I have to say, I agree with both Music of the Spheres‘ Bruce Irving and David Grinspoon about Pluto. A dwarf planet maybe, but to state that this fascinating little frostball isn’t actually a real planet is a travesty. As I’ve said before, a dwarf star is no less of a star, so why should a dwarf planet be considered any less of a planet? Nonetheless, I’m increasingly bored with the planet-or-not debate, and I’m sure that by now I’m not the only one.
All the same, Pluto is a curious little thing, whichever way you classify it. In just a few years, we should finally get a close-up view of this place. We’re bound to find out masses of information on the eccentric oddball when we do… but how much do we already know about Pluto and its near-twin world Charon?
For a start, this tantalisingly vague (but true colour) image is the best view we can get of Pluto’s surface. The interesting part is how remarkably uneven it is. Charon (shown below at different angles, alongside Pluto) is much the same. Right now, the features behind the dark patches and light spots are anyone’s guess. Cryovolcanism? Liquid nitrogen lakes? Tiny plate tectonics? The aftermath of a catastrophic event? It starts to get speculative at this point… Some have also conjectured that buried inside Pluto might be a subsurface ocean. That seems almost obligatory these days for small icy planets. Given examples like Enceladus, the possibility certainly can’t be ruled out.
What we do know about Pluto’s surface is that it’s overwhelmingly covered with frozen nitrogen. Traces of methane and carbon monoxide ices also mingle with all the nitrogen ice. In fact, the tiny planet contains so much ice, it’s estimated to be between 30% and 50% frozen volatiles! All of those ices could well be reacting together to make tholins in the weak ultraviolet light received from the Sun. It would do a good job of explaining the brown colour. So far out into the night, the light from other stars probably drives chemical reactions out there too.
Perhaps the most bizarre thing about Pluto though, is its atmosphere. A thin layer of the same stuff that covers its surface, Pluto’s atmosphere varies drastically with its orbit. Orbiting between 29.7 AU and 49.3 AU from the Sun causes quite a lot of variation on the surface this tiny world. From a summer maximum pressure of 0.3 Pa (about a million times lower than Earth’s atmosphere), it gets progressively colder as its orbit swings it farther from the Sun. So cold, in fact, that the entire atmosphere starts to freeze and fall to the ground as snow. On Pluto, the sky will quite literally fall twice every orbit.
Eventually when there’s scarcely any atmosphere left, Pluto starts to draw closer to the Sun again, causing those ices to sublime into gasses once more. This evaporation causes massive global cooling, keeping the little planet’s tenuous atmosphere at a frigid 43K (on average). Well below the melting point of nitrogen ice. No liquid nitrogen lakes here then, I suppose.
All the same, it’s quite exciting to think about what they might discover out there. I know it’s incredibly unlikely, but wouldn’t it be fascinating if the entire place was covered in some kind of life. Secretly, I’d quite like them to discover a lush garden of cryogenic lichen of some kind. I’m not holding out any hopes, mind you.
Even if the reality is somewhat less exotic (as is most likely), the arrival of New Horizons on 14th July 2015 is still something to look forward to!