It’s a Cold World

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…

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in Imported from Livejournal and tagged . Bookmark the permalink.

15 Responses to It’s a Cold World

  1. Pingback: Planetary postcards | Supernova Condensate

  2. Pingback: Astrotropes: Habitable Moons | Supernova Condensate

  3. invaderxan says:

    Re: Mission to Neptune..
    Wow. That’s really quite exciting — thank you very much!
    Just the prospect that we could get another close-up glimpse of Neptune and its entourage within the next 20 years is heartening… Though I agree, an orbiter would be infinitely preferable. An in-depth study of Triton, the way Cassini has been studying Titan could give some fascinating insights!

  4. Anonymous says:

    Re: Mission to Neptune..
    Forgot the Argo link

  5. Anonymous says:

    Mission to Neptune..
    There is a tentative proposal for a Neptune fly-by mission called “Argo”, launch 2019, flyby 2027.
    I would prefer to see a NEP orbiter – using RTGs and ion drives – its just about possible to do.

  6. invaderxan says:

    Probably. Still an interesting idea though…

  7. ryttu3k says:

    True… I suppose a return trip would be tricky, too?

  8. invaderxan says:

    Now there’s an interesting idea… Though presumably they’d have to be careful not to contaminate Europa if anything goes wrong!

  9. ryttu3k says:

    Yeah, true. Ooh, I just thought of an awesome experiment! Fly some Deinococcus radiodurans and Tardigrades up there and see how many come back alive! Conan the Bacterium on Jupiter ^_^

  10. invaderxan says:

    Heh… Lot’s of places could have life. If we’re honest, we don’t really know what we’re looking for.
    I agree though — Enceladus is interesting. Though Cassini is still investigating that one. I’d also agree that it would be nice to get a closer look at Io — though Jupiter’s radiation belts make that rather difficult to do…

  11. invaderxan says:

    Re: Negligible Age
    Fascinating. I hadn’t seen that paper before, thanks a lot! :)

  12. Anonymous says:

    Negligible Age
    Hi InvaderXan
    I so love Triton. It’s truly the Meltdown Moon, with that weird terrain and who-knows-what causing all that evident cryovolcanism. Schenk & Zahnle’s paper in 2007 should be reason to reconsider Triton/Neptune as serious targets for the next big OutPlanet mission. Let them send balloon probes to Titan, but don’t forget Triton!

  13. ryttu3k says:

    Heh XD I just think there are better cadidates to look at first. Titan and Europa could possibly have life! Enceladus is an absolute mystery – how can something so small be so active? Io… admittedly, that’s just my volcano-loving heart wanting to go there XD Iapetus and Hyperion probably aren’t important, but I’d say Titan, Europa, and Enceladus are better candidates for now.

  14. invaderxan says:

    That’s just it. Everyone would love to go back to Triton. Right after we go back to Titan. And Europa. And Io. And… ;)
    I have a few things I’ve been meaning to write about, but I’m sure I’ll get the chance to write about Saturn’s lesser talked about moons sometime soon.
    Heheh… Phobos Grunt. I forgot it had a silly name! :)

  15. ryttu3k says:

    Dude, I’d be SO for a mission to Triton! It’s a fascinating little (well – big) snowball, who knows what we’d find there? The fact that what is essentially a lump of ice has tectonic activity and cyrovolcanoes is fascinating :)
    There are a few moons I’d want to look at first (TITAN, Europa, Enceladus, Io, Iapetus (WALNUT MOOOON), possibly Hyperion (it looks like a sponge! Why does it look like a sponge?)… …I’m quite a fan of Saturn’s moons XD But yeah, Triton is up there. (I’m also very much looking forward to Phobos Grunt! Planetary Society is sending up a panspermia experiment :D)
    Any chance you could talk about Iapetus or Hyperion next? ^_^

Comments are closed.