The largest of Neptune’s 13 moons and the only one in our solar system that orbits in the opposite direction of its planet's rotation, Triton, has fascinated scientists since its discovery.
This 2,700 kilometre (1,680 mile) wide ice world that also shoots out massive, dark plumes of icy material out from the moon’s surface has now been earmarked by NASA as a potential target to study up close as part of its Discovery Program mission.
Running alongside NASA's larger “flagship” planetary science explorations, the agency’s Discovery Programme makes use of smaller missions that use fewer resources and shorter development times to unlock the mysteries of our solar system.
Mars Pathfinder and the current martian inhabitant InSight, are two such examples of the program’s past successes.
If selected, Trident, the name given to the mission to study Triton, would be the first spacecraft to get close to Neptune and its moons since the Voyager 2 spacecraft flew by three decades ago; an encounter that left a lasting impression on planetary scientists who were left puzzled over Triton’s many unusual features.
In addition to the retrograde orbit and icy plumes – a feature which on other moons is thought to be caused by water from the interior being forced through thick, icy crusts – images returned by Voyager 2 showed two further peculiar traits not expected from an ancient moon far out in the dark depths of the Solar System.
Firstly, Triton has a sparsely cratered surface with smooth volcanic plains, mounds and round pits formed by icy lava flows. All of this points to a young landscape that has been resurfaced over and over with fresh material. How could this be so? Is there something in its interior that is still warm enough to drive this activity?
More curious still is Triton’s atmosphere. The moon’s ionosphere is filled with charged particles and is 10 times more active than that of any other moon in the solar system.
Considering Triton and Neptune are 30 times farther from the Sun than Earth and ionospheres are generally charged by solar energy, it would indicate some other energy source must be at work; but what?
"Triton has always been one of the most exciting and intriguing bodies in the solar system," said Louise Prockter, director of the Lunar and Planetary Institute/Universities Space Research Association in Houston.
As principal investigator, she would lead the proposed Trident mission, and NASA's Jet Propulsion Laboratory in Southern California would manage it. "I've always loved the Voyager 2 images and their tantalising glimpses of this bizarre, crazy moon that no one understands," Prockter added.
As weird as Triton is, clues about the moon can be deciphered from its orbit around its host planet, Neptune.
With an extreme tilt and offset from Neptune's equator by 23 degrees, Triton was once likely a Kuiper Belt object that migrated inwards from a region beyond Neptune and was subsequently captured by the ice giant’s gravity.
If this is the case, then it is also likely that if Triton does have an ocean, it probably developed after it was captured by Neptune; something that so far is believed to be unique among other known ocean worlds.
Understanding how interior oceans form is therefore key to the Trident mission.
Called Trident for its three-pronged approach, if approved, not only will Trident seek to understand where we might find water beyond Earth, the mission aims to map the largest unexplored solid surface in the Solar System this side of the Kuiper Belt.
Like our own moon, Triton is locked in synchronous rotation with Neptune―one side faces the planet at all times. But because of its unusual orbital inclination both polar regions take turns facing the Sun.
So far though, scientists have only seen 40 percent of Triton’s surface and they want to see the rest, as well as taking a better look at the plume-rich area on Triton’s ‘dark side’ that Voyager 2 imaged as it whizzed by.
To achieve this Trident will use its full-frame imaging camera and take advantage of the Sun’s reflected light as it illuminates Neptune to capture this mysterious region.
Trident's third major goal is to understand how Triton’s crust of frozen nitrogen over an icy mantle with a stack of unusual landforms keeps renewing itself; a feat that means the moon has the second-youngest surface in the Solar System.
"Triton is weird, but yet relevantly weird, because of the science we can do there," said Karl Mitchell, Trident project scientist at JPL. "We know the surface has all these features we've never seen before, which motivates us to want to know 'How does this world work?'
If approved, Trident would begin its journey in October 2025 (with a backup in October 2026); a date that takes advantage of a once-in-a-13-year window, when Earth is properly aligned with Jupiter.
The spacecraft would use the gravitational pull of Jupiter as a slingshot straight to Triton for an extended 13-day encounter in 2038.
The timing would also benefit research into the moon’s eruption activities because if Trident arrives any later than 2040, the Sun will not be in a favourable position to study potential plume mechanisms as it will have moved too far north - a path it will keep on for the next hundred years!
"As we said to NASA in our mission proposal, Triton isn't just a key to solar system science - it's a whole keyring: a captured Kuiper Belt object that evolved, a potential ocean world with active plumes, an energetic ionosphere and a young, unique surface,” Mitchell concludes.
Trident is one of four concepts being considered for NASA’s Discovery Program mission; the remaining three are VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus) and the Io Volcano Observer (IVO).
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