Trident: A Mission to Explore Triton, a Candidate Ocean World

Triton: Scientific Motivation

Neptune’s moon Triton is one of the most interesting and surprising targets observed by Voyager. During its distant flyby in 1989, Voyager 2 captured a series of images, mostly of the southern, sub-Neptune hemisphere, establishing Triton as one of a rare class of solar system bodies with a substantial atmosphere and active geology. It is believed that Triton began as of a Dwarf Planet originating in the KBO, and subsequently captured by Neptune. It nor exists as Neptune’s icy satellite, and is subject to a tidal, radiolytic, and collisional environment. It is this duality as both captured dwarf planet and large icy satellite that has undergone extreme collisional and tidal processing that gives us a unique target for understanding two of the Solar System's principal constituencies and the fundamental processes that govern their evolution. Thus, comparisons between Triton and other icy objects will facilitate re-interpretation of existing data and maximize the return from prior NASA missions, including New Horizons, Galileo, Cassini and Dawn.

Triton’s surface and atmosphere are remarkable, but poorly understood. They hint at on-going geological activity, suggesting an active interior and a possible subsurface ocean. Crater counts suggest a typical surface age of <10 Ma (Schenk and Zahnle, 2007), with more conservative upper estimates of ∼50 Ma on more heavily cratered terrains, and ∼6 Ma for the Neptune-facing cantaloupe terrain. This implies Triton’s surface is young, almost certainly the youngest of any planetary body in the solar system (except Io). The lack of compositional constraints obtained during Voyager, largely due to the lack of an infrared spectrometer, means that many of Triton’s surface features have been interpreted as possibly endogenic based on comparative photogeology alone. Candidate endogenic features include: a network of tectonic structures, including most notably long linear features which appear to be similar to Europa’s double ridges (Prockter et al., 2006); several candidate cryovolcanic landforms (Croft et al., 1995), best explained by the same complex interaction among tidal dissipation, heat transfer, and tectonics that drives resurfacing on Europa, Ganymede, and Enceladus; widespread cantaloupe terrain, unique to Triton, is also suggested non-uniquely to be the result of vertical (diapiric) overturn of crustal materials (Schenk and Jackson, 1993); and several particulate plumes and associated deposits. Despite an exogenic solar-driven solid-state greenhouse effect within nitrogen ices being preferred initially (Kirk et al., 1990), some are starting to question this paradigm (Hansen and Kirk, 2015) in the context of observations of regionally-confined eruptive on the much smaller Enceladus.

Endogenic heating is likely, with radiogenic heating alone possibly providing sufficient heat to sustain an ocean over ~4.5 Ga (Gaeman et al., 2012). Orbit capture (e.g. McKinnon, 1984; Agnor and Hamilton, 200)) would have almost certainly resulted in substantial heating (McKinnon et al., 1995). The time of capture is not constrained, but if sufficiently recent some of that heat may be preserved. Triton’s orbit is highly circular but with a high inclination that results in significant obliquity, which should be sufficient to maintain an internal ocean if sufficient “antifreeze” such as NH­₃ is present (Nimmo and Spencer, 2015). Confirmation of the presence of an ocean would establish Triton as arguably the most exotic and probably the most distant ocean world in the solar system, potentially expanding the habitable zone.

Even if activity or an ocean is not present, Triton remains a compelling target. Its atmosphere is thin, ~1 Pa, 10^-5 bar, but sufficient to be a major sink for volatiles, and sufficiently dynamic to play a role in movement of surface materials. Its youthful age implies a highly dynamic environment, with surface atmosphere volatile interchange, and potentially dramatic climate change happening over obliquity and/or season timescales. An extensive south polar cap, probably mostly consisting of nitrogen which can exchange with the atmosphere, was observed.

A north polar cap on Triton was not detected (in part due to a lack of high northern latitude coverage). However, not even the outer limits of such a structure were detected, implying a north/south dichotomy. Methane in Triton’s atmosphere and on its surface makes possible a wide range of “hot atom” chemistry allowing higher order organic materials to be produced in a similar albeit slower manner to Titan. The presence of such materials is of potential importance to habitability, especially if conditions exist where they come into contact with liquid water.

Trident Mission Concept

We were selected by NASA for Phase A study in February 2020, baselining a New Horizons-like fast flyby of Triton in 2038. The mission concept uses high heritage components and builds on the New Horizons concepts of operation. Our overarching science goals are to determine: (1) Determine if Triton has a subsurface ocean or had one in recent history; (2) Understand the mechanisms by which Triton is resurfaced and what energy sources and sinks are involved; and (3) Investigate the diversity, production, and distribution of organic constituents on Triton’s surface. If an ocean is present, we seek to determine its properties and whether the ocean interacts with the surface environment. To address these questions, we propose a focused instrument suite consisting of:

(1) a magnetometer, primarily for detection of the presence of an induced magnetic field which would indicate compellingly the presence of an ocean;

(2) a camera, for imaging of the mostly unseen anti-Neptune hemisphere, and repeat imaging of the sub-Neptune hemisphere to look primarily for signs of change;

(3) an infrared imaging spectrometer with spectral range up to 5 um, suitable for detection and characterization of surface materials at the scales of Triton’s features.

References

Agnor and Hamilton, Nature 441, 192–194, 2006.

Croft et al., Neptune and Triton. Univ. Arizona Press, pp. 879–947, 1995.

Gaeman et al., Icarus 220, 339–347, 2012.

Hansen and Kirk, In Lunar Planet. Sci. XLVI, Abstract #2423, 2015.

Kirk et al., Science 250, 424–429, 1990.

McKinnon and Leith, Icarus 118, 392-413, 1995.

McKinnon, Nature 311, 355-358, 1984.

Nimmo and Spencer, Icarus 246, 2-10, 2015.

Prockter et al., Geophys. Res. Lett. 32, L14202, 2005.

Schenk and Zahnle, Icarus 192, 135–1492007.

Schenk and Jackson, Geology 21, 299–302, 1993.