By David F. Salisbury
Oct. 26, 2000

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.

Previous 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 be carrying.

The 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).

The 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.

“There’s 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.

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.

These 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.”

Description of SQUID:
http://www.vanderbilt.edu/lsp/abstracts/jenks-eap-1997.htm

Living State Physics Laboratory:
http://www.vanderbilt.edu/lsp/index.htm

SQUID Microscope design:
http://www.vanderbilt.edu/lsp/index.htm