A dark red spot discovered on dwarf planet Haumea appears to be richer in minerals and organic compounds than the surrounding icy surface say scientists presenting their research at the European Planetary Science Congress today.

Haumea is a member of the dwarf planet family and orbits the Sun in the Kuiper Belt, which lies beyond the orbit of Neptune. Haumea is unusual because of its highly elongated shape – it measures approximately 2,000 by 1,600 by 1,000 kilometres – moulded by its extraordinary fast rotation rate of just one revolution every 3.9 hours. This characteristic is attributed to a massive impact collision more than a billion years ago.

Previous studies of the dwarf planet imply that it has a density 2.5 times that of water. Spectroscopic observations reveal that Haumea is covered in water ice, so the high density suggests it must possess a rocky interior.

Astronomers discovered a mysterious spot on Haumea's surface by measuring changes in the tiny planet's brightness as it rotated. The resulting light curve, which describes variations in brightness over time, is not exactly the same shape in all wavelengths – small but persistent differences indicate that the dark spot is slightly redder in visible light and slightly bluer at infrared wavelengths.

“Our very first measurements of Haumea told us there was a spot on the surface," says Pedro Lacerda of Queen's University Belfast. "The two brightness maxima and the two minima of the light curve are not exactly equal, as would be expected from a uniform surface. This indicates the presence of a dark spot on the otherwise bright surface. But Haumea’s light curve has told us more and it was only when we got the infrared data that were we able to begin to understand what the spot might be.”

The scientists speculate that the spot could represent an enrichment of minerals and organic compounds, or that it contains a higher fraction of crystalline ice than previously though. Or, in keeping with the recent impact of a possible cometary object with Jupiter, if the Haumea's spot is a scar of a recent impact then the spot material might resemble the composition of the impactor, perhaps mixed with material from the inner layers of Haumea.

Further observations of this spot are planned for early 2010 using the ESO Very Large Telescope. “Now we will get detailed spectroscopy of the spot to hopefully identify its chemical composition and solve the puzzle of its origin,” says Lacerda.