[meteorite-list] Mysterious Bright Patches on Asteroids Explained
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Sep 6 14:11:40 2006
Mysterious bright patches on asteroids explained
06 September 2006
Asteroids gradually become coated with iron dust in space, becoming
darker and redder with time, close-up observations of the asteroid
Itokawa suggest. This confirms long-held suspicions about why asteroids
look different than the space rocks that land on Earth as meteorites.
Chunks of asteroids that were broken off during collisions are thought
to make up the most common kind of meteorite found on Earth - called
ordinary chondrites. But strangely, the light spectra of these
meteorites look different from the spectra of the most common type of
asteroid, called S-type asteroids.
Some scientists have suggested that this is because the S-type asteroids
have been "weathered" in space. In this process, fast-moving dust grains
or energetic particles from the Sun could vaporise chunks of iron in the
asteroid. The resulting cloud of iron particles would then rain back
down on the space rock, leaving a reddish coating on its surface. Over
time, the surface would become darker and redder.
Observations of meteorites support the idea. When they are cut open,
their interiors look fresh and relatively light in colour, presumably
because they have never undergone any space weathering.
Earth-based observations have also shown that families of older
asteroids - whose ages are calculated by tracing the orbits of their
individual members backwards in time to determine when they broke apart
from a single, parent body - tend to be darker and redder than younger
But astronomers had never found clear evidence of another expected
effect - that an asteroid would show variations in space weathering
across its surface because collisions with other space rocks would
expose fresh, unweathered material in some places.
Now, observations of asteroid 25413 Itokawa by Japan's Hayabusa
spacecraft have unambiguously found such variations. Researchers led by
Takahiro Hiroi of Brown University in Providence, Rhode Island, US,
analysed the spectra of infrared light from the 500-metre-long space
rock, which appears to be an S-type asteroid.
The spectra look like what astronomers would see if a type of ordinary
chondrite meteorite called an LL chondrite were coated with fine iron dust.
The researchers compared a relatively dark patch on the asteroid's
surface with one that is significantly lighter. The darker patch appears
to contain more than twice as much iron dust than the lighter patch,
suggesting that it has undergone more space weathering, the researchers say.
"Until now we haven't had these kinds of close-up images and actual
numbers on space weathering," Hiroi told New Scientist.
He says the relatively unweathered material in the lighter patch may
have been exposed more recently when a meteorite struck the asteroid.
Such an impact could expose fresh material by triggering a landslide or
by blasting rocks off into space, he says. Itokawa's low surface gravity
means that anything moving at more than about 10 centimetres per second
can escape the asteroid forever.
The Hayabusa results show that space weathering occurs as expected from
artificial weathering experiments on Earth, says William Bottke of the
Southwest Research Institute in Boulder, Colorado, US, who is not a
member of Hiroi's research team.
"It suggests we're really getting a handle on this problem," he told New
Scientist. "If you put something in space long enough, it can be
Richard Binzel of the Massachusetts Institute of Technology in
Cambridge, US, agrees. "The space weathering idea clears another hurdle"
with Hayabusa's observations of bright and dark patches, he told New
Hayabusa was launched by the Japan Aerospace Exploration Agency (JAXA)
in 2003 and reached Itokawa in September 2005. It was supposed to return
a sample of the asteroid to Earth, but the sample collection attempt
appears to have failed.
Still, a small amount of material from the asteroid may have made it
into the collector by chance, and mission controllers hope to return the
spacecraft to Earth by 2010 using only two or three of its four ion
engines (see Hayabusa does have enough power to fly home).
Journal reference: Nature (vol 443 p 56)
Received on Wed 06 Sep 2006 02:11:33 PM PDT