CALTECH NEWS RELEASE
Magnetic
microscopy image of a 1 mm by 2 cm by 1 cm slice of martian meteorite
ALH84001, overlayed on top of a visual photo of the same slice.
The colors give the field intensity, with red and yellow (blue)
corresponding to upward (downward) magnetization. The fusion crust
on the upper left side of the sample (visible as a thin black
rind in the visual photograph) has been remagnetized in the Earth's
field, while the interior of the meteorite retains the weaker,
heterogeneous magnetism it acquired on Mars. Designed by Francis
Macdonald, Caltech
According
to one version of the "panspermia" theory, life on Earth could
originally have arrived here by way of meteorites from Mars, where
conditions early in the history of the solar system are thought
to have been more favorable for the creation of life from nonliving
ingredients. The only problem has been how a meteorite could get
blasted off of Mars without frying any microbial life hitching
a ride.
But new research
on the celebrated Martian meteorite ALH84001 shows that the rock
never got hotter than 105 degrees Fahrenheit during its journey
from the Red Planet to Earth, even during the impact that ejected
it from Mars, or while plunging through Earth's atmosphere before
landing on Antarctic ice thousands of years ago.
The famed ALH84001 Martian meteorite. Photo: NASA
In the October
27 issue of the journal Science, Caltech graduate student Benjamin
Weiss, ndergraduate student Francis Macdonald, geobiology professor
Joseph Kirschvink, and their collaborators at Vanderbilt and McGill
universities explain results they obtained when testing several
thin slices of the meteorite with a new state-of-the-art device
known as an Ultra-High Resolution Scanning Superconducting Quantum
Interference Device Microscope (UHRSSM). The machine, developed
by Franz Baudenbacher and other researchers at Vanderbilt, is
designed to detect microscopic differences in the orientation
of magnetic lines in rock samples, with a sensitivity up to 10,000
times greater than existing machines.
"What's exciting
about this study is that it shows the Martian meteorite made it
from the surface of Mars to the surface of Earth without ever
getting hot enough to destroy bacteria, or even plant seeds or
fungi," says Weiss, the lead author of the Science paper. "Other
studies have suggested that rocks can make it from Mars to Earth
in a year, and that some living organisms can live in space for
several years. So the transfer of life is quite feasible."
The meteorite
ALH84001 has been the focus of numerous scientific studies in
the last few years because some scientists think there is tantalizing
evidence of fossilized life within the rock. The issue has never
been conclusively resolved, but Weiss says the matter is not important
to the present result.
"In fact,
we don't think that this particular meteorite brought life here,"
says Weiss. "But computer simulations of ejected Martian meteorites
demonstrate that about one billion tons of rocks have been brought
to Earth from Mars since the two planets formed." Many of these
rocks make the transit in less than one year, although ALH84001
took about 15 million years.
"The fact
that at least one out of the 16 known Martian rocks made it here
without heating tells us that this must be a fairly common process,"
says Kirschvink.
The famed
ALH84001 Martian meteorite. Photo: NASA
The sample
the Kirschvink team worked with is about 1 mm thick and 2 cm in
length and somewhat resembles the African continent, with one
side containing a portion of the original surface of the meteorite.
Using the UHRSSM, the team found that the sample has a highly
aligned and intense magnetic field near the surface, which is
to be expected because the surface reached a high temperature
when it entered Earth's atmosphere.
The reason
this is important is that any weakly magnetized rock will reorient
its magnetization to be aligned with the local field direction
after it has been heated to high temperatures and cooled. This
critical temperature for any magnetic material is known as the
blocking temperature.
Thus, the
outer surface layer of meteorite ALH84001 reached a high temperature
well above the blocking temperatures of its magnetic materials,
which caused the materials at the surface to realign with Earth's
magnetic field.
However, the
interior portions of the slice were found to have randomly oriented
magnetization, which means that some of the materials inside the
meteorite never reached their blocking temperatures since sometime
before they left the Martian surface. Further, when the researchers
gently heated another slice taken from the interior of the meteorite,
they discovered that the interior of the rock started to demagnetize
at temperatures as low as 40 degrees Celsius--or 105 degrees Fahrenheit--thus
demonstrating that it had never been heated even to that level.
Thus, a radiation-resistant
organism able to survive without energy and water for a year could
have made the journey from Mars to Earth. Examples of such hardy
organisms, like the bacteria bacillus subtilis and deinococcus
radiodurans, are already well known.
"Realistically,
we don't think any life forms more complicated than single-celled
bacterial spores, very tough fungal spores, or well-protected
seeds could have made it," Kirschvink says. "They would also have
had to go into some kind of dormant stage."
Though the
study does not directly address the issue of life in meteorites,
the authors say the results eliminate a major objection to the
panspermia theory--that any life form reaching Earth by meteorite
would have been heat-sterilized during the violent ejection of
the rock from its parent planet, or entry into the atmosphere.
Prior studies have already shown that a meteorite can enter Earth's
atmosphere without its inner material becoming hot.
"ALH 84001
has stimulated a remarkable amount of research to test the hypothesis
that life exists elsewhere than on Earth. The present study indicates
that the temperature inside the meteorite could have allowed life
to persist and possibly travel to Earth from Mars," says Nobel
Prize-winning biologist Baruch Blumberg, who is director of NASA's
Astrobiology Institute.
The famous
microscopic view of ALH84001 that some say pointed to fossilized
life within the meteorite. Photo: NASA
The results
also demonstrate that critical information could be lost if rocks
brought back from Mars by a sample return mission were heat-sterilized,
as has been proposed. Thermal sterilization would destroy valuable
magnetic, biological, and petrological information contained in
the samples.
If life ever
evolved on Mars, it is likely to have jumped repeatedly to Earth
over geological times. Because the reverse process--the transfer
of Earth life to Mars--is dynamically much more difficult, it
may be more important to instead protect any Martian biosphere
from Earthly microbes.
According
to Kirschvink, "The Martian biosphere, if it ever evolved, would
most likely have been brought to Earth billions of years ago,
and could have participated in the evolution and diversification
of bacterial life here.
"So there
is at least a chance that we are in part descended from Martian
microbes," Kirschvink says.
The ALH84001
research was funded in part by NASA's Astrobiology Institute,
an international research consortium involving academic, non-profit
and NASA field centers, whose central administrative office is
located at NASA's Ames Research Center in California's Silicon
Valley. A group from the Jet Propulsion Laboratory in Pasadena,
CA, which sponsored the Caltech research, is one of the 11 lead
teams of the institute.
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