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.
"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 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 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.
Contact: Robert Tindol (626) 395-3631