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Re: Sky & Telscope News Bulletin - March 13, 1998

Ron Baalke schrieb:

> SKY & TELESCOPE'S NEWS BULLETIN
> MARCH 13, 1998
>
> ANCIENT CRATER CHAIN ON EARTH
>
> The Earth already has many visible scars of cosmic collisions. Now
> researchers have linked five impact features and suggest that they all
>
> formed at the same time as a shattered comet or asteroid struck the
> Earth
> -- much as the pieces of Comet Shoemaker-Levy 9 hit Jupiter in 1994.
> In the
> March 12th NATURE, David Rowley (University of Chicago), John Spray
> (University of New Brunswick), and Simon Kelley (The Open University)
> explain how after moving the drifting continents back to their
> arrangement
> 214 million years ago, impact scars in France, Canada, Ukraine, and
> Minnesota lined up. The largest of the craters is 100 km across. These
>
> impacts are a likely influence on the mass extinction of life at the
> end of
> the Triassic period, where 80 percent of the species then living on
> the
> Earth disappeared.

------- snip --------

Sky & Telescope, January 1987, pp. 16-17:

Crater Chains on Earth?

Ever since they chanced upon the strung-out remains of Comet
Shoemaker-Levy 9, astronomers have pondered whether similar salvos
occasionally pepper other solar-system surfaces. For an answer, they
first looked to the Galilean satellites, where a dozen "crater chains"
had been pounded into the Jupiter-facing half of Callisto and three into
that of Ganymede. A search of Earth's Moon turned up only two: a fresh,
compact string inside the large near-side crater Davy Y, and a longer,
much older one near the crater known as Abulfeda.
Thanks to its dynamic geology and erosion, Earth itself has never been
very good at preserving impact scars. However, earlier this year Michael
R. Rampino and Tyler Volk (New York University) drew attention to an
alignment of eight large depressions - which includes two confirmed
impact craters - stretching across 700 kilometers of Kansas, Missouri,
and Illinois. In addition, what may be a multiple-crater cluster has
turned up in Space Shuttle radar imagery of northern Chad.
But were these created by single objects ripped apart by gravitational
stress, as Comet Shoemaker-Levy 9 was? A new analysis by William E
Bottke Jr. (Caltech) suggests the answer is probably no. Together with
Derek C. Richardson (Canadian Institute of Tbeoretical Astrophysics) and
Stanley G. Love (Caltech), Bottke has been exploring the fate of weakly
bound "rubble piles" that venture too near the Earth or Moon. They find
that projectiles heading directly for us can't come apart soon enough,
and thus create widely separated pieces, before hitting. But they can be
pulled into pieces as they pass the Earth before striking the Moon - or
vice versa.
Complete disruption, it turns out, occurs only under special
circumstances: during very close approaches at relatively slow speeds,
with the intruder spinning rapidly in a prograde direction. In fact, the
team's computer modeling reveals that total breakups are quite rare -
once every 210,000 years for Earth-grazing objects, every 530 million
years for those skirting the Moon. Using these statistics, Bottke and
his colleagues find that the lunar surface should bear only a single
crater chain and the Earth's none at all.
A more common result of these near misses is "mass shedding," in which
only a fraction of the passing object is lost; this material sometimes
becomes a satellite. 'Ihe computations confirm a previous result by
Bottke and H. Jay Melosh (University of Arizona) that 15 percent of the
asteroids in Earth's vicinity should be double, nicely matching the
terrestrial cratering record: three of the Earth's 28 largest craters
are paired.
Bottke stops short of claiming that the two suspected crater clusters on
Earth are illusory. But he notes that, if real, they might be the
consequence of debris spraying out from larger (and as yet unidentified)
impacts elsewhere on Earth. Details were presented in October at the
annual meeting of the American Astronomical Society's Division for
Planetary Sciences.

Regards, Bernd
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