For years, Saturn’s small icy moon Enceladus has drawn attention for the erupting jets at its south pole. Those dramatic plumes proved that a deep saltwater ocean sits under the crust. New research now shows that the opposite pole holds its own secret. When scientists examined data from NASA’s Cassini spacecraft, they discovered the north pole gives off more heat than sunlight could ever provide. That quiet but steady warmth reveals that the moon’s interior may be far more active than anyone expected.

A Better Look At the Winter Pole

Cassini took a long infrared “stare” at the northern terrain in July 2005. This area was deep in winter and had not seen the sun in years. All warmth recorded was felt to be rising from below the frozen surface of the moon. Researchers analyzed the radiance spectrum taken by the spacecraft’s Composite Infrared Spectrometer and saw an unusual signature.

The surface temperature spectrum was best fit to a temperature of approximately 38 K or about minus 235 C. While cold by Earth standards, it is still several degrees warmer than the pole would be during a sunless period, which sets the stage for the next expected cold temperature.

A new study has constrained the Enceladus’s global conductive heat flow by studying its seasonal temperature variations at its north pole (yellow). (CREDIT: University of Oxford / NASA / JPL-CalTech / Space Science Institute (PIA19656 and PIA11141))

Even when including the thermal glow from Saturn and faint reflected sunlight, we still could not account for the temperatures that Cassini observed. Something from inside the icy shell of Enceladus was producing additional heat.

An Unusual Emissivity Signal

Then, the team led by researchers from the University of Oxford, Southwest Research Institute and the Planetary Science Institute looked at emissivity, which tells how well the surface radiates as infrared light. The data found low emissivity at the north pole, and also that emissivity changed with temperature. When compared to the fitted blackbody temperature, the emissivity displays a detectable trend with a high correlation of 0.85. That trend is also present in narrower fields of view. 

This temperature dependent behavior does not occur in the warmer regions of Enceladus. There are suggestions for several sources for this measured emissivity. Carbon dioxide frost might accumulate during the long winter. Seasonal deposits or clathrate hydrates might lower the emissivity. For the very cold water ice, the behavior could change at the micro-scale, which would also then impact the ability to radiate heat. 

Even after accounting for the effects from those sources in their models, the researchers could not quantify the observed temperature without an internal heat source. 

CIRS FP1 observations of Enceladus’ north pole. Temperatures correspond to (A) a single temperature blackbody fit and (B) (improved) gray body temperature joint fit with emissivity to the individual spectra, with the location of the average stare FOV shown by the dashed line. The Saturn-facing hemisphere is centered at 0°W at the bottom of the panels. (CREDIT: Science Advances) Testing the Heat Flow

The team determined that additional energy was needed. Therefore, they developed several thermal models. The team explored various assumptions for surface and subsurface properties consistent with previous studies. One model was consistent for winter but not summer, while another model was consistent in sunlit regions but not in dark poles. A coupled model almost matched winter, but still not enough to match both seasons. 

It was not until adding an internal heat source that the modeled temperatures would match real temperatures. This quantification needed was around 46 mW/m2. While that does not sound like much, that quantity is on par with two-thirds of the heat escaping through the continental crust per unit area on Earth. 

This is interesting because the northern surface does not currently exhibit any plumes or apparent cracks. The dramatic geysers of the south pole have long been associated with tidal heating and a warm ocean under the ice. The northern area appears quiet on the surface, but there is still heat leaking from the deep interior.

What the Heat Reveals About the Ice Shell

The team used the heat flow they measured to estimate how thick the ice shell could be above the ocean. They tested both models of solid ice and models with a porous upper layer. By incorporating porosity, thermal conductivity was lowered, which improved the fit to several prior studies.

Projected detector response and modeled surface gray body temperature at the time of observation. (CREDIT: Science Advances)

The estimates for the thickness of the shell at the north pole were mostly between 20 – 23 kilometers thick, with a global average of 25 – 28 kilometers. These values are a little deeper than some previous estimates, but not out of the range of what we expect.

There was one discrepancy with an older shape-model study that the team noted. If that estimate is entirely correct, then or north pole would have to have a higher heat flow than Cassini found. Regardless of that, the new work allows far less doubt that the internal heating is active far from the famously hot south pole.

A Changing Perspective of an Ocean World

The Cassini mission ended in 2017, but the data shape your view of this little moon. New results suggest that Enceladus is not only active in one place. There would be heat coming from closer to the center of the moon above a much larger area of the ocean. This hints at a world with a dynamic interior and ice that acts over longer timescales.

Enceladus is already one of the most favorable places in the solar system for life beyond Earth. It has a deep ocean, heat from tidal forces, and the appropriate chemical ingredients (phosphorus and organic molecules).

This new study only strengthens that case. They find that, when accounting for a heat flow at the south pole, the total heat flow from the moon averages to ~54 gigawatts. “Enceladus is a key target in the search for life outside of Earth, and understanding the long-term availability of its energy is a key part of whether coastal conditions are suitable for life,” said Dr. Georgina Miles, Southwest Research Institute and Visiting Scientist at the Department of Physics, University of Oxford.

Ice shell thickness and conductive heat flux are shown for the north polar region (A and C) and for the global mean (B and D). (CREDIT: Science Advances)

Dr. Carly Howett, a co-author, is encouraged by the energy balance. “It is really exciting that this new result supports the long-term viability of Enceladus as a systems ability to support life. This is key for life to develop”.

The research team now hopes to find out how long the ocean has existed, which they do not know.

Practical Applications of the Research

These findings point to the Enceladus ocean being sufficiently stable for millions of years, and they lend credence to the suggestion that conditions may promote the development or maintenance of life.

The ability to derive estimates on ice thickness from thermal data will help plan future missions, especially those missions likely to land or sample the ocean. Understanding internal heat will also guide the design of landers, probes, or robotic submersibles.

A clearer picture of the moon’s energy budget could support concepts for instruments that will explore habitability in one of the most promising places off the planet Earth.

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