoyaban la teoria de un origen biologico;
creo recordar que la idea era que habian mirado las diferentes
proporciones de siotopos del carbono en los carbonatos. Bueno,
pero es que los carbonatos tambien contienen oxigeno, y cuando miran
a las composiciones de isotopos del oxigeno se encuentran con que es
dificil que los carbonatos se hubiesen formado a temperaturas 
adecuadas para la vida.
	Os mando los detalles, y quien lo entienda que me lo 
explique :)

	Santi

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[1] New UCLA research casts doubt on ancient life on Mars (Forwarded)
Followup-To: sci.space.policy
Date: Thu Mar 20 12:05:01 EST 1997
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Originator: daemon en yerevan
 
University of California NEWSWIRE
 
3/19/1997
 
Contact:
Stuart Wolpert (stuartw en college.ucla.edu)
Harlan Lebo (harlanl en college.ucla.edu) (310) 206-0511
 
NEW UCLA RESEARCH CASTS DOUBT ON ANCIENT LIFE ON MARS
 
     New research by UCLA scientists who studied pieces of the
meteorite from Mars that landed in Antarctica -- using UCLA's
high-resolution ion microprobe -- raises serious questions about
whether the meteorite contains any signs of ancient life.
 
     "We have come up with three possible scenarios, and none of
the three looks especially conducive to life," said Laurie
Leshin, a UCLA geochemist in the department of earth and space
sciences, who will discuss her research at the international
Lunar and Planetary Science Conference in Houston on Wednesday,
March 19.  "If you stretch the imagination, you may be able to
argue that one of the three scenarios may be consistent with
life, but even under the most charitable scenario, you have to
stretch the imagination pretty far."
 
     UCLA's ion microprobe enables scientists to learn the exact
composition of samples. The microprobe shoots a beam of ions --
charged atoms -- at a sample, releasing from the sample its own
ions that are analyzed in a mass spectrometer.  Scientists can
aim the beam of ions at specific microscopic areas of a sample
and analyze them. The microprobe was used in recent months to
determine that life on Earth began at least 3.85 billion years
ago and that Mount Everest and the Himalayas evolved as the
highest mountain peaks in the world some 15 million years later
than scientists had believed.
 
     Supporters of ancient life on Mars argue that evidence of
primitive life is associated with crystallized carbonate globules
in the meteorite.
 
     Studying bulk samples of the meteorite, advocates concluded
the carbonates could have formed at temperatures cool enough to
sustain life.  Other scientists have argued, based on the mineral
chemistry of the carbonates, that a higher temperature could not
support life.
 
     "We carefully correlated the chemical composition of the
carbonates with their isotope composition, which cannot be done
in bulk samples where they are mixed together," said Leshin, a
Rubey faculty fellow at UCLA.
 
     Leshin and her colleagues -- Kevin McKeegan, a UCLA research
geochemist; and Ralph Harvey, a research scientist at Case
Western Reserve University -- are the first scientists to
individually pinpoint a wide range of carbonate compositions from
the meteorite and analyze their oxygen isotopes.
 
     "What we found," Leshin said, "is that these two seemingly
unrelated data sets -- the chemistry of the carbonates and their
isotope composition -- are in fact related.  Any theory that
explains the carbonate formations must also explain the variation
in isotopes -- oxygen-18 to oxygen-16, and the calcium content.
 
     "When we placed the samples in the ion microprobe, we found
strong evidence that the first formed calcium-rich carbonates
contain the lowest ratios of oxygen-18 to oxygen-16," she added.
 
     Leshin said the findings provided hints to the conditions
that prevailed when the carbonates formed billions of years ago.
The scientists have produced three theories, which will be tested
over the next several months, to explain the findings.
 
     The first theory  -- which would explain the isotope
variations detected by the ion microprobe --  shows that the
environment where the rock was located on Mars when the
carbonates formed contained a very limited amount of fluid, which
consisted largely of carbon monoxide rather than water.  If this
theory proves to be correct, it virtually rules out
the possibility that the meteorite contains any signs of ancient
life because water is necessary to support life, Leshin said.
 
     Under a second theory, which also seems to be plausible, the
environment from which the carbonates were formed on Mars
contained a substantial amount of fluid that interacted with the
meteorite.  If that is correct, then the variation that the ion
microprobe detected in the isotope ratio of oxygen-16 and
oxygen-18 would most likely be explained by temperatures that
were variable, rising above 200 degrees Celsius -- far higher
than could support life, Leshin said.
 
     "Under this scenario, even if the carbonates were at 0
degrees when they neared final crystallization, the temperature
was boiling when they started forming," she said.
 
     Under the third theory, the fluid on Mars that interacted
with the rock was largely carbon dioxide when the carbonates
started forming, and largely water when they were fully
crystallized. This theory does not seem conducive to life either,
but makes it more difficult to exclude the possibility entirely,
said Leshin, adding that under this theory, the argument for
ancient life is "conceivable, but not persuasive."
 
     For more information about UCLA's programs, people and
resources, visit the university's new home page on the World Wide
Web at http://www.ucla.edu.
 
 
---
Andrew Yee
ayee en nova.astro.utoronto.ca