Plumes erupting from a frozen moon

In 2005, NASA's Cassini spacecraft made an astonishing discovery around Saturn. It found that its icy moon Enceladus was firing water from a subsurface ocean into space. This ocean could be habitable – and sampling the plumes of water could reveal the existence of life.

Cassini's mission came to an end in September 2017, but before then it flew through the plumes on several occasions. As these had not been expected, the spacecraft was not designed to search for life. Nonetheless, in April 2017 it was able to discover that hydrothermal activity was likely taking place at the bottom of this ocean.

The ocean of Enceladus is encased under ice that is several kilometres thick. While devoid of sunlight, it's thought the hot core of the planet could be powering hydrothermal activity at the bottom of the ocean, essentially underwater vents. On Earth, we think life may have begun around these deep-sea vents. Is the same happening on Enceladus?

Cassini left us with more questions than answers about this moon. If there is life there, it is almost certainly microbial in nature – but it will be up to a future mission to find out the answer to that for certain.