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Friday 5 November 2010

Worlds on the edge of knowledge: Triton.

As always, links are numbered and listed in order at the end.

Following the excitement of yesterdays Deep Impact flyby I thought I'd try to provide something to decompress to. And what better place to decompress and chill than the outer solar system?

Far out in space, where only a handfull of robots have dared to venture, a  huge group of icy [1]protoplanets and [2]planetesimals have survived in great numbers since our solar systems earliest days.
These worlds are the protoplanets of the [3]Kuiper belt; a wide and deep doughnut of icy objects beyond the orbit of Neptune, surrounding the worlds of our solar system.

Image left: A map of our solar system showing the Kuiper belt. Image courtesy of

The Kuiper belt is the solar systems deep freeze storage, home to many [4]weird ancient relics from the sunless far edge of the [5]protoplanetary disk, some of which reached the size of a a small planet before their growth was stunted. But how can we learn about this historical treasure trove when it lies so far away? To get a feel for these distant worlds there is one place we can look that has had the kind of detailed look only an on site spacecraft can bring: [6]Triton.

An enigma writ in pink

Image above: Neptune and Triton, in false colour to bring out Neptune's weather patterns. Image courtesy of the Palomar observatory.

The largest moon of [7]Neptune (2700 km in diameter), Triton orbits in the opposite direction to its planetary hosts rotation, suggesting that it does not come from the Neptune system at all- it is instead a wanderer from further out, in the black depths of the Kuiper belt.
This strange orb of rock and ice has been visited by just one human spacecraft, [8]Voyager two. Voyager two is the grandaddy of deep space probes, [9]still making ground breaking discoveries from the farthest edge of explored space.

Image above: Triton, one of the enigmas of the outer solar system, one of the solar systems most enticing moons, and one that is very very difficult to get to and study. This makes the data gathered so far truly precious. Image courtesy of NASA.

Voyager sent back pictures of Triton. They are images, not of a frozen chunk of ice that died eons ago, but of an active world of alien geology, based on processes we little understand and using [10]materials we only encounter as liquids or gasses. Triton, despite being cold enough that Earths atmosphere would freeze solid on its surface as snow, is active.

Triton surface tells a story of of intense geological history: Based on its density, and [11]spectroscopic analysis of reflected light from its surface, the best guess to date is that it consists of a [12]fat rocky core, maybe making up as much as two thirds of its internal volume, wearing a mantle of ices. The presence of the large core is based on [13]Triton's strangely high density, 2.062 g/cm3, which is far too high for an ice world. Yet the surface is composed of water and carbon dioxide ices, with a cap of frozen nitrogen studded nitrogen geysers that greet the dawn of far distant sol with fountains of gas and dust.  Triton appears to be some kind of hybrid world.

The surface is bitterly cold, only 38 kelvin, but the surface suggests that in the geologically recent past this might not have been so: [14]the surface is very free of craters, indicating that unlike most worlds the same size its [15]geology has been active as recently as a few million years ago, and could even be still running hot ([16]at least less cold) today. The large size of the core makes it possible that there could still be enough residual heat, [17]from radioactivity or tidal heating,  to power [18]convection in the mantle, as occurs on Earth with rock, and possible even sustain a [19]layer of liquid water deep beneath the surface.

Aside from the infrequent impact craters there beautiful and complex valleys and ridges seem to be formed by icy analogues of Earths own tectonic processes.  There are [20]massive high plains in the east, believed to have been formed by a massive eruption of [21]cryolava, and are dotted with pits, such as [22]Leviathan Patera, from which the molten material emerged. Four huge walled plains ave also been found and are thought to be due to volcanic eruptions.

Video above: Candy Hansen, a member of the Voyager team, demonstrates how tectonic activity has altered parts of Triton. Courtesy of NASA.

Image right : Leviathan Patera, the smooth floored depression center left, is believed to have been a source of massive cryovolcanic eruptions. Image courtesy of JPL

One pole is covered entirely with a [23]bright pink cap of frozen nitrogen. Nitrogen, which is a gas on Earth, is usually colourless to human eyes. But mixed in with the nitrogen ices is methane, which  breaks down under the suns ultraviolet light and other space radiation to give unknown carbon compounds, giving Tritons  surface a pinkish hue.

But Tritons most spectacular surface feature by far must be its [24]nitrogen geysers: These geysers are larger than any found on Earth, and are based in the polar cap of solid nitrogen. Right now the leading theory for how they function is that when the sun is directly over heat its radiance is able to pas through the top layer of nitrogen shell, and weak though it is, heat is trapped beneath the surface in a sort of 'solid state greenhouse effect'. Over time this vapourises enough nitrogen ice to produce a buildup of gas pressure that blasts through the ice in a pillar of nitrogen gas, carrying dust from the surface and subsurface up to 8000m high.

Image above: The geysers of Tritons polar cap, visible here by the dark streaks of wind-blown dust they deposit. Image courtesy of NASA.

These geysers may last for up to year, a river of gas to the black sky, and Tritons ultra thin nitrogen atmosphere helps to transport the dust about, slowly changing the face of the little world over time: it lofts the dust particles down wind producing spectacular ribbons of black against the pink nitrogen shell.

Triton geology had one last surprise for the team working on the Voyager images as they flashed back to Earth: The [25]cantaloupe terrain, a huge area of Triton western hemisphere, contorted by strange depressions and folds, making up circular dimples 30 to 40 km wide. Despite its lack of old craters it is thought to be the oldest ground on the mysterious moon, and theories for how it formed are [26]wide ranging, from cryovolcanic flooding, to subsurface collapse, to diaperism, where blobs of warm material rose like sea monsters from the core. This terrain exists only on Triton, and is made mainly of water ice with ...... Many of these causes would have had an incredible effect on Triton, spreading cryolavas of liquid water across the surface, and releasing gasses that could have left the place with a [27]short lived thick atmosphere.

Yet Tritons surface and interior aren't the only interesting things about it: Unusually for such a small world [28]Triton boasts an atmosphere. Not much of one, barely a hundredth of a millibar of pressure, but enough. Tritons surface sees a troposphere with weather patterns like Earth or Mars, and tenuous clouds were imaged by Voyager.

Image left: Tenuous clouds creep over Tritons surface, The image is a composite courtesy C. Hamilton.

These clouds are vaguely analogous to [29]cirrus on Earth, composed of crystals of nitrogen, and there are hazes, where the thin methane component of the atmosphere reacts under UV light to produce Triton [30]tholins, complex carbon based materials related to the [31]organic chemistry believed to have precipitated life on Earth.

How Triton got its lumps:

Video above: A lecture by Dr Gary Peterson of San Diego State University, on Triton and its geological history. Video courtesy of San Diego State University.

The blatant scars of active geology across Tritons surface give mute testament to its origins elsewhere in the universe: Some of the most plausible explanations for how Triton remained so active for so long are rooted its, possibly devastating, [32]capture by Neptune. There are three main mechanisms for this proposed:

1: Protoplanetary pile up.
One leading theory has Triton actually smashing into a preexisting satellite of Neptune to slow itself down, but that theory has problems- the hit satellite must be just the right size, too big and Triton gets totally pulverized, too small and it won't loose enough speed.

2: Skimming off of Neptune:
A second theory has Triton skimming off of the early accretion disk around Neptune, or even off of a vastly extended Neptune atmosphere. Neptune would have been puffed out like a cheek after a tooth extraction from accretion heat- but Triton is big, and this is a scenario that works much better for a smaller object

3: Sundered twins:
A third theory is that [33]Triton was part of a twin planet system, and that when these twins passed by Neptune one was captured, and the other was shot off into space, taking Tritons excess kinetic energy with it.

Whatever the mechanics of the union, Triton would have wound up in an elliptical orbit, which would have let Neptune gravity set up [34]huge tidal forces within its crust, flexing the whole moon. Back in the [35]Jupiter system this effect drives the terrifying [36]volcanoes of Io, and warms the [37]ocean of Europa and Ganymede.
In Tritons case eventually the very tides themselves forced its orbit to become more circular, and the heating died down, leaving behind a shell of bitter ices. Models suggest that Triton could have remained active or up to a billion years after its capture- and some heat left in its rock core could [38]drive residual activity even today.

Tritons immense distance from Earth makes putting a probe in orbit to do a detailed study difficult with today's engines, and there are no flyby missions planned for the Neptune system.

What do these places tell us about the deep past?

Triton represents one possible path an icy protoplanets can take we, know from our analysis of the Kuiper belt that there are almost as many paths to follow as their are worlds in the belt. Places like Makemake, [39]the Haumea system (a very odd place), [40]Sedna, the Pluto system; each has its own character. Tritons ath ahs ben that if a world that remained 'alive' long after the protoplanetary disk dissipated, and continued to evolve. There have probably many different geological epochs on Triton: Times when water slush lava was common on the surface, the entry into the Neptune system and the long icy days before it, leading back to birth at the edge of the pre-solar nebula, watching our star system grow from a distance. Unravelling Triton's mysteries tell us about how an icy protoplanet can evolve, and what kind of weird world it can grow into.
Next in this series of articles we'll look at one believed to be almost pristine from the ancient solar system...

How do we know these things?
There are two main ways of learning about distant, oddball little worlds: Use advanced telescope technologies to view them from afar, and put a robot space ship with cameras and sensors up close to one.
Image left: A model of Voyager 2. image courtesy of NASA.

In Triton's case, advanced telescope technologies is where its at for the foreseeable future- a pity given the fascinating world Voyager 2 gave us a glimpse of. On of the major tools for the study of Triton is our old friend [41]spectroscopy. The [42]European southern observatory is [43]very active in this field, making use of [44]adaptive optics in instruments that adjust the shape of the telescopes main mirror to cancel out the distortions of Earths atmosphere. [45]The Hubble has also spent a lot of time looking at Triton, and has seen [46]changes on its surface as its seasons have progressed.
But for the real goodies, the data that only a spacecraft like Voyager could reveal to us, we're stymied right? Not quite. Although there are no plans for a mission to the Neptune system, there is a spacecraft on its way to visit a world that is thought by many to be Triton's near twin: the [47]New Horizons mission to Pluto. Pluto and Triton are often held to be closely related objects, and the fun doesn't stop at Pluto. New Horizon's is planned to investigate other Kuiper belt worlds as it hammers out of our solar system. It will provide us with a larger picture of the Kuiper belt story, and we will be able to apply much of what it teaches us to our studies of Triton, this distant, slightly garish, and very mysterious world.

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