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Science - SPACE.com - updated 9:17 AM ET Oct 16
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Tuesday October 16 09:17 AM EDT

Birth of Uranus' Provacative Moon Still Puzzles Scientists

By Andrew Chaikin
Editor, Space & Science, SPACE.com

  
Some of the most bizarre sights in the solar system have been seen — by robotic eyes — on the moons of the giant outer planets. Eruptions of liquid sulfur on Jupiter's moon, Io. A global shell of ice, laced with cracks and ridges, on nearby Europa. Ice volcanoes on Neptune's satellite, Triton. And an opaque layer of orange smog hiding Saturn's moon, Titan. But nothing stranger than the tortured, jumbled surface of a tiny satellite of Uranus called Miranda.

In the minds of many planetary scientists, Miranda is the solar system's strangest moon. But the main theory about how it formed, after being blown to bits, is being questioned by recent thinking.

Miranda’s discovery in 1948 by astronomer Gerard Kuiper was an accomplishment: Only 290 miles across and almost 2 billion miles from Earth, the tiny moon appears as a mere speck of light in the largest telescopes. Almost nothing was known about it until the Voyager 2 spacecraft made its historic reconnaissance of Uranus and its satellites in January 1986. As Voyager swept past the seventh planet, it passed close to Miranda, and sent back a series of stunning, high-resolution images, some of which showed features less than half a mile across.

Anyone who witnessed those photographs as they appeared on monitors at the NASA (news - web sites)/Caltech Jet Propulsion Laboratory on Jan. 25 will never forget the moment.

The first closeup, of Miranda's curved horizon, showed a sawtooth pattern of ridges, along with an unintelligible collection of light and dark rectangles. Then came a large, bright streak shaped like a chevron, set within a patch of dark, tortured ground; it had caught scientists' attention in earlier, less detailed pictures but now was revealed in all its strangeness. This in turn was surrounded by a battered expanse of craters resembling the lunar highlands.

Along the edge of Miranda's sunlit face, a wide band of ridges and grooves cut across the surface like a racetrack and made a right-angle turn before disappearing into the darkness. Finally, there was a range of forbidding mountains, sliced by a towering cliff several miles high.

Planetary scientists could only stare in disbelief. Some had predicted that Miranda would be far too small to show any signs of geologic activity. And yet, its surface had turned out to be a collection of the strangest landforms the solar system had to offer. Geologist Larry Soderblom of the U.S. Geological Survey (news - web sites) called Miranda "a bizarre hybrid of the valleys and layered deposits on Mars, combined with the grooved terrain on Ganymede, and the compressional faults of Mercury."

At first, it seemed impossible to make any sense out of the bewildering variety of terrains. What created the jumbles of light and dark, such as the "chevron" pattern? What were the bizarre "racetrack" patterns? Nothing like Miranda’s surface had ever been seen before, and explaining it all was a tall order.

However, as scientists debated the findings, an explanation emerged: Billions of years ago, Miranda had been the victim of a cosmic collision, an impact so violent that it had broken the moon into pieces – some icy, some rocky. Later on, said the theory, these fell back together into a hodge-podge of chunks. Relatively pure ice would show up as bright areas, while dark areas would consist of ice mixed with carbon-rich compounds darkened by exposure to high-energy cosmic radiation.

The idea of a moon being blown apart and reassembled wasn’t new.

In fact, scientists had already debated the possibility after Voyager photographed Saturn’s moon Mimas in 1980. One face of Mimas was scarred by a crater so big that the impact must have come close to shattering the tiny moon. In all likelihood, Mimas had sustained even larger impacts during the solar system’s early history. It might have been broken up and reassembled not just once, but several times.

After seeing Voyager 2's images, some scientists believed the same thing could have happened to Miranda.

And the re-assembly process, they proposed, explained Miranda's present appearance: Once the moon's fragments coalesced, rocky chunks, being denser than ice, would have tended to sink toward the center. The resulting friction, said the theory, would have warmed the moon's icy interior and created circulating currents of icy material within the interior, like a frigid, slow-motion version of a pot of boiling soup.

Above these slowly churning currents, the crust would have been compressed in some places, producing the "racetracks" of ridges and grooves. In other places within the dark areas, fresh, light colored ice would have erupted to the surface, producing the "chevron."

Next page: Broken moon theory too simple?

With the mystery apparently solved, Miranda slowly faded from most scientists’ view. But is the "broken moon" theory really the answer? Not to planetary geologist Bob Pappalardo of the University of Colorado at Boulder. "People think it's cool," Pappalardo says. "But the blown-apart story is over-simplified."

Pappalardo, who did his PhD thesis on the satellite, says there are too many unanswered questions to call Miranda's case closed.

Pappalardo discovered the problems when he tried to understand how the coronae — the formal name for Miranda's tortured bullseye patterns of ridges, grooves, and jumbled terrain — could have formed by sinking blocks of the reassembled moon.

For one thing, when Pappalardo studied the racetracks of concentric ridges and grooves, they didn't look like features formed by compression. Instead, it looked as if the moon's crust had been ripped apart.

The sawtooth patterns of ridges, so striking in Voyager's first closeups, were likely created when blocks of icy crust fractured and tipped, like books falling over on a bookshelf. And a close look at the ridges by Pappalardo and others indicated that some are actually icy volcanoes.

Suddenly the whole picture changed. Instead of dense blocks sinking into the crust, Miranda's features seemed to be formed by something rising up from below.

Pappalardo says he sees evidence of rifting in the crust, much like that caused in East Africa by upwelling of hot material in the Earth's mantle (to picture this, think of the slowly rising blobs of molten wax inside a "lava lamp"). In the case of Miranda, however, where present temperatures hover around a frigid -335 degrees Fahrenheit, the rising material would have been relatively warm ice, possibly a mixture of frozen water and ammonia or methane.

Canyons formed by the rifting of Miranda's crust created the towering cliffs visible in Voyager's images, says Pappalardo. And the fractured crust allowed fresh ice to reach the surface, producing the bright chevron-shaped feature.

With internal heating now the culprit, theorists realized it no longer was necessary to invoke the process of destruction and re-assembly to explain Miranda's bizarre features. In fact, even though such catastrophes likely took place on Miranda and many other outer-planet satellites, they have probably done little to shape their present-day appearances.

"I don't think we see any evidence of these events today," says Geologist Bill McKinnon of Washington University of St. Louis. "The bodies just fall back together, and you get a cratered ball" resembling Saturn's Mimas.

But that left the question of what else could have heated Miranda to create its tortured landforms. The problem, says McKinnon, is that Miranda's small size and relatively large surface area makes it difficult to heat up. "It's hard to keep a sparrow warm," McKinnon says.

For that reason, McKinnon and others doubt that Miranda's geologic activity could have been powered by the decay of radioactive elements, the heat source that has helped fuel the Earth's geologic "engine."

For another possible source of heat, scientists look to tidal heating, caused by a condition called a resonance, in which two satellites whose orbits are synchronized so that they regularly line up on the same side of their planet. Resonances create a gravitational tug of war among Jupiter's large inner moons, causing their interiors to flex and heat up.

This tidal heating is what makes the Jovian moon Io a volcanic powerhouse, and may have created an ocean of liquid water beneath the crust of Europa.

There are no such resonances between Miranda and the other Uranian satellites today. But sharp-eyed theorists have calculated that resonances could have existed in the distant past, as Miranda's orbit changed. The result: A temporary heat-pulse that heated Miranda's interior just long enough to produce the observed features. And then, nothing. After this burst of activity, Miranda was left a work unfinished.

Or, as Pappalardo says, "Miranda is a world caught in the act of differentiating."

How long ago did all of this happen?

Based on the number of craters counted within the youngest terrains (at the centers of the coronae) Miranda's geologic activity may have continued until as recently as half a billion years ago. But there is debate about that, and about much else associated with Miranda. Even Pappalardo concedes that a catastrophic impact might have played an important role in Miranda's evolution. As McKinnon says flatly, "We don't really know what's going on."

A closer look at Miranda, on some future spacecraft mission, could change that for good.

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