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 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.”
Anatomy of Ultrahigh Resolution Scanning SQUID MicroscopeVanderbilt’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 better.”
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 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.
Materials provided by Vanderbilt University. Note: Content may be edited for style and length.
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