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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