Scientists have suggested that Enceladus, one of Saturn’s moons, has the long-term stability required for life to develop.

A new study has provided the first evidence of significant heat flow at Enceladus’ north pole, overturning previous assumptions that heat loss was confined to its active south pole.

This finding confirms that the icy moon is emitting far more heat than would be expected if it were simply a passive body, strengthening the case that it could support life.

Highly active world

Enceladus is a highly active world, with a global, salty sub-surface ocean, believed to be the source of its heat. The presence of liquid water, heat and the right chemicals (such as phosphorus and complex hydrocarbons) means that its sub-surface ocean is believed to be one of the best places in our solar system for life to have evolved outside the Earth, according to scientists.

The study led by researchers from Oxford University, Southwest Research Institute and the Planetary Science Institute in Tucson, Arizona, also underlines that this sub-surface ocean can only support life if it has a stable environment, with its energy losses and gains in balance.

This balance is maintained by tidal heating: Saturn’s gravity stretches and squeezes the moon as it orbits, generating heat inside. If Enceladus doesn’t gain enough energy, its surface activity would slow down or stop, and the ocean could eventually freeze. Too much energy, on the other hand, could cause ocean activity to increase, altering its environment, according to a press release.

Key target in the search for life outside the Earth

“Enceladus is a key target in the search for life outside the Earth, and understanding the long-term availability of its energy is key to determining whether it can support life,” said Dr Georgina Miles (Southwest Research Institute and Visiting Scientist at the Department of Physics, University of Oxford), lead author of the paper.

The team also pointed out that until now, direct measurements of heat loss from Enceladus had only been made at the south pole, where dramatic plumes of water ice and vapour erupt from deep fissures in the surface. In contrast, the north-pole was thought to be geologically inactive.

Using data from NASA’s Cassini spacecraft, the researchers compared observations of the north polar region in deep winter (2005) and summer (2015). These were used to measure how much energy Enceladus loses from its “warm” (0°C, 32°F) subsurface ocean as heat travels through its icy shell to the moon’s frigid surface (–223°C, –370°F) and is then radiated into space.

“Understanding how much heat Enceladus is losing on a global level is crucial to knowing whether it can support life,” said Dr Carly Howett (Department of Physics, University of Oxford and Planetary Science Institute in Tucson, Arizona), corresponding author of the paper.

“It is really exciting that this new result supports Enceladus’ long-term sustainability, a crucial component for life to develop.”

Researchers also highlighted that the next key step will be to determine whether Enceladus’ ocean has existed long enough for life to develop. At the moment, its age is still uncertain.

Thermal data can be used to estimate ice shell thickness

The study also demonstrated that thermal data can be used to independently estimate ice shell thickness, an important metric for future missions planning to probe Enceladus’ ocean, for instance using robotic landers or submersibles. The findings suggest that the ice is 20 and 23 km deep at the north-pole with an average of 25 to 28 km globally – slightly deeper than previous estimates obtained using other remote sensing and modeling techniques, as per the release.

The study also provides a previously unidentified constraint for models of tidal heat production, shell thickness, and the long-term evolution of Enceladus’ ocean.