Heat is escaping from Saturn’s moon Enceladus at its north pole, as well as the south, a reanalysis of data from the Cassini spacecraft reveals. This discovery suggests the moon is in rough energy balance, which in turn increases the chance that its subsurface ocean is a long-term feature, not a temporary aberration. If so, one of the major obstacles to life flourishing beneath the icy surface – and perhaps escaping through its geysers for us to collect – may be removed.
The discovery that Enceladus has a subsurface ocean, with geysers that spew ice particles into space, hit the planetary science world like a thunderbolt in 2005. Decades before, the discovery of an ocean inside Europa had sparked hopes that life might flourish there, but also major doubts about our capacity to reach it for testing. Enceladus’s ocean might be coming to us, such that Cassini flew through the geyser’s output, which would allow a future mission to collect the output and test the chemistry for signs of life.
Since then, hundreds of papers have been published on the prospect. Many have pointed to optimistic signs, including phosphates and complex organic molecules in the ocean, but some have sounded notes of caution. Perhaps the biggest challenge to the prospects for life in an Enceladean ocean is the possibility that the moon’s innards have not been liquid for long, making new evidence on that score particularly crucial.
“Enceladus is a key target in the search for life outside Earth, and understanding the long-term availability of its energy is key to determining whether it can support life,” said lead author Dr Georgina Miles of the Southwest Research Institute in a statement.
Key to this question is whether Enceladus is in thermal balance, in other words, if the heat it absorbs and releases match up. Thermal balance would indicate a sustainable state that might be very old. On the other hand, if more heat is either being absorbed or escaping, then the small moon must be heating up or cooling down. Its current circumstances, apparently suited to life beneath the surface, would then be likely to be new; potentially too new to give time for life to appear.
Cassini detected considerable heat escaping from Enceladus’s south polar region, where large “tiger stripes” break the insulating shell. Nevertheless, at an estimated 19 gigawatts, this was still less than the moon was absorbing. Unless heat was escaping elsewhere, it would not be in thermal balance; many planetary scientists suspected this was the case.
However, reanalysis of Cassini’s observations of Enceladus’s north polar region in 2005 and 2015 shows the region is 7° C (12° F) warmer than anticipated. The measurement dates were picked because they coincide with winter and summer for the region as its hemisphere shifts in line with Saturn’s. Such consistent extra heat across seasons almost certainly comes from below, indicating that Enceladus radiates not only from its southern polar region, but from the north as well.
The heat loss is just 46 milliwatts per square meter; not enough to be useful to any future Enceladus mission, and much less than in the south polar region. Across the polar region, it equals 1.7 GW. Nevertheless, this does indicate some geological activity is occurring. Moreover, if this is typical of the entire surface of Enceladus outside the south polar region, it would release about 35 GW, coincidentally about what Italy gets from solar panels on a sunny day.
Adding in the 19 GW already observed to escape from the south polar region provides a total of 54 GW. With the energy generated inside Enceladus thought to be between 50 and 55 GW, thermal balance looks very plausible indeed.
The heat escaping from Enceladus closely matches the best estimates for how much heat is created by the tidal flexing of other objects’ gravitational fields.
Image credit: University of Oxford/NASA/JPL-CalTech/Space Science Institute
“Understanding how much heat Enceladus is losing on a global level is crucial to knowing whether it can support life,” said corresponding author Dr Carly Howett of the University of Oxford. “It is really exciting that this new result supports Enceladus’s long-term sustainability, a crucial component for life to develop.”
Thermal balance relies on the rest of Enceladus radiating heat at similar rates per unit area to the north polar region, and that hasn’t been confirmed. Nevertheless, the prospects for balance now look much higher than before. The work also provided an estimate of the thickness of the ice above the ocean at the north pole and elsewhere: 20-23 kilometers (12-14 miles) and 25-28 kilometers (16-17 miles) respectively, in between previous estimates made with different methods.
Why thermal imbalance was suspected
As with Europa, and many other moons now known or suspected to have internal oceans, Enceladus is stretched and squeezed by the changing gravitational tugs of its planet and larger neighboring moons. This process generates heat, which is sufficient to melt some of its internal ice, while the outer shell insulates the relatively warm ocean from the cold of space, like ice on a winter pond.
However, changing dynamics amongst Saturn’s moons, and the varying eccentricity of Enceladus’s orbit, mean that the amount of heat generated changes over tens of millions of years. Therefore, some planetary scientists have proposed that, during periods of lower heat production, Enceladus may have frozen solid. Unless life could survive eons trapped in ice, only to recover when the ocean was restored, there might be little time for it to appear and flourish during phases when the ocean existed.
The study is published in Science Advances.
Source Link: Enceladus’s North Pole Is Leaking Heat, Indicating Its Ocean Is Ancient And Boosting Prospects For Life
