Some of the moons of gas giant planets have a unique property: They are worlds that have a source of heat other than the Sun. The huge tidal forces imparted by their host planet and fellow moons cause their crust to grow and shrink, creating huge friction and heating the rock. It means that even if these moons are far from the warmth of the Sun, they can still host liquid water. There are two moons in our own solar system that are especially enticing: Jupiter’s second moon Europa, and the icy moon of Saturn known as Enceladus. Each host icy surfaces that shift and bend above subsurface oceans of liquid water. New research suggests that Enceladus hosts hydrothermal activity similar to that seen in the depths of Earth’s oceans, and could provide an ideal environment for the development of life.
Since 2005, the Cassini spacecraft has discovered nearly 100 cryovolcanoes on Enceladus, spewing a constant stream of water vapour, sodium chloride crystals, and ice crystals. Some of this material is thought to return to the moon as snow, while the rest escapes its gravity where it feeds onto the E ring of Saturn. The discovery of the cryovolcanoes led scientists to speculate on the possibility of hydrothermal activity below the surface, and with close passes of the moon, the Cassini spacecraft was able to sample tiny fragments of material.
After four years of analysis, computer simulations, and laboratory experiments, researchers concluded that the tiny grains most likely formed when water from the subsurface ocean cooled dissolved materials rising from the moon’s rocky interior. Temperatures required to produce the grains sampled by Cassini have to be at least 90 degrees Celsius. Hydrothermal activity on earth occurs when seawater infiltrates the hot rocky crust and emerges as a heated, mineral-rich solution, and this is the first evidence suggesting a similar process occurs on Enceladus.
“It’s very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on — and beneath — the ocean floor of an icy moon,” said the paper’s lead author Sean Hsu, a postdoctoral researcher at the University of Colorado at Boulder. Data from Cassini on the gravity of Enceladus also suggests that the rocky core of the moon is porous, giving a large surface area for hydrothermal processes.
The prospects for life are amazing, as a source of heat is considered one of the key ingredients. Some of the other ingredients that Enceladus possesses are salt water from its liquid subsurface ocean, and the presence of simple hydrocarbons such as Methane, Formaldehyde, Propane, and Acetylene.
So where do we go, if we have the money for one mission? Europa or Enceladus?