So distant we’ve only been there once, Neptune’s giant moon, Triton, could well be the coldest object this side of the Kuiper Belt. Despite it’s intensely frigid conditions, however, Triton is one of only 4 planets we’ve seen to be geologically active! Voyager 2, on its flyby of the Neptunian system, spotted a number of geysers erupting from Triton’s surface – some of them up to 8km high! Imagine Yellowstone Park, only with less gravity. And colder. Much much colder. So cold in fact, that those geysers are thought to be spewing out liquid nitrogen. Which might make you feel slightly better if this has been a particularly cold Winter for you so far…
With temperatures ranging between 35 – 40 Kelvins, nitrogen dominates this world. Nitrogen ices encrust the planet’s surface, with some evaporating to form a thin nitrogen atmosphere. Surprisingly, there’s a little more in common with Earth here than you might realise. Specifically, Triton’s atmosphere has a troposphere — a region with weather. Rising a mere 8km high, this region is thought to have prevailing seasonal winds. Clouds of nitrogen ice particles form here, and a haze of nitrogen rich hydrocarbons (such as nitriles) has also been found, alongside clouds of condensing nitrogen gas lower down. This starts to paint a nice little picture of nitrogen snow falling amid the gassy plumes. Triton could be said to have a nitrogen cycle, with a tenuous parallel to the cycle of water on Earth. Above the troposphere, the rest of Triton’s atmosphere extends for another 800km above the surface, forming a neatly structured thermosphere, ionosphere and exosphere.
Triton’s atmosphere is practically non-existant by Earth standards, though it’s still unclear how dense (or otherwise) it truly is. Voyager showed that the atmosphere was extremely thin, but some observations from Earth since then have disagreed with those data, suggesting a denser atmosphere. This could be something to do with the fact that, also akin to Earth, Triton’s undergoing some global warming. It’s temperature seems to have increased by 5% between 1989 and 1998. In fact, because Triton’s atmosphere is so dynamic, a NASA-funded programme called Triton Watch is in place to look out for any changes. Sadly, despite the variable temperature, the hopes of surface liquid nitrogen look slim. At the pressures found on this moon, liquid nitrogen would quickly turn to vapour, although a slushy surface is still, perhaps, not too much to hope for.
Being a frosty little world, Triton’s surface is largely made of ices. Water, nitrogen and carbon dioxide ice form the bulk, with some amount of carbon monoxide and methane. Possibly ammonia too. All of that ice could help explain why Triton is so reflective, in some places reflecting up to 95% of the sunlight that falls on it. It also explains that hydrocarbon haze in the lower atmosphere. Planets like Triton are like global laboratories for studying the effects of ultraviolet on ice.
Fascinatingly, at these cryogenic temperatures, all those ices act a lot like rocks do on Earth, causing Triton’s surface to be zig-zagged with canyons, rift valleys and ridges, probably caused by tectonics (damn… seems I’ve been wrong about that twice now!). Much more prevalent than tectonic plate activity, however, is Triton’s cryovolcanism. Caused by subsurface layers retaining solar heat (a kind of solid-state greenhouse, if you will). Cryovolcanoes pepper the world’s landscape, causing extensive flows of icy slush which, by the looks of things, have reshaped the surface of the place a few times. Combining the planet’s geological activity with the possibity of snow gives some idea why the planet has such a youthful appearance, with a surface as young as 6 million years old in places. It has scarcely any impact craters (at least that we know of). Most of it’s craters are actually volcanic.
One thing unique to Triton, however, is its so-called “cantaloupe terrain”. Consisting of dirty comet-like ice, it’s thought to be the oldest part of the icy moon’s surface. The cantaloupe effect comes from a huge series of bumps, some 30-40 km in diameter, which mottle the surface. Why it looks like melon skin is still not certain, though many scientists believe it’s due to diapirism — big chunks of less dense material rising up through a denser layer. Somewhere between a giant slushie and a lava lamp.
As might be obvious, Triton is on my long list of “places I hope we’ll go back to”, though being so far away, who knows when we might. No one seems to be the least bit interested in going to Neptune, so it’s doubtful that humanity will head back that way anytime soon. Which is a pity. Voyager only imaged around 40% of Triton’s surface. Who knows what secrets this frozen wonderland may still hold…