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Radar evidence shows that Enceladus geysers eject water that turns into snow. The snow falls not only on the surface of Enceladus, but also towards the neighboring moons, Mimas and Tethys, which makes them more reflective. Researchers call this a "snow cannon".
The Cassini probe was equipped with a powerful radar designed to penetrate the thick and opaque atmosphere of Titan, the largest moon of Saturn. But it has also been reported from other moons, including Mimas, Enceladus and Tethys, to measure their albedo and characterize their surfaces.
New results obtained by a team of scientists working with Cassini radar data indicate that some of these data have been misinterpreted and that some of Saturn's moons are much brighter than expected. These results, as well as a model developed to explain them, show that Enceladus is the source of snow that falls on two other satellites of Saturn, thus increasing their reflectivity.
These radar measurements were intended to measure the albedo of many moons of Saturn. Enceladus had the highest albedo and Titan, hazy, the lowest. Two of the other moons, Mimas and Tethys, also had a high albedo. As for the orbits, Mimas and Téthys frame Encelade. Thus, the three brightest moons were all close to each other.
The results are featured in a poster presented by Dr. Alice Le Gall at the joint EPSC-DPS 2019 meeting in Geneva. The high albedo of the three moons Encelade, Mimas and Tethys indicates a fresh and pure water ice in the basement of the moons, as well as the presence of "diffusion structures" which "are particularly effective at sending back waves in the meaning of backscatter. according to the poster.
Dr. Le Gall, of LATMOS-UVSQ, Paris, explained: "The ultra-bright radar signals we observe require a layer of snow at least a few tens of centimeters in thickness. However, the composition alone can not explain the extremely brilliant levels recorded. Radar waves can penetrate transparent ice up to a few meters and thus have more opportunities to bounce off buried structures. The sub-surfaces of the inner moons of Saturn must contain extremely effective retro-reflectors that preferentially reflect radar waves towards their source. "
But researchers still do not know what these structures are. Enceladus has various surface and subsurface characteristics related to impacts and thermal stresses on its icy surface. There are pinnacles, ice blocks and areas of dense cracks. For now, there is no evidence that these features can cause backscatter.
Icy landforms can form other types of more exotic structures that could be responsible for reflectivity. The Penitentes are thin slices of snow or elongated ice, very close together, pointing towards the sun. Suncups are open depressions on a snowy surface that are also very reflective. But they need a lot of solar energy to train, and we do not know if they have enough.
Dr. Le Gall and his colleagues have developed models that will allow them to check whether specific characteristics contribute to the high albedo rate or if other random events are the cause. It is possible that straight fractures on the icy surfaces of these inner moons are the cause.
"Until now, we have no definitive answer," said Dr. Le Gall. "However, a better understanding of these radar measurements will give us a clearer picture of the evolution of these moons and their interaction with Saturn's unique ring environment. This work could also be useful for future missions to the moons. "
The rings and moons of Saturn are constantly exchanging matter. This occurs not only with Enceladus, the electronic ring, nor Mimas and Tethys. This also occurs with the Phoebe ring, which covers the leading edge of the Iapetus moon with a dark material, thus reducing its luminosity.
The rings are a source of fascination both visual and scientific, new research sometimes showing that the rings are very old and sometimes showing that they are very young. Although the Cassini mission has answered many questions about the Saturn system, research like this shows that there are still many mysteries to be solved.
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