New evidence that Martian meteorite could have harboured life
Geologists studying Martian meteorite ALH84001 have found new support for the possibility that the rock could once have harboured life. Moreover, the conclusions of California Institute of Technology researchers Joseph L. Kirschvink, Altair T. Maine, and 捆绑SM社区聮s Hojatollah Vali also suggest that Mars had a substantial magnetic field early in its history, and that any life on the rock existing when it was ejected from Mars could have survived the trip to Earth.
In an article appearing in the March 13 issue of the journal Science, the researchers report that their findings have effectively resolved a controversy about the meteorite that has raged since evidence for Martian life was first presented in 1996. Even before this report, other scientists suggested that the carbonate globules containing the possible Martian fossils had formed at temperatures far too hot for life to survive. All objects found on the meteorite, then, would have to be inorganic.
However, based on magnetic evidence, Kirschvink and his colleagues say that the rock has certainly not been hotter than 350 degrees Celsius in the past four billion years and probably has not been above the boiling point of water. At these low temperatures, bacterial organisms could conceivably survive. "Our research doesn聮t directly address the presence of life," says Kirschvink. "But if our results had gone the other way, the high-temperature scenario would have been supported."
Kirschvink聮s team began their research on the meteorite by sawing a tiny sample in two, and then determining the direction of the magnetic field held by each. This work required the use of an ultra-sensitive super-conducting magnetometer system, housed in a unique, non-magnetic clean lab facility. Their results showed that the sample in which the carbonate material was found had two magnetic directions: one on each side of the fractures.
The distinct magnetic directions are critical to the findings, because any weakly magnetized rock will reorient its magnetism to be aligned with the local field direction after it has been heated to high temperatures and cooled. If two such rock fragments are attached so that their magnetic directions are separate, but are then heated to a certain critical temperature, they will have a uniform direction.
The igneous rock (called pyroxenite) that makes up the bulk of the meteorite contains small inclusions of magnetic Fe-S minerals that will entirely realign their field directions at about 350 degrees Celsius, and will partially align the field directions at much lower temperatures. Thus, the researchers have concluded that the rock has never been heated substantially since it last cooled some four billion years ago. "We should have been able to detect even a brief heating event over 100 degrees Celsius," Kirschvink says. "And we didn聮t."
These results also imply that Mars must have had a magnetic field similar in strength to that of the present Earth when the rock last cooled. This is very important for the evolution of life, as the magnetic field will protect the early atmosphere of a planet from being sputtered away into space by the solar wind. Mars has since lost its strong magnetic field, and its atmosphere is nearly gone.
The fracture surfaces on the meteorite formed after it cooled, during an impact event which crushed the interior portion. The carbonate globules which contain putative evidence for life formed later on these fractures surfaces, and thus were never exposed to high temperatures, even during their ejection from the Martian surface nearly 15 million years ago, presumably from another large asteroid or comet impact.
A further conclusion one can reach from Kirschvink聮s work is that the inside of the meteorite never reached high temperatures when it entered Earth聮s atmosphere. This means, in effect, that any remaining life on the Martian meteorite could have survived the trip from Mars to Earth (which can take as little as a year, according to some dynamic studies), and could have ridden the meteorite down through the atmosphere by residing in the interior cracks of the rock and been deposited safely on Earth.
"An implication of our study is that you could get life from Mars to Earth periodically," Kirschvink says. "In fact, every major impact could do it." Kirschvink聮s suggested history of the rock is as follows: The rock crystallized from an igneous melt some four and a half billion years ago and spent some half a billion years on the primordial planet being subjected to a series of impact-related metamorphic events, which included formation of the Fe-S minerals. After final cooling in the ancient Martian magnetic field about 4.0 billion years ago, the rock would have had a single magnetic field direction. Following this, another impact crushed parts of the meteorite without heating it, and caused some of the grains in the interior to rotate relative to each other. This led to a separation of their magnetic directions and produced a set of fracture cracks. Aqueous fluids later percolated through these cracks, perhaps providing a substrate for the growth of Martian bacteria. The rock then sat more or less undisturbed until a huge asteroid or comet smacked into Mars 15 million years ago. The rock wandered in space until about 13,000 years ago, when it fell on the Antarctic Ice Sheet.
In a multidisciplinary effort involving other scientists (microbiologists, biochemists and physicists) Vali and Kirschvink are currently investigating the significance of the formation of magnetite by thermophilic bacteria for recognition of biological activities in Martian meteorites.
