A possible new mission called Trident aims to explore Neptune's strange moon Triton.
Neptune's largest moon, Triton boasts an uncommon icy mixture on its surface, among other unique characteristics that could help scientists learn more about how bodies in the solar system evolved. Trident is one of four mission concepts competing in a current round of NASA's Discovery program, which develops relatively low-cost robotic-exploration efforts.
The proposed Trident mission — named for its three-pronged approach to solving Triton's mysteries, inspired by the three-pronged spear carried by the ancient Roman sea god Neptune — plans to visit Triton and study the moon's entire surface in greater detail, according to a statement from NASA.
Related: NASA unveils best map ever of Neptune's moon Triton (images, video)
"Triton has always been one of the most exciting and intriguing bodies in the solar system," Louise Prockter, director of the Lunar and Planetary Institute of the Universities Space Research Association in Houston, who leads the Trident proposal team, said in the statement. "I've always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands."
NASA's Voyager 2 spacecraft launched in 1977 to study the outer planets. Voyager 2 is the only spacecraft ever to have flown past Neptune, the most distant planet in the solar system. The mission captured stunning views of the planet and its moons, but also left a lot of unanswered questions about the distant planetary system. If Trident is selected, it will be Voyager 2's first successor to Neptune.
Under NASA's Discovery program, up to two of the concept missions currently in development will be selected by summer 2021 for launch later in the decade. (Previously selected missions in the Discovery program include the InSight Mars lander, the Kepler Space Telescope and the Messenger spacecraft to Mercury.)
Triton is the seventh largest moon in our solar system. Scientists believe it was born in the Kuiper Belt — the region beyond Neptune full of icy bodies left over from the early solar system — before it migrated to its current position orbiting the gas giant.
Triton has many other unusual characteristics of interest. It is the only moon in the solar system that orbits in the opposite direction of its planet's rotation, and its path around the planet lies at an extreme tilt, offset from Neptune's equator by 23 degrees.
The moon also has an unusual atmosphere and a dynamic climate. 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. Generally, ionospheres are charged by solar energy. However, considering Triton and Neptune are 30 times farther from the sun than Earth is, there must be another energy source here, according to the statement.
"Triton is weird, but yet relevantly weird, because of the science we can do there," Karl Mitchell, Trident project scientist at NASA's Jet Propulsion Laboratory in California, said in the statement. "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?"
The Trident mission's first goal is to study Triton's strange icy plumes — a feature scientists believe is caused by water from the interior being forced through the moon's thick, icy crust. If Trident finds a subsurface ocean is the source of the plumes on Triton, the discovery would expand scientists' understanding of where water might be found beyond Earth.
The Trident mission also aims to map the largest unexplored solid surface in the solar system this side of the Kuiper Belt. So far, scientists have seen only 40% of Triton's surface. Using a full-frame imaging camera, Trident would aim to map the rest by taking advantage of the sun's reflected light as it illuminates Neptune.
Lastly, the mission aims to study the moon's surface in greater detail. Triton has almost no visible craters, suggesting the moon's surface is a relatively young landscape that has been resurfaced over and over with fresh material.
"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 said in the statement.
If selected, Trident would launch in October 2025 (with October 2026 as a backup date). At this time, Earth will align with Jupiter so that the spacecraft can use the gravitational pull of Jupiter as a slingshot. Trident would then arrive at Triton in 2038 for an extended 13-day encounter. Scientists are calling this timing "a once-in-a-13-year window," according to the statement.
With the Trident mission, scientists hope to learn more about how Triton has changed over time. Such knowledge, in turn, would provide a better understanding of how planets and moons in the solar system have evolved.
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