Pluto Is Slowly Erasing Its Face

After waiting nearly a decade, NASA scientists will soon get a chance to capture the clearest images of Pluto's surface ever taken. There's only one problem: Pluto's face is disappearing.

On July 15, NASA's New Horizons spacecraft will arrive at Pluto after over nine years of travel. It will make one single flyby, screaming past the not-quite-a-planet at some 30,000 miles per second per hour. Here, 32 times farther from the Sun than the Earth, with sunlight diminished to just a thousandth of what we experience here, the $700 million spacecraft will gather as much data as possible in what amounts to an extended blink, observing Pluto and its satellites up close for the first time in history as the craft approaches, whips by, and recedes from the dwarf planet.

One of many things the New Horizons mission is scheduled to observe and measure is the size-frequency distribution of craters on the Plutonian surface. This data may then be used to make some crucial calculations of the size and distribution of Kuiper Belt Objects, that extended disk of dwarf planets, asteroids, and trojans just outside the Solar System. To probe Pluto is then to probe the dark and mysterious zone beyond.

There may be something of a catch to this plan: the surface of Pluto is erasing itself. This is according to a new paper from a trio of New Horizons mission scientists led by the project's principal investigator, Alan Stern.

According to Stern and his team, it's less a statement of gloom and doom—though hardly ideal—than it is an updated guide for interpreting the forthcoming New Horizons data, which will likely reveal a surface much smoother, blanker, and, most of all, younger than first anticipated.

The basic problem is that Pluto doesn't have much of an atmosphere. It's a small, gravitationally-challenged rock, for one, but it also has a fairly hot upper-atmosphere that hovers around 100 degrees Kelvin on average. A hotter atmosphere, Stern told me, implies that its constituent molecules will have more energy to escape. The net effect is that Pluto's atmosphere is escaping at a rate of 10^27 to 10^28 molecules per second.

The result of this escape rate is that, over geologic time, the dwarf would have lost anywhere from one half to "several" kilometers of ice from its surface.

"Because this range of depths of surface [nitrogen] loss is comparable to or greater than most expected crater depths on Pluto," Stern and company write, it is possible that many craters on Pluto's surface may have been largely erased by the loss of surface material. What's more, Plutonian surface temperatures may be affecting the viscosity of surface nitrogen in such a way as to cause its craters to, well, slump.

Asked if the potential loss of historical material from Pluto was disappointing, Stern said, "Not to me, it's all wonderful, a higher escape rate tells us other things about the system, it's all good science!"

As New Horizons cruises past Pluto and heads onward to its very-near neighbor Charon, it should be able to use the data from each sorta-planet comparatively to come up with a pretty good estimate of the actual escape rate of material from Pluto.

Charon's surface and atmosphere haven't been well-characterized—which is part of New Horizons' mission—but it appears to have a surface dominated by water rather than nitrogen, which makes up about 98 percent of Pluto's surface composition (in ice form), according to past spectrographic analyses. The upshot is that structures on Charon will have "relaxed" less than those on Pluto, though the relative amounts of slumpage on Charon and Pluto is an unsettled issue.

Charon might have an undetected heat source; frozen nitrogen on Pluto might be less abundant than assumed. The outer limits of the Solar System are themselves unsettled. In any case, we'll know a whole lot more in July.