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Eyewitness Moon Impact Debunked, Tagish Lake: Mystery Meteorite

SKY & TELESCOPE'S NEWS BULLETIN - MARCH 16, 2001

AN EYEWITNESS IMPACT DEBUNKED

Were a small asteroid to hit the Moon, could we see the impact with the naked eye? In his chronicles of medieval life, Gervase of Canterbury described a dramatic event witnessed on the evening of June 18, 1178:

"Now there was a bright new Moon . . . and suddenly the upper horn split in two. From the midpoint of this division a flaming torch sprang up, spewing out . . . fire, hot coals, and sparks . . . The body of the Moon which was below writhed . . . throbbed like a wounded snake. Afterwards it resumed its proper state. The phenomenon was repeated a dozen times or more. [Finally] the Moon . . . along its whole length took on a blackish appearance."

In 1976 geologist Jack B. Hartung (State University of New York) proposed that this passage describes the creation of Giordano Bruno, a relatively young, 22-kilometer-wide crater near the Moon's northeast limb. Hartung reasoned that, seen from Earth, this brightly rayed crater appears near the midpoint of the young crescent Moon. Astronomers were quick to counter that on the date in question the Moon was only 1.3 days past new and thus too near the Sun to be easily visible at all. Also, Gervase's witnesses claimed to have seen the "flaming torch" many times, which sounds a lot more like the ordinary atmospheric distortions often seen near the horizon. Still, Hartung's hypothesis has made its way into many astronomy books and articles. It proved difficult to confirm or refute because data on Giordano Bruno and its surroundings were limited.

Now a new analysis demonstrates that a cratering event could not have happened in 1178. Paul Withers (University of Arizona) finds that an impact large enough to create a 22-km crater would likely have showered Earth with 10 million tons of ejected fragments -- perhaps a trillion bright meteors in all -- during the days that followed. "A meteor storm as impressive as this and lasting for a week would have been considered apocalyptic by all medieval observers," Withers comments. Yet no mention of such displays appears in English, European, Arabic, or Asian chronicles of the era.

Laser-ranging experiments during the 1970s revealed that the Moon nods back and forth by a tiny amount ("free libration"), suggesting to Hartung's supporters that the globe was still reverberating from the impact. But Withers notes that a reanalysis of the laser-ranging data later showed that the slight oscillation arises instead from fluid motions deep in the lunar interior. Furthermore, while Giordano Bruno is indeed the youngest crater of its size anywhere on the Moon, multispectral images from the Clementine spacecraft show that this impact site has to be much older than 800 years. Details of Withers's analysis will appear in the April issue of Meteoritics.

TAGISH LAKE: MYSTERY METEORITE

At last year's Lunar and Planetary Science Conference, held each March in Houston, Texas, meteorite specialists were salivating over the Tagish Lake meteorite, which had dropped as a hail of fragments onto the Yukon's winter wilderness just two months before. Within days of the fall, local outdoorsman Jim Brook carefully collected nearly a kilogram of icy fragments and stashed them in his freezer. Later a team of Canadian geologists and volunteers scoured the lake's frozen surface to collect as much of the fragile interplanetary material as possible before the spring thaw swallowed up the remaining pieces. Remarkable as much for the rapid, textbook recovery effort as for the stones' black, carbon-rich texture, Tagish Lake was hailed as the most important find in some 30 years.

A year later, the Tagish Lake fall is still causing a scientific buzz because its unique composition, forged at the very beginning of the solar system, defies easy explanation. For example, some of its dark, crumbly interior is riddled with carbonate minerals created when liquid water percolated through the rock multiple times. Yet adjacent sections bear no carbonates or other traces of water's influence at all. And though chemists would have bet money that the black stones would have teemed with exotic hydrocarbon compounds, analyses turned up a disappointing yield -- a thousandth the organic content of Murchison, a similarly carbon-rich meteorite that fell in 1969. "We were hoping to find all these amino acids," laments Iain Gilmour (Open University), "and they're just not there."

What Gilmour and others have identified are puzzling clues to the meteorite's origin. Some of the organic components mimic the nitriles and other aromatic species known to exist in molecular clouds. So might Tagish Lake have an interstellar origin? Or, as Takahiro Hiroi (Brown University) speculates, are these pieces of one of the dark, carbon-rich "D-type" asteroids that lurk in Jupiter's vicinity? More than one specialist openly questioned whether this find could represent chunks of a comet's nucleus. "There are no real conclusions yet," says Sandra Pizzarello (Arizona State University). "This meteorite is extremely difficult to study."


Copyright 2001 Sky Publishing Corporation. S&T's Weekly News Bulletin and Sky at a Glance stargazing calendar are provided as a service to the astronomical community by the editors of SKY & TELESCOPE magazine. Widespread electronic distribution is encouraged as long as these paragraphs are included. But the text of the bulletin and calendar may not be published in any other form without permission from Sky Publishing (contact permissions@skypub.com or phone 617-864-7360). Updates of astronomical news, including active links to related Internet resources, are available via SKY & TELESCOPE's site on the World Wide Web at http://www.skypub.com/.

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