The surface of the largest known 'plutoid' appears to have changed in recent years, according to new measurements of how elements are layered on its icy surface. But astronomers cannot explain the cause of the apparent change.
Eris is the largest known object beyond the orbit of Neptune, weighing nearly a third more than Pluto. It travels on an elongated path around the Sun that takes about 560 years to complete.
Astronomers think the distant world is covered by a layer of frozen methane and small amounts of nitrogen ice. When it comes near the Sun, these ices are thought to vaporise from sunlit portions of the surface and condense onto regions in shadow.
Eris is now near its farthest point from the Sun, so it is expected to be cold and inactive. But a new study suggests the dwarf planet's surface may have changed in the last few years.
"We're really scratching our heads," says author Stephen Tegler of Northern Arizona University in Flagstaff.
Pass through
Tegler and colleagues probed Eris's supposedly inactive surface by measuring how methane ice absorbed the Sun's light.
Methane absorbs some wavelengths of light more strongly than other wavelengths. Weakly absorbed light can therefore pass through the methane layer and reach greater depths beneath the surface, although the exact depth is difficult to estimate.
By studying different wavelengths - or 'bands' - of light in Eris's spectrum using the 6.5-metre MMT observatory in Arizona, the researchers concluded that the concentration of nitrogen seems to increase with depth.
That result, based on observations of five wavelength bands in 2007, contradicts observations made in 2005 with the 4.2-m William Herschel Telescope in Spain. The 2005 observations, which measured two bands of light, suggested that nitrogen is more abundant closer to the surface.
Icy weather?
Both sets of observations are valid, say researchers, but they can't yet come up with an explanation for the difference.
One possibility is that Eris experienced some recent change in weather that altered the top several centimetres of its surface, Tegler says.
But changes in weather are difficult to explain when Eris is so far from the Sun. "It's very hard to imagine that something that dramatic would be happening on a relatively short time scale", says Mike Brown of Caltech, who was not involved in the study.
Another possibility is that methane and nitrogen vapour erupted from Eris's interior, eventually condensing down to form a new layer of ice, says co-author William Grundy of Lowell Observatory in Flagstaff, Arizona.
Volcanic eruption
No one is sure whether Eris is warm enough to boast this kind of 'cryovolcanism', but "an eruption isn't out of the question", Grundy told New Scientist.
NASA's New Horizons mission, which is set to fly past Pluto in 2015, could help determine whether the erstwhile planet ever boasted similar eruptions. "If a shrimpy little body like Pluto can do it, Eris can too," Grundy says.
Alternatively, the two teams might have observed different parts of the dwarf planet. New measurements suggest Eris boasts an Earth-like day, rotating on its axis once every 26 hours.
Future observations could track the planet's appearance over multiple rotations to determine whether Eris has a patchy composition, Grundy says
