A new study led by researchers from Oxford University, Southwest Research Institute and the Planetary Science Institute in Tucson, Arizona 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.
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.
But 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.
"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.
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" (0C, 32F) subsurface ocean as heat travels through its icy shell to the moon's frigid surface ( - 223C, - 370F) and is then radiated into space.
By modelling the expected surface temperatures during the polar night and comparing them with infrared observations from the Cassini Composite InfraRed Spectrometer (CIRS), the team found 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 metre) may sound small, but this 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: a figure that closely matches 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.
"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."
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 modelling 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," added Dr Miles. "Our study highlights the need for long-term missions to ocean worlds that may harbour life, and the fact the data might not reveal all its secrets until decades after it has been obtained."
Research Report:Endogenic heat at Enceladus' north pole
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