Shadowy Quaoar is a distant mystery.© NASA and G. Bacon

Ice crystals have been discovered on Quaoar, a mysterious object in the outer reaches of the Solar System. The discovery suggests that Quaoar and many similar objects could be much warmer and more geologically active than had been thought.

Apart from Pluto, Quaoar is the largest known object in the Kuiper belt. This region lies beyond Neptune's orbit and is filled with icy rubble that formed during our Solar System's birth more than 4.5 billion years ago.

Because Kuiper-belt objects lie more than 4.5 billion kilometres from the Sun, astronomers had thought that they were unlikely ever to get warmer than about -223°C, which is just 50°C above absolute zero, the coldest temperature possible.

"At that temperature, the ice on Quaoar should be amorphous, but it is not," says David Jewitt, an astronomer from the Institute for Astronomy in Honolulu, Hawaii. He and his colleague Jane Luu of the Massachusetts Institute of Technology, Lexington, have carried out infrared observations of the rock, which show that the ice has a repeating crystalline pattern, similar to that seen in snowflakes. They report their discovery in this week's Nature¹.

Getting jiggly

The crystalline pattern means that when the ice formed, Quaoar must have been warmer than -163°C. Below that temperature, water molecules stick together in random patterns to form amorphous ice. But if it is warmer than that, the molecules can jiggle around enough to fall into a regular pattern that makes for a more stable arrangement.

<mediar rid='m1'><?xm-replace_text {mediar}?></mediar>Mike Brown, the astronomer at the California Institute of Technology, Pasadena, who discovered Quaoar along with colleague Chad Trujillo in June 2002, says he is not surprised by the result. "We seem to find [crystalline ice] everywhere we look in the outer Solar System," he says. "That tells us that the process creating it is ubiquitous."

But where is the heat coming from? Quaoar has a nearly circular orbit that never brings it near Neptune. So the orbit has probably been stable since the Solar System formed, ruling out the idea that Quaoar was heated by moving closer to the Sun, says Jewitt.

Radioactive heating

Another possibility is that the surface could have been warmed by the impact of tiny meteorites, but Jewitt and Luu think this is unlikely because they found ammonia trapped in Quaoar's ice. Because this chemical is more volatile than water, surface bombardment that heated the ice would eventually remove any ammonia that was present.

<mediar rid='m2'><?xm-replace_text {mediar}?></mediar>So Jewitt believes that the crystalline water-ammonia mixture is more likely to have been formed inside Quaoar, warmed by the radioactive decay of elements such as uranium and thorium. Such heating could also trigger 'cryogenic volcanism'. Such volcanism involves explosions of gas and liquid that bring underground reservoirs of the ice crystals to the cold surface.

"I think it was radioactive heating during the earliest history of the Solar System," agrees Brown. "But I think it would not be active today."

Jewitt, however, believes that the ice formation is likely to be an ongoing process. Crystalline surface ice would be unlikely to survive bombardment by the solar wind and cosmic rays for more than about 10 million years, he points out.

Comet nursery

Jewitt and Luu spotted the first Kuiper-belt object in August 1992, and have since found dozens more. Calculations predict that there are at least 70,000 objects in the belt with diameters greater than 100 kilometres, providing a pool of icy objects that may supply the Solar System with short-period comets.

So far, the researchers have only identified water and ammonia on Quaoar, but Jewitt believes that the object probably also contains silicate rocks and some carbon-containing materials. 

Institute for Astronomy, Honolulu, Hawaii

• References

  1. Jewitt D. C., Luu J., Nature, 432. 731 - 733 (2004). | Article |