Allan Hills 84001 (commonly
abbreviated ALH 84001) is a meteorite that was found in Allan Hills, Antarctica
on December 27, 1984 by a team of US meteorite
hunters from the ANSMET project. Like
other members of the group of SNCs (shergottite, nakhlite,
chassignite), ALH 84001 is thought to be from
Mars. On discovery, its
mass was
1.93
kg. It made its way into
headlines worldwide in 1996 when scientists announced that it might
contain evidence for microscopic fossils of Martian bacteria.
History
This rock is theorized to be one of the oldest pieces of the
solar system, proposed to have
crystallized from molten rock 4.5 billion years ago. Based on
hypotheses surrounding attempts to identify where extraterrestrial
rocks come from, it is supposed to have originated on Mars and is
related to other
Martian
meteorites.
In
September 2005, Vicky Hamilton of the University of
Hawaii at Manoa
presented an analysis of the origin of ALH 84001
using data from the Mars Global
Surveyor and Mars Odyssey
spacecraft orbiting Mars. According to the analysis, Eos Chasma
in the Valles Marineris
canyon appears to be the source of the
meteorite. The analysis was not conclusive, in part because
it was limited to parts of Mars not obscured by dust.
The theory holds that ALH 84001 was shocked and broken by one or
more meteorite impacts on the surface of Mars some
3.9 to 4.0 billion years ago, but remained on the
planet. It was later blasted off from the surface in a separate
impact about 15 million years ago and impacted
Earth roughly 13,000 years ago. These dates were
established by a variety of
radiometric dating techniques, including
samarium-neodymium
(Sm-Nd),
rubidium-strontium (Rb-Sr),
potassium-argon (K-Ar), and
carbon-14.
It is hypothesized that ALH 84001 originated from a time period
during which water may have existed on Mars. Other meteorites that
have potential biological markings have generated less interest
because they do not originate from a "wet" Mars. ALH 84001 is the
only meteorite collected from such a time period.
Possible lifeforms
On
August 6, 1996 ALH
84001 became newsworthy when it was announced that the meteorite
may contain evidence for traces of life
from Mars, as published in an article in Science by David McKay of NASA
.
The electron microscope revealed chain
structures in meteorite fragment ALH84001
Under the
scanning electron
microscope structures were revealed what may be the remains—in
the form of
fossils—of
bacteria-like
lifeforms. The
structures found on ALH 84001 are 20-100
nanometres in diameter, similar in size to the
theoretical
nanobacteria, but smaller
than any known
cellular life at the
time of their discovery. If the structures are in fact fossilized
lifeforms, they would be the first solid evidence of the existence
of
extraterrestrial life,
aside from the chance of their origin being terrestrial
contamination.
The announcement of possible extraterrestrial life caused
considerable controversy at the time and opened up interest in
Martian exploration. When the discovery was announced, many
immediately conjectured that the fossils were the first true
evidence of extraterrestrial life—making headlines around the
world, and even prompting
U.S. President Bill Clinton to make a formal televised
announcement to mark the event.
Several tests for organic material have been performed on the
meteorite and
amino acids and
polycyclic aromatic
hydrocarbons (PAH) have been found. The debate over whether the
organic molecules in the meteorite are in fact of
exobiologic origin or are due to abiotic
processes on Mars or contamination from the contact with Antarctic
ice on Earth is still ongoing.
Early on,
Ralph Harvey of Case Western Reserve University
and Harry McSween of University of
Tennessee
reported evidence that the carbonate globules found
in the meteorite were formed at high temperature (above 650°C) by
volcanic or impact processes on Mars. At such high
temperatures, it would be very unlikely that the morphology of the
globules could have had any kind of biological origin. Later,
however, the same authors published papers supporting a hypothesis
in which the globules formed at low temperature from an aqueous
solution. Most scientific papers published in the past 10 years now
accept that carbonates on Mars formed this way.
Evidence continues to grow that nanobacteria do exist in nature, in
spite of the initial skepticism based on the idea that the
particles were too small to contain RNA. Futhermore,
microbiologists have succesfully cultured nanobacteria in the lab,
with sizes within the range of at least some of the purported
microfossils in ALH 84001.
As of 2006, some experts still argue that the
biomorphs found in the meteorite are not indicative
of life on Mars, but instead are caused by contamination by earthly
biofilms. But McKay argues that likely
microbial terrestrial contamination found in other Martian
meteorites do not resemble the texture of the biomorphs in ALH
84001. In particular, the biomorphs look intergrown or embedded in
the indigenous material, while likely contamination do not.
It has not yet conclusively been shown how the features were
formed, but similar features have been recreated in labs without
biological inputs.
In
November 2009, scientists at Johnson Space Center
, including David McKay, reasserted that there is
"strong evidence that life may have existed on ancient Mars" after
having reexamined the meteorite using more advanced analytical
instruments now available, as well as the objections that had been
made since the biogenic hypothesis for the biomorphs first had been
put forward.
Student participation
The analysis of ALH84001 was unusual in that an undergraduate
student, Anne Taunton of the University of Arkansas, performed much
of the SEM work used to correlate the suspected nanobacterial
fossils with known terrestrial nanobacterial fossils. NASA's David
McKay hired Anne Taunton for a 10-week student internship to
perform the SEM analysis, but did not inform her about the nature
of what she was investigating. This technique is known as a
single blind. Taunton reported the
morphology of biomorpgs in ALH84001 to be very similar to
terrestrial samples without knowing that she was describing a
Martian meteorite.
See also
References
- Birthplace of famous Mars meteorite pinpointed.
New
Scientist article. URL accessed March 18, 2006.
- Harvey, R. P., and J. H. Y. McSween, "A possible
high-temperature origin for the carbonates in the Martian meteorite
ALH84001", Nature, 382(6586), 49-51, (1996).
-
http://www.nasa.gov/centers/johnson/pdf/403089main_7441-1.pdf
- http://www.agu.org/sci_soc/eisromanek.html
-
http://www.nasa.gov/centers/johnson/pdf/403089main_7441-1.pdf
- Taylor, Michael Ray, 1999. Dark Life. 0684841916, p. 90.
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