Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.
What Is a Volcano?
On Earth, a volcano is a place where magma from below the surface can escape from the interior of the Earth and flow onto the surface as lava. Simple enough, and it doesn’t have to be explosive like we visualize it to be (but face it, it’s pretty cool when it is).
These outflows can happen where plate tectonics meet or in the middle of one, known as a mantle plume. The heat source for all of this comes from within via the radioactive decay of elements, stemming all the way from the formation of the solar system.
While other places in the solar system also got these elements too, for many reasons most are not active. And yet, other places have what we would call volcanoes but would be shocked by how they operate. It is unlike anything we have on Earth… (Starkey 44-5)
Venus and Mars
The key element for an object to have volcanism like we do is size. There simply has to be enough radioactive material to generate the heat required to make materials molten. Mercury is too small to have this, and Mars is, too. But once long ago it did have volcanism and currently has the largest known volcano ever.
Olympus Mons is 25 kilometers tall (that is, 2.5 Mt. Everests in height) and likely erupted 20 to 30 million years ago. But Venus, practically being the same size as Earth, has volcanoes going to this day and in fact, helped drive its runaway greenhouse effect (45-6)
And past the rocky planets, we have the asteroid belt and the gas giants. When the solar syst3em formed, lighter elements were out here and so nothing really gathered sufficient radioactive materials to drive an internal molten core. Or so we thought…
This dwarf planet was first visited by the Dawn spacecraft in 2005 and offered plenty of mysteries right away. One of the most intriguing was Ahuna Mons, a 3-mile-high mound of material that looked strangely like a pyramid from Egypt. It just seems to have been plopped there in the middle of a flat piece of terrain, not really hinting at a remnant from an impact crater or from internal heat pushing outward.
But then scientists started to think of Ahuna Mons not as a volcano but a cryovolcano, which doesn’t release hot material but cold (Grenoble, Betz "Dawn" 47, JPL "Dawn's First," Klesman "The Case," Wenz "Ceres," Coral 31-2, Starkey 47).
Other cryovolcanoes likely existed on the planet but as the liquid material froze into a solid over the years, the ground beneath it began to shrink into the dwarf planet. Ahuna Mons was just one of the last ones to die away (last erupting roughly 200 million years ago), hence why we still see it. As far as what could have heated Ceres out there, no one knows (Ibid).
When the Voyager probes went out and explored the gas giants, scientists knew they were in for a treat. Finally, so many things seen from the small scope of our planet would be revealed in new detail. But not many expected these places to be so active.
Take Io for example. Voyager found this moon of Jupiter to have 400 volcanoes on it! Io’s surface is constantly being regenerated because of this, with some of the eruptions extending to 100 kilometers above the surface of the moon. Loki Patera is a 200-kilometer wise lava lake that still gets fed new material (Starkey 46).
It’s a hellish place, but surely it has to be too small to have enough radioactive material to heat it. This is where a new heat source called tidal heating comes into play. Its frictional forces are created by gravity tugging on an object, and in Io’s case, it has Jupiter and all its other moons pulling on it in different directions. This generates tremendous amounts of heat and drives its amazing volcanism (Ibid).
Then the Voyager probes visited Enceladus and found a nice pristine surface, something that an active object would have. Decades later, the Cassini probe reveals this moon’s secret: Huge plumes emanating from the south pole containing water, salt, ammonia, propane, methane, and other organic material (pointing to a hydrothermal source of energy delivering the organic material).
But unlike traditional volcanoes, these emanations come from areas on the surface where the ice sheets on the surface act like the tectonic plates on Earth which overlap. Once again, tidal heating provides all the energy I need to get a liquid material (via hydrothermal vents on the ocean bottom), with the ammonia allowing the water to exist as a liquid in the -124 F environment it exists in (Grant 12, Johnson "Enceladus", Douthitt 56, Betz "Curtains" 13, Postberg 41, Scharping, Klesman, Starkey 46-7).
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Voyager 2 was the only probe so far to visit Uranus and Neptune, and while at the latter ice giant observations were taken on Triton, an odd retrograde moon. Imagine everyone’s surprise when this distant object was spotted to be ejecting nitrogen plumes up to 8 kilometers in height! And the surface of Triton was found to be very young, around 10 million years old.
Spotted on the surface was Leviathan Patera, an 80-kilometer-wide volcanic dome with connections to long-dead cryolava lakes. Also, it was mottled like a cantaloupe, possibly showing signs of diapir action, caused by a weak surface being intruded upon by a hot interior. At first, it was theorized that the Sun heated Triton’s surface and material sublimated into space, but solar radiation is too weak out there. It is also too small for those radioactive elements to be in large quantities, too (Starkey 47, Redd).
Based on its chemical composition and weird orbit, Triton is likely a captured Kuiper Belt Object, and the tidal forces it experienced during this must have been great. But this was billions of years ago, so that heating should have long since dissipated. Well, it could actually be factors of all the above. Maybe some heat exists from radiation, maybe some exist from the tidal forces, and maybe some exists from solar radiation.
That latter source did get more steam after dark translucent material was spotted just below the surface of Trion. This could absorb solar radiation and potentially help drive an internal greenhouse effect. This, along with the other heat sources, would cause nitrogen gas to sublimate below the surface, building pressure up until released into space as the plumes (Ibid).
The distant dwarf planet was first visited by the New Horizons probe in 2015, and no one really knew what we would find once we arrived there. And wow, did it not disappoint. I have talked about many of the fascinating features in my other hubs, but amongst the intriguing things were smooth plains of nitrogen ice in the Sputnik Planun. Where did it come from? Well, Pluto could have intercepted comets, but the amount needed to impact would require 100’s of tons to be deposited (for that is the rate at which it sublimates off the surface). Besides, we would see more craters around Pluto than we do (Lewin, (John Hopkins 02 Jun, Timmer "Pluto's Sputnik").
But what if that material came from within? Something is driving nitrogen to the surface of the dwarf planet, where it cools and actually can sink back into the surface (owing to the interesting bouncy features of liquid and solid nitrogen). But some places in the plain are over 50 meters tall, and models show the plain can rise as much as 1 centimeter each year. This surface feature is at least 500,000 years old, meaning Pluto is a geologically active object (Ibid).
So where are the cryovolcanoes that would be further evidence of such activity? Pluto has several possible candidates including Wright Mons, Kubrick Mons, and Piccard Mons. They all have features reminiscent of shield volcanoes here on Earth and on Mars. Wright Mons, at 150 kilometers wide and 4 kilometers tall, has a rounded base that is also shallowed, as if the material below it condensed once the volcano went extinct (NASA "Pluto May Have," Berger "Volcanoes," Talcott "New," Stern "Hot" 32, Purdue, Stern "Puzzled" 26, Starkey 47).
So nitrogen plains indicate a young surface and collapsed volcanoes indicate extinction, but what is going on below the surface of Pluto to drive all of this? Well, it is too small to have enough radioactive material for the heat, and while it is in a practically double planet orbit with its major moon Charon, insufficient heat is created from tidal heating (Wenz "Pluto Becomes", Klotz "Pluto Has," Wenz "Pluto's Icy," Haynes, Wenz "Pluto's Slushy Heart," Starkey 48).
It may be possible that Pluto has a subsurface ocean with a high concentration of ammonia (5%, allowing for a viscosity 100,000 greater than that on Earth) which allows the material to have held onto any heat from Pluto’s formation, radioactive decay, and tidal heating to slowly drive a convective model. A roughly 124-mile-long dent in Pluto seems to have been filled with this material, supporting this idea, and density readings taken by New Horizons also point to a subsurface ocean at least 62 miles below (Ibid).
So what is the moral of this story? It would seem as though our traditional idea of a volcano is a rarity in our solar system. Most objects with it operate until chilly temperatures and use some nitrogen-ammonia-water mixture as the medium of transport. And some objects don’t even have traditional mounds upon which the material leaves. Clearly, what makes a volcano a volcano is a tough, evolving concept.
Earth Isn't the Only One With Volcanoes
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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Leonard Kelley