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THE surface make-up of an object from the Kuiper Belt—a source of comets beyond Neptune—has come to light. The object seems to harbour methane ice and tar-like hydrocarbons, a finding that adds weight to the idea that Pluto and one of Neptune's moons began their lives as the largest objects in this deep-frozen region.

The Kuiper Belt, thought to be the source of short-period comets, contains many thousands of objects no more than a few hundred kilometres across. Because these tiny worlds are so far away, they appear extremely faint. But by taking long exposures using the 10-metre Keck Telescope in Hawaii, Robert Brown of the University of Arizona in Tucson and his colleagues have at last managed to analyse the spectrum of a Kuiper Belt object.

They report in last week's Science (vol 276, p 937) that the object, called 1993SC, reflects long infrared wavelengths more strongly than short ones. They say this is "consistent with the presence of long-chain hydrocarbons". These could be tarry compounds like those sometimes found in meteorites.

1993SC also seems to absorb light in the same infrared bands as Pluto and Triton, the larger of Neptune's two moons. This absorption is probably caused by methane ice, Brown says. "Methane is one of the spectroscopically most active molecules in the Solar System, so it's the first thing you find," adds Leslie Young of Boston University.

The similarity with Pluto and Triton backs the idea that the two began their lives as the largest members of the Kuiper Belt family. The composition of 1993SC hints that these frozen objects have hardly changed since they formed about 4.5 billion years ago, Brown says: "They are some of the most primitive things in the Solar System."

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