Content prepared in collaboration with Discovery Space partner Lowell Observatory.
Triton is a mysterious world. Orbiting the gas giant Neptune, the planet's biggest moon was imaged up-close for the first time during Voyager 2's flyby of the outer reaches of the Solar System in 1989. Since then, no other robotic explorer has strayed close enough to catch a glimpse.
What makes Triton an interesting moon for me is the fact that it wasn't born from Neptune's debris disk when the planet was forming; Triton was kidnapped from the Kuiper Belt. Neptune is a gravitational bully, stirring up the Kuiper Belt population, nudging Kuiper Belt Objects (KBOs) into "resonances". However, Triton wasn't nudged, it was plucked from this icy region of asteroids and dwarf planets to become the oddball of Neptune's moon system.
Triton, if it wasn't the victim of planetary abduction, is actually an impressively-sized world itself. With a diameter of 2700 km (1680 miles), the is 40% larger than Pluto! It also orbits Neptune the wrong way, its retrograde orbit an obvious sign that it doesn't really belong there.
As one would expect with such a distant moon, it is actually rather icy, and planetary scientists and astronomers have been intrigued by Triton's composition for some time. In a publication soon to appear in the journal Icarus, a team of researchers have carried out a decade-long detailed infrared analysis (NASA Infrared Telescope Facility's SpeX instrument) of the various ices on the surface of Triton and discovered something peculiar. The different types of ice do not mix and they appear to be distributed separately, varying in concentration during the seasons.
"The expectation is that nitrogen, methane, and carbon monoxide should all go together," said Will Grundy, a Lowell Observatory planetary scientist and lead author of the paper. "They are volatile enough to move around even on the stately seasonal pace of Triton time scales. What we've observed, however, is that carbon monoxide and nitrogen do go together nicely, but the methane is doing its own thing."
Grundy suspects that methane has a different distribution due to the molecule's less volatile behavior at Triton's extremely cold surface temperatures.
The researchers also noticed a significant enhancement of nitrogen ice on Triton's Neptune-facing hemisphere. Also, carbon monoxide ice exhibits similar behaviour as nitrogen concentrations, indicating they occupy the same location. However, the methane ice has a completely different spatial distribution; it appears to be located in regions facing away from Neptune.
Over the 10-year study, the team also noticed that water and carbon dioxide ices (non-volatile ice species) are evenly distributed over the moon's surface.
"This type of long-term, detailed analysis would be equally valuable for small icy planets like Pluto, Eris, and Makemake, all of which are similar to Triton in having volatile ices like methane and nitrogen on their surfaces," said Grundy. "We have been monitoring Pluto's spectrum in parallel with that of Triton, but Eris and Makemake are quite a bit fainter. It is hard to get time on large telescopes to monitor them year after year. We expect that Lowell Observatory's Discovery Channel Telescope will play a valuable role in this type of research when it comes on line."
A special thanks to Steele Wotkyns, Public Relations Manager at Lowell Observatory for providing the material for this article.