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Eris' nitrogen-ice
surface revealed

Posted: 06 October 2010

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By marrying up laboratory studies of ice to telescopic observations of dwarf planet Eris, scientists have found that the planet's surface composition bears striking resemblance to that of Pluto.

Eris is the largest of the known dwarf planets, and at 2,500 kilometres wide is larger than Pluto. It is a trans-Neptunian object orbiting beyond the stretch of the Kuiper Belt, and at certain points in its orbit, is three times as far from the Sun as Pluto.

Dwarf planet Eris and its moon Dysnomia as seen by the Hubble Space Telescope in 2006. Image: NASA, ESA, and M. Brown (California Institute of Technology).

The findings, presented by Northern Arizona University (NAU) professor Stephen Tegler at the American Astronomical Society’s Division for Planetary Sciences meeting held in Pasadena this week, are the result of two years work conducted at the NAU ice laboratory. “There are only a handful of such labs doing this kind of work in the world,” says Tegler. “By studying surfaces of icy dwarf planets, we hope to get a better understanding of the processes that affect their surfaces.”

The team grew samples of ice of varying compositions – methane, nitrogen, argon, methane-nitrogen mixtures and methane-argon mixtures – at temperatures down to minus 200 degrees Celsius. By passing light through the samples to reveal their chemical fingerprints – spectra – the team could compare the compositions with telescopic observations of sunlight reflected from the dwarf planets' surfaces. The astronomical observations of Eris were conducted using the Multiple Mirror Telescope Observatory in Arizona, while Pluto observations came from Kitt Peak.

“By combining the astronomical data and laboratory data, we found about 90 percent of Eris’ icy surface is made up of nitrogen ice and about 10 percent is made up of methane ice, which is not all that different from Pluto,” says co-author David Cornelison of Missouri State University.

Studying these distant worlds will provide a better understanding of how objects in the most distant reaches of the Solar System formed and evolved, a theme that will be continued with the arrival of NASA's New Horizons mission at Pluto in 2015.