A radical view of the planets

The Hubble telescope's sharp pictures are changing our knowledge of our solar system, writes Peter Bond

Next week new photo-graphs of our own solar system taken by the Hubble space telescope (HST) will be unveiled at a meeting of the American Astronomical Association in Pittsburg. It is believed that the telescope may have detected several hundred hitherto unknown comets lurking within the Kuiper belt, a region of the solar system extending beyond the orbit of planet Neptune.

But even in advance of these latest pictures, it is already clear that the HST is rewriting the textbooks about our solar system.

Unlike all ground-based observatories, Hubble is unaffected by air currents which make planets appear to shimmer like some mirage. Since the flaw in its optics was overcome during a highly publicised series of spacewalks in December 1993, the space telescope has been operating to its full potential.

Lurking on the edge of the solar system are Pluto, the smallest planet, and its moon, Charon. The space telescope has obtained the clearest view yet of these remote worlds, although even Hubble cannot see any surface features on them without computer image enhancement. The pictures show that Charon is bluer than Pluto, indicating that the two bodies are very different. A bright highlight on Pluto suggests that its icy surface is acting like a smooth mirror.

At present, Neptune has temporarily supplanted Pluto as the furthest planet, 2.8 billion miles from the sun. Until the Voyager spacecraft flew past in 1989, very little was known about this gaseous giant. Despite a temperature several hundred degrees below zero, astronomers were surprised to find a world of hurricane-force winds and huge atmospheric storms. One of these, labelled the Great Dark Spot, was sufficiently large to swallow the earth, and was thought to be similar to Jupiter's Great Red Spot, which is hundreds of years old.

As it turned out, any similarity was purely coincidental. When the refurbished Hubble took its first look at Neptune, the planet's entire appearance had radically altered. The Great Dark Spot in the southern hemisphere had disappeared. Instead, a brand new dark spot had emerged in the northern hemisphere. Surrounded by bright, high-altitude clouds of methane ice, it seems to be a mirror image of the original Voyager feature. Still more surprising was the discovery that Neptune's cloud features can change radically over a matter of days, leading to speculation that some internal heat source is driving the dynamic atmosphere.

The same cannot be said of the other blue giant, Uranus. Voyager images taken in 1986 revealed a disappointingly bland atmosphere, resembling a pale blue snooker ball. Since then, no one has been able to observe any features, small inner satellites or narrow rings, because Uranus is so far away.

Now Hubble has been able to confirm the presence of a high-altitude haze over the South Pole and found two enormous cloud plumes in the southern hemisphere. It has also photographed the motions of the tiny chunks of rock, none more than 50 miles across, which orbit just outside the ring system.

Hubble's sharp vision means that changes in the atmospheres of the closer gas giants, Saturn and Jupiter, are much easier to see. Last year, astronomers were astonished by its images of Jupiter's collision with the fragments of comet Shoemaker-Levy 9. Since then the space telescope has watched as the brown smudges of material dredged up from the depths by the explosive impacts have spread out around the multi-coloured cloud tops.

In the past five years, Hubble has also imaged two vast storms which have erupted in the pale yellow clouds of Saturn. The most recent, discovered in September last year, was as wide as the earth, while the 1990 storm was even larger. Only five such storms have been seen over the past two centuries.

One of the most intriguing places in the solar system is Saturn's moon, Titan. The Voyager showed the planet-sized satellite to be covered in a dense orange blanket of hydrocarbons. Using near-infrared wavelengths to peer through the haze, Hubble has obtained the first pictures of large- scale features on the moon's surface. At present, the nature of the light and dark patches in the images is unknown, although astronomers have long believed that Titan has continents separated by an ocean of ethane and methane.

Just as bizarre is Vesta, a rocky asteroid 325 miles across which orbits the sun between Jupiter and Mars. By detecting features as small as 50 miles across, Hubble has shown that Vesta's surface is ancient and bears the scars of four billion years of bombardment. Among the unexpected features are ancient lava flows and a gigantic impact basin which resembles a bite taken out of its side. Among the chunks which have been broken off Vesta are a number of meteorites which plunged to earth.

Still closer to the sun are Mars and Venus. No spacecraft have visited Mars for many years, but Hubble is maintaining a close watch on the Red Planet. In March, scientists reported that Mars is much cooler and drier than normal, with large clouds of ice crystals seen over the huge volcanoes and impact basins.

Venus, on the other hand, is the hottest planet in the solar system due to its all-embracing cloud cover and intense greenhouse effect. Although Hubble is unable to penetrate the cloud blanket, observations in ultraviolet light do show faint markings. More significantly, Hubble's spectrograph has shown that amounts of sulphur dioxide in the atmosphere are steadily falling, firm evidence that the planet's numerous volcanoes have reduced their emissions of this noxious gas and are now relatively dormant.

Further discoveries are expected over the next 10 years, and astronomers are already looking forward to even better pictures when more advanced cameras are installed during the telescope's next service in 1997.