Pluto: The Saga Continues

Yes, it's time for another installment of big Pluto news. New Horizons is still sending back data, and will be for several months. As that data comes in, we learn more and more about this surprisingly complex little world. A group of five studies was recently published in the journal Science about Pluto, so let's take a look at what scientists have learned.

The first piece of news is that Pluto's surface has a lot of variation in which type of ice is dominant. There are areas which are mostly methane ice, areas which are mostly nitrogen ice, and areas which are mostly water ice. Nitrogen ice is dominant in the flat, craterless plains, while water ice is dominant in the mountainous regions, reflecting their characteristics at the very cold temperature Pluto sits at. Water ice is very rigid and behaves almost like rock does on Earth, while nitrogen ice is much less tough and is able to flow, much like our glaciers on Earth. Indeed, this sort of distinct differentiation in material most closely resembles Earth, where the surface is separated by areas dominated by water and by rock.

Another study focuses on the geology of Pluto, especially Sputnik Planum, the vast, nitrogen ice dominated region in the southern hemisphere. The plain is located on an ancient impact crater, and its smooth, craterless surface contrasts sharply to areas with a heavy crater density scientists were likely expecting all over Pluto, and to areas with an intermediate crater density. Small bodies like Pluto should have frozen solid all the way through billions of years ago, but something is causing Sputnik Planum and much of the surface to refresh itself, though what that mechanism is remains unknown. The same study also demonstrates that Cthulhu Regio is covered in tholins, an organic molecule that has turned the area a deep red. The tholins likely drifted down from the atmosphere and the mountains Wright Mons and Piccard Mons. These are big mountains, Wright Mons is 2.5 miles high and Piccard Mons is 3.7 miles high, and likely formed through cryovolcanism.

Pluto's atmosphere was the focus of a third study. It's a bit less interesting, but it turns out that the Plutonian atmosphere is significantly colder than what was predicted before New Horizons' visit. Because of this, the particles of the upper atmosphere are much less energetic, which in turn means the atmosphere is losing gas at a much slower rate than what was anticipated. Initial estimates were off by about a factor of 5,000.

Don't worry, Pluto's five moons weren't forgotten in all this new research. Pluto's biggest moon Charon is actually quite a bit different from Pluto, and closer in line to what scientists were expecting. There is some very dramatic topography, but on the whole, Charon is a dead world, and has been for about 4 billion years. Its surface is mostly water ice, and is lacking significant amounts of methane and nitrogen ice like Pluto has. Why this is the case is unclear. An additional study examined the 4 smaller moons, which are much more reflective than more typical Kuiper Belt objects, and move very chaotically. This bolsters the theory that the moons are bits of debris that were thrown off in an enormous collision early on in Pluto's history, and the 4 moons are survivors that have managed to escape being sucked up by either Pluto or Charon.

Credit: NASA/John Hopkins University Applied Physics
 Laboratory/Southwest Research Institute

In news unrelated to the Science studies, scientists recently announced that as recently as 800,000 years ago, Pluto likely had a much thicker atmosphere, more substantial than even Mars. Right now, the Plutonian atmosphere has a pressure about 1/100,000 that of Earth's, and this is likely what it usually is. However, Pluto has a axial tilt of about 120 degrees, and as the planet's tilt slowly undergoes procession, there are certain times when the amount of solar radiation in areas with lots of volatile ices goes up dramatically. The resulting outgassing would thicken the atmosphere so that the pressure would increase to about a tenth of Earth's. This is significantly thicker than the Martian atmosphere, and potentially thick enough to allow liquid nitrogen to flow freely on the surface. A thicker atmosphere with liquid nitrogen streams and ponds would explain a series of unusual surface features, such as empty channels similar to those seen on Mars, and a flat, icy area that resembles a frozen pond. The possibility of a thick atmosphere and the existence of a nitrogen cycle similar to Earth's water cycle, even if temporary and limited, on such a tiny, cold world seems pretty far-fetched, but ever since New Horizons flew by, Pluto has surprised us. This batch of new science has made Pluto that much more interesting.

It's still not a planet though.

Pluto: The Story so Far

On July 14, the New Horizons space probe flew past the dwarf planet Pluto, more than 50 years after Mariner 2 successfully completed a Venus flyby, the first time a space probe visited another world. It's been a long time coming for Pluto. Voyager 1 was nearly sent to Pluto after its Saturn visit, but instead it performed a close flyby of Titan. Honestly, considering how little Voyager was able to learn about Titan given the presence of its thick, inscrutable atmosphere, Titan may not have been the best choice for Voyager. But finally, Pluto has gotten its first visit, and as is tradition with cutting edge science such as this, Pluto gave us quite a few surprises.

One of the most unusual features is actually a lack of a particular feature, namely craters. There just aren't any there. Charon, Pluto's most significant moon, has a few craters, but not nearly as many as one would expect. These are two cold, small bits of ice and rock 3 billion miles from the sun. There's no gas giant to exert any sort of tidal flexing as is the case with some moons orbiting Jupiter and Saturn. Based on what we know, Pluto and Charon should both be dead worlds. But they're not. Pluto's surface seems to be less than 100 million years old, very young when it comes to geology.

Typically, worlds made up primarily of ice don't have big elevation changes. Look at Europa, the surface there is so smooth that a cue ball, expanded to Europa's size, would have greater elevation changes. So, it would make sense then for Pluto and Charon to be pretty much flat, right? Well, apparently not, because both have some pretty impressive topography. Charon has a canyon that could be up to 6 miles deep, just one of many canyons in a vast network that span for hundreds of miles. Charon also has a plain near the northern pole that scientists have been informally calling Mordor. I hope that name sticks.

Not to be completely outshone, Pluto has its fair share of interesting geography. Of course, there's the heart, which I believe has officially been termed Tombaugh Regio after Pluto's discoverer. Then there's the mountains. Now at the cold temperatures we're familiar with, water ice is pretty soft and not really capable of getting very high. Pluto is an entirely different sort of cold, cold enough for

strange and unusual ices like methane ice and nitrogen ice. At those temperatures, water ice because solid enough to build mountains. These are real mountains too, with the highest peaks soaring up over 10,000 feet. Sure, we've got higher here, but for a little chunk of basically ice, Pluto didn't do too badly for itself.

It's going to take a long time for New Horizons to send back all the data it collected during its brief visit, more than a year. Right now, we still barely know anything. I'm sure I'll be writing more posts about Pluto in the future as the data slowly trickles in. To wrap things up today, I just want to say that Pluto is not a planet, it just isn't big enough. That said, that doesn't make this achievement any less spectacular.