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Space Topics: Neptune

Neptune's Moon Triton

Neptune's Moon Triton
Triton's colorful, frosty surface was a surprise to Voyager 2. Credit: NASA/JPL/A. Tayfun Oner

Diameter: 2706 kilometers
Orbital distance: 354,800 kilometers from Neptune
Orbital period: 5.88 days
Orbital inclination: 156.8 degrees (retrograde, and inclined 33.2 degrees from Neptune’s equatorial plane)
Discovery: 1846 by William Lassell, only a few weeks after the discovery of Neptune

Triton is overwhelmingly the largest of Neptune’s moons and the only large moon in the solar system to travel in a retrograde (backwards) direction around its primary.  It’s an oddball for many other reasons as well. It is denser, and thus rockier, than most outer planet satellites.  Its frigid surface is covered with a very bright coating of methane frost, and it has a tenuous atmosphere of nitrogen and methane.  Although these characteristics do not match other icy planet satellites, they do match the characteristics observed for Pluto and other Kuiper Belt Objects.  Triton may well be a captured Kuiper Belt Object, perhaps the only one seen by a spacecraft.

Triton's South Polar Geysers
Triton's South Polar Geysers

Seen up close by Voyager 2, Triton revealed itself to be a weird world.  First, it has a polar cap, covered with pink-stained nitrogen frost.  Poking through the cap were several apparent geysers, with long, dark plumes extending “downwind.”  The geysers could result from a “solid-state greenhouse,” in which translucent frozen nitrogen produces a subsurface greenhouse effect, warming material enough that it boils and erupts.  But Triton’s lack of craters indicates that it has been geologically active relatively recently so the geysers could also result from an internal heat source.  Some researchers have even suggested that the geysers are Triton dust devils!

Cantaloupe Terrain on Triton
Cantaloupe Terrain on Triton
Credit: NASA/JPL
The trailing hemisphere of Triton is covered with a terrain so far unique in the solar system: a puckered surface that immediately became known as “cantaloupe terrain.”  The cantaloupe terrain is cut by a few, widely spaced, paired ridges or troughs.  There are very few impact craters.  Some areas are as smooth as lakes, and may represent volcanic flows of water, which would behave on Triton as rock does on Earth.