Meteorite is traced to moon; trail could lead on to Mars
Meteorite AHLA 81005 is about to become a star, of sorts. Scientists at the 14th Lunar and Planetary Science Conference in Houston are expected to confirm Thursday what many have suspected for months: that this fragment is actually a piece of the moon.
Of some 10,000 meteorites now in the collections of researchers, it is the first and so far the only one in history to have its place of origin identified. But AHLA 81005 may soon have to share some of the limelight. Armed with solid evidence that meteorites can be bits of planets, an increasing number of specialists are seriously considering a new theory. Nine rare SNC (shergottite, nakhlite, and chassignite) meteorites, collected over the last seven years, may well be chunks of Mars, they speculate.
Such discoveries are already causing scientists to ponder questions which they have not previously considered. The significance of this development has not been lost on those whose field stands to be the most enriched by it - planetary scientists.
''When you go back through history and rank the big events in the history of extraterrestrial material,'' says astrogeologist Bevan French, head of planetary materials research at NASA, ''you'd have to go back to the first discovery of meteorites as extraterrestrial, the return of the Apollo mission and its lunar samples, and now this discovery that meteorites came from the moon and perhaps Mars.''
Adds Johnson Space Center geochemist Donald Bogard, ''This is a hot topic in planetary science.''
Scientists say AHLA 81005 itself, discovered at Antarctica's Allen Hills region in January of 1982, will be a welcome addition to the lunar collection brought back by six Apollo missions. Such a find, adds Dr. French, is not unlike ''a free landing mission.''
It is improbable, they add, that the rock came from the small area covered by the Apollo missions (5 percent of the lunar surface). University of New Mexico geologist Richard Keil says the fragment is characteristic of the lunar highlands region covering most of the far side of the moon facing continually away from Earth.
Project scientists are convinced that comparison of the Apollo lunar samples with AHLA 18005 confirms the meteorite's lunar origin, despite slight differences in the chemical makeup of the rocks.
Over 20 laboratories in the United States and four foreign countries have been performing detailed chemical and geological analysis of the 31-gram specimen since its discovery. Not only did the rock appear similar to a lunar sample. But the experimental results were, according to University of Chicago chemist Robert Clayton, ''indistinguishable from that of the lunar samples brought back by the Apollo missions.''
Adds Keil: ''If it doesn't come from our moon, it comes from an object that looks like just our moon.''
If the meteorite did indeed come from the Earth's moon, scientists agree that it appears to have been blasted from the crust by a colliding asteroid.
A number of theoretical studies have purported to show the extreme difficulty or impossibility of such an event taking place. The massive amounts of kinetic energy transferred from the incoming asteroid to any surface fragment would be great enough to vaporize them on impact.
Thus the dynamicists who attempt to explain these forces are being sent back to the drawing boards to try to understand the mechanisms which allow particles to be sprung off planets without being destroyed.
The determination of AHLA 81005's origin is a boon to those who advocate the Martian origin of the SNC meteors.
Many characteristics of these rocks have hinted strongly at a Martian origin. First, they are thought to be volcanic, since they solidified 1.3 billion years ago. This ruled out the moon, which became inactive 3 billion years ago, or asteroids, which were geologically dead even before that.
At the same time, researchers found traces of volatile gases fused in so-called melt pockets within the SNCs. Those gases matched data collected by Viking I and II on the composition of the Martian atmosphere.
The shergottite, they add, underwent a massive shock a few hundred million years ago, indicating the occurrence of one cataclysmic event.
''For lack of a better alternative,'' says Blanchard, ''we picked Mars.''
Yet many scientists couldn't accept that alternative for one simple reason: ''We found it hard to accept the possibility of meteorites from Mars when there were none from the moon,'' recalls University of Pittsburgh geologist William Cassidy.
''So now that we've decided on this meteorite's lunar origin, the major stumbling block for accepting it as a definite possibility has disappeared.''
Researchers caution that regardless of how compelling the data may seem, theories on SNC origin are ultimately speculative.
NASA's French says, ''People are going to be confident and convinced that the meteorites came from Mars, but there will always be skepticism and uncertainty. The only way to remove that will be to bring back some Martian rocks. . . .''
Study of the SNC meteorites even without absolute assurance may be instructive. Pittsburgh's Cassidy sees SNC's will provide ideas ''for what we might be looking for when we actually go out there. They might help us keep our experiments from spinning off into fruitless areas.''
As the lunar origin study has boosted tremendously the theories of the interplanetary transport of matter and the Martian origin of the SNCs, it has also raised the expectations of those studying meteoritics.
Researchers are sure that much more remains to be tapped in the meteorite-laden glacial fields of Antarctica. Two especially promising locations have been marked for exploration in the next two years.
With the constraints of increasingly scarce federal funds closing in on their activities, meteorists are hoping that Thursday's announcement will bring attention and perhaps even money to their Antarctic exploration projects and the bounty of meteoritic treasures they've provided.