on the Secrets of Pluto

At this point, the New Horizons space probe has uploaded about half the data it took back in July 2015*. Given a few months to mull over the images and other data coming back, planetary scientists have found out a lot about Pluto. And it is WEIRD.

To start with, the whole reason for this mission was that the atmosphere of Pluto was supposed to be temporary, but it turns out not to be. We’ve known it HAS an atmosphere for a while now, from watching stars pass behind it (they dimmed slightly as they passed behind the atmosphere, rather than simply cutting out sharply as they passed behind the surface), but it was thought that Pluto was more like a giant comet – it was so small it couldn’t possibly hold onto an atmosphere, so whatever it had must be transitory and only there because it was much closer to the Sun now. I remember one bit of speculation that the atmosphere might stretch out to and encompass Charon – two worlds, one shared atmosphere, which would be absolutely wild. New Horizons had special observations meant to sample the giant tail of atmosphere leaking off into space that… found nothing. Pluto isn’t leaking.

Pluto, centered on the “Heart”. The “Whale” is the dark portion to the left of the”Heart”

To be clear, Pluto’s atmosphere is really, really thin. For a comparison, Mars’ atmosphere is 100 times thinner than the Earth’s, and Pluto’s atmosphere is 1000 times thinner than Mars’ – it MIGHT get as thick as the atmosphere on top of Olympus Mons (the largest volcano in the Solar System). Pluto’s atmosphere turns out to be about 40% thinner than that of Neptune’s moon Triton; a world Pluto was expected to resemble… so that at least went as expected. Triton’s atmosphere may actually be capable of hosting clouds… Pluto’s is apparently not. There are strage haze layers though:

Haze on Pluto. The Sun is almost exactly behind the dwarf planet, which means the only parts illuminated are the atmosphere. The stripey parts pointed out with the arrows are not image compression artifacts, they’re haze layers.

As for the surface, it’s not a heavily-cratered wasteland either. I’m inclined to try to compare it to Triton, the moon of Neptune that it’s supposed to resemble. Triton isn’t a perfect case, because it’s being powered and heated from within by tidal friction with Neptune… but it’s got weird terrain (that looks like a cataloupe!), an atmosphere, clouds, and even volcanoes (ice volcanoes, but still). Pluto may indeed have all of those too.

Possible ice volcanoes on Pluto. This is based on not much more than noticing their volcano-like shapes on this terrain map, and it’s not proof that they’re still active today.

There are indeed heavily-cratered sections, but they’re not the entire planet – there are also chewed-up plains, dune-like formations (which are usually formed by wind, and you really can’t have wind without an atmosphere), some cracked terrain that’s being called the “Spider” where the entire surface seems to have shrunk forming miles-high ledges (that’s already tall, and on an object that’s only 1/6 the diameter of Earth, it’s comparatively gigantic), and even what appear to be dry riverbeds. I went to a talk recently that referred to Pluto as being a lot like Mars, right down to the brown color.

The surface of Pluto – this is the northern boundary of the dark “Whale”, where it meets the white “Heart”. The bottom region is heavily cratered, but then it ends up as more generic mountains, and finally at the top there are jumbled blocks the size of mountains.


The “Spider”: A series of cracks that are thought to be ice upwelling ad pushing the surface OUT. There are a lot of other weird things here. The image looks weirdly blurry because it was on the edge of Pluto as New Horizons passed and had to be stretched back out to approximate what it would really look like.

A lot of recent interest has been directed at the “Heart” area, which was unusually bright (which really means unusually reflective). The “Heart” turns out to be almost entirely nitrogen ice. It’s an ancient sea, still flowing… slowly. “Sea” is a bad comparison, though. It’s really got more in common with the lunar seas, which are solid plains of what used to be magma.

The Edge of Pluto’s “Heart”. Craters (left) give way to jumbled rocky terrain (actually made out of water ice, frosted with methane snow), which give way to the smooth “Heart” region, which definitely looks like ice to me. The “cell” pattern in the “Heart” is upwellings of nitrogen ice, as if it were boiling slowly. Or they were tectonic plates. The dark shapes in the “Heart” are water-ice icebergs, floating in the nitrogen sea like… pumice, I guess.

See, we have to do a temperature adjustment. On Pluto, it’s so cold that water ice is like rock. Pluto never gets hot enough to even get close to melting it, so it’s basically a stable building material. You can build things out of it and it’s not going to melt. It will float though, which is weird to imagine. Methane, which is a gas on Earth, and a liquid on Saturn’s moon Titan (where it can be found in rivers, lakes, and probably rain) is a solid on Pluto, too. Nitrogen, which is a gas on both Earth and Titan, is in an equilibrium state where it exists both as surface ice and atmospheric gas.

So the Heart is made of nitrogen ice, but it’s moving at speeds we’d associate with tectonic plates. It’s possibly concealing a layer of liquid methane and water, which would be analogous to magma beneath the surface.

The heart is doubly weird, though. It happens to be at the point exactly opposite the one that faces Charon. See, like the Earth and the Moon, Pluto and Charon are tidally locked – the same side of Charon always faces Pluto. Unlike the Earth and the Moon, the same side of Pluto always faces Charon, too. If you’re sitting on the Charon-facing side of Pluto, Charon will ALWAYS be in the same spot in the sky, ALWAYS showing the same face. If you were on the other side of Pluto, you wouldn’t even know Charon existed. And if your Plutonian house was set up at the center of the Heart, you’d have to drive a quarter of the way around Pluto to finally see it. Of course, Pluto is tiny; that trip would be only slightly longer than driving from Boston, MA to Atlanta, GA.

But why is the heart there? The theory I heard at the talk was that this is no accident. The Heart is, like (again) the lunar seas, a giant impact crater. Material was gouged out of Pluto, and thus that part of the planet became lighter, and thus rotated AWAY from Charon. Imagine a bead on a horizontal wire, like an abacus. If you take a chunk out of the bead, that part will be lighter than the part opposite it, and it will naturally rotate to the top, AWAY from the pull of gravity. That’s what happened to Pluto.

Pluto and Charon, with the correct sizes and colors relative to each other, but not to scale in terms of separation. Notice how much darker (less icy) Charon is.


And that giant impact? Was probably responsible for the creation of Charon, Nyx, Hydra, Styx, and Kerberos. It’s basically the same mechanism that formed the Earth’s moon, and planetary scientists believe it happened here for the same reasons. If the Earth and Moon formed at the same time, they would have been drawing on the same pool of material in the solar nebula, and would have the same density, and same ratio of rock to metal. Instead, the moon appears to be made almost entirely of rock, which suggests that the Earth formed, the iron sank to the center, and THEN the impactor (Theia) slammed into it, blowing off mostly rocky bits. On Pluto, the impactor was a little more successful, and knocked off a huge chunk of the rocky center of Pluto. The rocky parts became Charon (which is denser than Pluto), and the less dense water/methane/nitrogen crust became the smaller moons, which are less dense than Pluto.

Unfortunately, as fascinating as this is, this is all we get (pending the other 50% of New Horizons’ data). We don’t know what’s happening on the other side of Pluto in anything like this detail, and there are parts of the south pole that are still completely unknown. There aren’t any other scheduled missions to the outer solar system planned, and even if one launched tomorrow it would still be 2025 before it would reach Pluto (probably longer, if it would have to make a longer side-trip to Jupiter to get a gravitational assist).

Cylindrical projection of the surface of Pluto.


If you want more, New Horizons is on its way (NASA funding pending) to a KBO provisionally named 2014 MU69, which it will pass on January 1, 2019. It’s a tiny iceball only about 40 kilometers across (the size of Manhattan), but who knows what we’ll find there?


*why only half? The transmission speed to Earth from 40 AU away is downright pitiful – 100 bytes per second.

Other posts about the edge of the Solar System: Pluto was Never a Planet, on the Exploration of Pluto, in which Pluto is Revealed, on the Most Distant Object in the Solar System, on the Saga of Planet Nineon the Search for Planet Nine (Part 2)

6 Comments Add yours

  1. Dragallur says:

    Amazing post! Well researched!


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