杏吧原创

Update 29 April 2010: The research has now been published in Nature ().

Two independent teams have found what may be the first direct evidence of water ice on the surface of an asteroid. The discovery lends support to the idea that asteroids could have helped deliver water to the early Earth.

Asteroids are generally considered to be rocky, and comets icy. That鈥檚 because ice in the early solar system is thought to have formed beyond a 鈥渟now line鈥� lying somewhere between Mars and Jupiter. Asteroids forming beyond that boundary could contain ice.

But it is not clear how common ice might be in the main asteroid belt, because sunlight is expected to quickly vaporise ice on the surfaces of airless bodies that fly closer to the sun than Jupiter.

In 2008, however, Andrew Rivkin of Johns Hopkins University in Laurel, Maryland, and Joshua Emery of the SETI Institute in Mountain View, California, found hints () that the asteroid 24 Themis, which sits in the main asteroid belt between Mars and Jupiter, could have water ice on its surface. The team found the water signal by measuring the spectrum of infrared light radiated by the object.

Now a second team has found the water ice signature using the same telescope, NASA鈥檚 in Hawaii.

The new observations suggest water ice, mixed with organic molecules, is 鈥渨idespread on the surface of the asteroid鈥�, of the University of Central Florida in Orlando at a meeting of the American Astronomical Society鈥檚 Division of Planetary Sciences in Fajardo, Puerto Rico, last week.

Icy parent

That is surprising, since the asteroid鈥檚 distance from the sun means it should lose about 1 metre of ice each year. 鈥淭his ice is unstable, and therefore we need a process to explain why there is ice on the surface now,鈥� Campins said at the meeting.

One possiblity is that an icy object might have collided with 24 Themis, leaving behind a layer of ice. Alternatively, the ice could have been part of the asteroid鈥檚 parent body, which is thought to have broken up about 2.5 billion years ago, producing a family of asteroids with orbits similar to that of 24 Themis.

In that case, the ice seen on the surface now would once have been buried beneath dust and rock that insulated it from the sun鈥檚 heat and prevented it from escaping into space. Impacts could then have churned the surface to expose the ice.

Watery hints

Until now, the possibility of present-day water ice on asteroids has only been hinted at. The density of the mammoth main-belt asteroid Ceres suggests it contains a large amount of water ice, but no clear-cut sign of ice has been found on its surface.

鈥淭his Themis result is really nice because it鈥檚 been confirmed by two independent groups,鈥� says of Queen鈥檚 University Belfast in the UK, though he notes that the spectral evidence for ice may not be entirely water-tight.

鈥淚t鈥檚 a messy region [of the spectrum] that鈥檚 hard to observe,鈥� says of Southwest Research Institute in Boulder, Colorado. 鈥淚n some sense, it鈥檚 a matter of judgment whether it鈥檚 a hint of water ice or an almost certain detection or somewhere in between.鈥�

Still, Chapman says, he would not be surprised to see water ice on asteroids, adding that the distinction between comets, traditionally considered to be icy, and asteroids, which have been largely thought of as rocky, is becoming increasingly blurred.

Comet tails

In recent years, Hsieh and others have discovered a handful of main-belt asteroids with comet-like tails that could be created by sublimating ice.

These so-called 鈥渕ain-belt comets鈥� are too distant and faint for astronomers to scan their spectra for signs of water ice. Two of the four detected so far are part of the same family of asteroids as 24 Themis, which is the family鈥檚 largest member.

鈥淲hat is most likely the case is that the parent body [of this family] was water-ice-rich, was broken up, and now the surface of its largest fragment, Themis, has been impact-excavated, revealing the ice that was once deep in a larger object,鈥� says Britney Schmidt of UCLA, who was not affiliated with Campins鈥檚 study. 鈥淚t is exciting to finally get a glimpse inside a water-rich body.鈥�

Mismatched isotopes

of UCLA, co-discoverer of the first main-belt comets, agrees. 鈥淭hese objects may hold ice and preserve a record of conditions at formation between the orbits of Mars and Jupiter,鈥� he told New 杏吧原创.

The detection could also go a long way towards explaining how the Earth got its water. Soon after the solar system鈥檚 formation, the sun鈥檚 heat is thought to have broken apart most water molecules that lay close to the sun.

Comets, which formed far from the sun, have been proposed as an alternate way of delivering water to the Earth, but the relative amounts of different isotopes of hydrogen in the water on comets do not match up with Earth鈥檚. 鈥淚f there was water in the asteroids, it could be enough to fill the oceans,鈥� Hsieh says.