Uranus: new details, new enigmas

Press, 4 February 1986, Page 20

Uranus: new details, new enigmas

From ‘The Economist,’ London

William Herschel’s suspicions — reported to the Royal Society in 1781 — were wrong. What the English musician had found with •the telescope he built in his back garden was not a comet but the giant gas planet Uranus — the seventh planet from the sun and the first to be discovered since ancient times.

Uranus, 15 times heavier than the earth, eluded ancient astronomers because the methane in its atmosphere traps red light, leaving the planet to reflect dim greens and blues. Herschel’s telescope discovered that it existed, but not much more.

Now Voyager-2, one of a pair of American spacecraft launched on a safari through the solar system in 1977, is producing images of the Uranian world that Herschel could never have imagined. Uranus is nearly two billion miles from earth. On Friday, January 24, Voyager sailed within 50,600 miles of its cloud-tops with cameras rolling. Man is profoundly ignorant about Uranus. The planet is so deep in space that its thin rings were not spotted until 1977, and then only by indirect “occultation”: astronomers saw a bright star blink several times just before Uranus passed in front of it.

ON Tuefday the i3tli of March, between ten and eleven in the evening, while I was examining the fmall ftars in the neighbourhood of H Geminorum, I perceived one that appeared vifibly larger than the reft: being ftruck with its uncommon magnitude, I compared it to H Geminorum and the fmall ftar in the quartile between Auriga and Gemini, and finding it fo much larger than either of them, fufpeded it to be a comet.

Its moons were also slow to emerge. Herschel sighted two — Oberon and Titania — six years after first sighting Uranus. Ariel and Umbriel were discovered in the nineteenth century. Tiny Miranda was not discovered until 1948.

Voyager’s flypast is producing a Copernican leap in knowledge about the planet. Even when it was still a long way off, Voyager began to discover a flock of hitherto unknown moons and moonlets — an additional seven for a total of 12 at least

But its instruments have embarrassed scientists by failing to detect several things they confidently expected it to confirm, such as evidence of a strong magnetic field. Even the nondiscoveries have made existing textbooks instantly out of date.

Counting moons is Voyager’s easiest job. With luck, Voyager will also help discover what they are made of. That, in turn, might begin to solve Uranus’s biggest mystery: its eccentric posture. While all other planets spin on an axis perpendicular to their orbit around the sun, Uranus lies on its side: its axis during the Voyager encounter points directly at the sun. The most exciting hypothesis is that the planet was toppled over by another object. Scientists reckon a collision with something the size of earth, travelling at about 40,000 miles per hour, could have done it. It might also have heaved Uranian rocks and gases into space where they formed a disc around the planet which eventually condensed into moons.

Some clues about the moons have come from ground observations. Comparisons between the wavelengths of the sunlight re-

fleeted from the moons and the spectral signatures of known chemical compounds suggest that, like the moons of Saturn and Jupiter, they are wrapped in ice.

There is also something darker — probably methane decomposed into dark organic polymers by sunlight or galactic radiation. Far more detail about the lunar surfaces will emerge after careful study of Voyager’s photographs of Miranda, taken from a mere 18,000 miles.

Voyager’s photographs may also provide hints of what is ' inside the moons, not just of what lies on their surface. The best clue to the composition of a body in space is its density, which can in turn be deduced

from its mass and diameter.

Voyager’s photographs give scientists their first chance to make accurate measurements of the moons’ diameters. Wobbles in Voyager’s trajectory as it sweeps through Miranda’s gravitational field will also sharpen earlier estimates of Miranda’s mass. Armed with revised estimates of the density of the Uranian moons, scientists will be better placed to judge one tantalising fragment of evidence that supports the collision theory. Dr Christian Veillet, of the Centre d’Etudes et de Recherches Geodynamiques in Grasse in France, recently calculated the density of the known moons. The data is tentative but preliminary results suggest that Ariel and Umbriel, the inner moons, have thick icy layers and small rocky cores while Titania and Oberon, the outer ones, are made mainly of rock with a thin coat of ice. If the moons had formed at the same time as Uranus, by the condensing out of a swirling protoplanet, the opposite would be expected: more heavy material would have condensed near the planet and the proportion of rock would therefore be greater Inside the inner moons. That is how Jupiter’s moons are organised. Dr Veillet’s calculations, if they are confirmed, strengthen suspicions that Uranus’s moons are indeed the debris of a collision.

Until Voyager began to close in on it, most scientists thought

that Uranus had a strong magnetic field. In 1982, an astronomical satellite in orbit around earth detected strong ultraviolet radiation from Uranus. Scientists jumped to the obvious conclusion: that the source of the radiation was a bright aurora created within an enormous Uranian magnetosphere — a planetary envelope of charged particles. This conclusion has crumbled with Voyager’s approach. A large magnetosphere should long ago have generated radio noise for Voyager to hear. Yet the first faint peeps were not picked up until days before the encounter with Uranus. The planet’s stubborn radio silence has flustered scientists. One new theory is that Uranus’s gassy bulk contains a' core no bigger than earth’s; it generates an earth-sized magnetic field, but one which tapers' sharply off at Uranus’s cloudtops. The absence of a shielding magnetosphere could explain the blackness of Uranus’s rings and moons. It would allow the solar wind to remove ice, leaving carbonaceous material behind.' But if there is no magnetosphere, where does Uranus’s ultraviolet radiation come from? Ideas abound, although some scientists are still committed to a strong magnetosphere. Perhaps, they plead, it is so strong that if is beyond the sensitive range of Voyager’s radio receivers. Copyright — The Economist.