A volcano is defined, by NASA, as an opening on the surface of a planet or moon that allows material warmer than its surroundings to escape from its interior. While volcanoes are a generally thought of as a feature of Earth, there are plenty of other planets and even moons with a range of extinct, dormant, and active volcanoes in our solar system, and presumably in others as well. Once a feature has been officially defined as a volcano, it is then further classified into one of three designations, mons, tholus, and patera, based on their external presentation as defined by the International Astronomical Union, or IAU. Montes are any mountains, while tholi are generally hills or small domical mountains, and paterae are irregular or complex craters with scalloped edges. The term volcano is pretty vague and many different variations on the feature can be found on a variety of different celestial objects throughout the solar system.
There are four main types of volcanoes on Earth: cinder cone, composite, shield, and lava dome. Cinder cones are the simplest and happen when, over time, the lava that is spewed into the air rains back down and forms a cone with a bowl-like crater at the top. These volcanoes rarely peak much higher than 1,000 feet. Composite volcanoes are also known as stratovolcanoes and can reach thousands of meters tall. They have a system that allows for magma to be pulled from the mantle itself up to the surface, and often erupt violently, like Mount Saint Helens. Shield volcanoes are large and broad and resemble shields from above hence the name. The lava that pours out of these is thin, which allows it to travel easily down the shallow sides of the volcano, building up into many layers over thousands of small eruptions. Lava domes, or volcanic domes, are also caused by build up of lava over time, but this lava is thick, and the domes grow from the lava expanding within. These types are generally more specific to terrestrial volcanoes, but they also serve as a base point for understanding extraterrestrial volcanoes, especially shield volcanoes.
Volcanoes can be found on all four of the terrestrial planets, but most of those found outside of the Earth are presumed to be dormant or extinct. Mars hosts the largest known volcano in the solar system, Olympus Mons, which is 624 km wide, which is comparable to the size of the state of Arizona, and 25 km high, whereas Mount Everest is a little under 9 km. Most volcanoes on Mars are significantly larger than those on Earth, which can be attributed to its lack of tectonic activity. Terrestrial volcanoes are created by the shifting, and subsequent crashing, of Earth’s tectonic plates over top of a hot spot within the somewhat liquid mantle, and as the plates move over the hot spot multiple volcanoes are fed. Mars does not have tectonic plates or a liquid mantle, and so significantly more magma is spewed out of a singular volcano, which then piles up into a larger and larger volcano. Martian volcanoes are assumed to be extinct since the estimate for the most recent eruption on the planet was from Arsia Mons around 50 million years ago, at around the same time as the Cretaceous??“Paleogene extinction. Volcanic activity is an integral part of Martian history and composition, even if there has not been any recent activity.
Olympus Mons, the largest volcano in the solar system
Artistic Rendition of a volcano erupting on Mars
Arsia Mons, Mars’ last active volcano
Venus has three main types of volcanoes, two of which are found on other celestial bodies and one that is exclusive to Venus. The most prevalent volcanoes are shield volcanoes, but they tend to be wider and shorter than those on Earth and Mars, likely because of its dense atmosphere. Venus also has lava domes, but they are often referred to as pancake domes because they are significantly larger than those found on Eart, sometimes up to 100 times so. The volcanic feature that is found exclusively on Venus is called a corona, which is an upwelling of material from the mantle that contains and is surrounded by volcanoes and lava flows. There is plenty of evidence of past volcanic activity on Venus, but is also the only other planet in the solar system, aside from Earth, thought to be actively volcanic. Current volcanic activity on Venus would explain the frequent flux in sulfur dioxide above the clouds, as well as the bursts of radio energy that occasionally observed coming from the planet, but until some volcanic activity is observed, Venus cannot be classified as active.
Nasa Image of Sapas Mons, a Venusian Volcano
An Image of Venus’s Pancake Domes
A Computer generated view of a Venusian volcano from 1995
Mercury has not been volcanically active for around 3.5 billion years, but there is proof on the planet of volcanoes in its past. Mercury has a very small mantle, which is the part of terrestrial planets that produce the energy needed for volcanic activity through radioactive decay, and so it lost its internal heat much faster and its volcanism much earlier than the other planets. However, there are extensive smooth plains on Mercury, similar to the Luna’s maria, where, billions of years ago, lava flowed across the surface and filled in the depressions. These smooth plains prove that at one point there was volcanic activity on Mercury, even if it has long since ended.
The volcanoes on Luna, Earth’s very own moon, seem to have been dormant for about a billion years, but the maria covering a significant amount of the near side of the moon is proof that there once was volcanic activity. These seas are similar to large parking lots, paved over and flattened out by lava flows earlier in the Moon’s life. Their location solely on the near side of the Moon implies that the volcanic activity was impacted in some way by the Earth, and adds another layer to the relationship between the two celestial bodies. Recently there has also been the discovery of Irregular Mare Patches, or IMPs, which are smaller spots on Luna that appear to be the product of volcanic activity, but some seem to be as young as 50 million years old. These IMPs mean that there could have been active volcanoes on the Moon during the time of the dinosaurs, and changes our perspective on the evolution of Luna. This also means that the interior of the Moon is likely warmer and possibly less dead than we previously believed.
The darker sections are the maria, evidence of a volcanic history for Luna
A close up of a volcano on Io, taken from the Galileo spacecraft
Jupiter’s moon Io has a very colorful exterior due to its volcanism
Io is the most volcanically active object in the Solar system and thereby has the youngest surface, which is ironic when considering that its fellow Jovian moon Callisto has the oldest. Io’s volcanism is a product of its location, being the closest of the Galilean Moons to their host planet Jupiter. Tidal forces from Jupiter are competing with tidal forces from Ganymede and Europa, two of the other Galilean Moons that are in resonance with Io, and these fighting forces cause Io’s orbit to be highly eccentric. This eccentric orbit causes Io to flex as it revolves around Jupiter, and this flexing heats the moon in a process known as tidal heating. This heat is what allows for Io’s volcanism, although the specifics of how it works are still being researched.
There are also some places in space that have ice volcanoes, known as cryovolcanoes, that instead of spewing out molten rock erupt volatiles, which are chemical compounds and elements with low boiling points. Cryovolcanoes happen on items further out from the sun, where it is cold enough for their surface to freeze, and there are several theories on the different methods these objects use to keep their internal temperature high enough to contain liquid. One seen in most moons with cryovolcanoes is tidal friction that stretches the planet enough to warm its core and give it a liquid interior, but there are other possibilities as well.
Enceladus is a moon of Saturn, a possible home for life, and a host of what seems to be at least one cryovolcano. On its southern pole, the moon spews out a plume of mostly water from spots within its Tiger Stripes, which are four large, linear depressions on the surface of the planet. This liquid water is believed to come from an internal ocean under the icy outer layer of Enceladus, and it is believed that Saturn’s tidal friction is keeping it warm enough to remain in a liquid state. This cryovolcanic activity is actually the cause of Saturn’s E ring, as the water freezes into chunks of ice and begins to orbit Saturn along with Enceladus.
Enceladus, Saturn’s 6 largest moon
A false-color view of Enceladus’s plumes
Enceladus’ eruption as captured by Cassini
Titan’s thick haze and nearly opaque atmosphere make it seem featureless
A topographic computer model of Sotra Facula, an apparent cryovolcano on Titan
The south pole of Triton, Neptune’s largest moon
An artist’s rendition of volcanoes on Triton
Titan is Saturn’s largest moon, the second largest moon in the solar system, the only moon in the solar system with a substantial atmosphere, and is also believed to have cryovolcanic activity. The thick haze surrounding the planet and its nearly opaque atmosphere make Titan’s surface very difficult to study, but recent flyby missions have suggested that volcanic activity is a highly likely possibility. Volcanic activity would explain the continuous supply of fresh methane found in Titan’s atmosphere, and what seems to be lava flow surrounding mountains on the moon. Although there is currently no concrete evidence that Titan has volcanic activity, or that it is of the cryo variety, there is no concrete evidence to prove the reverse, either.
Triton is the largest moon of Neptune, and when Voyager 2 did a flyby of the moon in 1989 it discovered that it, too, has cryovolcanoes that are spewing nitrogen and ice 8 km into space. Little is known about the internal workings of these volcanoes because of the moon’s distance from Earth, but it has been observed being actively cryovolcanic.
When the New Horizons space probe was launched in 2006, astronomers expected to find Pluto and its moon Charon to be icy wastelands, just hunks of frozen rock floating out in the Kuiper Belt. They were shocked when they found a high level of geologic activity on the two objects, and evidence of possible cryovolcanoes. There are two obvious spots on Pluto that are being looked into as volcanoes, Wright Mons and Piccard Mons, which appear similar to Mars’ volcanoes. This is the first time features like this have been found on celestial bodies that are not terrestrial planets, and if they are indeed cryovolcanoes, this could change everything we know about them.
Pluto, taken by the New Horizons space probe
Wright Mons, a possible cryovolcano on Pluto
Charon, as taken by the New Horizons space probe
An Artist’s rendition of a cryovolcano on Charon
Volcanoes are found on celestial bodies all throughout the solar system, and as space continues to be explored, new types of volcanoes are discovered. They differ in every possible way, from size to eruption composition, to formation, but they all tell a story about an active and constantly evolving solar system, and by extension, universe.
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