In September 2007 a fireball flew in from space and hurtled
though Earth's atmosphere, smashing into the ground in Peru while awestruck
witnesses watched. It was the first time people have witnessed space debris
forming an impact crater live.
But the witnesses'
reports and the geological aftermath stunned scientists. This meteorite
seemed to have flown in much faster than scientists thought possible for an
object of this kind, and it apparently survived entering Earth's atmosphere
intact, rather than splintering into bits as experts thought it should have.
"Many people thought this was a
fake," said Peter Schultz, a Brown University planetary geologist who
traveled to Peru
to analyze the crater. "It just didn't make sense with what we understand
of collisions with this type of fragile rock. Coming through the atmosphere they
get stressed so highly that they typically break apart. But this one didn't do
that."
Let's go look
A few months after the impact, Schultz went to investigate
the crater along with Peruvian scientists and government officials. He
presented his findings today at the 39th annual Lunar and Planetary Science
Conference in League City, Texas.
Schultz found fractured lines in sand grains and compressed
mixtures of earth and meteorite around the 49-foot-wide crater
near the village of Carancas. These, along with widespread debris from the
meteorite's crash landing, told him it landed at high speed, likely around
15,000 miles per hour at the moment of impact.
The meteorite was a common type, a chunk of silicate rock
called a stony meteorite. Usually a projectile such as this would be slowed
down by the drag of Earth's atmosphere. By the time it landed, it would be
traveling at the normal terminal speed of any object falling from the sky, and
would probably dint a hole in the ground, but not a crater.
"Essentially Carancas threw us this high-speed curveball,"
Schultz told SPACE.com. "The mystery
is why it didn't slow down and how did it make it all the way to the Earth intact
to form a crater? These are questions we have to resolve."
New thinking
Scientists have several hypotheses about what might have
happened. Perhaps as the meteorite hurtled through the atmosphere it melted and
morphed, becoming more of an aerodynamic needle-shape that could resist stress
and survive in one piece. Plus, this shape would help it hold on to its speed,
since the surface area exposed to atmospheric drag forces would be reduced.
"But the mystery is, why wouldn't all objects reshape?"
Schultz said. "Maybe it requires special circumstances, like the angle of
entry."
The unique event could change scientists' thinking about how
meteorites act, and how other craters on Earth were formed, especially ones
similar to the Carancas site, where water has since collected in the hole.
"What does this mean for other small water-filled
ponds?" Schultz asked. "You just wonder how many of these have
powdered remains of a stony meteorite at the bottom, something that would be
difficult to find after time."
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