Mysteriously muddled Miranda.

Mysteriously muddled Miranda

"Miranda is unlike any satellite we've ever seen before," says Bob Brown of Jet Propulsion Laboratory (JPL) in Pasadena, Calif.

"If you can imagine taking all the bizarre forms in the solar system and putting them on one object," says Laurence Soderblom of the U.S. Geological Survey in Flagstaff, Ariz., indicating one of the Voyager 2 spacecraft's photos of the fifth-largest moon of Uranus, "you've got it in front of you."

"It's among the most enigmatic objects in the solar system," says Voyager imaging team leader Bradford Smith of the University of Arizona in Flagstaff, who rates it "a toss-up," even in comparison with Io, the Jovian satellite whose active volcanism has been one of the major discoveries in the history of planetary exploration by spacecraft.

Miranda is a little moon, barely 300 miles across. Not only many years ago, scientists would have expected such a small object to have had a fairly dull geologic history, born with too few heat-producing radionuclides to have generated much in the way of internal tectonic thrashings, let alone actual volcanism. Such views were wrenched out of shape in 1980 and '81 by Voyager 1 and 2, whose close-ups of the moons of Saturn -- some no bigger than Miranda -- revealed not only vast grooves and fractures but also huge, smooth areas on otherwise crater-ridden surfaces. It was as though the ice-clad terrain, hard as rock at Saturn's distance from the sun (and overlying interiors that density calculations indicated to be largely ice themselves, with even less likelihood of buried radionuclides), had been somehow softened, erasing the evidence of past scarrings.

Other hypotheses were offered, such as tidal heating (cited by many as the key factor driving Io's volcanoes) due to gravitational tug-of-wars that may have stressed a given satellite between Saturn itself and one or another of its nearby moons. The effect could be enhanced, it was proposed, if the affected objects included ices such as methane and ammonia, which could be softened with less heat. Planetary scientists were having to learn whole new disciplines.

But Miranda is stranger still. Voyager 2's precicous treasure-trove of pictures of Miranda numbers barely a dozen frames (many of which have been combined into the preliminary photomosaic above), but they are more than enough to provide clear evience of Miranda's exciting -- and mystifying -- past.

From nearly 900,000 miles away, the day before the spacecraft's closest approach and with most details still all but unrecognizable, one photo revealed a large dark feature bearing a brighter V-shape that was promptly dubbed "the chevron." A closer look provided no easy answers when the dark patch turned out to be nearly rectangular. (Whatever happened to "Nature abhors a straight line?") "Mosaicking" the images together further showed two sides f the rectangle to be aligned with a pair of seeming fracture zones that extend all the way to the horizon, nearly 150 miles away, at approximately a right angle.

Also conspicuous on the face of Miranda are at least two other huge patches (Voyager 2's close-ups cover only about half the surface), similar to the one containing the chevron but more rounded at the corners. And all three are bordered by families of grooves or lineaments, nested one outside the other like the lanes of a gigantic racetrack.

Voyager's past images of the satellites of Jupiter and Saturn provided no precedent for such' a phenomenon, even among the multiple groove-families on the Jovian moon Ganymede. A typically stuck for a bit of descriptive jargon, the imaging-team scientists have adopted the tentative term "circus maximus," after the great chariot-race course of acient Rome.

Having had barely three weeks to study the images, the researchers have agreed on no common explanation for the strange patterns. At a team meeting at JPL on Feb. 5, says Smith, the discussion of Miranda "just went around and around and around." According to assistant team-leader Soderblom, "We're back in the 19th century -- all we're doing is classifying things."

One of the most curious aspects of the "circi maximi," notes Smith, is that the terrain outside them seems so unaffected by their presence. Compared with the stark contrast between the circi and their intervening ordinariness, in fact, he says, even a spectacular like the chevron "is not so bothersome."

Some of the team members have observed that the three circi seem to suggest the same phenomenon with different degrees of evolution. The one containing the chevron could be the least refined, while the one in the lower left corner of the photomosaic may show the most worn appearance, though no one is going so far out on a limb as to assert one circi to be older than the other. Even the less-disturbed-looking "inter-circus plains," however, are anything but dull, bearing intricate patterns of intersecting faults and fissures that one team-approved photo caption describes as "bewildering."

"Isn't it wonderful?" says Soderblom of Miranda's complexities. Says Smith, "It looks like a satellite designed by a committee."