Mimas – the smallest and innermost of Saturn’s eight main moons – has either a rugby ball-shaped rocky core or an underground ‘life-friendly’ ocean, according to a study published in the October 17 issue of the journal Science.
Mimas’ surface, like the surfaces of most of the other major Saturnian moons without atmospheres, is not pure ice but contains some dark impurities; the relatively dark markings appear along the lower portion of the walls of the 130-km-wide Herschel Crater (the peak in the middle of the crater is about as tall as Mount Everest; the impact that made it likely almost destroyed the moon) and some of the smaller craters; scientists interpret this darkening as evidence for the gradual concentration of impurities from evaporating icy materials in areas where the dark impurities slide slowly down the crater wall. Image credit: NASA / JPL / Space Science Institute.
Mimas was discovered in 1789 by the English astronomer William Herschel and named for one of the Giants of Greek mythology.
Along with Enceladus, Tethys, Dione and Rhea, it is classified as a mid-sized icy moon; the origin of these moons is still being debated.
Mimas is 396 km in diameter and revolves around the planet in a prograde, near-circular orbit at a mean distance of about 189 thousand km.
Because of tidal interactions with Saturn, the moon rotates synchronously with its orbital motion, always keeping the same hemisphere toward the planet and always leading with the same hemisphere in orbit.
“After carefully examining Mimas, we found it librates – that is, it subtly wobbles – around the moon’s polar axis. In physical terms, the back-and-forth wobble should produce about 3 km of surface displacement. Instead we observed an unexpected 6 km of surface displacement,” explained study lead author Dr Radwan Tajeddine of Cornell University, IMCCE-Observatory and the University of Pierre and Marie Curie in Paris, France.
Dr Tajeddine and his colleagues have measured the wobbles of Mimas by using stereo-photogrammetry on Cassini Image Science Subsystem (ISS) images.
“We’re very excited about this measurement because it may indicate much about the satellite’s insides. Nature is essentially allowing us to do the same thing that a child does when she shakes a wrapped gift in hopes of figuring out what’s hidden inside,” Dr Tajeddine said.
The scientists employed Cassini photographs taken at different times and from various vantage points to build precise 3-D computer models of the locations of hundreds of surface reference points. From these, they determined the moon’s shape and were able to notice that the satellite didn’t rotate smoothly but rocked back and forth a bit as well.
The amount of the to-and-fro motion indicates that Mimas’ interior is not uniform.
These wobbles can be produced if the moon contains a weirdly shaped, rocky core or if an underground ocean exists beneath its icy shell.
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R. Tajeddine et al. 2014. Constraints on Mimas’ interior from Cassini ISS libration measurements. Science, vol. 346, no. 6207, pp. 322-324; doi: 10.1126/science.1255299
