Pluto's Wright Mons cryovolcano. It's named after the Wright Brothers. Image credit: NASA/JHUAPL/SwRI
Try saying the words ice volcano without smiling a little at the wonders of the universe and how fortunate we are to live in an age of celestial discovery. The simple knowledge that such a thing as an ice volcano, or cryovolcano, exists would be reward enough, but it's doubly exciting to realize that such geologic marvels are now common knowledge, and that our children will no more blink in astonishment at the concept than the older generation does at a standard-issue lava-spewing volcano in the Pacific.
First observed in 1989 by the Voyager 2 spacecraft as it passed Triton, Neptune's largest moon, cryovolcanoes are again in the news because of data recently returned from the New Horizons spacecraft. Possible ice volcanoes—which are exactly what the name sounds like—have been discovered on Pluto, and they might have been active recently, in geologic terms.
WHAT IS A CRYOVOLCANO?
Think back to the classroom volcano diorama you made in grade school. Little mountain, maybe trees and plastic dinosaurs (because every grade school project is improved with dinosaurs). In our model, red food coloring, baking soda, and vinegar are meant to simulate what's going on when an Earthly volcano erupts. Magma, which is molten rock and volatile, builds up pressure until the ground gives way and it spews forth from vents in the Earth's surface.
This sometimes looks like the occasional, seemingly apocalyptic eruptions of Volcán de Colima in Mexico. And sometimes it looks like the gentle flows in the Pacific islands where you can hire a tour guide and observe lava streams as they roll along.
A cryovolcano isn't all that different. Like an Earth volcano, it results from pressure beneath a celestial surface. Rather than molten rock, however, as the name "ice volcano" suggests, cryovolcanoes are the eruptions of molten ice, sometimes called cryomagma. They can erupt violently or flow gently, just like the volcanoes on Earth. The gentle "tour guide" eruptions are believed to be like flowing slurries.
When you read about "plumes" emanating from planets—the plumes of Enceladus, a moon of Saturn, perhaps being the most famous—you're reading about a cryovolcanic eruption. These are extremely important to planetary science, as such plumes allow for the direct sampling of a celestial body's subsurface without the need to build a prohibitively expensive lander or rover. Last year, the spacecraft Cassini flew through one of one of Enceladus's plumes, taking samples for scientific analysis.
VOLCANIC SNOW AND OTHER AWESOMENESS
Plutonian ice volcanoes are exciting in part because of their size, but also because together they make up yet another surprising feature of a wildly complex, geologically active world. As if ice mountains and nitrogen glaciers weren't enough, one cryovolcano, called Wright Mons (named after the Wright Brothers, and pictured above) is two-and-a-half miles tall and spread across 90 miles. Piccard Mons is even taller.
Here is TomoNews's quirky animated take on Pluto's cryovolcanoes.
Wright Mons is thought to have been active very recently in geologic terms, as only a single impact crater has been found on the surface around the volcano. Craters help determine the age of a celestial object's surface. An old surface will be very heavily cratered; a young surface will not. It falls to planetary scientists to collectively play the role of Sherlock Holmes, taking in all of the geologic and cosmic evidence available to work out what processes are at work on a world to renew its surface.
Pluto, Enceladus, and Earth, of course, aren't the only bodies in the solar system with volcanoes. Titan, one of Saturn's moons, has possible cryovolcanoes that are shaped much like those on Earth. The reigning champion of active volcanism in the solar system is Io, Jupiter's moon, which spews Earth-like silicate lava. Io's volcanism is the result of the massive tidal forces of the giant planet it orbits. These tidal forces cause internal friction that, in turn, produces tremendous heat. Because Io is so cold, its volcanic eruptions sometimes produce "volcanic snow," which is exactly what it sounds like.