Triton and Neptune: 20 years since Voyager, 6 years before New Horizons – with Dr. Paul Schenk
Editor’s Note: It is really a pleasure for me to watch the Beyond the Cradle family getting bigger…
Our most recent collaborator is, currently, a staff scientist at the Lunar and Planetary Institute in Houston Texas. Paul Schenk has been mapping the icy satellites of the outer planets since graduate school days at Washington University in Saint Louis and is currently assisting the New Horizons team plan Pluto encounter observations for July 2015. Read his full profile back at the Collaborators Lounge, now…let us follow Dr. Schenk on a ride to Triton…
This August will mark 20 years since Voyager 2 ended its main mission with a hair-raising pass over the north pole of Neptune. While Voyager continues to report on conditions far from the Sun, its 10 year tour of the Outer Planets ending at Neptune was a benchmark in human exploration. The last solid object that Voyager encountered was Neptune’s exotic moon Triton. Slightly smaller than Earth’s Moon, Triton could not be more different. Coated in alien ices of frozen carbon monoxide, nitrogen, methane, and corbon dioxide, among others, Triton also has a geologic history just as alien and youthful as Europa and Enceladus. I have generated new topographic data from these old images, and shown here as a simulated fly over of the surface, give us a fresh perspective on this bitterly cold moon, chilled to a lonely 35 degrees above absolute zero near the edge of the Solar System:
Triton has been largely forgotten, if only because Voyager is receding in our racial memory and the prospects for a return are even dimmer due to the technical challenges and potential expense (several NASA team are working on developing affordable mission concepts but we also need a renewed interest in Neptune). The Voyager map of Triton was rather limited, with only about 35% of the surface seen at high resolution.
Triton may loom large in the middle of the next decade, however. As New Horizons approaches Pluto, scientists will glance backwards at Voyager’s views of Triton. Why? Triton may be a twin of Pluto. It is a very close twin in terms of size, density and overall composition. Both are 20 to 50% ices by mass, but it is likely that they they are estranged twins, different in appearance and with very different histories. Comparisons and contrasts between the two should be very illuminating.
Triton is Neptune’s largest moon, but was probably once in Solar orbit and captured by Neptune early in its history. The process of capture and the circularization of its backwards orbit was likely very traumatic, causing massive global heating and melting of the interior and completely remaking the surface. Triton is likely geologically active today, and Voyager saw faint columns of what may be nitrogen gas or dust rising from the surface. Voyager also saw large dormant volcanic calderas and pit chains on its icy crust. The infamous convoluted cantaloupe terrain may have formed when the icy crust overturned, a process known as diapirism. Large areas of triton remain uninterpretable to this day, however, especially in the southern hemisphere.
Pluto on the other hand, remains in Solar orbit. It also has one very large moon and two very small ones. It is believed that the large moon, Charon, formed during a violent collision involving the early Pluto. What effect this and subsequent events may have had on Pluto’s thermal and geologic evolution remains to be seen. One thing is clear, the dusky surface splotches seen on the surface by the Hubble Space Telescope are complex and possibly variable. We will not know Pluto’s geologic history until 2015, but we will definitely be looking for comparison’s with Triton’s volcanoes, vents, and diapir fields to better understand both bodies.
The thin atmospheres of both bodies are also a major question. What keeps them from blowing off into space? What do Pluto’s moon’s look like? Charon is similar in size to Uranus’ and Saturn’s icy moons, which Voyager and now Cassini are mapping in great detail and showing to be complex and compelling bodies. Where will Charon fit in this melange of moons? Is the ammonia apparently identified on Charon a sign of an active geology? History shows us that when we look at objects for the first time we are usually surprised. Of the more than 30 icy moons we have surveyed in the past 30 years, none are identical. Even these smaller moons are likely to surprise us.
Both Pluto and Triton are present or former members of a vast family of ice- and carbon-rich objects orbiting beyond Neptune called the Kuiper Belt. This Belt forms a second Asteroid Belt in our Solar System with the larger planets scattered in between. New Horizons will begin the first direct exploration of the objects populating this huge expanse of space, home to relics of our Solar System’s birth. Triton will be remembered once again.
About the Movie:
The flyover movie of Triton was generated using topographic data obtained using a shape-from-shading technique. Although Triton is locally rugged, with small hills, pits, ridges, and scarps scattered over wide areas, the movie also highlights the fact that Triton does not possess any prominent mountain ranges or deep depressions. Most topography is restricted to within 1 kilometer of the global mean surface. We start over cantaloupe terrain, moving eastward toward smooth plains, featuring a large 80-kilometer wide volcanic caldera, which includes a smaller dome near the center, before moving north over a series of volcanic pits and smooth deposits. Vertical exaggeration is roughly a factor of 10.