|Advertising|Jobs 転職|Shukan ST|JT Weekly|Book Club|JT Women|Study in Japan|Times Coupon|Subscribe 新聞購読申込|
|Home > Life in Japan > Environment|
Monday, Jan. 1, 2001
Life? Martian meteorite keeps its cool
By ROWAN HOOPER
"X-Files," eat your heart out. Supporters of the theory that life on Earth was "seeded" by organisms from deep space got a boost recently, as analysis of the famous Martian meteorite ALH84001 indicated that it was transferred from Mars to Earth without being heated above 40 degrees Celsius. This relatively low temperature is not enough to sterilize bacteria or microorganisms. This means that Martian microorganisms, if present, could hitch a ride to Earth unscathed on similarly cool meteorites ejected from the Martian surface.
ALH84001 is well-known for providing the best evidence yet of extraterrestrial life. In 1996 microfossils of possible alien bacteria were discovered in the meteorite, yet scientists have argued about the conclusions ever since.
ALH84001 was formed in a magma chamber on Mars 4.5 billion years ago. After a minor planetary impact on Mars about 15 million years ago, it was launched into space, landing on Earth in Antarctica about 11,000 years ago.
Scientists thought that meteorites that are blasted loose by impact events may undergo significant heating. Benjamin P. Weiss and colleagues at California Institute of Technology used a new, ultrasensitive scanning microscope to measure the intensity of some magnetic features in two major magnetic minerals in the meteorite. Their results, published in the journal Science, indicate that ALH84001's temperature did not rise above 40 degrees before or during the big impact that ejected it from Mars.
The new research suggests that meteorite bio-transfer is a viable means of inter-planetary transport for microorganisms. Alien enthusiasts who "want to believe" that we are descended from Martians will only be encouraged by the results.
Still, the matter is a long way from being settled. ALH84001 might be the best evidence we have for extra-terrestrial life, but it is by no means undisputed, or even, it turns out, particularly good evidence.
Meteorites from Mars are known as SNCs, after the locations where the three most famous ones were found: Shergotty in India, where a 4-kg rock fell in 1865; Nakhla in Egypt, where a 40-kg rock killed a very unlucky dog in 1911; and Chassigny in France, where a 4-kg rock fell in 1815.
The SNC known as ALH84001 was first analyzed by NASA scientists in the mid-'90s. Using state-of-the-art microscopes and laser chemical analysis, the researchers claimed that they found microfossils: tiny rods and ovoids in carbonate mineral globules in the meteor.
The carbonate minerals were probably formed when carbon dioxide in the Martian atmosphere dissolved in water. The NASA team claimed that as the carbonate globules expanded, they trapped microorganisms, which became fossilized. The process is analogous to how some insects on Earth were trapped in blobs of tree sap which fossilized into amber.
The meteorite also contains polycyclic aromatic hydrocarbons, organic molecules found on Earth when microorganisms decay. These molecules are not just found on the meteor surface, but in the interior of the rock too, suggesting that they are not a contaminant from Earth.
All these things, said NASA in the original Science paper, showed evidence of the chemical signature of life.
Since then other scientists have claimed that either the sample was somehow contaminated, or that the chemical signature is in fact the result of inorganic processes, and does not provide evidence for biological processes.
Bacteria on Earth are generally between 5 to 20 microns long (a micron is .001 mm). But the Martian rods and ovoids found in ALH84001 are far smaller, measuring between 20 and 100 nanometers (a nanometer is one-thousandth of a micron).
"You're closely approximating the limit of size to accommodate the genetic material required for living systems," William Schopf told New Scientist magazine. Schopf, a paleobiologist at the University of California Los Angeles, is a leading skeptic of NASA's claims for ALH84001.
Another of the arguments against the possibility that ALH84001 could have harbored life is that the temperature at which the microfossils formed was higher than NASA scientists suggested. Bruce Jakosky and Kevin Hutchins of the University of Colorado, Boulder, reported that the mineral globules in the meteor were formed between 40 and 250 degrees. NASA suggested that the globules formed between 0 and 80 degrees. The upper limit for life is around 150 degrees.
"My personal feeling," Hutchins told New Scientist, "is that the higher range is more believable, which adds a lot of uncertainty to the life issue."
If the globules formed above 150 degrees, then the new research showing that the meteor wasn't heated above 40 degrees during the transition from Mars to Earth is irrelevant to the argument over the alleged microfossils. The debate still has some distance left to run.
Nevertheless, there have been many winners from the story. Scientists in diverse fields related to Mars, meteors and extraterrestrial life have benefited from a huge increase in funding. The budget for NASA's astrobiology program grew from $9 million in 1996, the year that the research was published, to around $20 million in 1997. Some projections put future budgets at $100 million.
NASA also has plans for six major (unmanned) missions to Mars over the next decade.
Perhaps, however, the most lasting boost from ALH84001 will be to the UFO-watchers and apprentice Mulders who look to the stars for aliens.