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Wednesday, 27 June 2012

The Chaos of Miranda

A little while ago I posted about the Uranian system but, like Voyager 2 [1] when it visited, I was barely able to scratch the surface. To fix that a little, I thought I'd take a look at the most bizarre moon of Uranus.

Geologically it's, perhaps, the most bizarre moon in the solar system:

A moon with many faces: 

When Voyager 2 arrived at Uranus it was met with a blank sphere of cerulean blue, as if the god of the sky was refusing to give up it's mysteries. But it's moons....ah. They had stories they were willing to share with the visitor.

Image above: Uranus, hiding its secrets beneath a veil of opaque turquoise cloud. This is the view from Voyager two, and the ice giant seems clear it's giving nothing away easily. Image courtesy of JPL/NASA

And one of them, Miranda [2], the innermost large moon, had a story that was written on its face in scars and landscapes like nothing the team controlling Voyager had ever seen before....

 Image above: Miranda looms out of the darkness. The 'tick' feature near the middle top is called the Chevron and, like most of Mirandan geology, we have no idea how it got there. Image courtesy of NASA/JPL.

Voyagers cameras showed the 480km wide moon to be a mix of odd landscapes wedged together. The Voyager team saw...

.....compressed, crumpled, valleys and cliffs, reaching 5km into the black sky above the little moon...

Image above:  Miranda boasts the highest cliff known to exist: Fall off it, and between the low gravity and height you'd take ten minutes to hit the bottom. So at least you'd have time to work on your famous last words... Image courtesy of NASA/JPL.

.....massive parallel ridges, next to ancient rolling hills dotted with craters...

Image above: Ancient, heavily cratered, hilly terrain sits right next to a sea of massive parallel ridges, and a young (geologically young -  so still really hugely old) area of chaotic landscape. Landscape isn't supposed to do that. Why this one does....we simply don't know for sure. Image courtesy of JPL/NASA.

......fractures exposing different light and dark materials....

Image above: The deformed edge of Miranda, where huge rifts eat into the surface and expose subsurface layers of darker and lighter material. And, once again, we're not entierly sure what any of it means.... Image courtesy of JPL/NASA..

......Miranda looked like it had been shattered, the pieces squashed clumsily back together, and then told 'stay!'

And, at first, that's exactly what the geologists thought had happened: That Miranda had been broken by a massive asteroid or comet [3], and gravity had crudely bought the fragments back together. And that then it had been pulverised again. And again. And again. Five times in fact [4]. Five cycles of destruction, and rebirth, each time becoming more confused and twisted. Each time bits of the core wound up on the surface, so the theory goes,  and bits of surface wound up in the core.


There were a few problems with this idea - one being that the re-accretion process itself should have ground the surviving fragments down to no bits bigger that ten kilometres. So other explanations have been sought.
There is another explanation [5] for the jigsaw moon, one now more widely regarded than the first. It begins in a different part of the Uranian system: The moon Umbriel [6]. At one point in its history Miranda may have been in a 3:1 orbital resonance [7] with Umbriel - so for every orbit Umbriel made, Miranda did three. Because of the resonance,  the two moons were always closest to each other at the same point in the orbit of Miranda, and Umbriels gravity distorted Mirandas path.

Video above: I couldn't find a video showing the Miranda - Umbriel orbital resonance, so here's one with Titan and Hyperion instead, two moons of Saturn. Titan - the inner, faster moving moon - drags Hyperion along behind it. Since a faster moving object must orbit higher, the orbit of Hyperion orbit gets distorted into an egg shape.

That would mean Miranda was significantly further from Uranus - and feeling a different gravitational pull - at some points in its orbit than others. The changing gravitational pull would cause tides in the 'rock' making up Miranda, like those on the seas of Earth. Rocks that get stretched and squeezed by a huge changing force rub against each other, producing heat. Add up all that heat across the whole of a small moon.... and you can melt it.

Which is exactly what has happened to acne riddled Io, and this is a perfect excuse to put in a picture of the angriest moon in the solar system:

Image above: Io. The colours are due to sulphur compounds all over its surface. Io has volcanoes the same way that a car in a scrapyard has dings. After it's been through the crusher. Miranda may once have been a cooler, water-ammonia, cousin to the seething chaos of Io. Image courtesy of NASA/JPL. 

That melting turned Miranda into a pit of cryovolcanic [8] chaos, perhaps more wracked even than Io. Miranda started to belch its core onto its surface: Huge blobs of warm, ice called diapirs [9] would have forced their way out of the mantle, driving older terrain aside and compressing it. Water-ammonia slurry bursting out of fissures as cryolava, covered thousands of square kilometres. The changes in internal distribution of its mass made the whole moon shift on its axis [10], like a.......a 500 km wide ice and rock moon melting and shifting about, and that's impressive enough to need no metaphor. Imagine if a cluster of volcanoes popped up on Earth, a cluster so big the poles became the equator, because the plume of magma underneath the volcanoes was throwing the whole planet off axis.

But then Umbriel and Miranda drifted out of resonance, the tidal forces faded, and Miranda froze again - halfway through turning itself inside out like an old sock - leaving a surface that bears an odd resemblance to one in places.

Both theories might hold water - or cryolava - up to a point. The solar systems history is long enough to leave room for a lot of events.

Image above: A photo-montage of the moons of Uranus, revealing Miranda as the tiddler of the bunch. The huge unmapped regions on Ariel just serve to underline how much exploring we still have to do out there. Image courtesy of JPL/NASA.

Although we will have to wait a long time for answers, there has been a resurgence of interest in the ice giant [11] planets: New missions have been proposed [12], and the advent of new space drives like the solar sails [13] and ion engines [14] might ease the path to making them a reality. The New Horizons [15] probe to Pluto shows us again what Voyager did decades ago: That, even with conventional engines, the old gravitational slingshot manoeuvre can get a probe out further even than Uranus.

Oh, one other thing: Remember I posted a soundtrack of the radio wave 'song' of Uranus, recorded by Voyager as it passed by? Well, Miranda sings one to, but where Uranus's 'song' was kinda beautiful, Miranda's is...  frankly creepy.

Fitting, perhaps, for a world that has been racked by such violence and turmoil.....


Video above: The radio waves recorded by voyager two as it passed Miranda, made audible. It sounds... literally like nothing of this world. Image courtesy of NASA/JPL, audio taken from "Symphonies of the Planet vol.2 - Miranda", NASA Voyager Recordings, Brain/Mind Research, 1990.

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