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Jupiter Caught in Rare, Calm Period

Jupiter Caught in Rare, Calm Period

Dave Mosher
Staff Writer
SPACE.comTue Oct 9, 10:45 AM ET

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.

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."

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 lighting 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 779 million miles (1.25 million kilometers) away, can only do so much. So far, the leading theory is that radioactive materials in the planet's dense, rocky core are generating its warmth.

Another study led by Randy Gladstone, an astrophysicist 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 into the planet's atmosphere. Gas molecules excited directly by the sun's rays, however, create 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 almost certain 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 100 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.

Also:-

Spacecraft Surfs Jupiter's Magnetic Tail

Spacecraft Surfs Jupiter's Magnetic Tail

Ker Than
Staff Writer
SPACE.com2 hours, 30 minutes ago

NASA's Pluto-bound spacecraft, New Horizons, recently surfed a long tail of charged particles trailing behind Jupiter. Observations from that wild ride revealed enormous bobbing bubbles of charged particles, or "plasma," and showed that the structure of the planet's tadpole-shaped "magnetotail" is surprisingly varied.

The findings, detailed in two reports in the Oct. 9 issue of the journal Science, could help scientists understand the protective magnetic environment surrounding Earth and other planets.

"If we understand our Jupiter better, we will be able to further understand the extrasolar 'hot Jupiters' of other stars," Norbert Krupp, an astronomer at the Max Planck Institute for Solar System Research in Germany who was not involved in the studies, wrote in a related Science article.

In the space surrounding many of the planets in our solar system, there is an ongoing struggle between the magnetic fields of those planets and fast-moving charged particles of the sun's solar wind. The region around a planet where the magnetic field is strong enough to slow down or even repel the solar wind is called the magnetosphere.

The Jovian magnetosphere is enormous. It has a diameter 200 times that of Jupiter itself and is the largest cohesive structure in the solar system. Despite Jupiter's great distance from us, "If you could 'see' the magnetosphere of Jupiter from Earth, it would be about the size of the full Moon," said Ralph McNutt, a senior scientist at John Hopkins University's Applied Physics Laboratory and a lead author on one of the studies.

The side of Jupiter's magnetosphere facing the sun gets squashed by the barrage of oncoming solar-wind particles, but the opposite side is distended like the tail of a comet.

As part of a slingshot maneuver to shorten its journey toward Pluto, New Horizons entered Jupiter's magnetosphere in February 2007 and journeyed down the magnetotail for more than a hundred million miles—longer than any other spacecraft that had visited before.

Inside, New Horizons whizzed past relatively slow moving blobs of plasma, or "plasmoids," that bobbed along inside the magnetotail, guided along by Jupiter's magnetic field.

Scientists think the bubbles are formed from material ejected by Io, a Jovian satellite and the most volcanically active body in the solar system. Once ejected, the particles that make up Io's prodigious debris—the moon spews about 1 metric ton of material per second—are stripped of their electrons by particles in Jupiter's magnetosphere and become captured by the magnetosphere. The snared particles linger around Jupiter like a cloud.

McNutt and his teammates propose that Io's captured particles stretch Jupiter's magnetic field lines like rubber bands, and that occasionally, the field lines snap back into place in "magnetic reconnection" events.

Like elastic thread slicing through gelatin, scientists think the snapping field lines carve out huge chunks of the plasma blobs around Jupiter. These chunks are the plasmoids.

The snapping motion also imparts energy to these plasma bubbles and provides the acceleration needed to propel them down Jupiter's magnetotail.

New Horizons also detected another class of very hot charged particles hurtling down the magnetotail, which cooled and slowed as they moved away from the planet. Some of the particles originated from Io, but others came from the solar wind and Jupiter's atmosphere. The last source was a surprise to scientists.

"It's clear there's a significant escape of the material from the planet because the brightest burst we see turns out to be material that's largely from Jupiter, not from the solar wind or Io," said David McComas, the principal investigator of New Horizons' Solar Wind Around Pluto (SWAP) instrument and lead author of the other Science study.

The spacecraft also found that, in contrast to Earth's tail, the Jovian magnetotail is surprisingly structured, containing both gradual variations and sharp boundaries in the plasma density.

"There are reports of observations of the Earth's tail as far as about 1,000 Earth radii downstream by the Pioneer 7 spacecraft, but these were intermittent and the structure was certainly not as well ordered" as Jupiter, McNutt said.

Jupiter's magnetotail is long, but it is not infinite. At some point, the gas planet's influence is no longer felt and the magnetotail tapers out, blending into the solar wind.

The plasmoids likely lose their shape as well at those distances, McNutt said, and their particles probably merge with those from the sun.

Finally:-

Mystery of Saturn's Two-Faced Moon Solved

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No, seriously though ...

Mystery of Saturn's Two-Faced Moon Solved

Jeanna Bryner
Staff Writer
SPACE.comTue Oct 9, 8:45 AM ET

Saturn's moon Iapetus has virtually no gray. Rather, its features are all stark black and white. The appearance has long puzzled astronomers.

New detailed images suggest sunlight is melting ice on one side of Iapetus, leaving the moon's dark surface exposed, while the opposite half retains its reflective ice-mixed shell.

Since the moon's discovery by Giovanni Domenico Cassini in 1671, Iapetus' appearance has baffled astronomers. The leading edge of Iapetus, which faces the direction of its orbit, is black as asphalt, while its trailing side appears bright as snow. Iapetus is 907 miles (1,460 kilometers) wide and circles Saturn at a distance of about 2.2 million miles (3.6 million kilometers).

High-resolution images of Iapetus acquired last month by the Cassini-Huygens spacecraft during its low pass over the moon have uncovered telling details on its surface that may well yield the reason for its strange bright and dark patterns.

"While there are many details yet to be worked out, we think we now understand the essence of why Iapetus looks the way it does," said Carolyn Porco, the leader of the imaging team at the Space Science Institute in Boulder, Colo.

The new observations add support to a two-part explanation for Iapetus' appearance. First, as Iapetus treks around Saturn, its leading edge scoops up a thin coating of dark material, which amplifies sunlight absorption.

"Dusty material spiraling in from outer moons hits Iapetus head-on and causes the forward-facing side of Iapetus to look different than the rest of the moon," said Tilmann Denk, Cassini imaging scientist at the Free University in Germany.

Over time, as the black-ish surfaces warm, the rate of evaporation increases until finally all the surface ice in that region melts away. Infrared observations from the Cassini flyby confirm the dark dust material is approximately -230 degrees Fahrenheit (-146 degrees Celsius)--warm enough for the release of water vapor from the ice.

The water vapor formed then condenses on the nearest cold spot, such as along polar regions and icy areas at lower latitudes on the trailing side of the moon. In that way, the dark material loses the mixed-in ice and gets even darker, while the bright material accumulates more ice and gets brighter, in what the astronomers call a runaway process that leaves no gray area.

So there you have it.