Miranda, a small moon of Uranus, appeared in photographs from Voyager 2 today as a crazy quilt of disconnected and misshapen cliffs, valleys, canyons and glaciers that may force scientists to rewrite chapters of geology textbooks.
"If you took all the bizarre geology in the solar system and put it on one object, that would be Miranda," said Laurence A. Soderblom of the U.S. Geological Survey. "There is nothing like it in the solar system."
Miranda quickly emerged as the star of the Voyager 2 flight, which passed within about 50,000 miles of Uranus, the seventh planet from the sun, on Friday and used the planet's tremendous gravitational force like a slingshot to fling the spacecraft toward a 1989 date with Neptune.
As it passed Uranus, Voyager 2 discovered evidence of 10 new thin rings, which would double the number of known rings around the planet. It also tracked odd cloud formations in the atmosphere of Uranus, 1.8 billion miles from Earth.
Spectacular photographs of Miranda, one of five moons that had been discovered from Earth before Voyager 2 found 10 new smaller ones, showed a surface covered with geological oddities.
One region was dominated by massive oval-shaped cliffs that look like racetracks, another appeared to be an enormous glacier flow that disappeared beneath a huge mountain range. A third had the shape of a gigantic arrowhead or an upside-down chevron covered with frost.
Soderblom said he could see in the photographs 10 different kinds of terrain on Miranda, which measures 310 miles across. He said he could see an ice slide, ridges shaped like braided ropes, linear fault zones, stratified cliffs, at least one enormous scarp and the unmistakably steep walls of a canyon. He said features on the small moon were as high as 15 miles and as deep as 10 miles.
Miranda does not have the spherical shape of the 14 other Uranian moons. Soderblom said the moon is a "misshapen" sphere, bulging outward on one side and flattened on another.
"You name it," he said, "and we have it on Miranda."
Soderblom said he could not explain how such a strange moon could exist. He said one possible explanation is that Miranda had or still has a hot radioactive interior that could raise mountains, dig valleys, open cracks in the surface and move material out of its interior. Another explanation could be that tidal forces triggered by the moons closest to the mother planet might force Miranda to rearrange its surface.
"The trouble with an internal heat source is that Miranda is too far from the sun to have one and too small to keep one," he said. "The trouble with the tidal force explanation is that we have nothing else anywhere in the solar system to compare it to."
Soderblom said he thinks that any explanation of Miranda is tied to a suspected prehistoric collision that left Uranus tipped on its side. Uranus is the only planet except Pluto that goes about its orbit with one of its poles facing the sun.
"We know something cataclysmic happened to Uranus early in its formation that pumped up the heat in its environment," he said. "There's no question that as we move closer and closer to Uranus, we see an increasing ferocity in the way its moons have been worked over."
Soderblom said that scientists have to ask themselves numerous questions in an attempt to understand Miranda. "Was it part of a larger object? Was it exploded into fragments many times? We're kicking around embryonic notions about this. Give us at least 24 hours, and we'll try to come up with some answers."
The pictures revealed features less than a mile across with clarity never seen before. One reason for the sharp and detailed pictures of Miranda is that the spacecraft passed only 18,000 miles from the moon's surface. Another reason is that engineers devised a way to hold the spacecraft so still that its cameras were taking exposures of Miranda for as long as 16 seconds.
While the photographs of Miranda may be the highlight of the mission to Uranus, Vogager 2 was busy making other discoveries today.
An instrument called the photo polarimeter saw fragments of 10 new rings about 20 miles long curving around the planet in the same manner as the 10 known rings.