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article imageSaturn’s weird wobbly moon, Mimas

By Robert Myles     Oct 16, 2014 in Science
Ithaca - Saturn’s moon Mimas is already an oddity, being the most cratered object in the Solar System and likened to the Death Star of Star Wars fame. But new research this week points to Mimas being even weirder than previously thought.
Small oscillations in the movement of Saturn’s smallest moon may reveal an unexpected interior for the moon, according to a new study.
A team of researchers led by Radwan Tajeddine a Research Associate with Cornell University’s Faculty of Radiophysics & Space Research used images returned by the Image Science Subsystem aboard NASA’s Cassini spacecraft to analyze the rotation of the moon and its orbit around Saturn.
Astronomer William Herschel discovered Mimas in 1789. As well as being the smallest of Saturn’s moons with a diameter of just 396 kilometers (246 miles) it ranks modestly as the twenty-first-largest moon in the Solar System. It’s also the smallest astronomical body known to be rounded in shape because of self-gravitation.
By comparison our own Moon is a giant with a diameter of 3475 kilometers, almost nine times that of Mimas.
Mimas has also came to the attention of astronomers in the past as the so-called ‘Death Star’ moon due to its remarkable resemblance to that bearer of bad tidings from the Star Wars movies. Mimas’ gray surface is almost completely pockmarked with craters but one particular feature stands out.
The massive Herschel Crater is the most obvious evidence of other astronomical bodies having previously slammed into Mimas. The Herschel crater spreads across an enormous proportion of this Saturnian satellite. The crater measures 140 kilometers (87 miles) across so it covers nearly one-third of Mimas’ 396 kilometer diameter.
Pro rata, a hypothetical similarly sized crater on Earth would measure 4000 kilometers (2485 miles) in diameter.
In 2010 Cassini also discovered another odd feature about Mimas. The moon’s daytime temperature distribution is extremely uneven and a heat map compiled from data recorded by the Cassini spacecraft showed a highly unusual ‘Pac-Man’ figure looking as though it was about to gobble up the Herschel crater.
Schematic showing the odd Pac-Man like pattern of daytime temperatures found on Saturn s smallest mo...
Schematic showing the odd Pac-Man like pattern of daytime temperatures found on Saturn's smallest moon Mimas. The heat map was compiled from data recorded by the Cassini spacecraft during a Feb. 13, 2010 flyby of Mimas.
Now the latest research has revealed that Mimas has yet another weird feature. The moon’s rotational and orbital periods interact in such a way that Mimas appears to be rocking back and forth slightly when viewed from Saturn.
The part of this rocking motion that depends mostly on Mimas’ interior structure has an unusually large amplitude which suggests that there may be one of two things going on beneath the moon’s surface.
One possibility is that Mimas contains an elongated core, shaped by the moon’s formation within Saturn’s rings. Mimas is credited with having cleared a 4,800 kilometer (2980 miles) wide gap between Saturn’s two largest rings.
Named after the 17th/18th century Italian mathematician and astronomer Giovanni Domenico Cassini, this gap, known as the Cassini Division, comprises the large break situated between Saturn's A and B rings.
Alternatively, the unusual amplitude of Mimas’ ‘wobble’ could indicate the presence of a sub-surface ocean covered in a thick layer of ice. That could cause Mimas to adopt a motion similar to that of a balloon, part-filled with liquid, part filled with air, when rolled across the floor.
Whether it’s an oddly shaped core or a liquid interior, say the researchers, their findings suggest that Mimas is a much more complex moon than previously imagined.
The research paper, entitled "Constraints on Mimas’ interior from Cassini ISS libration measurements," is published this week in Science magazine.
More about Solar system, Saturn, saturn's moons, moons of saturn, Moons of the Solar System
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