Triton: Oceans of the Outer Solar System

The Voyager 2 spacecraft flew by Triton, a moon of Neptune, in the summer of 1989. Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, used Voyager data to construct the best-ever global color map of Triton. This map has a resolution of 1,970 feet (600 meters) per pixel. Image Credit: NASA/JPL-Caltech/Lunar & Planetary Institute

NASA recently released a new and highly detailed map of Neptune’s moon Triton. The map has a resolution of 600 meters per pixel and was created using ‘restored’ data from the Voyager 2 spacecraft’s 1989 encounter with Neptune. It is a beautiful example of the longevity of good data, and the value of using new technology and techniques to re-analyze archived data from past missions

The map of Triton provides a wealth of new information for comparative planetology and planetary geology studies, and scientists will use it to better-understand the diverse nature of solid bodies in the Solar System. But Triton also has some interesting things to teach astrobiologists.

Computer-generated montage of Triton and Neptune, using images from the Voyager 2 flyby. Image Credit: NASA/JPL/USGS

When studying life’s potential beyond Earth, Triton doesn’t immediately spring to mind as a promising location for biology. Earth, the only inhabited planet yet known, is roughly 149,600,000 kilometers away from the Sun and sits within our star’s habitable zone. In this region of space, the planet receives just the right amount of energy for liquid water to persist at the surface. Triton (and its host planet Neptune) are more than 4,503,443,600 kilometers away from the Sun’s warmth, orbiting in the dark depths of the outer Solar System.

But a star’s habitable zone is not the only measure of habitability in a solar system. With data from missions like Voyager, astrobiologists have identified cold, dark moons of giant planets that may still have the potential to support life.

In a 2012 paper in the journal Icarus, Jodi Gaeman and colleagues outlined a possible scenario for a subsurface ocean on Triton. They found that after Triton was drawn into Neptune’s orbit, the moon could have experienced tidal heating in a way similar to Jupiter’s moon Europa. Like Europa, energy from this heating could have melted areas of Triton’s subsurface, creating an ocean of liquid water beneath the moon’s icy shell.

The researchers determined that Triton could have a thin layer of liquid water rich in ammonia (NH3) today – but only if Triton started its life as a Neptunain moon with a highly eccentric orbit that slowly circularized over time.

An illustration of Voyager 2 setting its sights on Neptune and Triton in 1989 from Issue 4 of the Astrobiology Program’s graphic history of astrobiology. Available at: https://www.astrobio.net/nasa-astrobio-graphic-novels/ Credit: NASA Astrobiology Program, Artwork by Aaron L. Gronstal

Astrobiology Magazine asked Dr. Saswata Hier-Majumder, co-author of the 2012 study and Senior Lecturer in the Department of Earth Sciences at Royal Holloway University of London, if the new map of Triton could provide further clues about the moon’s subsurface.

“This high resolution map will be helpful in studying, among other geological features on Triton’s surface, the system of ridges in the Cantaloupe terrain and the smooth planes,” said Hier-Majumder. “Both of these features can provide clues regarding the presence of a subsurface ocean, and potential estimates of the thickness of Triton’s crust.”

In the case of Europa, images of the moon’s surface have been essential for identifying features that provide insights into what lies beneath the moon’s ice. The new, detailed map of Triton could  have similar applications. Dr. Hier-Majumder gave an example of the types of features he would look for:

“Given the young age of Triton’s surface (~100 Ma), cryovolcanic features on the surface would imply recent extrusion of fluid from the subsurface ocean. The mechanism by which such extrusions occur, however,  are still not completely clear.”

The Voyager 2 spacecraft flew by Triton, a moon of Neptune, on August 25, 1989. Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, used Voyager data to construct this video recreating that exciting encounter. Image Credit: NASA/JPL-Caltech/Lunar & Planetary Institute

Aaron L. Gronstal: