Triton (Neptune I)

 

 

Neptune's moon Triton, seen in images captured by Voyager 2 on August 25, 1989. Approximate natural color. 

Credit: NASA/JPL-Caltech/Voyager Imaging Team/Jason Major

Triton (Neptune I), the largest moon of Neptune, was discovered by English astronomer William Lassell (1799–1880) on October 10, 1846, just 17 days after the discovery of Neptune. Lassell spotted the moon with his self-built 61 cm (24 in) aperture metal mirror reflecting telescope (also known as the "two-foot" reflector) from his private observatory near Liverpool.

   Triton is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet, captured from the Kuiper belt. It orbits at a distance of 354,759 km from Neptune, a nearly perfect circle with an eccentricity of almost zero. It has an orbital period of 5.877 days, and the inclination of 156.9° to Neptune's equator.

 

A view of Neptune's moon Triton made from images captured by Voyager 2 spacecraft on August 25, 1989 from a distance of about 189,000 km. Note Triton's peculiar "cantaloupe terrain" at the top of the image. Color is approximate natural and was added from lower-resolution data captured by Voyager 2. Credit: NASA/JPL-Caltech/Voyager 2/Jason Major

   Triton has a mean diameter of 2,705 km, and it is the seventh-largest moon in the Solar System. During its 1989 flyby, Voyager 2 ― the only spacecraft to fly past Neptune and Triton ― found Triton has active geysers, making it one of the few geologically active moons in our solar system. Spacecraft images show the moon has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a core of rock and metal. The moon has a density about 2.061 g/cm³, twice that of water. This is a higher density than that measured for almost any other satellite of an outer planet (except Europa and Io). This implies that Triton contains more rock in its interior than the icy satellites of Saturn and Uranus.

   Only 40% of Triton's surface has been observed and studied. Analysis of crater density and distribution has suggested that in geological terms, Triton's surface is extremely young, with regions varying from an estimated 50 million years old to just an estimated 6 million years old. The Voyager 2 observed a handful of geyser-like eruptions of nitrogen gas and entrained dust from beneath the surface of Triton in plumes up to 8 km high. Triton's south polar region is covered by a highly reflective cap of frozen nitrogen and methane sprinkled by impact craters and openings of geysers. Little is known about the north pole because it was on the night side during the Voyager 2 encounter, but it is thought that Triton must also have a north polar ice cap. Triton's western hemisphere consists of a strange series of fissures and depressions known as "cantaloupe terrain", which is mostly dirty water ice.

 

This natural color image of the limb of Triton was taken early in the morning of Aug. 25 1989, when the Voyager 2 spacecraft was at a distance of about 210,000 kilometers from the icy satellite. The largest surface features visible area about 5 km across. The picture is a composite of images taken through the violet, green and clear filters. The color in this image is somewhat exaggerated: Triton is primarily a white object with a pinkish cast in some areas. Credit: NASA/JPL 

Limb clouds over Triton's south polar cap. Image is stretched to enhance the limb clouds and surface features. The image shows the cloud on the west limb that extends about 100 km along the limb and appears detached over much of its length. Credit: NASA/JPL 

   Triton's thin atmosphere is composed mainly of nitrogen with small amounts of methane. This atmosphere most likely originates from Triton's volcanic activity, which is driven by seasonal heating by the Sun. Triton's surface atmospheric pressure is only about 1.4–1.9 Pa (1.38×10⁻⁵ to 1.88×10⁻⁵ atm). A haze permeates most of Triton's troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton's atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from its surface. Triton is one of the coolest objects in our solar system. It is so cold that most of Triton's nitrogen is condensed as frost, giving its surface an icy sheen, with an unusually high albedo, reflecting 60–95% of the sunlight that reaches it. Voyager 2 found surface temperatures of 38 K (−235 °C).

   Triton is named after the Greek sea god Triton, the son of Poseidon (the Greek god comparable to the Roman Neptune). Until the discovery of the second moon Nereid in 1949, Triton was commonly known as simply "the satellite of Neptune."

 

The image of an erupting volcano on Triton, taken by Voyager 2 as the spacecraft approach the moon on Aug. 26, 1989, showd the geyser-like volcanic plume spewing an eight kilometer tall cloud of fine, dark particles into Triton's thin atmosphere. The cloud can be seen drifting downwind to the right for a distance of roughly 150 kilometers. The shadow of the lound cloud can be distinguished. Credit: NASA/JPL

 

Processed using calibrated orange, green, and blue filtered images of Neptune and Triton taken by Voyager 2 on August 31 1989. Credit: NASA/JPL-Caltech/Kevin M. Gill

   William Lassell, the discoverer of Triton, was a wealthy English merchant and astronomer. He constructed progressively larger reflecting telescopes, for which he pioneered the use of an equatorial mount for easy tracking of objects as the Earth rotates. In addition to Triton, he also discovered two moons around Uranus (Ariel and Umbriel) and co-discovered the eighth moon of Saturn (Hyperion).

 

William Lassell. Credit: Wikimedia Commons

References:

NASA Solar System Exploration: Triton

 

© 2025, Andrew Mirecki