Uranus: strange encounter looming.

Uranus: Strange Encounter Looming

On the surface, as an omen of things to come, it looks pretty dull. In less than 11 weeks, the Voyager 2 spacecraft will flash past Uranus, taking the first close-ups of a planet so distant that it is virtually invisible to the naked eye. Yet photos and other data from the rapidly approaching probe still reveal only a featureless fluffball, with no radio emissions (Jupiter's can be "heard' all the way to earth) and not even a trace of the planet's bizarre rings, which were discovered from earth eight years ago when they blocked the light of a star.

But the encounter will be anything but dull. And at last week's annual meeting of the American Astronomical Society's Division for Planetary Sciences in Baltimore, strange Uranus was looking more mysterious than ever.

"Let me give you a sense of our rising level of panic,' quipped Michael Kaiser of the NASA Goddard Space Flight Center in Greenbelt, Md., as he reported that Voyager 2 was still detecting "absolutely no sign' of radio emissions from the planet. Such signals would indicate that Uranus has a magnetic field. And perhpas it has none--except that the brilliant ultraviolet glow that a group of scientists has been monitoring for four years with the earthorbiting International Ultraviolet Explorer satellite would suggest otherwise. The glow, from the Lyman alpha emissions of molecular hydrogen, "is interpreted as an auroral emission from an active magnetosphere,' according to John T. Clarke of the NASA Marshall Space Flight Center in Huntsville, Ala., and his colleagues. In fact, the group notes, "this magnetosphere may be comparable in strength to those of Jupiter and Saturn.'

Such a magnetosphere, however, ought to be producing the radio signals that Voyager 2 has so far failed to find. Stephen A. Curtis of NASA Goddard suggests that they could be present but radiating only from the planet's nightside. On the other hand, according to Curtis and Goddard colleague Norman F. Ness, the lack of a magnetic field (though that would still leave the presumed aurora unexplained) could bear on another, very different problem: the planet's extremely dark rings. One thing a planetary magnetic field does is help keep out the solar wind; without such a field, the researchers note, the solar wind may be constantly eroding away the ices on the particles making up the rings, leaving the particles much less bright than those in the icy rings of Saturn.

Even so, the rings are so far completely absent to Voyager 2's cameras, leaving one of the encounter's major spectaculars yet to come. The earth-based measurements of occulted (blocked) starlight have suggested that there are nine principal rings. James L. Elliott of Massachusetts Institute of Technology and his colleagues report that two of the nine may be "double' rings, consisting of inner and outer components. Of course, the actual number may well be in the hundreds or thousands, given the myriad "ringlets' discovered by the Voyager spacecraft at Saturn. The Uranian rings, in fact, may well be more complex still. Some appear circular, others elliptical (or eccentric), all of them sharp-edged and narrow, and it has been suggested that many of them may be held in place by tiny satellites orbiting just inside and outside each ring.

One of the most unusual Uranus findings presented at the meeting was an infrared spectrum of the planet, reported by Glenn S. Orton of Jet Propulsion Laboratory in Pasadena--and he was anything but relaxed about the seeming implication of his result. "If you get the feeling I'm trying to make you feel as uncomfortable as I do,' he said as he introduced his brief talk, "you're right.'

After having gone over and over the spectrum itself, looking also for possible instrumental errors in the observations, Orton found himself confronted with the possibility that the atmosphere of Uranus may contain about 40 percent helium--5 to 7 times as much as Jupiter or Saturn. The solar system's four big, gassy planets (Neptune is the fourth) have been assumed to have at least similar ratios of helium to hydrogen, and 40 percent helium would require some explaining. It would seem to imply, for example, that the mass of Uranus is distributed more in the atmosphere and less toward its core. It could also radically affect models of how heat is produced in the planet's interior, and could throw off estimates of the planet's upper-atmospheric temperature based on stellar-occultation observations. There may be other explanations, says Orton, but "helium is the simplest thing which explains the shape of the spectrum.'

Voyager 2 is on the way.