Triton (moon)

Triton

Voyager 2 photomosaic of Triton's sub-Neptunian hemisphere
Discovery
Discovered by	William Lassell
Discovery date	October 10, 1846
Designations
MPC designation	Neptune I
Pronunciation	/ˈtraɪtən/
Named after	Τρίτων Trītōn
Adjectives	Tritonian
Orbital characteristics
Semi-major axis	354,759 km
Eccentricity	0.000016
Orbital period	5.876854 d (retrograde)
Average orbital speed	4.39 km/s
Inclination	129.812° (to the ecliptic) 156.885° (to Neptune's equator) 129.608° (to Neptune's orbit)
Satellite of	Neptune
Physical characteristics
Mean radius	1,353.4±0.9 km
Surface area	23,018,000 km2
Volume	10,384,000,000 km3
Mass	(2.1390±0.0028)×1022 kg (0.00359 Earths)
Mean density	2.061 g/cm3
Surface gravity	0.779 m/s2 (0.0794 g) (0.48 Moons)
Escape velocity	1.455 km/s
Rotation period	synchronous
Sidereal rotation period	5 d, 21 h, 2 min, 53 s
Axial tilt	0
Albedo	0.76
Temperature	38 K (−235.2 °C)
Apparent magnitude	13.47
Absolute magnitude (H)	−1.2
Atmosphere
Surface pressure	1.4 to 1.9 Pa (1.38×10−5 to 1.88×10−5 atm)
Composition by volume	nitrogen; methane traces

Triton, or Neptune I, is the largest moon of the planet Neptune.^[1][2] It is the seventh-largest moon in the Solar System. Triton is slightly smaller than Earth's Moon. Triton has a complicated geological history: it is thought to have a young surface compared to the age of the Solar System itself.

It was discovered by the British astronomer William Lassell on 10 October 1846, just 17 days after Neptune itself was discovered by the German astronomer Johann Gottfried Galle and Heinrich Louis d'Arrest.^[1]

Triton is believed to be a captured Kuiper Belt object,^[1] and is the coldest known body in the Solar System.^[1] The surface temperature of Triton was recorded by Voyager 2 as -235 °C (-391 °F).^[1] Triton has its own magnetic field and has a faint trace of an atmosphere.

Drag and tidal interaction cause Triton to fall slowly into lowered orbits. This process is very slow. 3.6 billion years from now, Triton will pass within Neptune's Roche limit.^[3] It will either collide with Neptune's atmosphere or break up, forming a ring system similar to that found around the planet Saturn.^[3]

The reason why Triton is thought to be a captured object is its orbit, which is unique in the Solar System. It is both retrograde (see diagram) and highly tilted. Although there are other satellites with retrograde orbits, they are much smaller than Triton, and much more distant from their host planets (their "primaries"). Triton, like our Moon, shows only one face to the planet: it rotates in synchrony to Neptune.

All we know about the surface of Triton comes from one fly-by of Voyager 2 in 1989. There are few impact craters. This suggests the surface is quite young in astronomical terms: estimates are six to 50 million years.^[4]

Despite its very cold temperature, the surface of Triton is geologically active. There are events which look like geysers, volcanos and earthquakes. All the materials involved are quite different from those on Earth. Most of the gases are frozen most of the time. Gases like nitrogen are frozen until something occurs to melt them, then it turns back into a gas. A temperature increase of just 4 K (7.2 °F) is enough to make this happen.

Like most TNOs, Triton is mostly icy. While Triton's orbit does help us understand its past, it is mostly unknown. It is thought that Triton formed in a binary system, like Pluto, and was moving at very high speeds. However, when it passed through Neptune's Hill sphere the other body will have been flung out, reducing Triton's speed and allowing it to be captured. Now, Triton is the most massive TNO to ever have existed, and it had effects on Neptune's moon system. All large moons were destroyed, flung out of orbit or crashed into Neptune itself.

• Triton Profile Archived 2011-02-19 at the Wayback Machine
• Triton Archived 2016-10-26 at the Wayback Machine