Enceladus' global subsurface ocean could host salt-driven currents
Scientists have predicted the presence and movement of ocean currents in the vast subsurface ocean of the Saturnian moon Enceladus. The moon, which is sheathed in a 20 km (12.4 mile)-thick shell of ice, is one of the most promising prospects in the ongoing search for extraterrestrial life.
Over the course of its 13-year mission, NASA’s now-retired Cassini spacecraft was tasked with exploring the secrets of the Saturnian system. During this time, the tiny, icy moon of Enceladus captured the attention of both the scientific community and that of the general population.
The solitary probe watched as plumes of material burst forth and leapt high into space from vast rents known as "Tiger Stripes" that mark the south polar region of the moon. This material was drawn from a global subsurface ocean that is thought to lurk beneath Enceladus’ icy shell. And this frigid realm is considered by many planetary scientists to be one of the best places in our solar system in which to hunt for extraterrestrial life.
However, this watery body shares little in common with the varied, nourishing oceans found on Earth. Beneath the thick shield of ice, Enceladus’ ocean is thought to be extremely deep compared to our home world's relatively shallow waters. It may extend over 30 km (18.6 miles) from the underside of the icy crust to the top of the solid core.
Its heating dynamics are also radically different. The moon’s hidden ocean does not receive warmth from the Sun’s rays, as is the case on Earth. Instead, the ocean near the underside of the ice shell is chilled, while the waters in the deepest layer of the ocean are warmed by heat from the satellite’s core.
Enceladus is also thought to be relatively homogeneous – or globally uniform in nature – with the exception of limited water-mixing prompted by temperature differences near the moon’s core. A newly published study has challenged this view, and has shed light on how currents may exist in Enceladus’ alien ocean, and how these currents can affect the prospects of life.
The Caltech scientists behind the study drew on measurements of the moon taken by the Cassini spacecraft, and on research relating to how interactions between liquid water and ice can drive ocean mixing.
When ocean water freezes, it releases its salt, which makes the surrounding water heavier and causes it to descend. This process is reversed in regions where ice is actively melting. The team took this logic and applied it to the waters of Enceladus, using computer modeling based on Cassini data. That data included details regarding how the moon’s ice shell varies in thickness, with the shell being thinner at the poles and widest at the equator.
Those variations indicate that the shell is melting in the polar regions, while the shell at the equator is being bolstered with freezing water. According to the researchers’ simulations, this could create a salt-driven circulation of water traveling from pole to equator.
The addition of such a movement of water, in addition to the core-heating-caused mixing proposed in previous studies, paints a more complex view of the dynamics taking place in Enceladus’ oceans.
Furthermore, the pole-to-equator circulating current could influence the global distribution of heat and nutrients, which in turn could help astrobiologists narrow down which parts of the subsurface ocean may be suitable for hosting life.
The paper has been published in the journal Nature.