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A Second Martian Meteorite Mystery
11 February 1997 8:00 pm
The controversy over whether a Martian meteorite bears evidence of ancient life is still raging, but two scientists think they've pinned down where on Mars the famed chunk of rock originated. The only problem is that the scientists--who conducted their searches separately and have both submitted papers on the topic to the Journal of Geophysical Research--don't agree on just where that location is.
Last August, soon after a NASA team announced possible evidence for Martian microfossils in a meteorite dubbed ALH84001, Nadine Barlow, a planetary geologist at the University of Central Florida, and Jim Rice, a graduate student at Arizona State University, set out to pinpoint where the meteorite came from. Although that may seem like an impossible task, they had several clues to go on.
Researchers believe that the softball-sized meteorite was blasted from the Martian surface when an asteroid slammed into the Red Planet. Based on physical changes in the meteorite caused by exposure to cosmic rays since it was ejected, scientists have calculated that this catastrophic event occurred about 16 million years ago. But 16 million years is just a tick of geologic time. So the crater where the rock came from had to be fairly "fresh," with sharp and uneroded edges, and a blanket of ejected material nearby. Additionally, the crater had to be fairly large; otherwise, there wouldn't be enough energy to fling rocks into space.
Then there is the age of the rock that comprises the meteorite. Tracking the decay of different radioisotopes, researchers gauged the rock to be about 4.5 billion years old. That means that it had to originate from an ancient area of the Martian surface, rather than a more recent lava outflow. This narrowed the search considerably, eliminating 60% of the area of Mars, says Barlow.
Using these and other clues, the two researchers drew up lists of the most likely craters. Barlow's prime suspect lies in a region between 10 and 15 degrees south of the equator called Sinus Sabaeus. Rice, on the other hand, believes the most likely culprit sits in a region called Memnonia just 5 degrees south of the equator. "I think his favorite crater is a little bit too small," says Barlow. And Rice counters that Barlow's crater show signs of aging. "A 16-million-year-old crater shouldn't have secondary impacts," he says. "Her crater is not that young."
The dispute doesn't surprise other astronomers. "We don't have a firm understanding about how things get ejected off planets," says Charles Wood, an astrophysicist at the University of North Dakota. "It's not too surprising to me that they disagree." Without being able to compare ALH84001 to rocks from the suspect craters, the researchers believe that the exact site of meteorite's origin is likely to remain a mystery--and one that only another mission to Mars is likely to solve.