Triton

Triton
True-color image of Triton's south pole, by Voyager 2
Date of discovery	October 10, 1846
Name of discoverer	William Lassell
Name origin	Demigod and son of Poseidon and Amphitrite.
Orbital characteristics
Primary	Neptune
Order from primary	7
Periposeidion	354,754 km
Apoposeidion	354,766 km
Semi-major axis	354,760 km
Orbital eccentricity	0.000016
Sidereal month
Inclination	157.345° to Neptune's equator
Rotational characteristics
Sidereal day	-5.876854 da
Axial tilt	0°
Physical characteristics
Mass	2.14 * 1022 kg
Density	2,050 kg/m³
Mean radius	1,353.4 km
Surface gravity	0.77955 m/s²
Escape speed	1.4526 km/s
Surface area	23,017,715 km²
Mean temperature	34.5 K
Composition	25% water ice, 75% rock
Color	Pinkish-white
Albedo	0.76

Triton, or Neptune I, is the seventh moon of Neptune in order of distance, the first moon of Neptune to be discovered, and the seventh largest moon in all the solar system. It is also the only moon that, while large enough to be called a dwarf planet were it not a satellite, revolves around its primary with a motion retrograde to the primary's own rotation about its axis. This remarkable object thus poses an interesting problem for uniformitarian astronomical theories.

Discovery

The British astronomer William Laskell discovered Triton on October 10, 1846, less than one month following the discovery of Neptune itself. Laskell observed what he at first took to be a ring, but which later proved to be an artifact of the distortion that his new telescope produced. However, Laskell also observed a large satellite, and that image was not due to distortion.

Triton is named after a demigod of the sea, son of Poseidon and his lover Amphitrite. The French astronomer Camille Flammarion is credited with suggesting this name. For more than a century, Triton was the only satellite of Neptune known to man.

Orbital characteristics

The most remarkable characteristic of this object is its retrograde motion. Triton takes 5.876854 Julian days to complete one orbit about Neptune—in a direction opposite to Neptune's own rotation. This makes Triton unique among moons large enough for their self-gravity to force them into hydrostatic-equilibrium (that is, spheroidal) shape. Yet Titan's orbit is nearly circular; its eccentricity is the least of those of the thirteen satellites of Neptune and one of the lowest of the orbital eccentricities of all solar system objects.

Rotational characteristics

Triton is tidally locked and thus always keeps the same face toward Neptune. Thus its sidereal month and day are the same length, and furthermore its sidereal day is also retrograde.

Physical characteristics

Triton's mass is 2.14 * 10²² kg, more than 29% that of Earth's Moon and the seventh-largest mass of all the moons in the solar system. With a density of 2,050 kg/m³, it has far more rock and far less water ice than the core of Neptune.

Triton is also the coldest object on record in the solar system, with a mean temperature of only 34.5 kelvins. This is probably due to Triton's unusually high geometric albedo, which causes it to reflect away energy from the Sun that it might otherwise absorb as heat. Yet in the decade since Voyager 2's flyby of Triton in 1989, Triton's trace atmosphere thickened, and its mean temperature rose by two to three kelvins, a radical proportional warming. This was probably due to Triton's unusual orbital inclination, and the exposure of its south pole to more solar radiation than usual.

Triton's surface is considered relatively young, not more than 10 million years by uniformitarian models. Geologists count 100 impact craters on the leading hemisphere (the one facing in the direction of its motion around Neptune) and suspect that debris from collisions involving the inner or outer moons of Neptune have struck Triton on this leading surface.

Gallery

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  Triton

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  Triton by Voyager 2

See also

John Gribbin, Companion to the Cosmos (Little, Brown & Company, 1996)