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.

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.