OUT IN THE COLD: TRITON, A HABITABLE OCEAN WORLD?

It has been a very long, dark, and cold night on the giant planet Neptune, the eighth planet in our solar system. Located almost 4.5 billion km from the sun, it receives just 0.1% of the light we receive on Earth. Neptune’s only rendezvous with a human-made object was in 1989, when the spacecraft Voyager 2 flew by, while on its long journey toward the edge of the solar system. 

Although brief, Voyager 2’s encounter with Neptune, and its coterie of rings and satellites, returned data that have kept planetary scientists busy to this day. Neptune’s moon Triton has proved to be particularly intriguing: discovered in 1846, by the 1930s astronomers had already hypothesized that it was a captured satellite, but its size and composition had been only a matter of speculation, and nothing at all was known about its geology until Voyager 2’s adventure to the outer solar system.

We now know that Triton has had a continuous and vigorous geological activity for at least the last tens of millions of years and that a potentially habitable ocean could exist under its cold surface. Many more questions have emerged, however, which is why scientists have been planning to return to Triton for decades. 

Now, the long wait may be over: in February this year, NASA announced that a mission called Trident, (led by Dr. Louise Prockter, Director of the Lunar and Planetary Institute in Houston, in collaboration with colleagues from JPL, Southwest Research Institute and other institutions), is a finalist for the Discovery Program. If selected, the mission will be launched in 2026 for a planned arrival to the Neptunian system in 2038. In this and following articles, I outline some of the most important steps in the exploration of Triton and briefly highlight what we know about this fascinating world.

Triton is the largest natural satellite of the outer solar system planet Neptune, located at a distance of 30 AU (approximately 4.5 billion km) from the Sun, and a little less than 4.4 billion km from Earth. Being so far away from us, and because of its proximity to the giant planet Neptune (orbital radius: ~ 355,000 km), it is very difficult to observe Triton using telescopes. Nonetheless, ground-based and space observations, as well as a spacecraft flyby (Voyager 2, in 1989), have provided scientists with enough data to paint an intriguing picture of this moon. Inevitably, this has also raised additional questions.  

Triton was first discovered in 1846 by British astronomer William Lassell, who used a 24 inch (about 61 cm) aperture reflecting telescope. Over the following 143 years, Triton could only be studied from Earth, using increasingly sophisticated telescopes. In spite of the limited power of observation from Earth of such a distant and small object, Triton’s orbital parameters were already well established by the 1930s: specifically, it was known by then that Triton’s orbit around Neptune is quasi-circular (eccentricity: e = 0.000016) and retrograde, in other words, both its orbital motion and axial rotation are opposite to those of Neptune. 

Additionally, Triton’s orbit is also inclined by approximately 157° relative to Neptune’s equatorial plane. All these characteristics suggested that Triton did not originate around Neptune, but was a captured object. Since the discovery in 1992 of the Kuiper Belt - a trans-Neptunian disk comprising solar nebula primordial material, comets and dwarf planets, revolving around the Sun at distances between 30 AU and 100 AU - astronomers have indeed concluded that Triton originated in the Kuiper Belt and that its heliocentric orbit was disrupted hundreds of millions of years ago, with it being eventually captured and bound in orbit around Neptune. 

Triton’s composition must thus reflect that of Kuiper Belt’s material, and must, therefore, comprise a mix of rock and ice. This model for the origin of Triton is indeed consistent with the results of infrared spectroscopy telescope observations carried out during the 1970s and 1980s, which detected methane (CH₄) and molecular nitrogen (N₂), leading to the conclusion that ices of these compounds exist on the surface. 

To determine the rock/ice ratio and the distribution of mass in the interior of this object, accurate determinations of its dimensions and albedo (i.e., the fraction of incident light reflected by an object) are needed. Various attempts at estimating the diameter and albedo of Triton using ground-based telescope data were made, but the results were all model-dependent. Even so, by the 1980s it was generally agreed that its radius should be in the range 1300-2100 km, and, because of the presence of ice on its surface, that its albedo should be higher than that of “common” satellites. It was not until the spacecraft Voyager 2’s historic flyby of the Neptunian system however, that the physical properties of Triton could be measured.

The first scientific reports of the preliminary analyses and interpretations from Voyager 2’s observations, published in 1989 in the 15^th December issue of the journal Science (vol. 246, issue 4936), had Triton’s provisional radius at 1,350 km (± 5 km), its mass at 2.141 x 10²² kg, and the density calculated from these parameters at 2,075 kg x m^-3. 

Further processing and refinement of the measurements led to the values of 1,353.4 (± 0.9) km (radius), and 2,059 (± 0.005) kg x m^-3 (density). Furthermore, data acquired during Voyager 2’s flyby showed a tenuous but extended nitrogen atmosphere (with minor contents of methane, possibly highly localised), exerting a surface pressure as low as 14 micro-bar (equivalent to 1/70,000th of the Earth’s atmospheric pressure) and ranging in temperature between 38 K (-235.15℃) close to Triton’s surface, and 95 K (-178.15℃) in the upper atmosphere. 

Almost a decade after the flyby, scientists at the Massachusetts Institute of Technology (MIT) and their colleagues, observed the occultation by Triton of star Tycho 651672 (also known as GSC6321-01030 and Tr180) using the Hubble Space Telescope (study published in 1998 in the June 25^th issue of Nature, vol. 393). They concluded that Triton’s temperatures and atmospheric pressure have increased slightly since its encounter with Voyager 2, thus supporting earlier predictions of significant seasonal changes on Triton.

Triton’s density is consistent with its captured origin. The reported value suggests its rock/ice ratio may be in the range 65-70/35-30, indicating this moon was not formed around Neptune, because the pressure around the latter would have favoured chemical reactions between nebular components leading to the formation of large quantities of ices (which would result in lower rock/ice ratios). Knowing Triton’s density also provides clues about the structure of its interior: models indicate that Triton has a stratified structure, with the rocky (denser) fraction concentrated in the centre of the satellite. 

Calculations based on a number of initial assumptions and plausible evolutionary scenarios suggest that Triton’s rocky spherical core has a radius of 950-1000 km, and is surrounded by a concentric ice mantle 353-403 km thick (subtle differences in the estimate of the depth of the rock/ice boundary depend on the model parameters that different groups of scientists adopt. Additionally, some models also suggest that a metallic core forms the innermost sphere of Triton). 

The models generally predict that the ice shell (primarily composed of H₂O) may be subdivided into external and internal layers, bound by a liquid NH₃-H₂O ocean potentially up to 130 km deep. High contents of ammonia in the ocean may prevent it from freezing at present, but the models also call for heating processes to explain the melting of ice and formation of the liquid layer. 

The exact timing and mechanism of Triton’s capture by Neptune has not been fully established, yet. The scientific consensus converges on two main models, however: collision with a Neptunian satellite, which would have slowed Triton binding it to Neptune’s orbit; or encounter with Neptune of a Triton-planetesimal binary system, with the planetesimal being replaced by Neptune and expelled from the binary (as proposed in a seminal paper published in 2006 in the 11^th May issue of the journal Nature, vol. 441).

Regardless of the specific capture process, Triton’s initial orbital path around Neptune would have been highly eccentric (i.e., elliptical), which would have resulted in catastrophic tidal forces generating extreme heat. The heat thus produced was probably responsible for partial melting and fractionation of the rocky component from the icy material, with the denser rocky fraction sinking at the centre of the body, forming the core, and the lighter components forming the outer icy shells. The dissipation of the tidal energy would have eventually caused the progressive decay of Triton’s orbital eccentricity and the circularization of its orbit, a process possibly aided by Triton’s destabilization of the orbits of pre-existing Neptunian moons and their consequent collision and mutual destruction. 

The timing of circularization of Triton’s orbit is not clear, however: this is a critical detail when attempting to model the existence, depth, thickness, and composition of Triton’s sub-ice global ocean, as well as to understand the geodynamic regime responsible for Triton’s intense geological activity (described in the following article). Unless the process of orbit circularization was completed in very recent (geological) times, the generation of tidal heat would have ceased a long time ago, and a different source of heat would have been necessary to melt layers of ice to form a subsurface global ocean. 

It has been suggested that the heat produced by radioactive decay of the radiogenic isotopes of the elements uranium, thorium and potassium, probably abundant in Triton’s rocky interior, could be responsible for maintaining a high heat flux, but all scenarios currently being explored are model-dependent: it is plausible that the formation of the global ocean and its persistence through time is due to a complex interplay of residual heat from tidal dissipation, radiogenic heat, thermal insulation by overlying ice, the heat released during crystallization and migration of the ocean through the icy planetary shell, tidal friction within the ice, and the presence of ammonia in the water.   

The presence of an ocean on Triton opens up the exciting possibility that this far away world may host life. It is indeed listed as a candidate ocean world by NASA’s Outer Planets Assessment Group (OPAG) Roadmaps to Ocean Worlds (ROW) group, whose brief is to establish the scientific framework for the definition and exploration of ocean worlds in the solar system, and their potential habitability (for more details on the ROW group’s roadmap, read the open-access paper by co-leaders Hendrix,  Hurford, and the ROW group at: https://doi.org/10.1089/ast.2018.1955). Ocean worlds and their astrobiological significance will be the topics of future articles in this series. 

In this feature, I have briefly outlined the principal characteristics of Triton that could be learned from telescope observation, complemented by data collected during Voyager 2’s flyby. It is clear that Triton’s origin as a Kuiper Belt object, and its subsequent capture into Neptunian orbit, have largely determined its evolution, including the possible formation of a subsurface ocean. New missions of exploration are needed to further our knowledge of the geological evolution and astrobiological potential of this moon, however, and it is possible that one may be launched in the near future. In the following article, I will outline what is presently known of the geological complexity of this remote world, and what we hope to learn from future space missions.

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