Looking back at Uranus: strangeness confirmed.

Looking Back at Uranus: Strangeness Confirmed

Only four days after the Voyager 2 spacecraft's Jan. 24 flyby of Uranus produced the first close look at that distant planet, world attention was abruptly wrenched earthward again by the explosion of the space shuttle Challenger. But while NASA struggles to get back up off the ground, the Voyager scientists have been poring over their data, learning their way around the strange Uranian system.

The first results of their studies -- as opposed to the "instant science" of the days immediately following the encounter -- were published in the July 4 SCIENCE. And not only have they markedly refined many of the quick-look details, such as the sizes of the planet's 15 known moons (10 of which were Voyager 2's discoveries), but Uranus and its attendant phenomena turn out to be at least as dramatic as the initial results had hinted.

The moons: Outermost Oberon, besides revealing a mountain that the images show to be protruding at least 20 kilometers above the horizon, bears a number of straight and curved escarpments that the Voyager scientists believe to be evidence of a "global-scale tectonic episode" in the past. Titania, too, shows an extensive pattern of faulting that seems to harken back to some process that caused a global extension of the crust, they report, such as might have resulted from "the late stages of freezing in the interior of the satellite."

Umbriel, much darker than the four other major moons and "strikingly" uniform in its somber complexion, leaves scientists puzzling over the question of where all the dark material might have come from on the presumably ice-rich satellite. One suggestion is that another object struck Umbriel and shattered, generating dark fragments that later settled all over the surface. A problem with such a scenario, however, is that unless such an impact were relatively recent, there ought to be evidence of later impacts that punched through the dark stuff to reveal lighter material below. Unless, of course, the surface and subsurface material are similarly dark, and "extremely uniform to a substantial depth on a global scale."

Far more complex-looking is Ariel, with valleys, cliffs and other scars in profusion. Yet the floors of many of its features are surprisingly smooth, as is an extensive, irregularly shaped plain. Whatever did the smoothing, the researchers maintain, "has clearly been emplaced, at least in part, as a flow or sequence of flows that overlaps and partially buries older craters." The nature of the flows on Ariel, and on Titania as well, is uncertain. Water ice, assumed to be the major component of the satellites, has a melting point about 200 K (360[deg.] c) higher than the ambient surface temperature, according to the Voyager team, though an ice mixture of ammonia and water could do the job with far less heat. Another possibility could be "tidal heating," caused by the same multi-satellite gravitational tug-of-war believed to drive the volcanism on Jupiter's active moon Io.

Strangest of all is Miranda (SN: 2/15/86, p. 103), with at least three large, closed patterns of light and dark bands, scraps and ridges, from about 100 to 300 km wide. At first look, the baffled Voyager imaging team dubbed them "circi maximi," after the ancient Roman racetrack; now they have been separately -- though not much more commitally -- named the "trapezoid," the "banded ovoid" and the "ridged ovoid," while the scientists wonder what caused Miranda's tortured geologic evolution.

"Miranda probably was catastrophically disrupted and reaccreted several times," they say, "by impact of objects large enough to produce a crater equal to or larger than the diameter of the satellite." In fact, they suggest, "Ariel was probably disrupted and accreted at least once; Umbriel may have been disrupted once. There is a fair chance that Titania was also disrupted."

The only one of the 10 newly discovered satellites to be even blurrily photographed, designated 1985U1, provided a different kind of surprise by turning out to be unexpectedly spherical. Most such small solar system objects are irregularly shaped, lacking the self-gravitation to pull them into roundness, yet 1985U1, about 170 km across, even survived an impact big enough to form a 45-km crater.

The rings: In addition to the nine rings previously known from earth-based observations of their blockage of starlight, Voyager 2 photographed "on the order of 10.sup.2 new ringlike features...interspersed within the main rings, as well as a broad, diffuse, low-optical-depth ring just inside the main ring system." Besides cameras, the spacecraft carried an instrument called a photopolarimeter, which racked the light of two stars through the ring system and recorded the resulting blinks. In addition to measuring the thickness (less than 150 meters) of the outermost of the nine previously known rings, the instrument showed that many of the rings vary not only in width but also in "optical depth," or density. It also revealed a number of "partial rings," or "ring arcs," a phenomenon that had been thought to be improbable at best until earth-based, stellar-occulation observations indicated what appear to be such arcs around the planet Neptune.

One striking characteristic of the Uranian ring system is that it turns out to have almost none of the extremely tiny, "smoke-to-dust-sized" particles that scientists expected to be common among the larger chunks. One inference from the missing dust has been the view of some scientists that various processes may be sweeping the finest particles away. This, combined with the observed variations in ring opacity and apparent ring arcs, suggests to some of the Voyager researchers that the rings may be a constantly evolving phenomenon, and perhaps quite young.

The magnetic field: The finding that Uranus even has a magnetic field resolved a baffling riddle, which had been fueled by earth-based satellite observations showing what seemed to be an aurora (presumed signs of a field) while Voyager's radioastronomy instrument kept falling to pick up radio emissions that should also have been present. Only five days short of Uranus, the device finally picked up the signals, and then found out the reason for the delay: The axis (dipole) of the Uranian magnetic field is tilted about 60[deg.] away from the planet's axis of rotation, presumably directing the signals in a different direction. Furthermore, the Voyager magnetometer team suggests, the radical tilt may be a sign that the polarity of the field is undergoing a reversal, a phenomenon long assumed to have taken place on earth, based on past geologic data, but never actually measured in progress for any planet.

The radioastronomy instrument was also able to determine the length of a Uranian day, 17.24 hours, based on the modulation period of the radio signals.

There is far more in the Voyager 2 data bank, and scientists are likely to study everything to the nth degree, given the uncertainty of when another spacecraft may pass that way again. Voyager 2, meanwhile, is due at Neptune in 1989.