Triton – Neptune's Captured Moon

Triton – Neptune's Captured Moon
The Solar System's large moons tend towards the weird and wonderful, and Triton is no exception. It has ice volcanoes, a strange “cantaloupe terrain”, and crazy seasons. Neptune has fourteen known moons, and Triton's mass accounts for about 99.7% of their total mass. Orbiting in the opposite direction to Neptune's rotation, it didn't form near Neptune. But where did it come from?

The header image is an artist's representation of the capture of Triton. (Mark Garlick/Science Photo Library)

Naming Triton
The planet Neptune was discovered in 1846 through its gravitational effect on Uranus. When he received news of the discovery, British astronomer John Herschel suggested to his friend William Lassell that he look for Neptunian moons. Just over a week later Lassell had discovered the moon we now call Triton.

It took some time to get agreement on a name for the new planet, but eventually Neptune, the name of the Roman sea god, was accepted. They just called the moon Neptune's satellite. In 1880 French astronomer Camille Flammarion suggested the name Triton for it. Triton, Neptune's son and messenger, was represented as a merman who could calm the waves by blowing through his conch shell horn.

But the moon didn't need a name until 1949 when Gerard Kuiper discovered a second Neptunian moon. He named it Nereid after the sea nymphs who were Neptune's attendants. Following this discovery, the largest moon was also given its name.

Comparing Triton with other moons
Triton is the seventh largest Solar System moon. A radius of 2700 km (1680 mi) makes it about the size of Jupiter's moon Europa, though Europa is 50% denser. Triton is a monster compared to Neptune's thirteen other moons. Proteus is the second largest, and its radius is only about 420 km (260 mi). The thirteen smaller moons account for only 0.3% of the mass of Neptune's moon system.

Triton is around the same distance from Neptune as our Moon is from Earth. As with other moons, Triton keeps the same side to its planet, making a day the same length as a month. However, Triton takes less than six days to whiz around Neptune. This is because of the stronger gravity of massive Neptune. If Triton had moved as slowly as our Moon, it would have been broken apart or pulled into its planet long ago.

Triton's orbit is bizarre for such a large object. Although the orbit is almost perfectly circular, it's highly tilted to Neptune's orbit. Its axis of rotation is also tilted so much that the polar and equatorial regions alternately point at the Sun, causing extreme seasonal changes.

The most surprising feature of Triton's orbit is that it's retrograde – it orbits in the opposite direction to Neptune's spin. Since the Solar System formed from a revolving debris disk around the Sun, the planets and their major moons orbit in the same direction as the rotation of the Sun. Triton came crashing into the Neptunian system, disrupting the existing moons, and going into its peculiar orbit as a captured object.

Geology and surface features
Triton is a reddish color, probably the result of reactions of methane ice with ultraviolet radiation. There are polar icecaps, but they're made of frozen nitrogen and methane, not water. Over half of Triton's surface is frozen nitrogen. The rest is water ice and frozen carbon dioxide with traces of methane and carbon monoxide.

A unique feature is the extensive cantaloupe terrain of Triton's western hemisphere. It's made up of of smoothly curved depressions 30-40 km in diameter, and named for its resemblance to the skin of a cantaloupe melon.

Data from Voyager 2's fly-by of Triton shows a geologically young surface with few impact craters. Its icy surface reflects most of the sunlight that it receives. We also know that Triton is still geologically active through Voyager 2's observation of eruption plumes of nitrogen gas and dust. These aren't the volcanoes we know on Earth spewing molten rock, but rather cryovolcanoes, ice volcanoes like those on Saturn's Enceladus.

Planetary scientists think that Triton must have a layered interior like a planet. There should be a solid core, a mantle and a crust. The core is probably rock and metal, while the mantle is water. There is enough rock for radioactive decay to occur, heating the mantle and creating convection currents. This happens on Earth, but the mantle is semi-liquid rock, not water.

Triton has a very tenuous nitrogen atmosphere with traces of methane and carbon monoxide.

Its high reflectivity, combined with its distance from the Sun and lack of blanketing atmosphere, make Triton the coldest body in the main part of the Solar System. Even Pluto isn't quite as cold. Triton's surface temperature averages only -235°C (-391°F).

The thin atmosphere varies seasonally, getting thicker when warmed. Obviously, even at the height of summer, Triton isn't warm. However there is enough heat from the Sun for some of the frozen surface nitrogen, methane and carbon monoxide to sublimate. In sublimation, instead of melting, a solid changes directly into a gas. You may have seen dry ice (frozen carbon dioxide) do this on Earth.

Pluto's cousin?
Triton's retrograde orbit shows that it didn't form as part of a Neptunian system. But where did this captured object come from?

Although Triton is very unlike the other moons, it's very similar to Pluto. Triton is only slightly larger than Pluto, and they're the same reddish color. Both bodies have a density of just over 2 grams per cubic centimeter and a similar composition. Triton appears to be a Kuiper Belt object.

You Should Also Read:
Pluto Is a Dwarf Planet
Literary Moons of Uranus
Volcanoes - Fire and Ice

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