Sotra Facula is the best candidate yet for an ice volcano on Saturn鈥檚 moon Titan
A potential new ice volcano has been found on Saturn鈥檚 moon Titan.
Named Sotra, the volcano is nearly 1 kilometre tall and has a 1.6-kilometre-deep pit alongside it. Surrounded by giant sand dunes, it is thought to be the largest in a string of several volcanoes that once spewed molten ice from deep beneath the moon鈥檚 surface.
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鈥淲e think we have found the strongest case yet for an ice volcano on Titan,鈥� said , a geophysicist at the US Geological Survey in Flagstaff, Arizona. 鈥淲hat we see is not just a flow like we see in other places, it鈥檚 like a volcanic field would be on Earth.鈥�
Titan is about the size of the planet Mercury but has an atmosphere thicker than Earth鈥檚. This makes it incredibly difficult for astronomers to know what鈥檚 happening on the surface. Planetary scientists, including Kirk, are using NASA鈥檚 Cassini spacecraft to map the moon, but so far only about half of Titan has been imaged.
Kirk and his team created a 3D mapping technique that patches together multiple images of the same area, so they were lucky that Sotra was in one of the rare places imaged twice.
鈥淭he classical volcano everybody thinks of when you say the word is a mountain with a crater on it and lava flows coming out of it,鈥� said Kirk. 鈥淭hat鈥檚 what we鈥檝e found on Titan.鈥�
鈥楾his is it鈥�
The team cannot be certain if the chain is active, but described the find as the best evidence found so far for a cryovolcano, or ice volcano. Previously, bright spots seen in low-resolution satellite images have been interpreted as volcanic flows and craters. However, once those areas were mapped in 3D, it became obvious they weren鈥檛 volcanoes.
鈥淲e had noted Sotra Facula as a candidate cryovolcano before,鈥� said Rosaly Lopes at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淏ut it was only when Randy got the topography done that we realised, wow, this is it.鈥�
Earth鈥檚 interior is divided into distinct layers of rock and liquid magma. When this molten rock erupts through the planet鈥檚 crust, it鈥檚 known as volcanism. Titan鈥檚 volcanism is more complicated because beneath the moon鈥檚 surface lies a layer of ice. Even a small amount of internal heat could create molten ices. Because the liquid would be less dense, it would force its way to the surface. The result would be a massive eruption of slushy liquid and gases similar to what scientists have seen on other icy moons.
鈥淚ce at outer solar system temperatures is very rigid,鈥� Kirk said. 鈥淚ce at close to its melting point is soft. What would be a glacier on Earth would be a volcano on a body that鈥檚 made of that same material. It鈥檚 the difference between the cake and the frosting.鈥�
Methane source
Some have theorised that volcanoes on Titan are the best way to explain the strange abundance of methane gas in its atmosphere. This gas is constantly being stripped from Titan鈥檚 upper atmosphere by the sun. Without a source to replenish it, all of the methane would disappear in a few million years.
However, if an ice volcano like Sotra were to erupt, it would release volatiles like methane and ethane from inside Titan. Kirk鈥檚 team calculates that it would take a Sotra-sized volcanic eruption every 1000 years to maintain the current level of methane in Titan鈥檚 atmosphere.
Others are sceptical about ice volcano claims and have proposed alternative theories to explain the methane abundance.
鈥淭here鈥檚 been this whole list of volcanoes (on Titan) that have been published and then subsequently shot down,鈥� said planetary scientist Jeffrey Moore at NASA鈥檚 Ames Research Center in Moffett Field, California. 鈥淭his new feature doesn鈥檛 make me change my tune that no one has unambiguously found a volcano on Titan.鈥�
Ice cube
Moore believes that unlike Earth鈥檚 well defined and separate layers, beneath Titan鈥檚 surface is a huge layer of mixed rock and ice, or what is called a partially differentiated interior. If this is the case, it would be much more difficult to heat ice enough to cause an eruption onto the surface.
Moore and others believe that Titan was once an enormous ice cube. According to their theory, as the sun aged and warmed, it heated Titan鈥檚 surface. This process could have put methane into the atmosphere and subsequently fuelled a rain cycle that erased all impact craters. Moore said this would also have given Titan the young appearance that many have attributed to volcanism.
鈥淚f you press forward in time, all the methane will be erased and (Titan) will have a blue sky and a nitrogen atmosphere with sand dunes of hydrocarbons,鈥� Moore said.
