Wednesday, April 28, 2010
Ice asteroids likely source of Earth's water: study
Astronomers have for the first time detected ice and organic compounds on an asteroid, a pair of landmark studies released on Wednesday says.
The discovery bolsters the theory that comets and asteroids crashing into Earth nearly four billion years ago seeded the planet with water and carbon-based molecules, both essential ingredients for life.
Working separately, two teams of scientists using NASA's Infrared Telescope Facility in Hawaii found that the 24 Themis, which orbits the Sun between Mars and Jupiter, is literally covered in a thin coating of frost.
It had long been suspected that the massive space rocks that bombarded our planet after the formation of the solar system contained frozen water, but the two studies, published in Nature, provide the first hard evidence.
Still, a mystery remained: How could frozen water persist over billions of years on an asteroid hot enough to vapourise surface ice?
Only if that layer of frost were continually replenished by the slow release of water vapour released from ice in the asteroid's interior, the researchers reasoned.
In other words, 24 Themis -- some 200 kilometres (125 miles) in diameter -- almost certainly contains far more water locked in its minerals than anyone suspected.
The findings also blur the distinction between rocky asteroids, once thought to be too close to the Sun to contain water, and comets, composed of much the same stuff along with ice and dust.
Comets form their signature tails when ice melts during the phase of orbit that brings them closest to the Sun.
"We had previously thought that only the comets could have brought a lot of water to Earth," said Andrew Rivkin, lead author of one of the studies and researcher at John Hopkins University in Laurel, Maryland.
"But we now have more reason to think that the asteroid impacts may also have brought a significant amount, especially if each one might have 20 to 30 percent water," he said in an email exchange.
"Much more water could have come in from asteroids than we thought.
Rivkin's team observed 24 Themis on seven dates from 2002 to 2008, analysing various wavelengths of reflected sunlight to determine the composition of the surface.
The study turned up a good match for water ice, with a modest helping of organic -- that is, carbon-containing -- material.
In a separate study, researchers led by Humberto Campins of the University of Central Florida arrived at the same conclusion using another technique.
"Knowing that Themis rotates once every 8.37 hours, they timed their observations to obtain spectra for the asteroid at four points in its rotation, creating a crude surface map," said Henry Hsieh, an astrophysicist at Queen's University in Belfast.
Not only did they find ice on the asteroid, they found that it was evenly distributed across its entire surface.
Taken together, the studies provide an unexpectedly window onto key aspects of our early Solar System, which came into being some 4.5 billion years ago.
"Our discoveries show that some objects can remain in the asteroid belt more or less unchanged since their formation," Rivkin said.
"We no longer have to infer the composition of ancient asteroid water from hydrated materials -- we can study the water directly because it is still around today," Hsieh noted in a commentary, also in Nature.
While life-giving water and carbon molecules are present on the asteroid, other factors will have discouraged new life forms emerging, Rivkin said.
"The lack of liquid water and an atmosphere, and the presence of very strong ultraviolet light from the Sun -- screened out by our atmosphere -- are all factors we think work against the formation of life," he said.