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Changing Face of an Icy Dwarf

2007 OR10 is an innocuous enough designation (discoverer Mike Brown calls it ‘an official license plate number’ based on date of discovery), but ‘Snow White’ isn’t. The dwarf planet that acquired the latter monniker from Caltech astronomer and KBO-hunter Brown seemed to deserve its name because at the time, Brown thought it was an icy chunk that had broken off from the dwarf planet Haumea. Ice in the outer system is almost always white, and that’s what you would expect on a world called ‘Snow White.’ But recent spectral analysis has revealed that while ‘Snow White’ is indeed covered in water ice, it’s not white at all. In fact, it is one of the reddest objects in the Solar System, about half the size of Pluto in its orbit at system’s edge.

What to make of this? It turns out that another dwarf planet fits the same characteristics, in being both red and covered with water ice. Although a bit smaller than Snow White, Quaoar is thought to have had an atmosphere and to have once been covered with ice-spewing volcanoes. As this Caltech news release points out, being smaller than the big dwarf planets Eris and Pluto, Quaoar could not hold on to volatile compounds like methane, carbon monoxide or nitrogen for long time frames. All that’s left at this point in our system’s history is methane, which over time and exposure to radiation would have been turned into reddish hydrocarbon chains.

Image: Caltech’s Mike Brown. Credit: California Institute of Technology.

So Quaoar, and possibly Snow White, are covered with irradiated methane, accounting for their hue. “You get to see this nice picture of what once was an active little world with water volcanoes and an atmosphere, and it’s now just frozen, dead, with an atmosphere that’s slowly slipping away,” adds Brown. It’s a view being pieced together with an instrument called the Folded-port Infrared Echellette (FIRE), used with the 6.5-meter Magellan Baade Telescope in Chile. The instrument’s spectral analysis revealing water ice told Brown just what he needed to know.:

“That combination—red and water—says to me, ‘methane,'” Brown explains. “We’re basically looking at the last gasp of Snow White. For four and a half billion years, Snow White has been sitting out there, slowly losing its atmosphere, and now there’s just a little bit left.”

Brown also talks about Snow White in a series of posts on Mike Brown’s Planets, from which this:

I love this spectrum of Snow White, since it tells a long complex history of a little icy world all in one glance. Snow White formed 4.5 billion years ago in the chaos that was the outer solar system. It had an evaporating atmosphere and a surface that was slowly gunking up from all of the frosts sitting in the sunlight on the surface. It would have been a cold, dark, uneventful place, until suddenly water burst out from the interior and began its slow slush flow on the surface before quickly freezing up. The volcanic period probably didn’t last long, and most of the atmosphere didn’t last much longer. The nitrogen went first, then the carbon monoxide. And finally, today, when we look at Snow White we see the very last gasps of a dying atmosphere covering a once dynamic but now dead and frozen world.

The methane finding will have to be confirmed as part of the larger study of volatile loss and retention on these distant objects. Volatiles have also been discovered on the surfaces of Eris, Makemake and Sedna. In fact, a model for volatile retention that has successfully explained the situation on these worlds seems to hold for every large KBO except Haumea, which is the parent body of a family of collision-born objects, and thus has a history more varied than most of its neighbors. From the paper on the spectral analysis, a clear view of where to go next:

While the size of 2007 OR10 has yet to be measured, the simple assumption that it has an identical albedo to Quaoar – the object whose spectrum its spectrum most resembles – places 2007 OR10 into a regime where it would be expected to retain trace amounts of methane on its surface. Such an object would be expected to have red optical coloration from methane irradiation, which both Quaoar and 2007 OR10 do have. In addition, such an object should have detectable signatures of methane if observed at sufficient signal-to-noise. Such methane signatures have been detected on Quaoar, but require higher signal-to-noise to positively identify on 2007 OR10. While additional measurements of the size and spectrum of 2007 OR10 are clearly required, we conclude that volatile retention models (Schaller & Brown 2007b) appear to continue to flawlessly predict both the presence and absence of volatiles on all objects in the Kuiper belt which have been observed to date.

Assuming that confirmation proceeds as planned, Snow White and Quaoar will stand apart from the vast majority of KBOs as being large enough to hold on to volatile compounds, a trait that helps us to analyze their subsequent history. The paper is Brown et al., “The surface composition of large Kuiper Belt Object 2007 OR10,” accepted by Astrophysical Journal Letters (preprint).


Comments on this entry are closed.

  • mike shupp August 24, 2011, 22:18

    So what happens to the material that once made up those volatile atmospheres — the molecules of water and methane and nitrogen and so on? They do go away, it seems, somewhere. At least it’s what I infer from statements that younger stars than ours, both larger (Fomalhaut) and smaller (e Eridani), have larger Oort Clouds than ours.

    My handwaving guess is that they’re in a diffuse cloud of gas and primordial “dust” — solid material of micron scale or less, most of which has never been combined in a larger body — dispersed, possibly with some clumpiness, throughout the Oort Cloud itself. Presumably at this distance outgoing solar radiation and incoming solar radiation would be of comparable effect, leaving gravity as the predominant force, so much of this gas would wind up on the surfaces of small bodies. I do not know if this “thinning out” would be quick or slow. Perhaps there are models?

    Or, alternately, perhaps over time the gases do disperse into the interstellar medium. Are there models for that? Or is this subject far from resolution or even far from being addressable by our current technology?

  • Laurel Kornfeld August 25, 2011, 23:59

    How far in AU is 2007 OR10 from the Sun, and how far is Quaoar? And how elliptical are their orbits? In losing atmosphere, 2007 OR10 resembles a comet, which while not really having an atmosphere, loses mass to outgassing with every orbit. This is not true for Pluto, whose atmosphere freezes back onto the surface as it recedes from the Sun, only to re-form once it approaches the Sun again. Is there a size threshold at which a Kuiper Belt planet will not lose atmosphere this way, and what would is that size threshold estimated to be?

  • Adam August 26, 2011, 0:48

    Hi Mike Shupp
    All good questions. In the case of methane and nitrogen, the two can remain on an object as photolysis products – tholins and cyanide based polymers – with a net loss of hydrogen to interstellar space. That’s the cause of the colour change from white/colorless to “red”/brown. There’s some evidence that the dark colour of Jupiter’s Callisto is from a long term build up of the heavy atomic mass remnants of a methane/ammonia atmosphere – ultimately a dark tarry substance forms, mixed with meteoritic dust.

  • ljk October 14, 2011, 21:28

    Pluto or Eris: Which is Bigger?

    by Ray Sanders on October 14, 2011

    The controversy between Pluto and Eris regarding their status as “largest dwarf planet” continues. During a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress last week in Nantes, France, new data was presented that may help settle the debate. The new findings regarding this size of Eris may be a surprise to some, and to others a confirmation of what was believed to be true.

    How were astronomers able to make the new measurements of Eris, and what implications will these new measurements have on the Pluto / Eris debate?

    Using a celestial alignment known as an occultation, Bruno Sicardy of the Paris Observatory (University of Pierre and Marie Curie, France) and his team were able calculate the diameter of Eris in 2010. The occultation was caused by Eris moving past a background star, which blocked the star’s light and cast a small shadow on Earth.

    When Sicardy and his team compared the shadow’s size at two different sites in Chile, the calculations provided a diameter of 2,326 kilometers for Eris. A previous study by Sicardy in 2009 placed Pluto’s diameter to be at least 2,338 kilometers.

    Full article here: