Triton - Icy Moon of Neptune

View from Triton - artistic impression

Triton is a truly fascinating world.

The Pov-Ray model above, of a frozen moon orbiting an ice giant, was rendered using NASA/JPL's Neptune map from their Solar System simulator (https://maps.jpl.nasa.gov/neptune.html).

Neptune fills some 7.85 degrees of arc in Triton's sky, making it appear about 14 to 15 times as large as the Sun typically does from Earth (this can be calculated using trigonometry). Sunlight on Triton has about 0.1% of the intensity of sunlight reaching the Earth's upper atmosphere (this can be calculated from the inverse-square law) which would be perceived by the human eye as twilight. Neptune itself will of course look bright in Triton's sky since it is only some 354 759 km away.

Triton is an icy world with an extremely low surface temperature of around 33 to 38 K. At these temperatures, nitrogen freezes on the surface, forming a layer of translucent nitrogen ice overlying darker material. Organic tholins appear to be abundant, forming when methane ice is bathed in UV light from the Sun, forming more orange-red material. This article forms part of a series looking at the coldest of worlds (see also Pluto).

Inner satellites of Neptune

Above: a representation of the orbits of Neptune's first 8 moons and their near circular orbits. Neptune has 14 known moons, the remaining 5 outer moons being in much wider and elliptical orbits, suggesting they are more loosely bound. The largest moon of Neptune, by far, is Triton, shown here at twice its relative diameter as the eighth satellite and outermost in this diagram. Moving towards Neptune we then encounter the orbits of the seven inner satellites (ring moons) in order from outer to innermost as follows: Proteus, Hippocamp, Larissa, Galatea, Despina, Thalassa and Naiad. (The three innermost orbits are very close together). The outer moons are, from inner to outermost: Nereid, Halimede, sao, Laomedeia, Psamathe and Neso.

Triton is the only moon of Neptune large enough to form a spheroid. The other satellites are irregular. The next largest, Proteus, at 420 km in diameter, is just too small to pull itself into a spheroid - it's gravity is not sufficiently strong. Neptune is about 18 times the diameter of Triton.

Triton is the only one of Neptune's moons that is planet-like and although its orbit has circularized it is orbiting in the opposite sense to Neptune's rotation, suggesting that it is a captured object, rather than an object that formed along with Neptune, presumably Triton was a planet of the dwarf class captured from the Kuiper Belt.

Inner satellites of Neptune

Above: the orbits of Neptune's inner moons, rings and Triton. the relationship between the inner moons and the rings can be clearly seen. Proteus and Hippocamp are slowly be accelerated outwards, spirally away from Neptune. Triton and Proteus are both tidally locked into synchronous orbits around Neptune, meaning that they always present the same side to Neptune. Energy dissipated by tidal interactions slowed down the rotation of these moons on their axes until it matched the orbital period. (Tidal bulges result in gravitational torques that slow rotation).

A good example of a synchronous orbit is the Earth-Moon system in which the moon completes one orbit of the Earth at the same time it completes one rotation on its own axis relative to the Sun: this is the synodic period of about 29.5 Earth days. The tidal bulge on the Earth due to the Moon's gravitational pull lifts up material, particularly water in the oceans to form a tidal bulge. However, since the earth is rotating eastwards and it takes time for the materials to respond and be displaced in the bulge, the bulge is maximum slightly eastward of the center-line between the centers of the Moon and Earth. The gravity of the Moon pulls on this bulge (generating a torque), dragging the Earth westwards. As a result, the rotation of the Earth is gradually slowing, by about 0.002 s per century: the days are slowly lengthening on Earth. Since angular momentum (momentum due to rotary motion) is conserved, as the Earth's spin slows down, the orbital angular momentum of the Moon must increase by a corresponding amount - the Moon is slowly accelerating and spiraling away from the Earth. Thus, we see that despite the apparent harmony, the orbits of celestial bodies are not fixed but constantly evolving.

Rings of Neptune

Above: The rings of Neptune shown to scale. The rings are, from innermost to outermost: Galle, le Verrier, Lassell (the bright outer edge of Le Verrier), Arago (the bright outer edge of Lassell), an unnamed (incomplete?) ring in the orbit of the moon Galatea and the Adams ring. These rings are all faint and obscured by the glare of reflected sunlight from Neptune. The inner moons are at least partly responsible for shaping these rings.

Tidal forces are causing Galatea to decelerate and spiral inwards. Galatea is perhaps a shepherd moon of the Adams ring; Despina is the shepherd moon of the Le Verrier ring and is also spiraling inwards; Thalassa is spiraling inwards and is in a 69:73 orbital resonance with Naiad, the innermost moon. This means that Thalassa completes 69 orbits to every 73 orbits of Naiad. Tidal interactions between orbiting bodies, whether interacting as pairs or more complex arrangements, tend to displace objects from unstable orbits, placing them into stable resonances in which the ratio of the orbits is a ratio of two whole numbers. Similar interactions enable shepherd moons to keep gaps in ring systems clear and/or constrain the orbits of particles in the rings. Tidal resonances are important when orbits precess (when an elliptical orbit itself slowly rotates in the plane of the orbit over time) since this ensures objects eventually return to identical positions relative to one-another and hence the orbits are stable against gravitational perturbations.

These inner moons are irregular and could be fragments of one or more previous moons that became disrupted by the gravitational interactions between Neptune and Triton. Those that are spiraling inwards will either collide and plunge into Neptune's atmosphere or be tidally disrupted to form new rings.

Volcanism on Triton

The surface of Triton is geologically young and chiefly formed by volcanic extrusion of a mixture of water and ammonia ice (it is not clear that this ice is erupted as liquid, solid or slush). Leviathan Patera is a caldera-like feature 100 km in diameter with a volcanic dome 2000 km along its long axis, evidently an ice volcano and one of the largest volcanoes in the Solar System.

Nitrogen geysers are also abundant and have been seen erupting. They eject gaseous nitrogen mixed with dark dust being carried by the flowing gas.The South Polar cap consists of frozen nitrogen and methane and is studied with geysers. (The North Pole has not been observed as of 2021). The geyser jets reach a height of about 8 km from vents. At 8 km altitude the plume spreads into an almost horizontal dark cloud extending for hundreds of km in the East-West direction. During the Voyager 2 probe's flyby the geysers erupted from a region between 30 and 60 degrees South, whilst the Sun was directly overhead at 45o S, suggesting that solar heating is an important driving force.

Each geyser is active for an estimated 5-10 earth years on average; a Tritonian year is 165 Earth years. Triton has a very thin atmosphere, 800 km deep with a surface pressure about 0.001 % of Earth's atmosphere. This atmosphere is almost entirely nitrogen, with traces of methane, carbon monoxide and other gases. During winter, nitrogen frosts form on the surface of Triton. Triton has significant wind, some high altitude clouds of frozen nitrogen particles and photochemical haze.

A model for geyser formation involves a greenhouse effect (Kirk, http://adsabs.harvard.edu/pdf/1990LPICo.740...22K): sunlight passing through the translucent nitrogen ice is absorbed by darker material which traps the heat, causing warming between the darker material and the overlying nitrogen ice in a porous layer where nitrogen gas flows along channels driven by the heat. Nitrogen freezes below 41 K at zero pressure, so only a modest amount of heating is required if the surface temperature is 38 K. This effect is more efficient if this porous layer is shallow.

An area serves as the collector, feeding flowing gas to the geyser fissure or forcing it through a weak point in the ice crust to form a new fissure. Models suggest that channels of several meters in diameter must exist in the porous layer if the collection zone is to be around 50 m radius as observations suggest. it is speculated that contraction of the nitrogen ice as it cools in water could fracture the nitrogen ice into blocks (nitrogen contracts on freezing unlike water which expands). It is estimated that a 100 m diameter collection zone could supply enough nitrogen to maintain a geyser for 5 Earth years, evaporating 20 kg of nitrogen per second. Alternative models factor in tidal heating which might help accumulate reservoirs of nitrogen gas to feed the geysers.

 

Moon size comparison

Above: a size comparison of some large moons and small planets.

Triton - NASA/JPL

Above: Triton's southern hemisphere, photo by NASA/JPL (Voyager 2). The black streaks on the Sun-facing hemisphere are due to geyser plumes releasing dark dust.

Kuiper Belt and the origin of Triton

Triton is thought to have originated from the Kuiper Belt, a belt of large asteroid-like objects and dwarf-class planets extending from 30 to 50 AU around the Sun (1 AU, astronomical unit, is the average distance between the Sun and the Earth). Note, this is distinct from the scattered disc of objects lying from 30 to 100 AU and the proposed Oort Cloud at 2000 to 200 000 AU (0.03 to 3.2 ly) of loosely bound objects at the very edge of the Solar System.

Neptune itself is at about 30 AU from the Sun and hence perturbs the Kuiper Belt with its gravitational field: ejecting objects from unstable orbits. Stable orbits are those in orbital resonance with Neptune: meaning those orbits in which the orbital period shares a common factor with the orbital period of Neptune. For example, Pluto and Charon (objects from the edge of the Kuiper Belt orbiting at 29.7 to 49.3 AU) are in a 2:3 orbital resonance with Neptune: for every 3 orbits of Pluto, Neptune completes two. About 200 other objects share this resonance (hence Pluto can be said not to have cleared its orbit though in fact few planets do since objects can exist in stable 1:1 resonances at the Lagrange points - see Roche Potential and http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/lagpt.html). Other objects may exist in stable resonances of 1:2, 3:4, 4:7 and 2:5.

Does Triton have a subsurface ocean?

The possible ice volcanoes and obvious geologically recent remodeling of Triton's surface raises the question of conditions beneath the frozen crust. Is their a mantle of slush or a subsurface ocean of molten liquid?

The graphic below, prepared by NASA/JPL, shows a model of Triton. We have already discussed solar-heating models of geyser formation, though other sources of heat, such as tidal heating, may contribute.

Structure of Triton - model by NASA/JPL

Let us consider the question of whether or not there could be a liquid subsurface ocean inside Triton. First of all, if the surface temperature on Triton was about 40K, how much heat would be radiated into space?

Triton heat model 1

Note that J/s is equivalent to Watts, W. Let us do another 'back of an envelope' estimate for heat absorbed from the Sun by an illuminated hemisphere of Triton.

Triton heat model 2

An additional heat source worth considering is heat generated by the decay of radioactive isotopes in Triton's core. Consideration of the main isotopes contributing to heat generation on Earth, we can obtain a conservative estimate of 10-11 W/kg of generated thermal energy in the Earth's mantle (ignoring other sources of heat such as energy released by sedimentation of denser materials etc.). However, these metal isotopes are probably scarce in the light icy crust of Triton. Triton has a density of 2.061 g/cm3 compared to 5.514 g/cm3 for the Earth. This suggests that of Triton's 2.139 x 1022 kg of mass, only about 25% is a rocky/metallic core, the rest consisting of ices. Thus, a reasonable first estimate of heat generated inside Triton is: 0.25 x 2.139 x 1022 kg x 10-11 W/kg = 5 x 1010 W. Of course, heavier isotopes may not be present to the same extent in Triton as they are in the Earth, but this contribution to Triton's heat budget is much less than that radiated into space, hence Triton is dependent on solar radiation to prevent it from freezing solid. Let us assume a total heat energy input (generated + absorbed) of 2.9 x 1012 Js-1.

Let us assume that Triton is in a steady-state, certainly our energy input is very close to that output, and take energy in equal to energy out, with 1012 J/s being gained and lost. Next, assuming the surface temperature of Triton to be a cold 40K (or thereabouts) what thickness of insulating ice would be needed to maintain an internal liquid ocean at 0 oC? In fact if Triton did accumulate more internal heat than it radiated, then this might explain the occurrence of ice volcanoes to vent off accumulated excess heat, particularly in the geological past when Triton would have had more radioisotopes and have heat left over from its formation.

Triton heat model 3

Thus we are predicting an ice crust thickness of 10 km, however, we assumed a value of thermal conductivity for solid water ice, for want of a ballpark value. What if the crust was porous and contained pockets of nitrogen gas? The conductivity of nitrogen gas is very low at low temperatures (see: https://www.engineeringtoolbox.com/nitrogen-N2-thermal-conductivity-temperature-pressure-d_2084.html) with a value quoted of 7.253 x 10-3 Wm-1K-1. This much lower would give us a required minimum thickness of only 22 m. Clearly, a porous ice crust filled with nitrogen gas would offer much better thermal insulation and could be much thinner than 10 km.

What is interesting, is that unless my crude calculations have failed to factor something in important, we would indeed expect a liquid subsurface ocean on Triton and that ocean may even be warm. An icy crust would drop the surface temperature enough to reduce heat loss by radiation into space.

The explorations of the far reaches of the Solar System have proven far more interesting than anyone ever guessed. Far from finding simple balls of ice and frozen rock, outer dwarf-class planets (and moons derived therefrom) exhibit a remarkably active geology. I suspect they will continue to surprise us.