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[escepticos] meteorito marciano
Mas sobre el meteorito marciano. Este articulo
-desgraciadamente en ingles- explica como en unos experimentos han
separado el carbono organico del inorganico en el meteorito. Me
Parte de la evidencia a favor de que el meteorito contenia
restos de vda marciana es que contenia sustancias organicas; mas
concretamente, "polycyclic aromatic hydrocarbons (PAHs)". Estas
moleculas son organicas y contienen atomos de carbono.
El meteorito en si esta formado por minerales que incluyen
carbonatos; estos carbonatos son moleculas inorganicas que tambien
contienen atomos de carbono.
Bueno, pues calientas una muestra del meteorito lo
suficiente como para que se evaporen los PAHs y obtienes una muestra
del carbono organico. Despues lo calcinas para asegurarte de que has
quemado todo el material organico y tomas una muestra de los
carbonatos -carbono inorganico.
Y entonces viene la sorpresa: las proporciones de carbono 12
y carbono 13 son diferentes en las muestras de carbono organico e
inorganico. A saber, la composicion isotopica de los carbonatos
coincide con la de Marte pero la del carbono organico se ajusta a la
de la Tierra. Es mas, resulta que parte del carbono organico es
carbono 14, cuya semivida es de unos 5000 an~os... a-jem...
Conclusion aparente: la roca es marciana, y los compuestos
organicos que contiene son terrestres y recientes. O sea, esta
contaminada. Pero digo aparente porque los analisis hechos sobre
aminoacidos en el meteorito no son tan claros; en particular, "Bada
dijo que no podia eliminar la posibilidad de que el meteorito
contuviese minusculas cantidades de aminoacidos extraterrestres,
como formas dextrogiras de alanina". Y es que si ese material
organico fuese producido por bacterias terrestres, toda la alanina
que contuviese el meteorito tendria que ser levogira.
Esperemos con paciencia a que se pongan de acuerdo... pero
la cosa esta poniendose mas clara cada dia.
sci.space.news (moderated) #11014 (1 + 8 more) (1)
From: baalke en brucie.jpl.nasa.gov (Ron Baalke)
 Martian Meteorite Bears Signs Of Life From Earth, Not Mars
Date: Thu Jan 15 12:32:00 EST 1998
Organization: Jet Propulsion Laboratory
Content-Type: text/plain; charset=US-ASCII
News-Software: VAX/VMS VNEWS 1.41
Originator: daemon en yerevan
>From The Planetary Society Home Page
Martian Meteorite Bears Signs of Life from Earth,
Scientists Report that the Martian Rock Is
Contaminated with Organic Material from Earth
The case for life on Mars seems to be becoming
more difficult to prove.
The meteorite that scientists thought contained
microscopic fossils of martian life may just be
contaminated with organic material from Earth. In
tomorrow's issue of Science, two teams of
researchers report that the organic carbon in the
martian meteorite Allan Hills 84001 (ALH84001)
comes from Earth and not from Mars.
In two separate papers, scientists from the
Scripps Institution of Oceanography at the
University of California, San Diego, and the
University of Arizona in Tucson conclude the
potato-size martian rock was contaminated by the
surrounding Antarctic ice in which it was found.
The scientists are the first to publish results of
tests of organic material contained in the
meteorite since research teams at NASA's Johnson
Space Center and Stanford University announced
their results in August 1996.
"This is bad news with respect to using these
meteorites to assess whether there ever was or is
life on Mars," said Jeff Bada, a professor of
marine chemistry who headed the Scripps team. "It
shows that the meteorites aren't going to give us
a definitive answer."
Possible Martian Microfossils
To understand this continuing scientific debate,
we should review the findings of scientists at
Johnson Space Center and Stanford University. In
August 1996, they reported that they had found in
meteorite ALH84001 the first organic molecules
thought to be martian in origin. Called polycyclic
aromatic hydrocarbons (PAHs), these organic
molecules were found in easily detectable amounts
in tiny globs of carbonate within the meteorite.
They also noted finding several mineral features
characteristic of biological activity and possible
microscopic fossils of primitive, bacteria-like
organisms inside the meteorite. Their findings
were published in the August 16, 1996, issue of
The scientists proposed that very primitive
microorganisms may have assisted in the formation
of the carbonate, and some of the microscopic
organisms may have become fossilized, in a fashion
similar to the formation of fossils in limestone
Questioning the Finding:
Amino Acids in the Martian Rock
Bada's team at the the Scripps Institution of
Oceanography analyzed amino acids contained within
a sample from the meteorite, while Timothy Jull's
team at the University of Arizona examined the
radiocarbon activity of the bulk organics.
"What we found," Bada said, "was that, yes, there
are amino acids in the meteorite at very low
levels, but they are clearly terrestrial and they
look similar to amino acids we see in the
surrounding Antarctic ice. How they got in there
is still an open issue."
Bada said he chose to focus his analysis on amino
acids within the meteorite because, unlike PAHs,
they play an essential role in biochemistry.
An expert in the analysis of amino acids, Bada
used high-performance liquid chromatography to
analyze amino acids in the meteorite to determine
their "handedness." He found that the bulk of the
amino acids consisted of the left-handed forms
similar to that seen in the Allan Hills ice in
Antarctica where the meteorite was found. Bada
said he could not rule out the possibility that
minute amounts of some extraterrestrial amino
acids such as right-handed forms of alanine were
preserved in the meteorite.
"What we and Tim Jull's team have shown is that
there is no evidence in our hands that the
meteorite contains any compounds that we could
definitely trace to Mars except maybe some tiny
mysterious component that we don't understand at
this point," he said.
Questioning the Finding:
Radiocarbon Dating the Martian Rock
A.J. Timothy Jull's group at Arizona used 14C and
13C tracers to determine the origin of the
carbonate minerals and organic carbon in the
meteorite. Their results indicated that the bulk
of organic material in ALH84001 is contaminated
material it acquired after falling to Earth.
"It looks like regular terrestrial organic
material," Jull said. "The 14C content of it
suggests that there were several episodes of
Jull's team burned samples of the meteorite at
different temperatures to separate organic carbon
and carbonate minerals in the meteorite. In four
separate such "stepped-combustion" experiments,
they collected the carbon dioxide gas produced and
prepared the carbon for isotopic analysis by
standard radiocarbon procedure. At the
university's Accelerator Mass Spectrometer
Laboratory, the scientists then measured how much
of the heavy stable carbon isotope, carbon-13, and
the radioactive carbon isotope, carbon-14, were
present in both the organic carbon and the
Jull's group is the first to report on the bulk,
or main part, of the organic material in a sample
of the ALH84001 meteorite. For the past three
years, Jull, a research geoscientist, has been
studying the isotopic composition of the Allan
Hills meteorite to get more information about the
isotopic composition of the early martian
atmosphere. Before Johnson Space Center and
Stanford University scientists announced in August
1996 the possible existence of bacterial fossils
in the meteorite, several scientists, including
Jull, had discovered that the carbonate minerals
of the meteorite were far richer in carbon-13 than
are any carbonates on Earth.
"This unusual signal (carbon-13 enrichment) tags
the carbonate minerals in the Allan Hills
meteorite as likely formed from a reservoir such
as the Mars atmosphere," Jull said.
He and his team now also have discovered that the
abundance of carbon-13 in the organic carbon in
the meteorite is an exact match to the abundance
of carbon-13 in Earth's organic carbon. "It looks
like regular terrestrial organic material, with
the exception of one small component in ALH84001."
The researchers say they suspect that this
component is some carbon indigenous to the rock,
possibly associated with a mineral phase, that
burns at higher temperatures.
The carbon-13 data alone are convincing evidence
that the organic carbon in the meteorite is
"regular terrestrial organic material," Jull said.
"Combining this with the carbon-14 evidence is the
clincher," he added.
Radioactive carbon is produced when cosmic rays
from space strike Earth's atmosphere and react
with nitrogen. Carbon-14 also can be produced in
minerals irradiated in space and on Mars, by
high-energy nuclear reactions. However, Jull and
his co-researchers show there is no mechanism to
produce carbon-14 in the organic material, as this
requires low-energy neutrons to interact with
nitrogen atoms. Thus, organic material which
originated on Mars would contain a negligible
amount of radioactive carbon before it fell to
Jull and his team discovered that the organic
carbon in the Allan Hills meteorite contains
enough carbon-14 to yield radiocarbon ages of
between 11,000 and 5,200 years. Jull previously
had determined by radiocarbon analysis of silicate
minerals in the meteorite that the rock fell to
Earth about 13,000 years ago.
"The carbon-14 shows conclusively that the
carbonates and the organics in the meteorite do
not come from the same source," Jull said. "It
also shows the organic carbon has a terrestrial
source, likely through several episodes of
"The organic material contains 14C and the
carbonate doesn't because the carbonate came from
somewhere in space, presumably Mars, and the
organic material is a recent addition which took
place while the meteorite was sitting on the ice,"
Jull said. "So, there is no connection between the
The Questions Continue
Jull said that although the scientific community
can be expected to make many more discoveries
about the Allan Hills meteorite, he would be
surprised if scientists got a definite answer on
the question of possible ancient life on Mars from
this or any other meteorite.
J. Warren Beck, an associate research scientist in
physics at the University of Arizona, agreed.
"Even if we ultimately find that all the organic
matter in this meteorite came from Earth, that
doesn't rule out the possibility that life may
have evolved on Mars. A meteorite represents only
a tiny fragment of an entire planet," Beck said.
Bada said scientists will have to wait until a
Mars mission scheduled for 2005 to bring back
samples from the Martian surface to determine
whether life ever graced the planet.
"In the meantime, we can throw any kind of
analyses that we want to at these meteorites and
we are not going to provide an answer one way or
another about whether life existed on Mars," he
Credits and More Information
Co-authors of the Scripps paper with Bada are
Daniel Glavin, a Scripps graduate student; Gene
McDonald, of NASA's Jet Propulsion Laboratory; and
Luann Becker, of the University of Hawaii.
Co-authors of the University of Arizona paper with
Jull and Beck are Christopher J. Courtney and
Daniel Jeffrey of the University of Arizona.