I just got back from the American Astronomical Society’s winter conference, and let me tell you, there was a lot of great science going on there, but I wanted to focus particularly on the latest findings about Pluto.
Pluto’s geology is more complicated than I ever could have expected, and New Horizons principle investigator Alan Stern said the same last week: “We expected a high school calculus-hard kind of planet, but instead we parachuted directly into graduate school.” It’s amazing that at 40 kelvins, Pluto can be anything but a solid ball of ice, but it’s actually more active than most of the warmer worlds in the Outer Solar System.
Pluto gets an advantage in the geology department because it’s colder than 63 kelvins–the temperature at which nitrogen freezes. Nitrogen ice is soft–“the consistency of toothpaste”, Dr. Stern said–meaning that it flows in glaciers and convection cells not seen on any of the moons of the gas giants. Interestingly, water ice, which forms the mountains of Pluto, floats in nitrogen ice. That means that what look like mountain ranges as high as the Rockies aren’t true mountains at all, but are enormous icebergs floating in a sea of nitrogen! Every time we turn around we see something new and strange on Pluto, from ice volcanoes to giant canyons to giant plains of constantly-resurfacing ice.
That we see so many unexpected things is all the more surprising because we can predict pretty well what chemicals we’ll find on Pluto, and there aren’t many of them. In fact, for Pluto, many other icy worlds, and even some exoplanets, we expect to see just a few major chemicals, and we can predict them pretty easily. Here’s how.
The six most common elements in the universe are hydrogen, helium, oxygen, carbon, nitrogen, and neon. The next few–magnesium, silicon, iron, and sulfur–are found in rock and so wouldn’t be found on an icy world like Pluto. Helium and neon don’t form chemical compounds, and even Pluto isn’t cold enough for them to freeze, so the vast majority of the molecules on Pluto’s surface will be made from hydrogen, oxygen, carbon, and nitrogen–and they’ll usually be simple molecules.
We can narrow down the list of molecules still further. Hydrogen by itself (H2) is too light and will evaporate into space. Oxygen by itself (O2) will react with other molecules and won’t stick around for long. And while there will occasionally be more complicated molecules with three elements, the most important being hydrogen cyanide (HCN), most of these molecules will be as simple as possible.
Within these rules, it turns out that almost all of Pluto’s surface and atmosphere are made of just six molecules: nitrogen (N2), water (H2O), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and ammonia (NH3). And from the properties of these molecules, we can also figure out what roles they play.
Water ice is rock hard at 40 K, so it forms the main structural component of Pluto’s surface. Nitrogen is a soft, flowing ice that sublimes easily and forms the basis of the “hydrological” cycle. Ammonia and carbon dioxide don’t do much because they both freeze at much warmer temperatures, but carbon monoxide freezes at 68 K, much like nitrogen, so you might expect that it would form a major component of the soft ice, and it does, but only in a limited area. Besides a whiff of the stuff in the atmosphere, the carbon monoxide on Pluto is concentrated mostly in the very young ice of Sputnik Planum, which is now thought to be a large impact basin filled with a sea of this soft material.
The remaining molecule, methane, is an interesting one. It forms only 0.25% of Pluto’s atmosphere, but that small fraction is vitally important. When exposed to ultraviolet light for millions of years–even the feeble UV light from the distant Sun–methane turns into a deep red, tar-like “snow” called tholin. We see tholins all over the Solar System, from Jupiter to the Kuiper belt, and they probably occur even in clouds of exoplanets that orbit very near their stars. Tholins slowly stain the older regions of Pluto and Charon dark red, like the large, ancient crater fields called Cthulhu Regio. Parts of Cthulhu Regio are over four billion years old, and to be so close to Sputnik Planum, which is filled with ice that is only ten million years old, speaks to some very complex geology that we don’t fully understand.
So we begin to see how we build up the complicated structure of Pluto from a few very simple molecules, but the fact remains that it is still much more complex than we expected, and there remains a lot to learn about this icy world.