The tides of Venus

The tides of Venus

When is Ski season on Pluto?

The icy mountains of Pluto tower above their surroundings, but would they be any good to ski on? NASA-JHUAPL-SwRI

Just think, this time last year we knew next to nothing about Pluto. It was a fuzzy blob, with even the Hubble Space Telescope struggling to make it out.

Fast forward to earlier today where in a press conference in Houston, the scientists behind the New Horizons’ mission talked of a world that potentially had liquid flowing across it. Gone is the thought that Pluto is a dead world. New Horizons has picked up evidence that the dwarf planet has been active throughout its 4 billion year life, with thoughts that areas on the the ‘heart’ of Pluto (the region informally called Sputnik Planum) could have seen activity as recently as 10 million years ago.

An initial geological map of Pluto, showing how the people studying it painstakingly separate each type of terrain into units. The trick then is to see how these all fit together. NASA/JHUAPL/SwRI

The composition of the surface hasn’t been the biggest of surprises - nitrogen, methane and water get very common as rock and metal get scarce in the outer solar system. But, the sheer complexity in the mix of these materials has been a shock, and point to a rather interesting geological history still to be unpicked. Reporting in last week’s issue of Science, ahead of presenting the results at the Lunar and Planetary Science Conference in Houston, the team present Pluto and it’s neighbour Charon as diverse and complex bodies.

There’s a multitude of interesting things tumbling out of the New Horizon’s data - but for me the towering mountains of H2O ice are really something to be marvelled at. Even more than that, it is the strange processes that are driving this marvellous landscape.

On Earth our great mountains have (mostly) been sculpted by volatile water - through it freezing, flowing and battering against the rock that builds them. But on Pluto, water freezes as hard as rock, and along with carbon dioxide it is thought to be the base behind much of the landscape that’s been seen. Like rocks on Earth, water it is thought builds builds these mountains on Pluto, towering kilometres above the plains.

Methane snow caps on the mountains of the Cthulhu region. This is inferred from the fact that the brighter areas on the caps coincide with the spectral signature of methane (shown in purple). NASA/JHUAPL/SwRI

If water is making up these mountains, then what sculpts them to the dramatic peaks that we’ve seen? Well that’s where Pluto’s volatile ices, nitrogen and methane, play the role that water does on Earth. New Horizons picked up lots of evidence of nitrogen and methane on the surface. From assessing where nitrogen and methane are found coupled with the atmospheric conditions the thought is that these ices moved about, and sculpt the water-ice mountains in their wake.

There also an interesting point that the state of nitrogen at Pluto temperatures. It is sat at a tipping point and a small increase in temperature would cause a good bit of the nitrogen to sublimate which could thicken Pluto’s atmosphere. The exciting thing is that this also could have happened in the past, and the thought is that liquid nitrogen could have once flowed on the surface.

So with mountains sculpted, and potential seasons progressing on Pluto the big question is…. When is ski-season on Pluto?

It seems to me that winter sports are a fight for or against friction - and the very physics of water plays a crucial role in how we slide and glide on it. But on Pluto it won’t be the water ice your skiing on, can you get good powder from nitrogen snow?

Strangely enough, not that many people have invested research time into the physics of nitrogen and methane snow (yet). Though I did find this great video of solid nitrogen from a cookery(!) website. Getting solid nitrogen here on Earth is pretty tricky. I’ve often used liquid nitrogen to solidify the hydrates I’ve studied, but to solidify nitrogen itself you’d need to use liquid helium - which is a little expensive. Instead these culinary investigators trap liquid nitrogen in a vacuum vessel and use the fact that this causes the nitrogen to bubble off more vigorously which has the effect of evaporatively cooling it to below its melting temperature.

Amazing video capturing solid nitrogen within a vacuum chamber.

They get a glassy material first, but as the cooling has stopped and the material is now warming up there’s a sudden snap of crystallisation and some pretty powdery crystals emerge. Though they incorrectly say this is the same hexagonal structure as water (it is quite a bit different) this is a way that solid nitrogen would form on the surface of Pluto, only there is would be slower by a few thousand years or so.

So the prospects for good powder on Pluto are pretty good! I hope I live to see the day that we land a skiing robot on the surface of this dwarf planet.

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