Cantaloupe Terrain

This week’s image comes from Triton, the largest satellite of Neptune. It was captured by the voyager two spacecraft and shows a region of “cantaloupe terrain”. Credit for this image goes to NASA’s Voyager team. Cantaloupe terrain is a good example of typical astro-geological naming conventions. When the voyager spacecraft imaged this region it was not immediately clear what was going on or how this distinctive terrain might have formed. It was thus named based on what it looked like, in this case the distinctive skin of a cantaloupe melon. This terrain is only found on this one moon, and consists of small depressions interspersed with ridges as illustrated above. Another view of this sort of terrain can be seen below. 

 

These depressions are 30-40 km across, and are believed to result from the overturning of Triton’s surface, which is largely icy in composition. The reason for the unusual surface texture is the way this overturn took place. Tidal heating from Neptune resulted in the formation of numerous diapirs. These are regions where ductile material rises up and forces its way through an overlying brittle layer. The term is derived from the Greek word for piercing, as the underlying material squeezes its way through the overlying layers. On Earth diapirs of magma tend to intrude into overlying rocky strata. On Triton ices of various different sorts make up the layers which are being deformed by the same processes. The result is a pitted and churned up surface, deformed by the rise of this material.   

Triton is one of only a few moons in the solar system to be geologically active, and it is believed that its interior is melted by tidal heating from Neptune. This moon is also unusual because it has a retrograde orbit. This means that it orbits in the opposite direction to the other neptunian satellites, and the planet itself. This anomalous orbit suggests that the moon didn’t form in its current position, but was rather captured by the gravity of the larger planet. 

Triton is believed to have originally been Kuiper Belt object, a dwarf planet like Pluto, which it is similar to in many ways. In fact Pluto’s orbit is elliptical, and actually crosses the path of Neptune bringing it closer to the sun for 20 years in every 248. Pluto’s orbit will never bring it into contact with the larger planet, as they are never in the same parts of their orbits when they cross. However Triton wasn’t as lucky. At some point its orbit brought it too close to Neptune and it was swept up by the ice giant planet, becoming a moon. This means that in some ways Triton is a warmed up Pluto, it is likely made of similar materials, but the tidal heating from its large neighbour keeps the interior much warmer, giving it a geothermal gradient which is probably lacking on Pluto. The diapirs that caused the cantaloupe terrain are one product of this warmer state, and cryovolcanism is believed to take place on the moon as well. 

Image Credits:

NASA/JPL/Universities Space Research Association/Lunar & Planetary Institute

 https://photojournal.jpl.nasa.gov/catalog/PIA12186

https://pds.jpl.nasa.gov/planets/captions/neptune/tritncnt.htm

Further Reading: http://gizmodo.com/this-is-the-most-detailed-map-of-neptunes-moon-triton-1625889935