Dwarf planet. Trans-Neptunian Object. Doggy dog world. Who would have ever guessed that Pluto would one day be the most interesting place in the solar system?
Perhaps Alan Stern did. After all, it was he who spent a decade advocating for a mission to Pluto before succeeding in getting New Horizons launched in 2006. But even he has to be surprised by the breadth of exciting data coming home from the expedition. New Horizons has brought astonishing resolution to a place that was pixellated less than a year ago. Past data dumps have teased us with features like Charonian ice volcanoes and slow flowing nitrogen ice mud. A quintet of new papers---all freely available---published today in the journal Science expounds on these to provide new details about Pluto, from its geology and surface ices, to its atmosphere and interactions with the solar wind.
Pluto's mysteries begin with its heart---known to Plutocionados as Tombaugh Regio. Discovered less than a year ago, it is already one of the most iconic geographic (plutographic?) features in the solar system. Right up there with the Big Red Spot (junography?) and Olympus Mons (areology?).
The two lobes might look roughly symmetrical in living color, but spectroscopic analysis has shown they are very different. The right lobe is highly eroded, and in fact has glaciers flowing into the lower, flatter left lobe (called Sputnik Planum). "Put together, it looks like somehow material from Sputnik Planum gets transported east, and condensed there, accumulating to the point where it feeds glaciers that flow back west to Sputnik Planum," says Will Grundy, astronomer at Lowell Observatory in Arizona, and co-author of the paper on Pluto and Charon's ices.
Sputnik Planum is in fact several kilometers deeper than the surrounding landscape, and filled with nitrogen ice. And it is relatively smooth---its lack of craters suggests it is less than 10 million years old, which is geologically spry! Not to say it has a mirror finish. Close up imagery of the northwestern portion of Sputnik Planum reveals surface texture that looks like cells. The centers bulge up to 150 feet higher than their rounded edges. "What we think is happening is heating below Sputnik Planum, so the ice convects, rises, and gets an edge to it like water in a saucepan," says Oliver White, a post-doctoral researcher at NASA Ames Research Center in Silicon Valley and co-author of the paper describing Pluto and Charon's geologies. The fact that the cellular lumps do not cover Sputnik Planum's entirety suggest that the subterranean heating is localized.
"Once you get outside the cellular plains, evidently you move away from the convective zone," says White. Without that constant renewal, the nitrogen ice develops pits---some up to several miles wide---likely caused by sunlight turning the ice directly into gas (a process called sublimation). The southern and eastern regions of Sputnik Planum are dotted with these pits.
The localized heating under Sputnik Planum is an example of a larger mystery. Pluto is covered with terrain. Not just craters: The world has plateaus, rocky scarps, mountains of all sorts. These tectonic features suggest large-scale inner heating. "So the thing about Pluto and Charon is unlike icy satellites around big planets, some of which have similar sizes, there’s no tidal heating of any significance," says White, referring to the way that strong gravitational forces squeeze and pull on a planet or moon's core. Pluto, it turns out, is too far from any other large body to have tidal heating. So New Horizons' geologists suspect some sort of radioactive decay is responsible for its inner fire.
But the heat is only one part of the landscape story. These new analyses provide a clearer picture of Pluto's different ices. Nitrogen, methane, and water ices melt at different temperatures, which means when one becomes fluid (or gaseous), it affects the structure of the harder, still frozen ices around it. "For instance, if there was only water ice, little or no new geology would be happening at such low temperatures, and we'd just have an ancient, cratered surface," says Grundy. Conversely, with only volatile ices like nitrogen and methane, all of Pluto would be smooth and flat like Sputnik Planum.
Those volatile ices aren't just moving mountains. When solar energy sublimates nitrogen or methane, the gas becomes an atmosphere. Pluto's is thin, but dynamic, with breezes, layers, and even hazes. "There are general circulation models that predict methane clouds could form in certain regions on Pluto, but they aren't common," says Randy Gladstone, planetary scientist at the Southwest Research Institute at the University of Texas, San Antonio, and co-author of the paper on Pluto's atmosphere. "I don't know if they would snow, but all the haze that settles onto the surface counts as precipitation!"
Not all the gas settles on the surface. A small portion escapes Pluto, creating a buffer zone with the solar wind. "The nature of this interaction tells us the amount of atmosphere that is as escaping and what happens to it when it leaves the Pluto system," says Fran Bagenal, astrophysicist and planetary scientist at the University of Colorado, and co-author of the paper on Pluto's space environment.
The flyby showed Bagenal that Pluto is losing atmosphere at a much slower rate than previously thought. "This means the solar wind interaction with Pluto's escaping atmosphere is somewhere between a comet and Mars," says Bagenal. A body's mass (hence, gravity) is one of the strongest predictors for how much atmosphere it holds against the solar wind's incessant blowing. "The comet-Pluto-Mars comparison allows us to see the effects of the object's mass as well as the influence of its distance from the sun." Surprisingly, Earth, Mars, and Pluto are all losing about a pound of atmosphere every second. Which is weird, considering their different masses and relative distances from the sun.
Some of that lost atmosphere could be arriving on Charon, which is either Pluto's moon or co-dwarf planet depending on your astronomical politics. Charon's north pole is covered by a big, red splotch. This is probably made out of tholins, ionized particles of nitrogen or methane. The general idea is that gas escaping from Pluto gets trapped by Charon's gravity. Most of those molecules escape into space, except those that get trapped in the moon's frozen, winter pole. While trapped, the molecules get zapped by a special kind of radiation, kicking off a series of reactions that leave behind a reddish hue. "According to the atmosphere paper, most of what is actually escaping Pluto is methane rather than nitrogen," says Grundy. "But that actually helps with the Charon red pole hypothesis because it is easier to make tholins out of methane."
Of course, Charon could also be supplying its own methane. The moon has a much less varied landscape than Pluto, but still has some interesting features. Particularly the huge equatorial rift and the big, smooth Vulcan Planum in the southern hemisphere. A few months ago, New Horizons geologists hypothesized that Vulcan Planum could have been caused by a big freeze. Charon would have contracted, causing its sides to split, releasing frozen gases within. "I think we compared it to the Incredible Hulk ripping through his clothing," says White.
Pluto and Charon share a center of gravity, and four, rocky, oblong moons orbit them. A final paper discusses how these lumps came to be. Each of the four moons is at least two times longer than wide, and all have highly reflective surfaces, suggestive of water ice. Which would make the Pluto system a great place for a fuel depot should our species ever build interstellar rockets. The moons' rugged composition and relatively rapid (or in one case, retrograde) rotations suggest that other objects in the Kuiper Belt have collided with each of them.
New Horizons is long gone from the Pluto system, but it's still sending home data, which will take years to fully analyze. Well before that task is complete, the probe will pass by its next target, the Kuiper Belt Object 2014 MU69. No pressure, MU69, but you have a lot to live up to.
