page_title: Triton (moon)
section_title: Polar cap, plains and ridges
hierarchical_section_title: Triton (moon) / Surface features / Polar cap, plains and ridges
caption_attribution_description: English: Voyager 2 image showing the southern hemisphere of Triton. At 2,700 km diameter, Triton is Neptune's largest satellite. This image was made using about a dozen Voyager 2 frames. The large, pinkish colored south polar cap is at the top of the image. North of the cap the surface is generally darker and redder in color. This area exhibits a plethora of unusual morphologic features, including the long lineations at the center of the frame.
context_page_description: Triton is the largest natural satellite of the planet Neptune, and the first Neptunian moon to be discovered. The discovery was made on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, an orbit in the direction opposite to its planet's rotation. At 2,710 kilometres in diameter, it is the seventh-largest moon in the Solar System, the only satellite of Neptune massive enough to be in hydrostatic equilibrium and the second-largest planetary moon in relation to its primary, after Earth's Moon. Because of its retrograde orbit and composition similar to Pluto, Triton is thought to have been a dwarf planet captured from the Kuiper belt.
  Triton has a surface of mostly frozen nitrogen, a mostly water-ice crust, an icy mantle and a substantial core of rock and metal. The core makes up two-thirds of its total mass. The mean density is 2.061 g/cm³, reflecting a composition of approximately 15–35% water ice.
  During its 1989 flyby of Triton, Voyager 2 found surface temperatures of 38 K and also discovered active geysers; Voyager 2 remains the only spacecraft to visit Triton.
context_section_description: 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.
  The high plains found on Triton's eastern hemisphere, such as Cipango Planum, cover over and blot out older features, and are therefore almost certainly the result of icy lava washing over the previous landscape. The plains are dotted with pits, such as Leviathan Patera, which are probably the vents from which this lava emerged. The composition of the lava is unknown, although a mixture of ammonia and water is suspected.
  Four roughly circular "walled plains" have been identified on Triton. They are the flattest regions so far discovered, with a variance in altitude of less than 200 m. They are thought to have formed from eruption of icy lava. The plains near Triton's eastern limb are dotted with black spots, the maculae. Some maculae are simple dark spots with diffuse boundaries, and others comprise a dark central patch surrounded by a white halo with sharp boundaries.  The maculae typically have diameters of about 100 km and widths of the halos of between 20 and 30 km.
  There are extensive ridges and valleys in complex patterns across Triton's surface, probably the result of freeze–thaw cycles. Many also appear to be tectonic in nature and may result from extension or strike-slip faulting. There are long double ridges of ice with central troughs bearing a strong resemblance to Europan lineae (although they have a larger scale), and which may have a similar origin, possibly shear heating from strike-slip motion along faults caused by diurnal tidal stresses experienced before Triton's orbit was fully circularized. These faults with parallel ridges expelled from the interior cross complex terrain with valleys in the equatorial region. The ridges and furrows, or sulci, such as Yasu Sulci, Ho Sulci, and Lo Sulci, are thought to be of intermediate age in Triton's geological history, and in many cases to have formed concurrently. They tend to be clustered in groups or "packets".
caption: Triton's bright south polar cap above a region of cantaloupe terrain
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