Volcanism on Enceladus

Observations

Situated on Enceladus’ south pole is a broad region around the ‘Tiger Stripes’ surface fissures. This region was measured to have a temperature of 145 K which is far above the average temperature of Enceladus’ surface which is 75 K. This heat is not emitted from the Tiger Stripes themselves but rather from the surrounding surface.[1][2].

These ‘Tiger Stripes’ are four, roughly parallel linear depressions that are, on average, half a kilometre deep, 130 km long and are spaced roughly 35 km apart. They are oriented at about 45° from the line connecting Enceladus and Saturn.[2]

Emanating from this region is Enceladus’ plume, which data from the Cassini probe suggests ejects between 120 and 180 kg of material every second! This is composed of 91.4% water, 3.8% an unknown chemical with a mass-to-charge ratio of 28 daltons—most likely either carbon monoxide or molecular nitrogen—3.2% carbon dioxide and 1.6% methane. It also contains trace amounts of acetylene, propane and ammonia.[3]

What is Cryovolcanism?

A volcano, in it’s broadest and simplest sense, is an opening in the surface of a body through which material is ejected. The most well-known type of volcano is the type that occurs on Earth and erupts molten rock, gasses and pyroclastic material; although this is not the only type of volcano on Earth—sedimentary volcanoes erupting sand and mud do exist.[4]

A cryo-volcano is an opening on the surface of an icy body, like the moons of our outer planets—that, instead of erupting molten rock—erupts chemicals with a low boiling point—otherwise known as volatiles—such as water and methane.[5]

But what drives this icy volcanism? Let’s once again return to Earth and examine what drives our own volcanoes. Magma rises because it is hotter and therefore less dense and more buoyant than the surrounding rock;[6] it gains this heat from the Earth’s interior which, in turn, is heated by radioactive decay and residual heat from formation.[7]

Icy satellites however, are too small to generate sufficient energy through radioactive decay and their large surface-area–to–volume ratio means they will lose their heat quicker than Earth does[8]—by that logic, they shouldn’t be hot enough to sustain volcanism.

While many ideas have been proposed as to how icy satellites get their heat, the most likely is that they are heated by tidal strains from the massive planets they orbit. As they orbit, the gravity of the planet pulls part of the moon towards it in the same way as our Moon pulls our oceans towards it and generates our tides.[9]

So cryo-volcanism on these icy satellites is driven by tidal heating and erupts volatiles such as water and methane. Because of the low gravity on these bodies, eruptive material can form huge plumes that can have the potential to escape the moons gravity into space.[10]

A model of how Enceladus’ volcano might operate with hot rock melting ice which vents to the surface as a cryovolcano.

Tidally Heated Rock

Liquid Water Under Pressure

Comparison to Volcanoes on Earth

The volcanism on Enceladus is very different to the volcanism that we would recognise from our experience of terrestrial volcanos. Perhaps the most obvious difference between the two is the material erupted; Enceladus’ watery cryomagma may look different to the molten rock we associate with terrestrial volcanoes—and indeed, chemically, it is—however it serves more-or-less the same function; bearing in mind the surface on Enceladus is entirely covered by ice much like the surface of Earth is covered by rock.

On Earth, magma is produced predominantly by tectonic processes—we don’t yet understand exactly how the ice on Enceladus melts, however the proposed mechanisms are markedly different to how our magma is formed. Collins and Goodman theorised that there might be an internal sea beneath the south pole, Nimmo et al. suggested that the ice could be being melted by heat generated from lateral faults caused by tidal forces and Kieffer et al. proposed that there may be a reservoir of chemicals that can reduce the melting point of the ice lying beneath the surface.[11]