Is Triton Hiding an Underground Ocean?

Voyager 2 mosaic of Neptune’s largest moon, Triton (NASA)

At 1,680 miles (2,700 km) across, the frigid and wrinkled Triton is Neptune’s largest moon and the seventh largest in the Solar System. It orbits the planet backwards – that is, in the opposite direction that Neptune rotates – and is the only large moon to do so, leading astronomers to believe that Triton is actually a captured Kuiper Belt Object that fell into orbit around Neptune at some point in our solar system’s nearly 4.7-billion-year history.

Briefly visited by Voyager 2 in late August 1989, Triton was found to have a curiously mottled and rather reflective surface nearly half-covered with a bumpy “cantaloupe terrain” and a crust made up of mostly water ice, wrapped around a dense core of metallic rock. But researchers from the University of Maryland are suggesting that between the ice and rock may lie a hidden ocean of water, kept liquid despite estimated temperatures of  -97°C (-143°F), making Triton yet another moon that could have a subsurface sea.

How could such a chilly world maintain an ocean of liquid water for any length of time? For one thing, the presence of ammonia inside Triton would help to significantly lower the freezing point of water, making for a very cold — not to mention nasty-tasting — subsurface ocean that refrains from freezing solid.

In addition to this, Triton may have a source of internal heat — if not several. When Triton was first captured by Neptune’s gravity its orbit would have initially been highly elliptical, subjecting the new moon to intense tidal flexing that would have generated quite a bit of heat due to friction (not unlike what happens on Jupiter’s volcanic moon Io.) Although over time Triton’s orbit has become very nearly circular around Neptune due to the energy loss caused by such tidal forces, the heat could have been enough to melt a considerable amount of water ice trapped beneath Triton’s crust.

Related: Titan’s Tides Suggest a Subsurface Sea

Another possible source of heat is the decay of radioactive isotopes, an ongoing process which can heat a planet internally for billions of years. Although not alone enough to defrost an entire ocean, combine this radiogenic heating with tidal heating and Triton could very well have enough warmth to harbor a thin, ammonia-rich ocean beneath an insulating “blanket” of frozen crust for a very long time — although eventually it too will cool and freeze solid like the rest of the moon. Whether this has already happened or still has yet to happen remains to be seen, as several unknowns are still part of the equation.

“I think it is extremely likely that a subsurface ammonia-rich ocean exists in Triton,” said Saswata Hier-Majumder at the University of Maryland’s Department of Geology, whose team’s paper was recently published in the August edition of the journal Icarus. “[Yet] there are a number of uncertainties in our knowledge of Triton’s interior and past which makes it difficult to predict with absolute certainty.”

Still, any promise of liquid water existing elsewhere in large amounts should make us take notice, as it’s within such environments that scientists believe lie our best chances of locating any extraterrestrial life. Even in the farthest reaches of the Solar System, from the planets to their moons, into the Kuiper Belt and even beyond, if there’s heat, liquid water and the right elements — all of which seem to be popping up in the most surprising of places — the stage can be set for life to take hold.

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Inset image: Voyager 2 portrait of Neptune and Triton taken on August 28, 1989. (NASA)

8 Replies to “Is Triton Hiding an Underground Ocean?”

  1. It is interesting here that Triton is still the largest KBO in evidence. But apparently not too large to have high pressure ices isolating the ocean from the rocky core.

    And according to the linked article the way tidal heating works it helps too:

    “While the concentration of tidal dissipation near the bottom of ice shells was known for some time, we believe our work is the first to demonstrate that it indeed controls the rate of freezing and sustainability of subsurface oceans,” says Saswata Hier-Majumder at the University of Maryland. “Radiogenic heating, in comparison, heats up the shell uniformly, and thus doesn’t have as disproportionate an influence as tidal dissipation does.”

    Access to minerals, and possible hydrothermal vent activity in association with the water mass, gives these moons a higher habitability potential than larger moons. Between tidal heated icy moons, and radioactively heated Ceres and large KBOs, we have a long list between certain (Enceladus, likely Europa) to possible (Ceres, Pluto, Sedna) oceans.

    From the astrobio article:

    Another more remote possibility is that Triton could host silicon-based life, assuming that silicon can actually be used as a foundation for life instead of carbon. Silanes, which are structural analogues of hydrocarbons, could be used as a building block for life under the right conditions.

    The main stumbling block is that the potential metabolite SiO2 is a solid under liquid water conditions.

    There are other sources for silicone “organics” such as the mentioned silanes, but our biosphere stopped relying on haphazard and external abiotic organic production long since. At the time of the RNA/protein world it seems the preferred carbon source was CO2. [“The Emergence and Early Evolution of Biological Carbon-Fixation”, Braakman et al, PLOS Comp. Biol. 2012.]

    Maybe silicon based protocells can evolve, you don’t need much according to Shoztak’s work on organic protocells, likely silane based membranes and some polymer analog to RNA.* Maybe they can couple to a silane based metabolism. But the long term viability of such life doesn’t seem large.

    * Already there we have a problem. The Si-Si bound is readily hydrolyzed, so we have shadows of “arsenic life” all over again.

    1. ipoteticaly life based on silicon if it could exist, don`t you believe it should have already been on Earth ? We have silicon in very large quantities and in all kinds of temperatures, on frozen areas and also near volcanic places.

    2. I suspect the relatively short length of stable silanes makes it unsuitable as the base of self replicating molecules. But it might still be invoked in quite advanced combinations that we are yet to discover, as could arsenic and perhaps a few other elements.
      Or to paraphrase evolution – “Whatever works”

  2. A planet-captured denizen of the Kuiper Belt? Or, an orphan moon, from world orbit of its parent, Sun-lit home, hurled-out? Outcast from ancient System-wide upheaval, was it later adopted from distant, long-wonder path, by a an ice giant on the frontier? — Simply, could it have been disrupted and flung-outwards from its warmer, INNER place of Solar family (swaddled in gaseous atmosphere?), rather than having been set-adrift, or nudged inwards from its outer, dim-lit edge?

    The Neptune-Triton coupling of chance(?), have longed intrigued me. Significant that retrograde orbit, and its relative close proximity, in magnificent charge of ice-hued master.

    Battered, atmosphere frozen–or stripped–moons; hints of interior “ocean”-volumes of water, hard-core metallic evidence, broken tales of mantle rocks, frozen stories suspended on surfaces of time: what common theme, the complex of these moons’ remains, underlies? A broad, interrelated case for astro-detectives to study and investigate.The answer of a common bond, hidden under frozen, Sun-blasted exteriors(?), or readily seen in surface shadows of Sunlight(?), when solved, might astound!

    A semi-liquid ocean beneath the Triton mass of Neptune’s grip, encased in frozen, fossil- atmosphere? Any connection to the geysers spewing-out shadow-casting plumes of nitrogen, I wonder. Encelodus, with its own unique, but similar(?) dynamic, as one consulted piece suggests, the secret may cryptically hold (like an unseen tiger concealed the forest).

  3. I’ve long suspected that when we do eventually get to see what Pluto looks like, courtesy of New Horizons, it will look a lot like Triton. Do we know if New Horizons is equipped, or will have time to, detect if any such sub-surface ocean might exist on Pluto too?

  4. I’d like to see Cassini-style orbiters for both Uranus and Neptune. Then we could find out for sure!

  5. Just a small nitpick on the title of the article: Underground implies to me a rocky surface with an aquifer. Maybe it should instead say ‘ice covered ocean’?

  6. Is Triton Hiding an Underground Ocean?

    I hope so. Anything to make this solar system and this existence more interesting!

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