Enceladus, a moon of Saturn, may be harboring a stable ocean that could support life, according to a new study. But here's where it gets controversial: the study reveals that Enceladus is emitting far more heat than expected, challenging previous assumptions about its heat loss. This finding not only strengthens the case for the moon's potential habitability but also raises intriguing questions about the balance of energy and the stability of its sub-surface ocean.
Enceladus is a highly active world, believed to have a global, salty sub-surface ocean that is the source of its heat. The presence of liquid water, heat, and the right chemicals (such as phosphorus and complex hydrocarbons) makes its sub-surface ocean one of the best places in our solar system for life to have evolved outside Earth. However, 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.
Until now, direct measurements of heat loss from Enceladus had only been made at the south pole, where dramatic plumes of water ice and vapor erupt from deep fissures in the surface. In contrast, the north pole was thought to be geologically inactive. But using data from NASA's Cassini spacecraft, researchers compared observations of the north polar region in deep winter (2005) and summer (2015), finding that the surface at the north pole was around 7 K warmer than predicted. This discrepancy could only be explained by heat leaking out from the ocean below.
The measured heat flow (46 ± 4 milliwatts per square meter) may seem small, but it is about two-thirds of the heat loss (per unit area) through the Earth's continental crusts. Across the whole of Enceladus, this conductive heat loss totals around 35 gigawatts, roughly equivalent to the output of over 66 million solar panels (output of 530 W) or 10,500 wind turbines (output of 3.4 MW). When combined with the previously estimated heat escaping from Enceladus' active south pole, the moon's total heat loss rises to 54 gigawatts, closely matching predicted heat input from tidal forces.
This balance between heat production and loss strongly suggests that Enceladus' ocean can remain liquid over geological timescales, offering a stable environment where life could potentially emerge. According to the researchers, 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.
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.
‘Eking out the subtle surface temperature variations caused by Enceladus’ conductive heat flow from its daily and seasonal temperature changes was a challenge, and was only made possible by Cassini’s extended missions,’ said lead author Dr. Georgina Miles (Southwest Research Institute and Visiting Scientist at the Department of Physics, University of Oxford). ‘Our study highlights the need for long-term missions to ocean worlds that may harbor life, and the fact that data might not reveal all its secrets until decades after it has been obtained.’
The study ‘Endogenic heat at Enceladus’ north pole’ (https://www.science.org/doi/full/10.1126/sciadv.adx4338) has been published in Science Advances (open access).
Astrobiology
