A new study, based on microwave observations by NASA’s Cassini orbiter, shows that the south polar region of Saturn’s frozen moon Enceladus is warmer than expected just a few feet below its surface. This suggests that Enceladus’ subsurface sea might be lurking only a few miles beneath — closer to the surface than previously thought.
This enhanced-color view of Enceladus is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image. Ancient craters remain somewhat pristine in some locales, but have clearly relaxed in others. Northward-trending fractures, likely caused by a change in the moon’s rate of rotation and the consequent flattening of the moon’s shape, rip across the southern hemisphere. The south polar terrain is marked by a striking set of ‘blue’ fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced. The mosaic was created from 21 false-color frames taken during Cassini’s close approaches to Enceladus on March 9 and July 14, 2005. Images taken using filters sensitive to UV, visible and IR light (spanning wavelengths from 338 to 930 nm) were combined to create the individual frames. Image credit: NASA / JPL / Space Science Institute.
Enceladus, the sixth-largest of Saturn’s moons, is an active world.
In 2005, Cassini witnessed for the first time water-rich jets venting from four anomalously warm fractures — so-called ‘tiger stripes’ — near its south pole. The salty composition of these jets points to an underground sea of liquid water that might interact with Enceladus’ rocky core, similar to the subsurface ocean that is thought to exist on Jupiter’s moon Europa.
Many of Cassini’s flybys of Enceladus have been dedicated to understanding the structure of the interior of this fascinating body and its potentially habitable water reservoir.
Now, a study based on data collected by Cassini’s RADAR system during a close flyby in November 2011 indicates that the moon’s subsurface sea might be closer to the surface than previously thought.
“During this flyby, we obtained the first and, unfortunately, only high-resolution observations of Enceladus’ south pole at microwave wavelengths,” said study lead author Dr. Alice Le Gall from the Laboratoire Atmosphères, Milieux, Observations Spatiales, and the Université Versailles Saint-Quentin, France.
“At a distance of 311 miles (500 km) from the surface, Cassini’s RADAR instrument acquired an image and recorded the 2.2-cm-wavelength thermal emission of an arc-shaped region, which is 311 mile long and 15.5 mile (25 km) wide, centered at 63° S and 295° W, and located 19-31 miles (30-50 km) north of the thermally active ‘tiger stripes’ identified as the sources of Enceladus’ jets,” the researchers said.
“These observations show that the first few feet below the surface of the area that we investigated, although at a glacial 50-60 degrees Kelvin, are much warmer than we had expected: likely up to 20 degrees Kelvin warmer in some places,” Dr. Le Gall added.
“This cannot be explained only as a result of the Sun’s illumination and, to a lesser extent, Saturn’s heating so there must be an additional source of heat.”
The detected heat appears to be lying under a much colder layer of frost, as no similar anomaly was found in infrared observations of the same region — these probe the temperature of the surface but are not sensitive to what is underneath.
“The thermal anomaly we see at microwave wavelengths is especially pronounced over three fractures that are not unlike the tiger stripes, except that they don’t seem to be the source of jets at the moment,” Dr. Le Gall said.
These seemingly dormant fractures lying above the warm, underground sea point to a dynamic character of Enceladus’ geology: the icy moon may have experienced several episodes of activity at different locations during its past history.
Even if the observations cover only a small patch of the southern polar terrains, it is likely that the entire region is warm underneath and Enceladus’ sea could be a mere 1.2 miles (2 km) under the icy surface.
Stereographic polar projection of active and passive RADAR observations of Enceladus’ south polar terrain. Left: RADAR image overlaying a color mosaic produced by Cassini’s Imaging Science Subsystem (ISS). Right: surface brightness-temperature map overlaying a visible-light ISS mosaic. Image credit: A. Le Gall et al, doi: 10.1038/s41550-017-0063.
The finding agrees well with the results of a 2016 study, which estimated the thickness of the crust on Enceladus. With an average depth of 11.2-13.7 miles (18-22 km), the ice shell appears to reduce to less than 3.1 miles (5 km) at the south pole.
Dr. Le Gall and co-authors think that the underground heating source is linked to the tidal cycle of the moon along its eccentric orbit around Saturn.
This induces stress compressions and deformations on the crust, leading to the formation of faults and fractures while at the same time heating up the sub-surface layers.
In this scenario, the thinner icy crust in the south pole region is subject to a larger tidal deformation that, in turn, releases more heat and contributes to maintaining the underground water in liquid form.
The findings were published online this week in the journal Nature Astronomy.
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A. Le Gall et al. 2017. Thermally anomalous features in the subsurface of Enceladus’s south polar terrain. Nat. Astron. 1: 0063; doi: 10.1038/s41550-017-0063
