[/caption]Eris, the largest dwarf planet beyond Neptune, is currently at its furthest point in its orbit from the Sun (an aphelion of nearly 100 AU). At this distance Eris doesn’t receive very much sunlight and any heating of the Plutoid will be at a minimum. However, two recent observations of Eris have shown a rapid change in the surface composition of the body. Spectroscopic analysis suggests the concentration of frozen nitrogen has dramatically altered during the two years Eris had been at this furthest point from the Sun. This is very unexpected, there should be very little change in nitrogen concentration at this point in its 557 year orbit.
So what is going on with this strange Plutoid? Is there a mystery mechanism affecting the surface conditions of this frozen moon? Could there be some cryovolcanic process erupting? Or is the explanation a little more mundane?
“We’re really scratching our heads,” says Stephen Tegler of Northern Arizona University in Flagstaff, author of the new Eris research (to be published in the journal Icarus). Tegler and his team analysed spectroscopic data from the 6.5 metre MMT observatory in Arizona and compared their 2007 results with a similar observation campaign by the 4.2 metre William Herschel Telescope in Spain two years earlier in 2005.
During that two year period, the scientists wouldn’t have thought there would be much difference in the two datasets. After all, the reflected sunlight off the surface of Eris should reveal a similar surface composition, right? Actually, the results couldn’t be more surprising. It would appear that within two years, having not changed its distance from the Sun significantly, the surface composition has changed a lot. Normally, this would be expected if a planetary body approaches or travels away from the Sun; the increase or decrease in solar energy would change the weather conditions on the surface. But this situation does not apply to Eris, there is little chance that the Sun could influence the weather on the surface of Eris to any degree (or, indeed, if Eris even has “weather”).
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So what have the researchers deduced from the comparison of the 2005/2007 data? It would appear the spectroscopic methane lines have become diluted by an increased quantity of nitrogen. This means that the 2005 results showed a higher concentration of nitrogen near the surface, whereas the 2007 results show a higher concentration below the surface. For a dwarf planet to demonstrate a very fast change in surface composition appears to show some very dynamic process is at work.
So what could have caused this change? In the case of a dynamic weather process, “it’s very hard to imagine that something that dramatic would be happening on a relatively short time scale,” says Mike Brown of Caltech, a scientist not involved with the research. Another possibility is that 2003ub313 is a cryovolcanic body. Cryovolcanoes can erupt on icy moons or bodies in the Kuiper belt, but rather than spewing molten rock (magma), they erupt volatiles like ammonia, water or (in this case) nitrogen and methane. The ejected cryomagma then condenses into a solid, thus changing the surface composition of the icy body.
But it is not known whether Eris is warm enough for such a process to work. More information on trans-Neptunian object (TNO) cryovolcanism will be examined when NASA’s New Horizons mission reaches Eris’ smaller cousin Pluto in 2015. “If a shrimpy little body like Pluto can do it, Eris can too,” said co-author William Grundy of Lowell Observatory in Flagstaff, Arizona.
However, there is a possibility that the surface composition of Eris hasn’t changed at all. The 2005 and 2007 observations may have been analysing two different regions on the dwarf planet, thus the difference in surface composition (after all, the Plutoid has a rotation period of 26 hours, they would have almost definitely have seen different parts of Eris). So the next step for the researchers is to carry out an extended campaign throughout an “Eris day” to see if the surface composition is in fact patchy, which would be an interesting discovery in itself.