The thought of a volcano conjures up images of exploding mountains belching fiery lava and emitting huge plumes of dust and steam, but there are worlds in the Solar System that are so cold that the volcanoes do not spew fire, but instead, ice! Most of these worlds are found beyond the orbit of Jupiter where temperatures struggle to warm up past -300ºF. How can a volcano be powered without fire?
The answer is heat, but it is a relative term. In the realm beyond the orbit of Mars, water ice is so cold that it be-haves like rock. Other compounds such as methane and ammonia remain liquid down to -300ºF and lower, especially if dissolved with salts. Most of the planets and moons retain some of their heat from creation, from the de-cay of radioactive compounds or even from feeble sunlight, but tidal forces between bodies are the key to generating heat internally. There are three worlds in the outer Solar System beyond Jupiter that are known to be active: Triton, Enceladus, and Titan. All three have volcanic activity intense enough to resurface them and affect their atmospheres.
Triton is a rather large moon that orbits Neptune and is about 1680 miles in diameter. It was always suspected of having vast lakes of liquid nitrogen on its surface, but when Voyager 2 flew by on 25 August 1989 it did not find any lakes, but it did make a stunning revelation. Triton is peppered with nitrogen geysers on its south polar ice cap. The geysers are impressive as they tower up to five miles above the surface and the plumes are carried over 100 miles downwind towards the night side of the moon. These geysers are powered by nitrogen beneath the vast methane-nitrogen polar ice cap. The surface temperature is a numbing -392ºF, but the feeble sunlight, 900 times dimmer than on Earth, is still strong enough to heat the surface and the ground beneath. The nitrogen ice beneath the surface changes to nitrogen gas due to the heating and since it occupies more space, pressure builds up. When the pressure becomes too great and the crust weakens, the liquid nitrogen erupts violently into a towering plume of nitrogen gas that also churns up dust and surface ices. Liquid cannot erupt and flow onto Triton’s surface because the atmosphere is hardly 1/100,000th Earth’s and is too thin to support liquid. These plumes may have confused astronomers into thinking there was liquid nitrogen on the surface. These geysers may be responsible for maintaining the deep, rarified atmosphere that is 500 miles thick. Triton is so dynamic that a mission to Neptune may occur before a return to Uranus. A flyby mission called Argo being considered for launch in 2019 would fly past Neptune and Triton in 2028.
Enceladus is an unusually bright, small, moon of Saturn at 311 miles in diameter. Nearly all of the sunlight is reflected, which keeps it unusually cold at -350ºF. When the Voyager spacecraft flew past it in 1980-81 it was found to be unusually smooth and covered with fresh frost, barely tainted by the damaging ultraviolet radiation from the Sun. Scientists suspected that geysers had to be the reason behind the smoothness and brightness of Enceladus, but evidence was not found. Hardly a year after the sophisticated Cassini orbiter arrived at Saturn in 2004, it discovered numerous geysers erupting from fissures at the south polar region of Enceladus. It also found that the fissures were much warmer than the surrounding terrain, although still well below -100ºF. Cassini flew within 12 miles of Enceladus in October 2008 through several of the plumes without harm. It made a huge discovery that the icy particles being emitted were made of water. This was not water tainted with nitrogen, methane, or ammonia, but pure water, and where there is water, there may be life. Now Enceladus joins Jupiter’s moon Europa as a world that will receive intense exploration in the decades ahead. Enceladus is small, so it should have very little internal heat, but the tidal flexing from Saturn warms the moon enough to create pools of liquid water that is kept under pressure beneath the crust. Just like Triton, when the pressure becomes too great, the water erupts, but in the vacuum of space, it quickly turns to vapor and ice particles that coat the surface to give it a smooth, bright appearance. The gravity is too weak for Enceladus to retain an atmosphere so many particles escape to form a vast, faint ring of water ice around Saturn.
There is one other world that is shaped by volcanoes and is not far from Enceladus. Titan is Saturn’s largest moon at 3193 miles in diameter and is enshrouded in perpetual smog of methane, nitrogen, and hydrocarbons. Huge lakes of liquid methane cover the north polar region where temperatures plummet to -290ºF. Like Venus, Titan cannot be imaged directly due to the smog, but Cassini can pierce it with infrared and radar imaging that has revealed areas where large amounts of material have erupted and flowed. The icy cold lava is a mixture of water and ammonia ice, almost the consistency of a thick slush that builds up to create domes and calderas much like a fiery volcano that oozes and flows onto the surface. These areas look like craters, but the flow features indicate that these are an exotic form of icy volcanism and are proof that Titan is still quite warm beneath the crust, thanks again to tidal heating from Saturn and its neighboring moons. The eruptions may be continuing today, but confirmation may have to wait until future missions, preferably a balloon mission that can image a lot of territory. All of this makes Titan an exciting world to explore, like a frozen, pre-biotic early Earth rich with organics and volcanism, and enveloped in an atmosphere 1.5 times thicker than Earth’s. Best of all, it is the only other world besides Earth with liquid on its surface.
It is possible that Europa may have had geysers at one time, but it appears that they are now dormant and completely frozen over. The moons of Uranus - Oberon, Titania, Umbriel, Ariel, and Miranda - were not really seen close up except for Miranda, so they cannot be totally ruled out for volcanic-geyser activity. There is one world left that may have active geysers, Pluto. It is much like Triton and has a thin atmosphere, so geysers are suspected, but will have to wait discovery until New Horizons flies by on 14 July 2015.
Volcanoes and geysers create and destroy and are a vital part of the creation and evolution of life. We owe our lives to them for the essential compounds that they dredge up from deep inside the Earth and for the atmosphere they help to create and maintain. We are blessed on Earth with just the right amount of volcanic activity; too much or too little and we could either be as blazing hot as Venus or locked into a global ice age like Mars. The energy of a volcano is the heartbeat of the Earth and through destruction there is creation.