Some families seem blessed with eternal youth, looking much younger than their years. Now, astronomers have found just such a clan of icy objects in the outer solar system. They appear puzzlingly fresh-faced, despite the fact that they probably formed in a collision more than a billion years ago.

The largest member of the family, a rapidly tumbling blimp-shaped object called 2003 EL61, was discovered in 2005. In 2007, astronomers found five smaller objects travelling in similar orbits. Their paths suggested they all formed a single object that was broken apart in a collision more than a billion years ago.

Now, a team led by David Rabinowitz of Yale University in New Haven, Connecticut, US, reports that the brightness of the large object and four of the smaller ones (the fifth could not be observed) changes little when observed from various points along Earth's orbit.
That suggests their surfaces are covered with fresh powdery ice no more than 100 million years old. The researchers also suspect it means the surfaces are bright, though they haven't directly measured how much light the objects reflect.

Odd compositionA fresh surface is understandable for the biggest object, 2003 EL61, because it is large enough to hold an atmosphere that vaporises and refreezes regularly. But the four smaller family members are too small to hold atmospheres, says Rabinowitz.

In fact, no other small objects in the outer solar system have been found with bright young surfaces. They are thought to darken over time as solar ultraviolet radiation and charged particles called cosmic rays break down carbon-rich ices such as methane. This 'space weathering' leaves behind dark, reddish carbon compounds.

"Something fishy is going on [with this family]," says Mike Brown of Caltech in Pasadena, US, who was not involved in the new study. "We sure have a lot to learn about what happens to ice in the outer solar system."
(New Scientist)