[meteorite-list] 123-Mile Asteroid Has Surface Water

[Sterling K. Webb]

http://www.space.com/scienceastronomy/water-ice-on-asteroid-100428.html

Water Ice Discovered on Asteroid for First Time 
By Clara Moskowitz
SPACE.com Senior Writer
28 April 2010

Water ice has been found on the surface of a nearby asteroid 
for the first time - a discovery that could help explain how 
Earth got its oceans, scientists announced Wednesday.

Two teams of researchers independently verified that the 
asteroid 24 Themis - a large rock hurtling through space 
in the asteroid belt between Mars and Jupiter - is coated 
in a layer of frost. 

They also found that the asteroid contains organic material, 
including some molecules that might be ingredients for life. 
But scientists have not found any evidence for life itself on 
this asteroid, or anywhere else in the universe beyond Earth.

While comets, which have characteristic tails and generally 
orbit farther out in the solar system, are known to have water, 
asteroids in that region were thought to be too close to the 
sun to contain water on the surface without it evaporating 
away. The largest asteroid in the solar system, Ceres, is 
thought to harbor a vast amount of frozen water, but scientists 
suspect all of it is buried beneath a rocky, dusty surface.

But in this new study, researchers found concrete proof of 
water ice on the surface of 24 Themis by measuring the 
specific characteristics of sunlight bouncing off the surface 
of the asteroid. They saw the tell-tale signatures of H2O 
coating most of the surface of the 123-mile (198-km) wide 
rock. 

"This is the first time we've actually seen ice - literally H20 - 
on an asteroid," said one of the study leaders, Andrew Rivkin 
of Johns Hopkins University. 

Previously, hints that water might be present on 24 Themis 
were found in the form of hydrated minerals, which were 
thought to have formed from the reaction of water with rock. 
But this time the researchers saw the direct signature of 
water itself, he explained.

Another science team, led by Humberto Campins of University 
of Central Florida, found the same thing. Both teams used the 
NASA Infrared Telescope Facility atop on Mauna Kea in Hawaii 
to make their observations, but conducted them on different nights.

"Our work and their work are very nicely confirming and 
complementary," Campins said.

Campins' team timed their observations so that they caught 
the asteroid at different points in its rotation, and combined 
these data to create a rough surface map, showing that not 
only is ice present on 24 Themis, but it coats much of the 
surface on all sides.

"To our surprise there was water ice, there were organic molecules, 
and they were more or less evenly distributed throughout the surface," 
Campins told SPACE.com. "We thought that was fascinating."

Both teams reported their findings in the April 29 issue of the 
journal Nature.

Another researcher - Henry Hsieh of Queen's University Belfast 
in the U.K., who was uninvolved in either study - noted surprise 
at the extent of ice coverage on the asteroid. 

"The average temperatures of asteroids (about 150-200 Kelvin) 
at this distance from the sun should cause surface ice to 
sublimate away in a matter of a few years or less, which is 
inconsistent with the billions of years that Themis is thought 
to have spent at its current location," he wrote in an 
accompanying essay in the same issue of Nature.

The discovery might even provide clues about the origin of water 
on Earth. 

Earth has had a violent history, having been bombarded with 
space rocks throughout much of its life. In particular, a large 
rock was thought to have crashed into Earth some 4.5 billion 
years ago, knocking off a chunk that became our moon. This 
collision would have heated things up so much, any water that 
was on Earth at that point was vaporized. So how did the oceans 
arrive?

Some scientists have suggested that most of it arrived via other 
asteroids that crashed into Earth later in smaller collisions. But 
for that idea to hold weight, asteroids would have to carry water. 
Comets aren't a good possibility for this scenario because the 
water they hold tends to be of a slightly different nature, with 
atoms in a different configuration, or isotope, than most of the 
water on Earth.

Though the recent measurements can't tell anything about the 
isotope ratio of the water on 24 Themis, the fact that there is 
water there at all is an encouraging sign.

"Our data are certainly at least consistent with the idea that 
you could bring in plenty of water form impacts," Rivkin said.

If it sounds surprising that the vastness of Earth's oceans built 
up from deposits of water by asteroids, Rivkin said it isn't that 
crazy an idea.

"We know that the rate of [asteroid] impacts was very high," he 
told SPACE.com. "If each impactor, each asteroid, were 20 to 
30 percent water by weight, then that could potentially add up."