Eris and Pluto Find Common Ground

They may have their differences, but it seems that dwarf planets Eris and Pluto have a lot more in common than just their planetary status.

The larger Eris orbits the sun at a distance three-times that of Pluto, so it may not seem possible that we'll ever get a glimpse of what its surface is made of. But scientists from Northern Arizona University (NAU) have shown that even this lonely object can be probed from afar.

At the American Astronomical Society's Division for Planetary Sciences conference held in Pasadena this week, NAU professor Stephen Tegler presented the results of a two-year study that combined laboratory ice work and observations from two U.S. telescopes.

Tegler's team grew samples of methane, nitrogen, argon ice, plus mixtures of methane-nitrogen and methane-argon ice in the NAU ice laboratory. All of these ices were chosen as they most likely exist on Eris. These ices were formed at temperatures of around -200 degrees Celcius (-328 degrees Fahrenheit, or 73 Kelvin) and light was shone through each. This process allowed the researchers to see the spectra of the samples.

Through a process of elimination, they were able to compare their lab results with the spectra of light reflected from Eris' surface using spectroscopic observations from the Multiple Mirror Telescope Observatory, Arizona. The team also gathered observations of Pluto's spectra from Kitt Peak National Observatory. Eris' surface ice composition could then be deduced and compared with that of Pluto.

"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," said co-author David Cornelison of Missouri State University.

Caltech's Mike Brown, who's Palomar Observatory team discovered Eris in 2005, attended Tegler's presentation on Tuesday and is confident about these new findings.

"My suspicion -- not having read the paper yet -- is that the answer is right, but only because it is the answer that we have always expected!" Brown told Discovery News.

"Eris and Pluto should have more or less the same surface composition based on some modeling a student of mine did a few years ago, but Eris is so far away that there is no way to directly detect the nitrogen like you can on Pluto."

Brown pointed out that to actually "see" the nitrogen signature in Eris' spectra, you have to understand how the hard-to-detect nitrogen ice affects the easy-to-detect methane ice.

"Tegler's method is clever: rather than try to find the nitrogen itself, which has proved impossible, look for the way the nitrogen affects the more easily detected methane." he said. "I think they make a very good case that the methane is affected, and I suspect (but haven't yet seen) that they make a good case in the lab that the effect is exactly that expected from nitrogen."

Of course, we'd only be able to confirm what the spectroscopic analysis shows if we can get a close-up look of a dwarf planet. Fortunately, that will happen in 2015 when NASA's New Horizons probe zooms past Pluto. Then we'll know for sure what it is made of.

As for Eris, we'll have to wait a lot longer (if ever) for Tegler's study to be confirmed by a space probe visit.

Image: Artist's impression of Pluto's icy surface (NASA)