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The moon Triton

TritonTriton. © based on NASA images

Triton is the largest of Neptune’s moons. Triton has a diameter of roughly 2,700 kilometers. It rotates and orbits Neptune every 5.9 Earth days. And it is the seventh largest moon in solar system. While Neptune has a dozen other moons, Triton’s mass is literally twenty times greater than the rest of the moons put together.

Triton is about two thirds the size of Earth’s moon. Triton is unique for being a large moon with a retrograde orbit and its exotic early history.

History of Triton

Triton may have originated in the Kuiper Belt, a region beyond Neptune filled with rocky bodies ranging from tiny asteroids to the Plutoids. The Plutoids include the dwarf planet Pluto and similarly sized objects like Sedna and Makemake. This theory is further supported by the melted and reformed surface of Triton, giving it a relatively smooth surface. This theory is also backed up by the fact that Pluto itself crosses Neptune’s orbit; this means that Triton could have been a similar dwarf planet that crossed Neptune’s orbit but was captured by its gravity.

Triton was discovered in 1846. William Lassell discovered Triton almost three weeks after Neptune itself was discovered by German astronomers. Triton is named for a son of the Greek god Poseidon or Neptune.

Triton's odd orbit

Triton is unique for being a large moon with a retrograde orbit
Triton is unique for being a large moon with a retrograde orbit. © NASA

Triton orbits Neptune every 141 hours. Triton also rotates once per orbit, so that the same side always faces Neptune. Triton is unique because it is the only large moon that has a retrograde orbit, an orbit opposite of the planet’s rotation. There are many small moons that have retrograde orbits which are known to be captured asteroids, including Neptune’s moon Nereid.

However, Triton is the only large moon with this trait. Because of Triton’s eccentric orbit, it is the only moon whose polar regions alternate in facing the sun. However, Triton is not the only large orbit with this habit. Uranus also rotates on its side, presenting rotating such that its North and South poles alternate facing the Sun.

Impact of Neptune’s pull on Triton

Triton has a faint atmosphere. The thin atmosphere contains traces of methane, but it is mostly nitrogen. Triton has volcanic activity in the form of geysers. Neptune exerts significant tidal forces on Neptune. This has slowed its orbit, pulling Triton closer to Neptune to its current location. Tidal forces also power the volcanic activity on Triton, though it spews nitrogen gases instead of molten rock like Jupiter’s Io. Triton‘s orbit is not stable. The gravitational pull of Neptune on the moon’s orbit will eventually pull Triton into Neptune or cause it to break up.

Surface of Triton

Kuiper Belt
Kuiper Belt. © NASA images

Triton has very few craters but several large cracks. This may mean that it is geologically very young, that the surface is very young, or that Triton drifted far out in space away from other objects that could impact it. If Triton is a captured dwarf planet, the tidal forces that currently drive its volcanic activity could have melted and reformed the surface, giving it the relatively smooth surface we see today. It has been theorized that the tidal forces of Neptune on the moon could have kept its surface liquid for up to a billion years. Triton could have also impacted one of Neptune’s earlier moons, putting it in an irregular orbit and melting much of the surface. This theory would also account for Neptune‘s faint ring and the four small moons within Neptune’s ring.

Triton is one of the coldest objects in the solar system. Its surface temperature has been recorded to be as low as -235°C or 34°° Kelvin. Other temperature measurements have found the temperature to average 38° Kelvin, which is just above absolute zero. Absolute zero is the temperature at which even molecular hydrogen comes to rest and these temperatures were recorded during Neptune’s spring, which reached its summer solstice in 2000. Triton is so cold that nitrogen becomes a solid, raining down as frost on the night side of the moon. Only Pluto and other Kuiper objects are thought to be as cold.

Triton’s surface would appear red to a casual observer. The methane ice on its surface has been converted to compounds called tholins. These red colored molecules are the result of hydrocarbons exposed to ultraviolet light.

Inside Triton

Tidal forces also power the volcanic activity on Triton, though it spews nitrogen gases instead of molten rock like Jupiter’s Io.
Tidal forces also power the volcanic activity on Triton, though it spews
nitrogen gases instead of molten rock like Jupiter’s Io. © NASA

Triton has a very low density. It is probably primarily composed of ice and nitrates, while about a quarter of the planet is rocky material. It has been theorized that Triton could have had a liquid water ocean under its frozen surface above the silicate rock core that formed shortly after it was captured by Neptune. When Triton was first captured, its orbit was initially very elliptical. This would have generated much higher tidal forces than Triton receives today, generating internal heat. Neptune’s gravitational pull could have kept ice under the surface in a liquid state for millions of years. If an underground ocean existed, it would have been laced with ammonia and nitrogen compounds. This would lower the freezing point of the water to -143°F. The tidal forces pulled Triton into its current, circular orbit. As the orbit changed, the tidal forces were reduced. Today, there is probably not enough tidal heating for a liquid water ocean to exist under Triton. Triton may have had radiogenic heating, heating caused by the decay of radioactive elements in the rocky core. This is how the Earth’s core is heated. However, there are no signs of a radioactive core today.

Exploration of Triton

Triton’s internal structure
Triton’s internal structure. © Wikipedia

Voyager 2 flew past Triton in 1989. Voyager 2 captured images of a liquid nitrogen geyser plume rising eight kilometers above Triton’s surface. The plume then extended 140 kilometers before falling to Triton’s surface. There are no future missions planned to Triton.

The European Union’s Very Large Telescope or VLT based in Chile has studied Triton. It discovered the traces of methane and carbon dioxide in Triton’s atmosphere. It also showed that Triton’s atmosphere thickens during Triton’s summer. This seasonal change means that the Sun does affect Triton, albeit very little. Most of the Sun’s light is reflected by Triton’s nitrogen ice and its faint atmosphere.

The New Horizons mission sent by NASA is expected to reach Pluto, an object similar to Triton, in 2015.