It's raining on Jupiter, according to the latest results from the Galileo spacecraft.
Although the probe that plunged into the giant planet's atmosphere two years ago found no trace of water, researchers at the Jet Propulsion Laboratory in Pasadena are now convinced that the miniature weather station merely hit a patch of unusually clear, dry weather--the Death Valley of Jupiter.
The rest of the enormous sphere of swirling gases, they believe, is a lot like Earth--wet in some places, bone-dry in others. The main difference is that storms on Jupiter can last for hundreds of years.
A ball of gas so huge that it holds dozens of moons in its orbit, Jupiter should contain copious amounts of oxygen and hydrogen--the ingredients of water. As the probe descended through surface clouds of frozen ammonia on Dec. 7, 1995, researchers expected the weather to be wet and wild, with layers of both hydrogen sulfide and water clouds.
"When we got there, we didn't see any of these," said Toby Owens, a planetary scientist from the University of Hawaii. Instead, the probe entered a massive hot, dry spot.
However, new images from instruments on the orbiting spacecraft have detected clear evidence of water lurking perhaps 60 miles beneath the impenetrable ammonia cloud cover. "There is weather on Jupiter," JPL's Robert Carlson said.
What's most surprising, said chief Galileo scientist Torrance Johnson, is the wild variation in wetness--from completely dry to almost total saturation. "It means that water in the atmosphere is much more spotty than anyone had expected," he said.
In saturated areas, "It's either about to rain or it's raining or snowing right now," Carlson said. In the dry areas, huge downdrafts suck out every drop of moisture.
Although no rain or lightning is visible directly in the images, patches of rapidly moving high white clouds are evidence of violent storm activity, said Caltech graduate student Ashwin Vasavada.
The smallest of the thunderstorms are comparable to large storms on Earth. They rise above the lower cloud layer about 40 to 60 miles. Unlike Earth's atmosphere, which coats our planet with a thin layer, Jupiter's weather is a seemingly bottomless pit. Rain doesn't fall into waiting oceans, but instead evaporates in the heat of the giant planet's interior, condensing into water again, like the inside of a very hot, humid shower.
Perhaps because Jupiter's weather systems are so deep and have so much inertia, storms on Jupiter can be predicted "many times better than we can on Earth," said Caltech planetary scientist Andy Ingersoll.
Along with water, Galileo found Jupiter's atmosphere is "enriched" by copious amounts of oxygen, sulfur and carbon that must have been carried to the giant planet by comets--perhaps, in crashes similar to the planet's spectacular encounter with Shoemaker-Levy in 1994. Earth probably acquired these essential ingredients for life through a similar shower of comets.
The main lesson scientists learned from the probe's failure to detect water on its pass through the Jovian clouds is that they can't get a clear picture of a giant planet from looking at a tiny sample. "I'd like to have more probes,"Ingersoll said. "NASA headquarters, are you listening?"