|
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.
In the
October 27 issue of the journal Science, Caltech graduate
student Benjamin Weiss, undergraduate 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.
The famed ALH84001 Martian meteorite. Photo: NASA
"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.
|
