Much of the outer Solar System has now been visited by robotic spacecraft from Earth, including the gas and ice giants Jupiter, Saturn, Uranus and Neptune, as well as Pluto. Many of the moons of these worlds have also been seen up close, sometimes by multiple spacecraft over the years. But as of yet, some of these moons have still only been seen once, and scientists are itching to go back and have a much closer look. One of these moons in particular stands out: Neptune’s largest moon Triton.
The mission, called Trident, is one of four competing in NASA’s Discovery Program. Up to two of the mission proposals will be selected for further development as full-fledged missions, which will be launched later this decade.
“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. “I’ve always loved the Voyager 2 images and their tantalizing glimpses of this bizarre, crazy moon that no one understands.”
Trident is named after the three-prong spear used by the ancient Roman sea god Neptune.
Just like the spear, the mission would take a three-pronged approach to exploring Triton. The first major goal would be to observe the plumes first seen by Voyager 2. It has been thought that they are composed of nitrogen gas, which escapes from deposits of nitrogen ice when it is warmed slightly by the distant sun. But could they also perhaps originate from a subsurface ocean, just like the ones on Saturn’s moon Enceladus and (likely) Jupiter’s moon Europa. Trident would investigate this possibility and help scientists determine just how they are produced. Trident will also specially look for evidence of a subsurface ocean. Such oceans are now known to exist on several small icy moons in the outer solar system.
The second major goal would be to observe and map the portions of Triton not yet seen. Voyager 2 was only able to see 40% of the moon’s surface during its brief flyby. What we have seen so far is very intriguing, but what does the rest of Triton look like? Trident would also re-photograph the same regions on Triton where the plumes were found, in full “Neptune-shine,” when the Sun’s meager reflected light illuminates the dark side of Triton. Those images could then be compared to Voyager 2’s to see what changes have occurred since that mission.
The third primary goal is to examine how the moon’s relatively young surface keeps renewing itself over time. There are almost no craters seen in the areas of Triton’s surface that have been observed so far, so what process is erasing them? Triton’s surface is estimated to be only about 10 million years old, which is very young compared to the 4.6 billion year age of the Solar System itself. Triton only has a very thin atmosphere, so scientists want to know how the surface keeps getting refreshed.
“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?'”
“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.”
And Triton is indeed a weird moon. It orbits in the opposite direction that Neptune rotates, which no other large moon in the Solar System does. It’s orbit is also extremely tilted from Neptune’s equator, by 23 degrees. Scientists also think that Triton originated from the Kuiper Belt, a region of icy bodies out beyond Neptune that includes Pluto.
Triton is large, about three-quarters the size of our Moon, and it’s very thin nitrogen atmosphere is filled with charged particles in its ionosphere layer, which is 10 times more active than that of any other moon. But scientists don’t know why that is, since ionospheres are typically charged by solar energy, and Triton is too far from the Sun for that process to be the only answer.
Triton’s surface has features not seen on any other icy moons, like the dimpled “cantaloupe terrain” and protruding “walled plains.”
Like Voyager 2, Trident would be a flyby mission, but it would last 13 days, allowing much more time to fully observe and map this bizarre world. If the proposal is approved, Trident would tentatively launch in October 2025 (with a backup date in October 2026 if needed) and arrive at Triton in 2038. An ideal launch window to Neptune/Triton comes once every 13 years, when Earth is aligned with Neptune in a way that a spacecraft can use Jupiter’s gravity along the way to slingshot itself to Neptune.
“The mission designers and navigators are so good at this,” said JPL’s William Frazier, project systems engineer of Trident. “After 13 years of flying through the solar system, we can confidently skim the upper edge of Triton’s atmosphere – which is pretty mind-boggling.”
The other three mission proposals are VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy), DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus) and Io Volcano Observer (IVO).
Voyager 2 has been active since 1977, and is the only spacecraft so far to have visited both Neptune and Uranus, as well as Jupiter and Saturn. It reached interstellar space on Nov. 5, 2018, and is expected to be able to send back data until at least 2025.
Even though it is Neptune’s largest moon, there is still a lot we don’t know about Triton. If chosen, Trident will be an exciting mission to an active and mysterious world that beckons for further exploration.