The Dallas Morning News June 22, 1999 Page: 7D By Alexandra Witze Hot rocks: Possible bacteria in 2 other meteorites stir caldron of debate about life on Mars.
HOUSTON - Spring has sprung a new crop of Martian bacteria claims. NASA scientists say they've identified structures that look like fossilized bacteria in two additional Martian meteorites. Under a special microscope, the meteorites called Nakhla and Shergotty appear to contain small round and sausage-shaped structures that resemble Earth bacteria. "There are some similarities here," said David McKay of NASA's Johnson Space Center in Houston. "We have not proven that they are fossil bacteria, nor have we proven that they are from Mars." In 1996, Dr. McKay's team announced finding possible fossil bacteria in another Martian meteorite, ALH 84001. That claim is still controversial. If any such meteorite turns out to contain Martian bacteria, it would be science's first example of extraterrestrial life. The Nakhla and Shergotty discoveries are more tenuous than the ALH 84001 claim, Dr. McKay said last week as he presented the findings to a meeting of lunar and planetary scientists in Houston. The researchers recently split open a piece of Nakhla, a Martian meteorite that fell near that Egyptian town in 1911, killing a dog on impact. Only 13 Martian meteorites are known on Earth; they all contain the chemical compositions that are known to occur on Mars. Inside Nakhla, the scientists found tiny spheres and wormlike features, scattered across clay surfaces within the meteorite. Some of the structures appear by themselves, while in other areas they are clustered or embedded within the clay. Under an electron microscope, the structures look much like bacteria on Earth, Dr. McKay said - including some his team grew in the laboratory last year, and others found in the Jemez Springs region of New Mexico. The Nakhla "bacteria" are substantially bigger than those Dr. McKay' s team saw in ALH 84001. That makes the Nakhla structures a lot easier to study, and puts them more in the size range of typical Earthly bacteria, Dr. McKay said. One criticism of the ALH 84001 "bacteria" has been that those structures appear too small for a typical cell - and, in fact, Dr. McKay's team has since retracted that part of the claim, that the wormlike structures in ALH 84001 are truly bacteria. The NASA team thinks it can even see the Nakhla features with an ordinary microscope, as tiny flecks peppering the meteorite. Other scientists at the meeting said they would need a lot more proof to believe that the structures were actually bacteria, and that they had formed on Mars, not Earth. In another presentation, Andrew Steele of the Johnson Space Center showed pictures of Earthly bacteria and fungi, such as staphylococcus and penicillium, on the surfaces of other meteorites. Almost every meteorite is contaminated with these terrestrial organisms, he pointed out. "Contamination is a serious issue from the moment a meteorite has the temerity to land on this planet," he said. Dr. Steele has found Earth fungi penetrating Nakhla, and other researchers have detected Earthly chemical traces saturating ALH 84001. Nakhla does have one thing going for it: It was picked up right after it landed on Earth and kept in strict preservation. ALH 84001 was found in 1984 in the Allen Hills region of Antarctica, where it had probably lain on the icecap for more than 10,000 years. Everett Gibson, a member of Dr. McKay's team, said the researchers took every possible precaution to get a fresh piece of Nakhla, free from contamination. Dr. Steele and Dr. McKay have joined forces to study Nakhla in more detail, with Dr. Steele assuming that the features are all terrestrial contaminants and Dr. McKay assuming they are all Martian bacteria. The two scientists will meet regularly and hope to eventually come to some agreement about what the structures actually are. Dr. McKay's team reported seeing similar, though fewer, shapes in the Shergotty meteorite. At just 165 million years old, Shergotty is the youngest of the three meteorites containing purported bacteria. ALH 84001 is more than 3 billion years old, and Nakhla 1.3 billion years old. "If this proves out, this will show that life has basically spanned the entire history of Mars," said Dr. McKay. Other researchers at the meeting weren't sure that proof was forthcoming. When Dr. McKay ran over his allotted presentation time, the packed room of scientists grumbled and hissed when they weren't allowed to ask him questions because of time constraints. Aside from the new meteorite findings, many questions remain about ALH 84001. At the meeting, Kathie Thomas-Keprta of Dr. McKay's team presented more work she hoped would bolster the ALH 84001 claim. The evidence concerned grains of magnetite, a magnetic mineral found in ALH 84001 that can also be produced by bacteria on Earth. The meteorite' s magnetite is very pure and shaped in elongated prismatic crystals, just like Earthly bacteria produced by magnetite, said Dr. Thomas- Keprta. But just because the magnetite looks like bacteria produced it doesn't mean that bacteria are the only possibility, critics pointed out. "The interpretation of uniqueness is very touchy," said Peter Buseck of Arizona State University. Dr. Thomas-Keprta plans to also look for magnetite in the Nakhla and Shergotty meteorites, to see whether they contain similar evidence of bacteria-produced crystals. Dr. McKay said he thought the Nakhla and Shergotty claims would be easier to prove than ALH 84001, simply because the structures are so much bigger in the new evidence. "In a year, we should know," he said. PHOTO: (Agence France-Preese file photo) Questions lingerabout ALH 84001, the first meteorite thought to hold evidence of Martian life. ______________________________________________ Meteorite-list mailing list [EMAIL PROTECTED] http://www.pairlist.net/mailman/listinfo/meteorite-list
