To human eyes, Saturn’s moon Enceladus seems relatively simple. Take the wavelength away from the optics, however, and Enceladus begins to look much more interesting, as new images amply demonstrate.
Although its surface is marked with chasms and deep gorges, Enceladus otherwise appears quite uniform, with a shimmering shell of white ice, like a giant snowball in space.
In infrared wavelengths, astronomers found that much of the ice on the entirety of the moon is fresh, suggesting that there may be global internal activity resurfacing on the moon.
We’ve known for a while now that Enceladus isn’t necessarily a quiet place. In 2005, Saturn’s Cassini probe discovered plumes of salt water gushing out of four massive chasms parallel to the moon’s south pole, dubbed “tiger stripes”. Cassini then mapped over 100 geysers in the tiger band fractures.
These fractures are generated by tidal forces on the moon as it makes its eccentric orbit around Saturn. The planet pulls and stretches Enceladus, causing internal heating and geothermal activity, and creating cracks in the surface ice at the south pole. Geysers spew water from within, kept liquid by internal heating; this water sprays over the surface and freezes, creating a new layer of ice.
So in infrared images from newly reanalyzed data generated by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) – the spacecraft’s mission ended in September 2017, but its legacy lives on – it was to be expected until you find a light consistent with fresh ice, reflecting the region around the tiger stripes.
Indeed, the highly detailed images, compiled from 23 close flybys, show persistent resurfacing. You can see it in the image above, and you can explore an interactive globe here – the bright red regions around the tiger stripes indicate the spectral signature of crystalline ice, in which molecules are ordered in a neat geometric lattice and repetitive; it reflects infrared light differently than amorphous ice, with messy, disordered molecules.
It is important. Almost all natural ice on Earth is crystalline – but almost all the ice we detect in space is amorphous. This is because temperatures in space tend to be very low, and at very low temperatures water molecules collide and freeze in place.
Crystalline ice, on the other hand, indicates that the water has been relatively warm, above about 110 Kelvin – even after freezing, the molecules retain enough thermal energy to change to a crystalline configuration. So when you see crystalline ice in space, you can draw certain conclusions about its thermal history.
Most of the ice on the surface of Enceladus is crystalline, but the level of crystallinity is significant. If we find ice that is more crystalline than the ice around it, we can assume that it formed from warmer water – like ice and water freshly spewed from the inside through of geysers in tiger stripes.
But that’s not all. What the team led by Rozenn Robidel from the University of Nantes in France did do not to expect to find was a spectral signature of crystalline ice distributed widely across the globe of Enceladus, including the north pole, which has no tiger stripes.
This unexpected discovery suggests that geologic activity occurred on both hemispheres and that the northern hemisphere underwent similar resurfacing to the south, although the mechanism may be different – more gradual crustal fracturing.
Since such activity is likely related to hotspots on the seafloor, and these hotspots likely only have a lifespan of a few million years, this allows us to infer the age of the surface in these regions.
“The infrared shows us that the surface of the south pole is young, which is not a surprise because we knew the jets blowing icy material there,” said astronomer Gabriel Tobie from the University of Nantes. .
“Now, with these infrared eyes, you can go back in time and tell that a large area in the northern hemisphere also looks young and was probably active not too long ago in geological timelines.”
The team plans to apply their analysis techniques to data obtained from the upcoming Juice and Europa Clipper missions, to see what they can learn about the icy moons of Jupiter, Ganymede and Europa.
The research has been published in Icarus.