Pluto-bound probe's Jupiter vista


Oct 10, 2007

Each time a probe visits, it does so with better instruments

Jupiter proved ready for its close-up when the New Horizons spacecraft flew by earlier this year.

New images and analyses of the massive planet have revealed surprising details of its atmosphere, rings and moons.

They include never-seen-before observations of Jupiter: lightning displays at the poles, mysterious clumps embedded in its rings, and the first movie of volcanic eruption on its moon Io.

Scientist took advantage of the flyby, designed to give New Horizons a gravity boost and shorten its journey to Pluto, to learn more about the Jovian system, and to follow up on previous missions to the gas giant.

"It was a very close flyby - three times closer than Cassini - and our first mission with really modern instruments," said Amy Simon-Miller, a planetary scientist at the Nasa Goddard Space Flight Center.

"The imaging was spectacular," she said.

The pictures, taken during the spacecraft's closest approach to Jupiter in February and March this year, were presented at the American Astronomical Society's Division of Planetary Sciences meeting in Orlando, Florida, in anticipation of their publication in the journal Science.

Big sparks

One result is a comprehensive picture of volcanism on Io.

Along with identifying new volcanic hot spots, the New Horizon movie camera captured a 350km-high plume spewing from Tvashtar, a volcano near the moon's north pole. It is the first observation of an eruption in motion on Io.

"We've seen glimpses of eruptions like this before," said John Spencer, from the Southwest Research Institute, "but we got a lot more detail than we were able to get with any previous spacecraft."

Dr Spencer suggested that some of the sulphur material gushing from Io might become charged, flow along magnetic field lines, and rain into the Jovian atmosphere.

The largest - and most turbulent - planet in the Solar System, the gas giant is 320 times more massive than Earth.

"It's a planet on steroids," said Harold Weaver, from Johns Hopkins University Applied Physics Laboratory. "It rules the Solar System."

New Horizons has provided insight into the forces driving Jupiter's atmospheric dynamics - in particular, its circulation, along with its thermal convection.

Its instruments captured the first-seen flashes of Jovian lightning at the poles, when the spacecraft sailed around the dark side of the planet.

"These are superbolts, a thousand times stronger than what we usually see on Earth," said Dr Simon-Miller.

Scientists had previously seen lightning at the planet's mid-section, but not at high latitudes. Lightning on Earth draws its energy, in part, from the Sun; the electric display at Jupiter's poles, where sunlight is weaker, suggests an internal heat source that is distributed throughout the planet.

Lumpy rings

Saturn's rings are iconic, but Jupiter has rings too; dusty and faint.

Two Jovian moons, Adrastea and Metis, travel along them, like marbles on a track.

New Horizons scientists failed to discover new moons in the ring system, as they had anticipated, but did detect two clumps of moon-like material embedded in it.

It is not known why they are clumps - and not spread out - and what keeps them in place.

"Somehow they got stuck in the system and can't escape," said Mark Showalter, a planetary scientist at the Seti Institute.

"We've seen similar phenomena in the rings of Saturn and Neptune," he said, "but in this case we can't explain why they're there."

He suggested that perhaps they were held in place by the gravitational tug of Metis.

As scientists continue to analyse New Horizons data, the spacecraft continues its journey to Pluto.

The $700m (£350m) probe was launched in January last year.

The Jupiter pass was needed to accelerate the spacecraft away from the Sun by an additional 14,500km/h (9,000mph), pushing it past 84,000km/h (52,000mph). This shortens the journey time to Pluto by four years.