When we think of volcanism on Earth, we tend to think of eruptions of hot flowing lava and volcanic ash, but deeper in our Solar System, on the icy moons, there are some unusual volcanoes. These volcanoes don't erupt molten rock, but instead, they erupt volatiles such as water, ammonia, or methane (Wikipedia). We're going to explore the wonders of cryovolcanism (what causes it, where it takes place, how it was discovered, etc). So get ready to explore some frozen space volcanoes!
What is the cause of cryovolcanism?
In order to understand the causes of cryovolcanism, let's begin with how volcanoes themselves form on Earth. This will give some understanding into the mechanisms that cause volcanoes on icy moons to erupt.
There are several layers that make up the Earth (as shown in the diagram below):
The rock from the mantle (mostly-solid area between the crust and the core) moves to the surface from the crust, causing an eruption. Essentially, a volcano is just an opening which causes warm material from the interior of the planet to escape onto the surface. The rock beneath the surface of the Earth is able to melt because of the extremely high temperatures and pressures that it experiences. This causes it to become "liquid rock" or better known as "magma". This process causes volcanoes to form (rising magma).
There are three ways that magma can rise and cause eruptions on the surface of the Earth (Image and information from NASA):
One way that magma can rise is when Earth's tectonic plates move away from each other. Once this happens, magma will rise up from below to try and fill in the space. This is actually how underwater volcanoes are formed!
Magma can also rise when tectonic plates move toward each other and one of the plates will be forced into the Earth's interior. The crust will melt due to the high temperatures and pressure and rise as magma.
The last way that magma can rise is due to hot-spots. Hot-spots are hot areas inside of Earth which will heat up magma which will cause it to become less dense and rise.
All of the ways the magma can rise are different, but they each can form volcanoes.
Now that we've talked a bit about volcanoes on Earth, let's get to what you really came here for: cryovolcanoes.
Since cryovolcanoes don't have magma, there must be a liquid layer under the crust that acts as the mantle (Sky at Night Magazine). I've previously mentioned many of the icy moons in the outer Solar System having subsurface oceans, and these come into play when discussing cryovolcanism. Planets and moons both exert a gravitational force on each other, which pulls the moons and prevents them from being frozen solid. Since the moons are not completely frozen, they have liquid water oceans many kilometers deep underneath their crusts. There are also additional ways that these moon could have additional heat, which are radiogenic heat (heat released from the decay of radioactive elements in the interior of a planetary body or moon), and primordial heat (heat leftover from the formation of the body).
Since there is some mechanism that is heating the subsurface of these bodies, the icy magma will be able to rise to the surface. This icy magma behaves just like the magma that we are used to seeing with volcanoes here on Earth. "As the icy-water magma rises, it behaves just like magma does. It could contain bubbles of gas, crystals and solid chunks of ice or rock that have partially dissolved into the fluid. It may rise straight up a conduit, or it may mechanically force its way through the upper crust of the body, and cause earthquakes"(Forbes). Additionally, if there are chlorine salts in cryovolanoes, then there will be larger amounts of the icy-watery magma beneath them because these salts help to lower the melting temperature of the ice mixtures.
How do you know that it's cryovolcanism? (From Sky at Night Magazine):
Water Plumes: Geysers of water ice mixtures that have been observed over the surfaces of several icy moons.
Geysers observed on Saturn's moon Enceladus [Credit: NASA]
2. Young surfaces: Usually, bodies that exhibit evidence of cryovolcanism are very smooth (free of craters) and highly reflective. This is because the ice lava will flow over and fill in craters.
Neptune's moon Triton [Credit: NASA/JPL/USGS]
3. "Tiger Stripes": Cassini spotted four rifts that were referred to as "tiger stripes" on Saturn's moon Enceladus. These were below where the plumes were initially spotted and something interesting is that they were 70 degrees Celsius warmer than the rest of the moon's surface, which means that warm water was most likely escaping from the interior.
Tiger stripes on Enceladus, as seen by Cassini [Credit: NASA/JPL, ESA, SSI, Cassini Imaging Team]
How was cryovolcanism discovered?
During the Voyager missions in the 80s, both spacecraft sent images back that looked very much like volcanoes with lava plains when flying past the moons of Jupiter and Saturn. Scientists questioned what it was, but couldn't quite confirm anything until Voyager 2 flew by Neptune's icy moon Triton and saw an icy plume rising 8 kilometers above the surface!
This is a picture of Triton's South Pole, the dark spots could potentially be the eruptions of many ice volcanoes [Credit: NASA/JPL].
"Astronomers coined the term cryovolcanism to explain the new phenomenon where ice acted like rock and water was lava" (Sky at Night Magazine).
Cassini was able to gave a better view of what cryovolcanism looked like on Saturn's moon Enceladus. On Enceladus, it found evidence of a liquid layer under the crust that acted like a mantle (sound familiar?).
The spacecraft took images of jets coming from the moon's surface, which proved that there was some sort of liquid layer underneath the moon's crust, which was being quickly shot into space high above the surface of the moon.
Plumes of water ice on Enceladus (captured by Cassini). [Credit: NASA/JPL-Caltech/SSI]
What is exactly going on with "ice volcanoes?":
It is apparent that cryovolcanism can be seen throughout the Solar System, but these processes do not occur on Earth or the inner Solar System, so this was a new discovery for scientists to try and uncover the mysteries of.
This is a region on Jupiter's moon Europa. It appears to be frozen "slush" and could be caused by the upwelling of icy lava from a subsurface ocean underneath the moon's crust. [Credit: NASA/JPL]
Icy lava erupts at the freezing point of 0 degrees Celsius, which may seem strange for Earth, but for these moons in the outer Solar System, it's quite "warm," since their average surface temperatures are usually a couple hundred degrees below this freezing point. These "icy lavas" are usually a combination of water and salt solutions with ice crystals inside of them as well. They're usually more slushy. There are also amounts of nitrogen, methane, and ammonia as well due to the chemistry of the moons. The moons of the ice giants have "thicker" lava than the mons of the gas giants with more amounts of ammonia and methanol in the water.
The importance of the icy moons (the possibility of life):
The icy moons have a great amount of mysteries that could lead to the potential for future explorations. We have been trying to search for extra-terrestrial life for many years, and after these important discoveries on these icy moons, scientists have concluded that the subsurface oceans have the great potential for life to exist. In fact, the subsurface oceans have the best potential for life to exist in the Solar System (beyond Earth of course). Scientists will have to find a way to be able to get underneath the icy crust in order to get to the ocean. Since that could be a bit tricky, cryovolcanism could be the key to exploring these icy worlds. According to Sky at Night Magazine, there was an early version of a mission in 2015 when Cassini flew through one of the plumes of Enceladus. Unfortunately, it was unable to sample the plumes, but it did find a high level of molecular hydrogen in the water, which could mean that there are potential hydrothermal vents on the ocean floor, which are potential hotspots for life to exist.
There is one such mission, the Europa Clipper, which is set to launch in 2024, with the primary objective of flying through the plumes and using special instruments to identify and better understand their chemistry. This mission would be able to pave the way for future missions to land on the moon and look for areas to obtain actual samples. “These lavas could be cycling organics between the surface and subsurface, in which case they could play a big role in creating habitable environments by bringing nutrients from the ocean to the surface and vice versa” (Sky at Night Magazine). The ESA will be launching another spacecraft to a couple other moons of Jupiter as well; Ganymede and Callisto will be studied by the Jupiter Icy Moons Explorer (JUICE) and has a proposed launch date in 2022.
These icy worlds hold so much potential to change the fields of Planetary Science and Astronomy once these missions are able to land and find out more. Frankly, I wish they were talked about more, instead of Mars (no shade to Mars, I just think the icy moons are MUCH cooler, pun intended). They will completely change where we think life may be able to arise, due to the surface temperatures and overall conditions that we've observed on these moons. I am very much looking forward to the future of the exploration of the outer Solar System and what we have the potential of uncovering about these worlds and life itself.
If you want to learn more, here are the two primary sources that I used when doing research for writing this blog post: