Jupiter Caught in Rare, Calm Period

Jupiter's atmosphere froths with violent winds and mega-storms as large as the entire Earth, but a recent spacecraft flyby captured the planet in an "unusually calm period," astronomers said. Calm on Jupiter, however, still makes terrestrial hurricanes look like breezes.

The New Horizons spacecraft, bound for Pluto on a nine-year journey, caught Jupiter off-guard in February 2007 and allowed astronomers to gather hordes of new information about the Jovian giant.

"Jupiter changed its attitude right before the flyby," said Kevin Baines, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Every other time we've looked at the planet with the Voyager, Galileo and Cassini spacecraft, we've seen a very traditional view of Jupiter."

Nine separate studies on Jupiter, three of which detail some of this new information about the planet's atmospheric phenomena, will be published in the Oct. 12 issue of the journal Science.

Wrinkles and burps

Jupiter's equator is usually one of its most violent places, where clusters of massive storms emerge just behind the planet's infamous Great Red Spot. During the recent flyby, however, the storms were diminished and a normally thick band of cloud cover along the equator was thinned out.

The calmed Jovian surface gave astronomers an unprecedented look at a strange band of wave-rippled clouds near the surface.

"These waves have been seen before, but we've never been able to measure their speed," said Dennis Reuter, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "We got lucky this time and clearly saw a whole train of them."

Reuter and his team used a 41-minute observation to discover that energy waves were traveling through the clouds at about 537 mph (240 meters per second). The clouds themselves traveled at about 224 mph (100 meters per second) on average.

"That's about a quarter of the speed of sound, which is pretty impressive," Reuter told SPACE.com. Category five hurricanes can't generate such speed on Earth; only extreme tornadoes and mountain wind gusts can compete.

The astronomers also saw enormous "burps" of ammonia gas breaking Jupiter's surface, just southwest of the Great Red Spot. "These clouds only lasted for about 40 hours, then dropped back down," Reuter said, but he noted that the clouds should help scientists better understand what is going on underneath Jupiter's thick outer atmosphere.

Reuter thinks the rippling activity and ammonia plumes are driven by heat from deep beneath the Jovian surface, which Baines said remains as one of the solar system's great mysteries.

"The sun can't account for all of the heat we see coming from Jupiter," Baines said. "It's putting out about twice the energy that it should be."

Polar lightning, at last!

To find out more about Jupiter's mysterious inferno, Baines led a search for long-sought lightning strikes at Jupiter's chilly poles. He found them.

"This is the first polar lightning we've ever seen on a non-terrestrial planet," he said. "Other spacecraft that visited Jupiter never saw it."

Some of the lightning strikes seen during the New Horizons flyby were about 10 times more energetic than Earth's strongest bolts and occurred about 50 miles (80 kilometers) below Jupiter's surface. Baines thinks that hot gases-including water vapor-are rising from deep within the planet to cause the lightning.

"On Earth we see most lightning near the equator, where warm moist air is rising up above colder, denser air," he said. "We now know that Jupiter's lightning occurs all over the planet, so some uniform internal heat source has to be driving the activity."

Baines said the finding is a relief for scientists trying to get to the bottom of Jupiter's heat source because the sun, about 484 million miles (779 million kilometers) away, can only do so much. So far, the leading theory is that the planet's crushing mass generates its warmth.

Come to the dark side

Another study led by Randy Gladstone, a planetary scientist at the Southwest Research Institute in San Antonio, focused on night-side observations of Jupiter to investigate its bright auroras and eerie "airglow."

Like auroras above the Earth's arctic regions, auroras above Jupiter's north pole are caused by magnetic fields slamming charged particles—from the sun or the magnetosphere—into the planet's atmosphere. Gas molecules ionized by the sun's rays, however, lead to airglow often seen high above the Jovian surface as well as above tropical regions on Earth.

"I was hoping to map out these bright emissions on Jupiter's night side, but we hardly saw any activity," Gladstone said, noting that Voyager observed the faint glow during its visit more than 25 years ago. Why the airglow wasn't detected on Jupiter's dark side continues to puzzle Gladstone and his team.

"We're sure New Horizon's instruments were functioning correctly, so Jupiter was probably acting up when we observed it," he said.

Baines said that while New Horizons has deepened some Jovian puzzles, such as the lack of night side airglow seen on the planet's night side, he noted that the flyby data will provide "calm" examples to compare to Jupiter's typical conditions.

"We observed the planet during its calmest period ever recorded," Baines said, noting such an event might happen only a few months every 40 years or so. "This gives us a baseline to compare to data in the future."

Baines, who was at the American Astronomical Society Division for Planetary Sciences meeting in Orlando, Fla., told SPACE.com that the calm period may already be over, as astronomers at the meeting recently reported observing Jupiter returning to normal.

"Jupiter has apparently stopped being calm and is back on track to being its usual, violent self," he said.

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