Galileo teases out Jupiter's secrets

时间:2019-02-27 01:10:01166网络整理admin

By Justin Mullins PICTURES from Galileo, the NASA spacecraft watching Jupiter, have revealed huge thunderclouds on the giant planet. Their existence may help to explain one of the strangest mysteries in the Solar System: why planetary winds are more ferocious the further a planet is from the Sun. Last week, NASA released photographs of Jupiter’s Great Red Spot, a gigantic hurricane bigger than Earth that has raged for at least a century. The pictures taken by Galileo show thunderclouds 100 kilometres wide rising 50 kilometres above the other clouds, just like the anvil-head clouds associated with storms on Earth. “We always expected Jupiter to have thunderclouds but this is the first time we’ve seen them,” says Andrew Ingersoll, a planetary scientist at the California Institute of Technology who has analysed the images. “They’re huge by terrestrial standards and create winds around 300 kilometres per hour.” Ingersoll believes the storms are powered by a layer of steam that lies unseen beneath the cloud tops. On Earth, thunderclouds form when evaporated water condenses and releases heat. But a probe dropped into the Jovian atmosphere last December failed to find water in the expected quantity. Ingersoll says the images may help to solve this mystery. Jupiter’s storms seem to be clustered to the north of the Great Red Spot. “Maybe that’s where the water is,” he says. Another possible explanation for the strong winds is that Jupiter may have far fewer thunderstorms—which dissipate energy—than Earth, says Ingersoll. “Without this dissipation the winds may just get stronger.” He says that although it sounds paradoxical, the reason why there are fewer storms might be because there is less energy from the Sun to trigger them. Ingersoll’s ideas could explain why winds are even faster on the more distant gas giants. For instance, Neptune receives only a twentieth the amount of sunlight that falls on Jupiter, yet has winds nearly three times as fast. “Perhaps Neptune has even fewer thunderstorms,” he says. NASA also released new images of the Jovian moons Europa and Io. The images of Europa are twice as clear as those taken by the Voyager spacecraft in the 1970s. They show that the moon’s icy surface may be older than scientists had thought. The surface of a moon can be dated by counting craters, which provide a record of how many meteors have hit it. Voyager’s images show only a few craters on the entire moon and this led scientists to believe that Europa’s surface may be continually renewed by liquid water from beneath. But the new pictures reveal many more craters along the moon’s terminator—the dividing line between night and day where the craters cast shadows that are easy to spot. The terminator is a few hundred kilometres wide and about 2000 kilometres long, and represents only a small fraction of the surface. “We’re seeing dozens of craters in this area,” says Ronald Greeley, a planetary geologist at Arizona State University and a member of the imaging team. Greeley has not yet estimated the date of the surface. “We’re doing an accurate crater count right now but the surface is certainly older than the Voyager data suggest,” he says. The question of whether liquid water lies beneath Europa’s icy surface—and whether it might harbour some form of life—remains unanswered. “There may be water but there may just be warmer ice,” says Greeley. Future images may help to resolve this question. Last week’s pictures have a resolution of only 1.6 kilometres per pixel, but in December Galileo should send pictures with a 22-metre resolution. The new pictures of Io may help to explain why the moon has red polar caps. Io is the most volcanically active body in the Solar System, heated by gravitational squeezing as it orbits Jupiter. Both Voyager and Galileo pictures show active volcanoes. But the recent images also show areas the size of Belgium that have changed from deep red to a pale yellow since the Voyager flyby 17 years ago. Scientists believe much of the debris in these areas is sulphur which comes in many forms and colours. “If the red areas were short-chain sulphur molecules of S3 and S4, the colour change may be due to phase transformations to more stable, pale yellow forms of sulphur,” says Alfred McEwen of the Lunar and Planetary Laboratory at the University of Arizona. But Io’s poles are colder than the rest of the moon. “Any change would be slower at colder temperatures,