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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)
 [1] Martian Meteorite Bears Signs Of Life From Earth, Not Mars
 Followup-To: sci.space.policy
 Date: Thu Jan 15 12:32:00 EST 1998
 Organization: Jet Propulsion Laboratory
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 Originator: daemon en yerevan

>From The Planetary Society Home Page

                  Martian Meteorite Bears Signs of Life from Earth,
                                       Not Mars
                      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."

                  The Finding:
                  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
                  on Earth.

                  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
                  carbonate minerals.

                  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
                  two things."

                  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.