An Analysis of Triton's Composition and Orbit

The gas giants of our solar system appear to have a specific gift of accreting or capturing large numbers of moons of varying sizes and characteristics. Coincidentally, the moons that are far more interesting than our own, having extremely interesting characteristics, such as atmospheres, constant volcanic resurfacing, and a myriad of others, are extremely far away and difficult to observe. One particularly intriguing moon orbiting the furthest gas giant, Neptune, has been given the name Triton. Since Triton has been observed mostly during fly-by, astronomers have to rely on observations using reflected light, pictures, and infrared observations to help make inferences as to what the moon is actually like.

In order to get somewhat of an idea as to what Triton may actually be like, a good place to start is to discuss where it came from. Triton is the largest moon orbiting Neptune and makes up for about 99.5 percent of the mass of all of Neptune’s satellites (1). Since most of the others have varying diameters of about 5km to 200km, one has the right to be curious as to where this comparatively massive moon, with a diameter of 2700 kilometers (2), came from. Since Triton has a very similar composition and size as the dwarf planet Pluto, most astronomers attribute them with the same origin in the Kuiper Belt (2). If the two bodies have a similar origin, it would not be out of the question to consider them to have relatively the same composition as most things in the Kuiper belt, being mostly frozen gas and ice.

The Kuiper belt lies beyond the orbit of Neptune, extending from 35 to around 50 AU (3). Some short period comets originate from here, having roughly 200 year orbits, while most other comets originate in the Oort cloud (extending to about 50,000 AU) (3). The Kuiper belt is home to many dwarf planets right around the size of our moon, whose compositions are mainly of frozen gases and ice. Due to the extreme cold, the ices present in the Kuiper belt behave much like rock, and can have similar properties as such.

Shown in the diagram to the right (4), Neptune’s orbit lies just on the inside of the Kuiper belt. This would lead astronomers to believe that Triton could very well have been picked up by Neptune during a close encounter. Triton’s retrograde orbit also proves to be a tell-tale sign of this occurrence (5,6).

Triton’s size may also give some insight as to what surface temperatures might be, whether or not it has a magnetic field, and even if it could possibly hold onto an atmosphere. There is most certainly some sort of seismic activity going on within Triton, as seen in its spectacular geysers and cryovolcanism (1). The cryovolcanism that hurls water and methane into space is a definite indicator of a relatively hot interior. Whether the interior is being churned tidally by Neptune’s gravity or collisions isn’t entirely clear, but more support is being placed into the categories of “tidal dissipation, heat transfer, and tectonics.” (7). The churning of a hot interior is definitely cause for speculation on the possibility of a magnetic field, which could lead to the possibility of an atmosphere. Luckily astronomers have been able to point out the presence of clouds in a picture taken by the Voyager 2 Satellite during a fly-by (8). These clouds that appeared just over the southern cap, certainly point to the presence of an atmosphere, one with 1/70,000 the density of Earth’s(9). This is surprising for an object of Triton’s size and distance from the Sun. The presence of an atmosphere would point toward the existence of a magnetic field in order to protect the atmosphere from being blown away from such a small object.

One of Triton’s most unique characteristics is its cryovolcanism. Triton lacks impact craters, which tells astronomers that Triton must be constantly resurfacing itself. The same processes such as plate tectonics, and volcanism that help resurface earth also take place on Triton(1). However, rather than resurfacing itself with liquid rock and metals, Triton resurfaces with water ice and ammonia(1). The way these ices behave gives some insight as to what sort of temperatures are present in this area of the solar system (the kinds of temperatures needed to make ice behave like rock). Due Triton’s extremely low temperatures, small fluctuations in temperature can set off the same sort of cryovolcanism, even as little of a change as 4 Kelvins (10). This temperature change can be made by the Sun’s energy, even at such an incredible distance.

If these temperature changes were to be made by the sun, then the surface layer would have to be extremely thin or even transparent to let the energy through. This is exactly what many astronomers speculate as to why some of the geysers exist where they do. It seems that there is a surface layer of semi-translucent frozen nitrogen overlaying a darker underlayer, creating a “solid greenhouse effect”(11). This would permit the solar radiation to penetrate the top layers to then heat up and trap deeper nitrogen against the sublevels. The resulting pressure is enormous and causes the geysers to eject material up to 8 km high and last up to a year(10).

The constant cryovolcanism leads to the [relatively] frequent resurfacing of Triton. Triton's surface is very distinguishably different around it’s western hemisphere. The very smoothe looking terrain known as “cantaloupe terrain” appears to have very few craters and serious imperfections, but it is in fact considered to be some of the oldest surface on Triton(12). The small and rounded imperfections are often attributed to flowing ice and lower density materials rising through to the surface. The terrain is thought to be comprised of water ice and is unique to Triton(12). Compared to the new terrain formed on Triton, the cantaloupe terrain appears to be very pure and homogenous, lacking black spots and build up of different ices due to its lack of geysers and violent seismic activity.

Surface temperatures on Triton are estimated to be right around the mid 30’s-40’s Kelvins(13). Observations in the infrared and the specific ice type on Triton’s surface continue to support this claim. The Nitrogen ice seems to be fluctuating between hexagonal and cubic Nitrogen ice, which occurs at around 36 K(13). However, the fluctuations in temperature and the extremely low pressure allows these ices to evaporate and contribute to the atmosphere, creating an almost seasonal difference in the density of Triton’s atmosphere(14)

Despite the vast amount of information gathered regarding Triton, much of the moon is still left undiscovered. NASA has conversed over the topic of Neptunian missions in which Triton will be studied extensively with a probe designed specifically for Triton(15). Not much has been confirmed as to whether or not this will be taking place any time in the near future, but due to its remarkable characteristics, Triton is still high on the list of potential bodies to observe.