Mapping the Martian home of life evidence.

A potato-sized meteorite hit the headlines this month when NASA scientists reported persuasive signs of ancient life in this fragment of Mars (SN: 8/10/96, p. 84). Astronomers believe that asteroid impacts on Mars threw the meteorite and others into space, and Nadine G. Barlow of the University of Central Florida in Orlando is pinpointing where the telltale rock originated.

Using clues from the meteorite's composition and structure, the planetary scientist winnowed its place of origin down to 2 out of 42,283 Mars impact craters in her computer database.

Barlow says the rock's 4.5-billion-year-old age indicates that it came from the impact-pitted highlands of the planet's southern hemisphere-the oldest terrain on Mars. Furthermore, microscopic signs of shock in the ancient rock reveal that the meterorite's home crater lay near or even atop an older, more eroded crater. Finally, laboratory test results place the asteroid impact on Mars at roughly 16 million years ago, clear evidence that the crater in question must be among the freshest visible on the planet.

Theoretical models indicate that an object 100 kilometers or larger striking Mars from straight above could have thrown the meteorite, ALH84001, off the face of the planet, but no large, round craters in Barlow's database fit the other requirements. Alternatively, a low-angle impact of a much smaller object, creating a distinctively elliptical crater, could have done the job.

Barlow was preparing to map the craters of origin for the 12 known Martian meteorites when the news broke about signs of life in one of them. So she focused on ALH84001. The 12 meteorites are considered to be Martian because their chemical signatures match readings taken by the Mars Viking landers in the late 1970s. Only ALH84001, however, is of sufficient age to date from a warmer, wetter, possibly life-supporting era 3.6 billion years ago, scientists say.

Barlow's most likely candidate lies in the Sinus Sabaeus region about 14#251# south of the Martian equator. The other candidate, smaller and less elliptical, is at 12#251#S in the Hesperia Planitia region. The famous rock's origin is of great interest to scientists planning to use robots to collect samples during future missions to Mars, says Allan H.

Treiman of the Lunar and Planetary Institute in Houston. Barlow, he adds, "far and away knows the Martian craters better than anyone."