Brown Scientist
Answers How Peruvian Meteorite Made It To Earth
(13 March 2008) It made news around
the world: On Sept. 15, 2007, an object hurtled through the sky and crashed
into the Peruvian countryside.
Scientists dispatched to the site
near the village of Carancas found a gaping hole in the ground.
Peter
Schultz, professor of geological sciences at Brown University and an expert in
extraterrestrial impacts, went to Peru to learn more. For the first time, he
will present findings from his travels at the 39th annual Lunar and Planetary
Science Conference in League City, Texas, in a talk scheduled for 2 p.m. on
March 11, 2008. Brown graduate student Robert "Scott" Harris collaborated on
the research, joined by Jose Ishitsuka, a Peruvian astrophysicist, and Gonzalo
Tancredi, an astronomer from Uruguay.
What Schultz and his team found is
surprising. The object that slammed into a dry riverbed in Peru was a
meteorite, and it left a 49-foot-wide crater. Soil ejected from the point of
impact was found nearly four football fields away. When Schultz's team analysed
the soil where the fireball hit, he found "planar deformation features" or
fractured lines in sand grains found in the ground. Along with evidence of
debris strewn over a wide area, the shattered sand grains told Schultz that the
meteorite had maintained a high rate of speed as it shot through the
atmosphere. Scientists think it was travelling at roughly 15,000 miles per hour
at the moment of impact.
"Normally with a small object like this, the
atmosphere slows it down, and it becomes the equivalent of a bowling ball
dropping into the ground," Schultz said. "It would make a hole in the ground,
like a pit, but not a crater. But this meteorite kept on going at a speed about
40 to 50 times faster than it should have been going."
Scientists have
determined the Carancas fireball was a stony meteorite -- a fragile type long
thought to be ripped into pieces as it enters the Earth's atmosphere and then
leaves little more than a whisper of its journey.
Yet the stony
meteorite that struck Peru survived its passage mostly intact before
impact.
"This just isn't what we expected," Schultz said. "It was to the
point that many thought this was fake. It was completely inconsistent with our
understanding how stony meteorites act."
Schultz said that typically
fragments from meteorites shoot off in all directions as the object speeds to
Earth. But he believes that fragments from the Carancas meteorite may have
stayed within the fast-moving fireball until impact. How that happened, Schultz
thinks, is due to the meteorite's high speed. At that velocity, the fragments
could not escape past the "shock-wave" barrier accompanying the meteorite and
instead "reconstituted themselves into another shape," he said.
That new
shape may have made the meteorite more aerodynamic -- imagine a football
passing through air versus a cinderblock -- meaning it encountered less
friction as it sped toward Earth, hitting the surface as one large
chunk.
"It became very streamlined and so it penetrated the Earth's
atmosphere more efficiently," Schultz said.
Schultz's theory could upend
the conventional wisdom that all small, stony meteorites disintegrate before
striking Earth. If correct, it could change the thinking about the size and
type of extraterrestrial objects that have bombarded the Earth for eons and
could strike our planet next.
"You just wonder how many other lakes and
ponds were created by a stony meteorite, but we just don't know about them
because when these things hit the surface they just completely pulverise and
then they weather," said Schultz, director of the Northeast Planetary Data
Center and the NASA/Rhode Island University Space Grant
Consortium.
Schultz's research could have implications for Mars, where
craters have been discovered in recent missions. "They could have come from
anything," he said. "It would be interesting to study these small craters and
see what produced them. Perhaps they also will defy our
understanding."
(source: Brown University)
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