Unless there are some surprising events (always a possibility), this will be my last post on what passes for volcanic activity on other solid bodies in the solar system. This series grew out of science post several years entitled Volcanoes as Heat Engines, which made the case that something that looks like a volcano or related thermally driven system (geysers, jets, hydrothermal systems, etc.) have been found on every reasonably large body with a solid surface in the solar system. If you include the comets, the number of these sorts of bodies expands vastly.
The larger bodies range from Earth and Venus down to the Galilean Moon Io and even to tiny Enceladus around Saturn. Systems that look like volcanic systems are found where it is really hot like Mercury and Venus, and very, very cold with a pair of structures that look like shield volcanoes on distant Pluto.
As you go farther from the sun, the erupted material changes from melted rock to melted ices. Indeed, as you get really far away from the sun, what we know of as water ice becomes the very rock which the farthest bodies are built of. We have seen this on Pluto, Saturn’s largest moon, Titan, and around 43 years ago with the first close inspection of Neptune’s largest moon Triton. Like every single other body, we have taken a close look at in the solar system, there were some surprises.
This post is about one of those surprises.
The Outer Solar System
The outer solar system is very far away, very cold, and very dark. But it is populated with numerous bodies, some of which are quite large. The closer group of bodies is referred to as Kuiper Belt or trans-Neptunian objects (KBOs or TNOs). The more distant population is referred to as the Oort Cloud.
The Kuiper Belt is thought to be a donut shaped ring of bodies, hundreds of thousands with diameters over 100 km, perhaps trillions of smaller bodies, all thought to be debris from the formation of the solar system. Its inner edge is at the orbit of Neptune, around 30 AU (AU = Astronomic Unit, the distance from the Earth to the sun) and extends to 50 AU. An extended disk may extend to 1,000 AU.
It is thought to be the source of most comets, and there seems to be a gravitational conveyor belt that delivers large bodies into unstable orbits between Saturn and Neptune. These are called Centaurs. As their orbits are unstable at that location, some are ejected from the solar system. Some are ejected into the inner solar system where they either pass through and return to the outer solar system or hang around for a while. Gravitational interactions with Jupiter, will occasionally inject these bodies into orbits that cross those of the inner planets, which becomes a potential problem for those of us who make this part of space our home. The last visitor from the Centaurs is thought to be Comet 2013 AZ60. It was 40 km in diameter.
These relatively large, icy bodies are frozen examples of what was going on in the outer regions of the solar system during its formation. As such they are prime targets for exploration. What are they made up of? What is their history? How did they come to be? How did they age?
To date, only three have been closely observed, Triton by Voyager 2 in 1989, Pluto – Charon by New Horizons in 2015, and Ultima Thule by New Horizons in 2019. All visits were high speed fly-bys with little time spent in the vicinity of each target. One of them, Triton was volcanicly active at the time of the visit.
Farther out is the Oort Cloud. This a theoretical collection of gravitationaly bound icy bodies stretching from 2,000 to as far as 200,000 AU (0.03 – 3.2 light years). The inner portion is a disk while the outer portion is a thick sphere around the sun. The small, dark, distant bodies are generally too far away to be found so far, but there may have been a couple discovered to date.
One interesting observation about the Oort Cloud is that if these bodies, this stuff is the leftover, the detritus of the formation of the solar system, similar structures should exist around other stars. Stars orbit around the center of the galaxy in individual orbits. It takes the sun about 250 million years to make one trip. Other stars have other orbits. This means from time to time; some stars get closer and then farther away. Today, the closest star is Proxima Centauri at 4.2 light years. The Algol system approached within 9.8 light years 7.3 Ma. Zeta Leporis was within 4.2 light years 861,000 years ago. Ross 248 is expected to get within 3 light years 33 – 42,000 years from now.
As Oort and Kuiper bodies are gravitationaly bound, a passing star will not only nudge their orbits, but the passing stars could exchange members of their Oort and Kuiper equivalents with ours. The more distant from the sun, the looser they are bound to the sun, and the less of gravitational nudge it will take to change its orbit. Get close enough, and a lot of these bodies have their orbits sufficiently changed, so they fall down the gravity well toward the sun. And this has happened on an irregular basis since the formation of the this and other solar systems.
KBOs and TNOs (other than Pluto) are a relatively recent discovery with the first search starting in 1987. The first object was discovered in 1992. To date, more than 1,000 of these objects have been discovered, at least one of which is larger than Pluto. One estimate is that there are at least 100,000 KBOs larger than 100 km in diameter. Spectroscopic analysis of these suggest some of the surfaces are similar to that of Pluto and Triton.
As an aside, the discovery of similar large bodies in the same part of the solar system led directly to the designation of Pluto as a minor planet. There are at least 8 known bodies with diameters larger than 600 km among the KBOs with more likely to be discovered. Pluto ends up being the first known member of this population rather than a unique planet.
There has long been speculation about the presence of a large body in the outer solar system. Indeed, the search that led to the discovery of Pluto itself was based on what at the time was unexplained variations in the orbits of Uranus and Neptune. It turned out that what was finally found, Pluto, was not massive enough to really move anything as large as the two outer gas giants and the orbital perturbations its orbit was predicted on were based on mass estimates for Uranus and Neptune that were not cleared up until the Voyager 2 flybys. Essentially, the discovery of Pluto in 1930 was an accident, though it had shown up on photographic plates multiple times since 1909.
But the search for large bodies in the outer solar system continues. The current search is for a hypothetical Planet Nine. This one is based on clustering of orbits of extreme TNOs. If it exists, it might be quite large at 5 – 10 times the mass of Earth. Earlier this year, there was a claim that the orbital clustering could be explained by simple gravitational interaction of the TNOs, meaning that this search may be unnecessary. As of this writing, both the search and the argument over dueling orbital simulations continue – which they should.
Neptune is the largest, most massive object in this part of the solar system. As such, its gravitational field governs what happens in the inner part of the Kuiper Belt. The larger objects have orbits that have interacted with Neptune over time. One of these is the Pluto – Charon system. Another is Neptune’s largest moon, Triton. An orbital resonance of 2:3 means that the inner body goes around the sun three times while the farther body only makes two trips. There is a group of trans-Neptunian objects with a 2:3 orbital resonance with Neptune called Plutinos. We know of at least 20 of them so far. There are other bodies with different orbital resonances with Neptune.
In order to set up a resonance with Neptune, each of these objects had to gravitationaly interact with it over time. The early solar system was a chaotic place, before these gravitational resonances had a chance to be created. For example, the Pluto – Charon system looks to be a double planet. Some of its smaller moons look like shards left over from some sort of a collision.
Triton is the seventh largest moon in the solar system at 2,700 km. It is the largest moon orbiting Neptune. Its orbit is almost circular but tilted in inclination from the plane of Neptune’s equator. It is also tidally locked, with one face always facing Neptune. Triton makes an orbit of Neptune in just under six days.
Strangely, it is in a retrograde orbit. It is the only large moon orbiting any planet in retrograde. This means that it was gravitationaly captured by Neptune in the early days of the solar system. There are other retrograde moons known, but for the most part these are distant moons of the giant planets. All are relatively small. All are irregular (non-spherical) in shape.
This is where things get odd, as Triton looks like an interloper, something captured by Neptune during or after a close encounter. Clues for this are everywhere if you know what to look for. One clue is the circular retrograde orbit. Another is the inclination of that orbit relative to Neptune’s other satellites and ring system. Final clue is the remaining moons around Neptune. Neighboring Uranus has 27 known moons and a regular ring system. Neptune has only 14 and the ring system appears to be very young and has at least two ring arcs. One of the moons, Nereid is in a highly elliptical orbit.
The capture and circularization of Triton’s orbit around Neptune is thought to have disrupted the original system of moons, colliding with one or more, ejecting some from Neptune’s orbit, and sending some into Neptune itself. Some simulations suggest that about 10% of today’s Triton’s mass comes from the original moons.
Orbital simulations for this work out best with a binary pair of large objects having a close encounter with Neptune. One of them continued on its way back into the outer solar system. The other was captured.
The process of capture made a mess of the original satellite system around the planet. The process of orbital circulation introduced tidal forces that flexed Triton, not unlike what we saw with Io, and liquefied some unknown amount of interior ices. Those ices are now erupting as the geysers of Triton.
Orbital dynamics are not finished with Triton just yet, as its orbit around Neptune is unstable. It is slowly spiraling into the planet and expected to get close enough to be tidally disrupted 1.4 – 3.6 billion years from today.
I previously noted that spectroscopic data of Triton looked similar to the Pluto – Charon system. This, plus the odd, unstable orbit led to the conclusion that Triton is a captured KBO / TNO.
Surface of Triton
Due to the distance and the instrumentation available at the time, Triton, Pluto and the other objects in the outer solar system were only visible as faint dots on photographic plates. Triton was the first one visited. And that visit found active volcanic activity, one of only three planetary satellites known to be active. All detailed knowledge of the surface of Triton comes from a single pass by Voyager 2 at 40,000 km at its closest approach. Only 40% of the moon was imaged during this encounter.
Triton is relatively flat, with vertical development on its surface being around a kilometer. There are a few impact craters mostly concentrated on the leading hemisphere. Analysis of crater distribution suggest that Triton’s surface is very young. Estimates of age range from 6 – 50 Ma.
Triton has a chemical composition similar to that of Pluto. 55% of its surface is covered with frozen nitrogen, 15 – 35% water ice, and dry ice the remaining 10 – 20%. Trace amounts of methane carbon monoxide and ammonia are believed to exist. There are deposits of tholins, which we have seen on the Pluto – Charon system and Ultima Thule. Tholins are complex organic solids formed by the action of ultraviolet light on gasses (methane, ethane, ammonia, water, hydrogen sulfide, molecular nitrogen and formaldehyde).
Its density suggests that the interior has differentiated between a rocky core, an icy mantle and a crust. There is enough rock in the core capable of radioactive decay which in turn powered some sort of convection in the crust. Tidal flexion after capture may have revived or extended that time of activity. This may or may not have been able to maintain a subsurface ocean for some unknown amount of time.
University of Maryland researchers suggested in 2012 that there is currently a subsurface ocean on Triton. There are several reasons for this. They include tidal heating, radioactive decay in the rocky core, residual energy from its encounter with one or more of Neptune’s original satellites, and the presence of ammonia and salts in the water which significantly lower its freezing temperature.
Large parts of Triton have been volcanicaly resurfaced. Smooth plains, volcanic calderas and irregular pits are visible on portions of the moon. Smooth plains and irregular pits are associated with volcanism on Io, Mercury and the moon.
The strangest and likely the oldest terrain is described as “cantaloupe” terrain, because it visibly resembles the skin of a cantaloupe. It is a large swath of closed depressions 30 – 50 km wide separated by ridges. One theory is that this is the surface representation of diapers of molten ice penetrating the surface layer, which in turn suggests that Triton has a layered crust.
Triton is considered as one of the coldest places in the solar system. This low temperature allows mixed ices to act a lot like what we see as silicic volcanism here in Earth. This is called cryovolcanism. Triton has two forms, large scale cryovolcanism that has resurfaced much of the surface photographed by Voyager 2 and active geysers.
It is the active geysers that are the biggest surprise. In these, some mixture of water, ammonia and nitrogen gas erupt from the surface sending black plumes up to 8 km above the surface of the moon.
The volcanic activity may renew a very thin nitrogen atmosphere 8 km deep. There is a haze composed of hydrocarbons and nitriles created by the action of sunlight on methane. We have seen a similar haze on Pluto and a much thicker one on Saturn’s moon, Titan. There are methane clouds from time to time between 1 – 3 km above the surface.
Active volcanism in the form of geysers was discovered on the southern hemisphere of Triton, where multiple active plumes were spouting black gas to an altitude of 8 km. The plume capped out there and the black smoke / gas / aerosols / dust were visible for several hundred kilometers downwind. The surface under the plumes was dusted with fall from whatever was erupted, not unlike what we see with ashfall here on Earth. The nitrogen plume eruptions are distinct from Triton’s larger scale cryovolcanic activity.
All observed geysers are located on the part of Triton’s surface that currently receives most sunlight. This led to the notion that solar heating even at this great distance, plays some sort of role, perhaps a crucial one, in triggering or powering the geysers. This has been modeled as a solid-state greenhouse, via which incoming sunlight penetrates some distance into the surface ice, boiling off nitrogen ice mixed with crustal ice which then makes its way to the surface.
Between 1977 and the Voyager 2 flyby in 1989, Triton shifted from a reddish color like Pluto to a far paler hue. This suggests that lighter nitrogen frosts either erupted or deposited from the atmosphere are actively changing the color of the moon.
Larger scale volcanism on Triton is complex and extensive. There are a great variety of volcanic types identified, suggesting a complex internal geochemistry and prolonged history of activity.
The most common volcanic units are at low latitudes on the leading hemisphere (mostly because the trailing hemisphere was not photographed). These include chains of rimmed volcanic pits, volcanic caldera-like complexes, and extensive smooth plains. These are related to linear fracture zones. The volcanic style appears to be similar to flood basalts observed on Earth, the moon and Mercury. Flows are visible, may be as wide as 80 km and 220 m or less thick. These overlap into the cantaloupe terrain and partly obscure it.
The region of overlap into the cantaloupe terrain has visible individual flows with distinct flow fronts. Width varies from 10 – 15 km to narrow flows 20 – 40 km long. There is at least one mountain that looks like a breached volcanic cone. There are at least two ring complexes that may be related to caldera formation. The largest of these is 100 x 130 km. There are narrow ridges and domes along the rings, similar to resurgent domes seen on Earth.
There are numerous flat-topped mesas within the transition zone. These appear to be older than the volcanic flows and may also be volcanic in nature.
Triton has been resurfaced to a considerable depth. Most craters are pristine and there are no examples of relaxed impact craters.
Linear ridges and troughs are visible over nearly the entire imaged face of Triton. Individual ridges extend from several hundred to at least 2,000 km. Widths vary from 10 – 30 km, and heights to a few hundred meters. They show up as parallel and single ridges. The interpretation is of a dike extrusion. There is at least one lobate flow connecting directly to one of the broad ridges.
There are four quasi-circular structures with smooth floors bounded with inward-facing scarps. Smooth material of the floor embays all irregularities, suggesting low viscosity of this material. They resemble large caldera like Kilauea or Olympus Mons. Circular potions of these are 150 – 200 km in diameter comparable to the theorized thickness of the underlying crust.
Collapse caldera are similar to terrestrial volcanic structures and associated with irregular pits and chains of pits. These are quasi-circular depressions with smaller pits and depressions within. They are surrounded by quasi-circular deposits of smooth materials that extend 100 – 200 km from the main depressions. There is a strong correlation between pit chains along extensions of visible surface fractures and failure zones, not unlike what we see on Earth. Some of these deposits have feathered edges and terrain softening, suggesting lofted deposits
Dark lobate flows are very dark that occur in small, sub-circular to elongated patches a few kilometers wide and up to 10 km long. Individual patches appear as dark domes and lobate tongues in the hollows of very rough cantaloupe and patchy smooth terrain. They are reminiscent of viscous dacite extrusions on Earth.
Dark spots, on the other hand have no parallels anywhere else in the solar system. They are oval to irregular spots with smooth, convex upturned edges scattered 20 – 40 degrees south. They range 30 – 80 km in diameter and look like shapes of water splashed across a oil-covered surface. Most if not all are associated with a bright border of lighter material. They are interpreted as being extrusive, with the borders of fairly high viscosity thermally modified crustal materials.
The four bodies we have seen up close in the outer solar system are surprisingly active. The three largest ones, Pluto, Charon and Triton all have evidence of some sort of tectonic activity. On Pluto and Triton that activity looks to be currently underway. All three of these bodies may have had some sort of subsurface ocean. Oceans may still exist on Pluto and Triton. When you have tectonic activity and a suspected subsurface ocean, there must be sufficient internal heat to power that activity. And that heat will in turn power volcanic activity. I would expect more surprises from the bodies in this part of the solar system as we learn more about them.