Scientists have
discovered a beach ball-sized meteorite a half-mile below a giant crater in
South Africa.
The
145-million-year-old meteorite, found in the Morokweng
crater, has a chemical composition unlike any known meteorite.
It is also an
unusual find because it was largely unaltered by the extreme heat from the
impact.
The study is
detailed in the May 11 issue of the journal Nature.
Oddball meteorite
Scientists have
collected thousands of various meteorites over the years and tell them all
apart by their various structural, chemical, and mineralogical compositions.
The specific concentrations of platinum group elements in the newfound 10-inch
(25 centimeter) meteorite place it in the "LL-ordinary chondrite"
group of meteorites.
But other
characteristics set it apart from the group, such as having silicate and
sulfide minerals rich in iron, but no metallic iron-nickel phase.
"So it is 'another
kind' of LL-ordinary chondrite that we do not have in
our collections," said study co-author Alexander Shukolyukov
of the Scripps Institution of Oceanography and the University of California,
San Diego.
A potential
implication of this odd meteorite, he said, is that the bombardment of
meteorites 145 million years ago was different than those crashing into Earth
more recently.
The researchers
can't say for sure why this fragment is preserved. Current models indicate that
no unaltered fragments can survive large impacts, which, Shukolyukov
suggests, implies the models are incomplete.
Just a remnant
It is also clear
that it was much, much larger than 10 inches in diameter when it smacked the
surface.
An
820-foot-diameter (250 meters) meteorite would slam into the planet and release
energy on the order of about 1,000 megatons--about 66,000 times the strength of
the 15-kiloton bomb dropped on Hiroshima. The impact would create a crater 3
miles (5 kilometers) across, Shukolyukov told SPACE.com.
In contrast, the Morokweng crater is a whopping 43 miles across (70
kilometers), so the meteorite that created it must have been substantially
bigger than 820 feet.
The raw energy
produced by the impact also generates a lot of heat--3,100 to 24,700 degrees
Fahrenheit (1700 to 13,700 Celsius)--so it's surprising that anything remains of
the rock at all, and even more so that it is unaltered. Generally the heat
completely melts or vaporizes meteorites, and if anything is left, it doesn't
resemble its original state.
"It may be that
this fragment was just a separate small meteorite that accompanied a 'big
guy," Shukolyukov said.
However, according
to the models, a trailing object would have completely melted.
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