Analysis of Martian meteorite using unique magnetic microscope supports claim that meteorites could have carried life from Mars to Earth
When Joseph L. Kirschvink heard about the capabilities of the new
magnetic microscope designed and built by scientists at Vanderbilt’s Living
State Physics Laboratory, he immediately had an idea for an important
experiment that the instrument was uniquely suited to perform.
The professor of geobiology at the
California Institute of Technology had samples of the famous Martian
meteorite, ALH84001, and he realized that he could use the Vanderbilt
instrument to gain important new information about the meteorite’s thermal
history, information that could provide valuable support for the popular theory
that, over geologic time, Martian meteorites may have carried microbial life
from Mars to Earth.
The subsequent collaboration between
Kirschvink and his colleagues and Vanderbilt scientists Franz J. Baudenbacher,
research assistant professor of physics, and John P. Wikswo, the A B Learned
Professor of Living State Physics, has resulted in an article that appears in
the Oct. 27 issue of the journal Science. In the article, “A
Low-Temperature Transfer of ALH84001 from Mars to Earth,” the scientists do not
claim that microbial life actually traveled from Mars to Earth aboard the
meteorite, but they do conclude that the famous meteorite’s interior remained cool
enough to allow such a thing to happen.
|Baudenbacher holding ALH84001 sample next to SQUID Microscope
studies have shown that spores and microorganisms can exist for a number of
years in deep space. Dynamic simulations indicate that a small, but significant
number of the meteorites that travel between the two planets do so in less than
a year. Further studies have shown that the process of re-entry into Earth’s
atmosphere does not heat the interior of even modest sized meteorites to levels
that would kill microscopic passengers.
The major remaining objection to the
hypothesis is that when the meteorites are initially blasted into space by
major meteoroid impacts, they are necessarily subjected to so much energy that
even their interiors become hot enough to sterilize any life-forms they might
Caltech scientists realized that they could map the weak magnetic fields frozen
in the meteoritic material with the Vanderbilt microscope and perform a simple
experiment that would reveal whether the meteorite’s interior had been
subjected to temperatures above 40 degrees Celsius (104 degrees Fahrenheit).
instrument that made this study possible is called the Ultrahigh Resolution
Scanning SQUID Microscope. It was designed and built by Baudenbacher and is the
only instrument in the world capable of measuring the extremely weak magnetic
fields within the meteorite with the precision required for the study.
“The Vanderbilt instrument is a
stunning advance with profound applications in the earth and planetary
sciences,” says Kirschvink.
no other instrument in the world like it,” agrees Baudenbacher. The device can
measure magnetic fields a million times weaker than Earth’s field with
sub-millimeter spatial resolution, allowing it to produce extremely detailed
maps of magnetic field variations at the level of a single grain in a rock. “We
designed it to study the magnetic fields generated by living tissue, like the
heart, brain, and even some plants. But it is also ideally suited to measuring
the weak fields found in meteorites.”
Material from ALH84001 is gray and
looks something like concrete. The samples are slices a little larger than a
fingernail and about a millimeter thick. By scanning the samples back and forth
underneath the microscope, the researchers successfully built up a detailed map
of the magnetic field that they possess. They found that the magnetic field in
the meteorite’s interior was jumbled and changed direction every few
millimeters. There are several possible causes for such a heterogeneous
magnetic field structure, but any of them would have occurred on Mars before
the meteorite was blasted into space, the Caltech scientists argue.
|Wikswo and Baudenacher with SQUID Microscope
To determine whether the meteorite’s
interior had grown hot enough on its voyage to sterilize any living passengers,
the researchers heated some of the samples to 40 degrees Celsius (104 degrees
Fahrenheit) for 10 minutes and let them cool down to room temperature in a
container specially designed so the magnetic field strength inside was zero.
When they did so, they found that a number of the features in the original
magnetic structure had been altered or erased.
The changes indicate that the
meteorite’s interior was not heated above 40 degrees Celsius when the rock was
ejected from Mars, the scientists say. If it had been heated to higher temperatures
and cooled in a region without a magnetic field, then the magnetic pattern
would not have changed when reheated. If it had been heated to a high
temperature and cooled in a region with a magnetic field, then only features in
one of two directions would have been affected rather than in both directions
as they observed.
results led the scientists to conclude that “conditions are appropriate to
allow low-temperature rocks—and, if present, microorganisms—from Mars to be
transported to Earth throughout most of geological time.”
Ultrahigh Resolution Scanning SQUID Microscope
Vanderbilt’s Ultrahigh Resolution
Scanning SQUID Microscope (URSSM) was built in 1998. Wikswo and Baudenbacher
wanted an instrument that could measure the magnetic field produced by a living
heart in enough detail to map the electrical currents that play a critical role
in cardiac function. But no magnetic field detector available at the time had
the required characteristics, so the scientists decided to design and build
such a device.
“We have been pushing for higher
spatial resolution and sensitivity for the past decade,” says Wikswo, who’s
been measuring biomagnetic fields with SQUID magnetometers for the past 30
years and looked at his first rock with a SQUID in 1992. “This microscope is by
far the best act in town, and the ones on the drawing board will be even
The entire microscope system stands
about six feet high. It has a cylindrically shaped body about the size of a
scuba tank. Most of the space inside the tank is taken up with containers of
liquid nitrogen and liquid helium. The liquid gases are used to keep the
critical electronic components at a frigid five degrees above absolute zero.
The heart of the microscope is a
SQUID, or superconducting quantum interference device. When cooled to extremely
low temperatures, this bit of microelectronic circuitry is the most sensitive
detector of magnetic flux known. The SQUID is connected to a pickup coil wound
from niobium wires that are a fraction of the thickness of a human hair on a
tiny sapphire bobbin only 500 microns (a fiftieth of an inch) wide.
“The key to getting superior
performance is getting the tip very close to the sample,” Baudenbacher says.
That’s easy to say, but hard to do:
The pick-up must be maintained at cryogenic temperatures while the sample
remains at room temperature. The scientists solve this problem by putting an
extremely thin sapphire window at the bottom of the instrument. The window is
only 25 microns (a thousandth of an inch) thick, yet on one side it is room
temperature and on the other it is more than 250 degrees Celsius below zero.
That is a temperature gradient of about 30 million degrees per foot.
Wikswo says, “If the window breaks,
there’s a hissing sound, a plume of cold helium gas forms, and the tail of the
microscope is covered instantaneously with ice. As long as nothing else breaks,
it’s only an afternoon’s work to get back running.”
The pickup and the sapphire window
are at the bottom of the cylindrical body of the microscope. To make a
measurement, the scientists carefully center the sapphire bobbin just above the
window and they position the sample beneath the window as closely as possible.
The sample sits on a scanning platform that moves it from side to side and
forward and back with extreme precision. This allows the scientists to produce
a detailed map of its magnetic field.
Baudenbacher and Wikswo are using
the microscope to search for a peculiar pattern of magnetic fields around the
heart that has been predicted but not yet observed. Other scheduled experiments
include measuring the magnetic fields of algae, developmental currents in
embryos, and injury currents produced by ischemic cardiac tissue, all of which
are difficult to detect with more conventional approaches. In addition,
Kirschvink and his colleagues are lining up microscope time to study more
Martian meteorites, lunar samples, and some of the oldest rocks on Earth.
Development of the UHRSSM was funded
in part by grants from the National Science Foundation and the National
Institutes of Health.
Read the Cal Tech news release
See more images
Contact: David F. Salisbury (615) 343-6803