Birth of Uranus' Provocative Moon Still Puzzles Scientists

By Andrew Chaikin
Editor, Space & Science
posted: 07:00 am ET
16 October 2001


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.

Mirandas 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 -->

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The bizarre surface of Miranda is revealed in this mosaic of Voyager 2 images. The bright chevron-shaped marking lies within a feature called Inverness Corona; the bullseye pattern along the bottom edge is called Arden Corona. Less than 300 miles across, Miranda's extensive geologic activity has baffled scientists.

The sawtooth pattern of ridges in Arden Corona may have formed from blocks of Miranda's crust that were fractured and rotated.

In this closeup, which covers an area about 140 miles across, the fractured ground of Inverness Corona is seen at lower left. At upper right, the "racetrack" pattern of ridges and grooves is part of Elsinore Corona. Features on Miranda are named for characters and places from Shakespeare's plays.

This cliff, which may be 3 miles or more in height, forms one wall of large canyon. Some scientists now believe this feature, and the network of jagged forms on the canyon floor, indicate rifting and fracturing of Miranda's crust in the distant past.

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Anyone who witnessed those photographs as they appeared on monitors at the NASA/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 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 Mirandas 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 wasnt new.

In fact, scientists had already debated the possibility after Voyager photographed Saturns 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 systems 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?

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