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The portion of Triton viewed by Voyager 2 in 1989 at high resolution is geologically unique in several ways. This photomosaic shows Triton's southern hemisphere - the image is centered near 20 degrees north latitude, and Triton's equator runs approximately through the bright swath across the middle of the image.
We do not yet have detailed photographs of the surface of Pluto, but we would like to predict what that surface might look like up close. When planetary scientists are faced with having to predict what the surface of a planet might look like, they look at another body in the solar system that has the same general properties as the body they're guessing about.
| The surface of a solid planet records the history of its interior evolution. The surface geology will also reflect the planet's composition. Icy worlds in our solar systems seem to have surface features that are a little bit different from the features we see on terrestrial planets like Earth. Therefore, when predicting what the surface of Pluto is like, we should scan the solar system for an object with a similar history of heating and impacts, and with a similar composition as Pluto.
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A likely candidate for comparison to Pluto is Neptune's strange moon Triton. The interiors of both Pluto and Triton are likely composed primarily of water ice over a rocky core. Their outer layers also have some amount of non-water ices such as methane, carbon dioxide and nitrogen, which are present on their surfaces and their thin atmospheres replenished from sublimation of ices from their surfaces.
Images of Triton returned from the Voyager spacecraft show plumes of gas erupting from the interior. Triton's surface has two types of terrain and its poles are brighter than its equator.
Triton is in an unusual orbit around Neptune. It orbits in the opposite direction from Neptune's spin - a retrograde orbit - and its orbit is inclined relative to Neptune's equator. If Triton had formed around Neptune, its orbit should have been in the plane of Neptune's equator, and it would rotate around Neptune in the same direction as the planet.
A Stray KBO
Several scientists have suggested that Triton may be a Kuiper Belt Object that wandered too close to Neptune and was captured into orbit.
Something had to slow the proto-Triton down as it wandered past Neptune; one theory is that proto-Triton impacted one of Neptune's existing satellites and leftover debris formed Triton. Another possibility is that "gas drag" caused proto-Triton to slow down as it grazed through Neptune's atmosphere. In either case, a tremendous amount of heat would have been deposited in Triton's interior. The heat inside Triton would cause a geologically active interior, leading to eruptions of material onto the moon's surface and a redistribution of mass in its interior.
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Pluto is a member of the Kuiper Belt, and the giant impact that formed Charon may have disrupted Pluto's interior, just as the interior of proto-Triton would have been disrupted when it impacted one of Neptune's satellites. As Pluto cooled down and the mass in its interior redistributed after the impact, interesting geology may have formed on its surface. Because Pluto's composition is likely similar to Triton's, the features we see on Triton (such as the "cantaloupe terrain" shown in the top of the figure above and wind streaks shown below) may also exist on Pluto. The dark wind streaks detected by Voyager 2 on Triton's surface were tens to hundreds of kilometers long and preferentially pointed northeast, away from the south polar cap. |
This portion of Triton's surface, viewed by Voyager 2 in 1989, may be the best preview of Pluto's surface - although Pluto is uniformly less bright than Triton and there are many other differences. The dark spots just left of center (called maculae) appear to be surface deposits left by evaporating ices blowing in from the south pole (upper left direction). Right of center is a roughly circular walled plain (Sipapu Planitia) and at the bottom is a complex of flat, dark spots. At bottom center is the nearly 17-mile (27-kilometer) impact crater Mozamba, the largest seen on Triton. The boundary between the light colored south polar material and the darker surface runs from top center toward the lower left side of the picture.
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