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Death Star moon may be 'wonky or watery' | Death Star moon may be 'wonky or watery' |
(4 days later) | |
The internal structure of one of Saturn's moons is either wonky or awash with water, according to a new study. | The internal structure of one of Saturn's moons is either wonky or awash with water, according to a new study. |
Mimas is nicknamed the Death Star because it resembles the infamous Star Wars space station. | Mimas is nicknamed the Death Star because it resembles the infamous Star Wars space station. |
It has a tell-tale wobble that is twice as big as expected for a moon with a regular, solid structure. | It has a tell-tale wobble that is twice as big as expected for a moon with a regular, solid structure. |
The researchers offer two explanations: either it has a vast ocean beneath its surface, or a rocky core with a weird shape resembling a rugby ball. | The researchers offer two explanations: either it has a vast ocean beneath its surface, or a rocky core with a weird shape resembling a rugby ball. |
The study appears in Science Magazine. | The study appears in Science Magazine. |
Its authors are astronomers in the US, France and Belgium, who based their calculations on high-resolution photos of Mimas snapped by the Cassini spacecraft. | Its authors are astronomers in the US, France and Belgium, who based their calculations on high-resolution photos of Mimas snapped by the Cassini spacecraft. |
Cassini was sent to Saturn in 1997 to explore the planet and its many moons, which so far number 62 (53 with names). | Cassini was sent to Saturn in 1997 to explore the planet and its many moons, which so far number 62 (53 with names). |
Exotic interior | Exotic interior |
The researchers built a detailed 3D model of Mimas using images taken from various angles, and tracked the movement of hundreds of reference points on its pockmarked surface. | The researchers built a detailed 3D model of Mimas using images taken from various angles, and tracked the movement of hundreds of reference points on its pockmarked surface. |
"After carefully examining Mimas, we found it librates - that is, it subtly wobbles - around the moon's polar axis," said lead author Dr Radwan Tajeddine, who works at Cornell University in the US and the Pierre and Marie Curie University in Paris. | |
Apart from these gentle "librations", Mimas otherwise presents the same face to Saturn throughout its orbit. | Apart from these gentle "librations", Mimas otherwise presents the same face to Saturn throughout its orbit. |
Our own moon has a similar motion, with a small wobble that offers us slightly different views of the satellite over time. | Our own moon has a similar motion, with a small wobble that offers us slightly different views of the satellite over time. |
But when Dr Tajeddine and his colleagues put all their measurements together, they found that the surface of Mimas swivels back and forth by 6km. | But when Dr Tajeddine and his colleagues put all their measurements together, they found that the surface of Mimas swivels back and forth by 6km. |
This is quite a wobble for a moon that measures less than 400km across. In fact, it is twice as much movement as expected, based on Mimas's size and its elliptical orbit. | This is quite a wobble for a moon that measures less than 400km across. In fact, it is twice as much movement as expected, based on Mimas's size and its elliptical orbit. |
"This is where we started thinking of more exotic interior models," Dr Tajeddine told BBC News. | "This is where we started thinking of more exotic interior models," Dr Tajeddine told BBC News. |
First, the team tested whether the extra rotation could be explained by a deformity underneath the enormous Herschel Crater, one-third the size of Mimas itself, which gives the moon its signature appearance. | First, the team tested whether the extra rotation could be explained by a deformity underneath the enormous Herschel Crater, one-third the size of Mimas itself, which gives the moon its signature appearance. |
But even a "huge mass anomaly" created by the wallop that left the crater would not deliver the amount of movement that Dr Tajeddine's team had observed. | But even a "huge mass anomaly" created by the wallop that left the crater would not deliver the amount of movement that Dr Tajeddine's team had observed. |
Instead, they wondered whether Mimas might be far from the simple, uniform sphere of ice and rock that most planetary scientists had previously assumed. | Instead, they wondered whether Mimas might be far from the simple, uniform sphere of ice and rock that most planetary scientists had previously assumed. |
"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. | "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. |
His team settled on two likely plot twists, wrapped beneath Mimas's icy crust. | His team settled on two likely plot twists, wrapped beneath Mimas's icy crust. |
Firstly, their calculations suggested that the wobbles could arise from a core that was squashed or elongated by 20-60km: a huge, central rugby ball of rock. | Firstly, their calculations suggested that the wobbles could arise from a core that was squashed or elongated by 20-60km: a huge, central rugby ball of rock. |
Alternatively, the moon could have a normal spherical core and crust, but separated by a "global ocean". That way, Dr Tajeddine explained, "the shell can wobble more easily, because it's not attached to another mass". | Alternatively, the moon could have a normal spherical core and crust, but separated by a "global ocean". That way, Dr Tajeddine explained, "the shell can wobble more easily, because it's not attached to another mass". |
Of the two explanations, he favours the subterranean sea. | Of the two explanations, he favours the subterranean sea. |
"When we saw this wobbling, the first thing we thought of was an ocean," Dr Tajeddine said. | "When we saw this wobbling, the first thing we thought of was an ocean," Dr Tajeddine said. |
Either possibility would make Mimas a much more interesting research subject: "This brings the spotlight back to this moon, which was a little bit ignored." | Either possibility would make Mimas a much more interesting research subject: "This brings the spotlight back to this moon, which was a little bit ignored." |
Prof Chris Lintott, an astrophysicist at the University of Oxford, was surprised to hear of the new results. | Prof Chris Lintott, an astrophysicist at the University of Oxford, was surprised to hear of the new results. |
"If you'd asked me before now, I would have said that Mimas is a boring, icy moon. | "If you'd asked me before now, I would have said that Mimas is a boring, icy moon. |
"If the ocean is really there, we're getting to the point where it's just completely standard for icy moons to have substantial bodies of water inside - and that could have interesting implications for how many of these things could support life." | "If the ocean is really there, we're getting to the point where it's just completely standard for icy moons to have substantial bodies of water inside - and that could have interesting implications for how many of these things could support life." |
Follow Jonathan on Twitter. | Follow Jonathan on Twitter. |
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