Meteorite hints at pounding of planets.

Nearly 4 billion years ago, a fusillade of debris clobbered the moon. Samples from the heavily cratered lunar highlands attest to an intense battering at that time, perhaps by stray fragments of an asteroid. Planetary scientists have assumed that this bombardment represented a solar-system-wide event, not a strictly local sideshow. But they lacked convincing proof.

Now, a detailed analysis of argon in the oldest identified meteorite of Martian origin suggests that the Red Planet suffered a similar pounding. Moreover, much of the solar system probably took a hit, says Grenville Turner of the University of Manchester in England. He and his Manchester colleagues Richard A. Ash and Stephen F. Knott presented their findings last week at the annual meeting of the Meteoritical Society in Washington, D.C.

The researchers measured the ratio of two isotopes, argon-39 and argon-40, in the meteorite ALH84001, recently shown to have come from ancient Martian crust (SN: 3/25/95, p.180). Argon, an inert gas, does not bind chemically to material in the meteorite and would have escaped easily if the rock grew hot during a catastrophic impact. The team's measurements show that a significant amount of argon left the meteorite 4 billion years ago, when it was still part of Mars--around the same time that debris pelted the moon.

The finding, Turner says, "provides the earliest evidence from Mars in support of the view that the...bombardment was a widespread event." But he notes that the final answer will require scientists to date the cratered Martian highlands.

Robert N. Clayton of the University of Chicago notes that argon might have leaked out of the meteorite slowly rather than all at once, as it would have done during a collision. But by measuring both argon isotopes, the team appears to have ruled out this possibility, he says.

Clayton adds that another meteorite, identified earlier this year as Martian, may shed further light on the bombardment. Keizo Yanai of Iwate University in Morioka, Japan, reported last week that the meteorite, Y-793605, ranks as the only known Martian fragment consisting of an amalgam of broken bits of different rocks. This object could have formed when debris struck Mars, heating and fusing the pieces.

No one knows the age of Y-793605 yet. But if it turns out to be as old as ALH84001, its structure could support the notion that Mars, as well as the moon, took a beating soon after the solar system formed, Clayton says.