Did asteroids bring water to Earth?

"It's a difficult region to work in," says Dr. Rivkin of the hunt for signatures of cosmic water at infrared wavelengths. Earth's atmosphere is laden with water vapor. It's presence swamps signatures of water from celestial objects in all but a few infrared wavelengths.

In their observations of 24 Themis, "there was something that looked kind of weird," he says. "We initially thought: Well, it kind of looks like it could be ice." They made several measurements over a six-year period "just to be sure we weren't being fooled." Once the team had collected its 2008 measurements, "we were confident enough to go: 'OK, this is ice,' " Rivkin says.

Although this and the parallel observations from Dr. Campins' team were firsts, Rivkin says the slap-the-forehead moment for his team came with the detection of organic compounds on 24 Themis.

The estimated lifetime for the surface ice – thought to exist as extremely thin coatings on dust grains – is on the order of thousands of years, Rivkins' team notes. But the team also calculates that enough water is present in the asteroid's top 2 kilometers of material that at current loss rates, it would outlast the age of the solar system.

Campins's team suggests that the surface ice gets replenished as micrometeoroids strike 24 Themis's surface, breaking off and vaporizing subsurface ice it encounters. That vapor refreezes around the dust grains at the surface.

The discovery also tightens a connection researchers have proposed between 24 Themis and a recently discovered class of comets in the main belt. Perhaps one-third of these main-belt comets trace their roots to a massive collision that spawned 24 Themis and its far-flung family of asteroids, explains Michael A'Hearn, a planetary scientist specializing in comet and asteroid studies at the University of Maryland.

"Finding the ice supports the family association," he explains. "Now, the key question is whether Themis and its family are a one-off, or the first of many."

And beyond the scientific interest in ice on the asteroid's surface lies a practical one, adds MIT's Binzel. If asteroids rich in accessible water ice are common, they hold the potential of serving as oases for future astronauts during interplanetary trips.

Related:

Blog: Amateur astronomer finds a hole punched in Jupiter

Blog: Protecting Earth from asteroids

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