Scientists say they have found evidence that Jupiter's largest moon, Ganymede, may have an ocean of water, a key ingredient of life.

The researchers said magnetic readings taken by the Galileo space probe suggested the presence of a vast body of water in liquid form beneath the moon's frozen surface.

On Earth, wherever there is water, there is usually life. The same is certainly not true in space, but this discovery will heighten expectations that one day extraterrestrial life may be found.

The University of California scientists stressed they could not be certain they had discovered water on Ganymede, which is larger than Mercury or Pluto.

Persuasive results

Margaret Kivelson of the University of California-Los Angeles said that readings taken in May this year were "highly suggestive" that a salty, liquid ocean existed there.

Ganymede: Larger than Mercury or Pluto

"It would need to be something more electrically conductive than solid ice," she said, explaining that a melted layer of water several kilometres thick about 200 km beneath the surface would fit the data if it were about as salty as Earth's oceans.

Other scientists have identified what appear to be salty minerals on the moon's crust, indicating that salt water was once found on the surface.

"They are similar to the hydrated salt minerals we see on Europa, possibly the result of brine making its way to the surface by eruptions or through cracks," said Thomas McCord, a geophysicist at the University of Hawaii, Honolulu.

The hypothesis of water on Ganymede has been bolstered by new, high-resolution images of Ganymede sent back by Galileo.

The images hint that water or slushy ice may have surfaced through the fractured crust to create smooth areas in between separated areas of crust.

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Listening to the 'sounds' of Jupiter

December 18, 2000 - BBC

Ripples in magnetic fields wafting through interplanetary space have been detected by a space probe and converted into sound.

The "noises" were picked up the Cassini spacecraft which is en route to Saturn in 2004 via Jupiter.

The probe is detecting waves in the thin gas of charged particles that fills the space between the Sun and its planets.

The waves are in low radio frequencies and can be converted to sound wave frequencies to make the patterns audible.

Listening to the "sounds" of near-Jupiter space has been done before by the Pioneer and Voyager probes of the 1970's, but this is the first time that Cassini's data have been used in this way.

Oscillations

The waves were detected by Cassini's radio wave and plasma science instrument on 8 December, at a distance of about 23 million kilometres (14 million miles) from Jupiter.

Scientists believe that they arise from interactions between Jupiter's magnetic field and the solar wind of particles streaming away from the Sun.

Researchers point out that valuable information can be gleaned from this data. There are oscillations discernible that are produced by so-called ion-acoustic waves, the result of electrons pushed around by the energy in Jupiter's bow shock region.

The bow shock is similar to a sonic boom from a supersonic jet flying through Earth's atmosphere. Near Jupiter, the bow shock is caused by the superfast solar wind being diverted around Jupiter's magnetic field. The solar wind is heated, slowed and deflected.

Cassini has not yet reached Jupiter's bow-shock - this should occur in a few weeks from now. But the shock is probably the source of energy driving the waves that are reaching the spacecraft.

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Giant storms collide on Jupiter

The Cassini spacecraft snaps Jupiter's Great Red Spot

October 25, 2000 - BBC

For the first time, astronomers have seen two giant storms on Jupiter collide and merge to form an even bigger storm.

The two swirling wind systems are each about half the size of the Earth.

Researchers speculate that a similar merger of storms produced Jupiter's famous Great Red Spot, a hurricane that has persisted in the planet's atmosphere for more than 300 years.

The collision between the storms will help scientists understand more about the dynamics of Jupiter's turbulent atmosphere.

"Usually when we've seen two (storms) approaching each other, they bounce back away from each other," said Dr Glenn Orton of Nasa's Jet Propulsion Laboratory in California.

Swirling vortex

The current giant storm began forming about 60 years ago as three "white oval" storms in a band of Jupiter's atmosphere south of the Great Red Spot.

In the following decades until 1998, the storms sometimes approached each other but never collided.

But in early 1998, astronomers witnessed two of the ovals moving towards each other as Jupiter went out of sight from Earth, behind the Sun. When the planet came back into view, the two had become one.

"We weren't able to see how they came together that time," said Dr Orton.

Last year, however, the newly formed storm approached the third remaining oval. Winds within it were swirling counter-clockwise at about 470 km (290 miles) per hour. One was about 9,000 km (5,600 miles) across, the other slightly smaller.

A third, darker oval, this time swirling clockwise, formed temporarily between the two white ovals.

Joining forces

That type of intervening system would usually keep white ovals from colliding, the team proposed. But the middle storm appears to have been pushed even further south and torn apart, as all three passed near the Great Red Spot last December.

The disappearance of the opposite-swirling storm from between them cleared the way for the two white ovals to meet.

Their collision began in March and lasted about three weeks. The storms circled around each other counter-clockwise then merged into a single oval.

This single storm was about one-third wider than either of the ovals had been beforehand.

A giant storm is formed from three 'white oval' storms

The ovals' approach and merger was observed by several telescopes including one at the Pic-du-Midi Observatory in France, Nasa's Infrared Telescope Facility in Hawaii, and the Hubble Space Telescope.

Meanwhile, the Cassini spacecraft, approaching Jupiter for a fly-by in December, has taken a new image of the Great Red Spot.

As it passes by, Cassini will make a series of measurements of the planet in several wavelengths of light as well as sampling energetic particles and magnetic fields.

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Io's wandering volcanoes

A dark lava lake at the summit of a volcano

May 19, 2000 - Journal Science

Astronomers have obtained their best view yet of the most volcanically active world in our Solar System: Jupiter's moon, Io.

Io, roughly the size of our own Moon, has a yellow and ochre surface covered in sulphur chemicals deposited by giant volcanic plumes that reach hundreds of kilometres in height.

A ribbon of light marks the edge of a lava lake

Its volcanism is driven by the extreme tidal energy it gets from Jupiter.

The satellite was first studied in detail in 1979 by the Voyager fly-by mission. Now, new images, taken from the Galileo spacecraft show Io's surface to be marked by volcanic calderas, plains, and fissures.

It is awash with fluid lava thrown up by fountains and pooled in lakes.

Ribbon of light

A lava lake was seen in the central caldera of Pele, a volcano 1200 km (746 miles) across that sends a gaseous plume 400 km (250 miles) into space. Viewed at night-time, the lava lake is seen as a glowing ribbon of light.

Prometheus: A wandering volcano

The most powerful volcano in the Solar System is located on Io and is called Loki Patera. Galileo's images show it to have a huge, dark lava lake cooling in its summit depression.

The images also reveal evidence of lava flows that may have streamed across the surface at hundreds of metres a second.

Analysing the spectra of the lava allows scientists to deduce its composition. Changes between the images show how the eruptions change over time, and how they help to create mountains and other landforms.

Scientists believe that Io's surface is very young, being continuously covered by sulphur deposits. The fact that no impact crater was seen in the images anywhere on Io means that the world must be completely resurfaced every million years or so.

One volcano even appears to have moved.

Galileo and the 1979 Voyager spacecraft observations show that one large volcanic plume called Prometheus may have been caught in the act of migrating across Io's surface.

Unlike any volcano on Earth, Prometheus appears to have wandered as much as 95 km (59 miles) over a volcanic plain in just a few decades.

Scientists think that the two vents are connected by a dark lava flow running over a so-called snowfield of sulphur dioxide. The pressure and heat of the lava flow from Prometheus melts the sulphur snowfield that makes it way to the surface through a wandering pipe.

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Jupiter's Moons in Focus

Europa's icy crust may conceal an ocean

March 8, 2000 - BBC

This mosaic provides the best view ever obtained of the side of Jupiter's moon Europa that faces the giant planet.

The 12 images of the ice-encrusted surface were taken by the Galileo spacecraft last November during its 25th orbit of Jupiter. Details as small as one kilometre (0.6 miles) can be seen.

A -- The gravity of Jupiter and its large moons play tug-o'-war with Io. As a result, the moon bulges on two sides like an American football. Right: B -- Jupiter and three Galilean satellites pull on the same side of Io, creating tidal bulges in Io's crust about 100 m high. [more information]

Astronomers believe that beneath the ice-crust lies a liquid water ocean which some think may be a good place to look for alien life.

Linear features visible in the centre of the mosaic and toward the poles may have formed in response to tides strong enough to fracture the moon's icy surface.

Some of these features extend for over 1,500 km (900 miles). Darker regions near the equator on the eastern (right) and western (left) limb may be vast areas of chaotic, broken terrain. Bright white spots near the western limb are the ejecta blankets of young impact craters.

Io is the most volcanic world in the Solar System

High-resolution images of Jupiter's volcanic moon Io have also just been released by Galileo mission scientists. They show the relationships between the different surface materials and the underlying geologic structures on Io.

The red material, seen around the margin of an erupting volcano called Monan Patera, is thought to be a compound of sulphur. A broad circle of bright, white material seen in the image is thought to be sulphur-dioxide being deposited from the plume called Amirani.

Sulphurous chemicals spill on to the surface

The lengths of the shadows cast by the mountains make it possible to estimate the mountains' heights - between 4,000 and 8,000 metres (13,000 to 26,000ft) high.

Another Io image shows the Zal Patera region. The edge of this volcanic crater is marked by black flows, coinciding with the edge of a plateau. In this image, the red material follows the base of a mountain, which may indicate that sulphurous gases are escaping along a fault associated with the formation of the mountain.

Io is the most volcanic world in the Solar System.

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Galileo's brush with volcanic moon

Passing close to volcanic Io although Galileo has a damaged antenna

February 23, 2000 - BBC

The Galileo spacecraft, in orbit around Jupiter, has made its third and closest flyby of Jupiter's dramatic moon Io, the most volcanic body in the Solar System.

Observations from Earth-based telescopes by the International Jupiter Watch suggest that a volcanic outburst was underway on Io as Galileo passed by.

Galileo flew only 200 km (124 miles) above Io's surface at 15:32 GMT on Tuesday. During its previous close flybys of Io Galileo has gathered a wealth of data as well as stunning pictures.

Io, the most volcanically active world in the Solar System, has volcanic vents that spew sulphur plumes over 100 km (60 miles) above the surface. The sulphur falls back and covers Io, turning it into a dramatic red, yellow and black landscape.

"Io's volcanoes are so active that the moon's surface is always changing, and with each flyby we get new and different observations," said Dr. Torrence Johnson, Galileo project scientist at Nasa's Jet Propulsion Laboratory.

"This time we expect to be able to observe the effects of the eruptions we saw in the October and November Io flybys."

Risky mission

Io orbits so close to Jupiter that it is bombarded by radiation from the giant planet's radiation belts. The radiation can disrupt Galileo's systems or even disable its instruments, but mission planners believe potential gains in scientific knowledge outweigh the risk.

Nonetheless, the encounters were planned near the end of Galileo's mission when the spacecraft has already returned volumes of pictures and information from Jupiter and its moons.

Galileo has been in orbit around Jupiter since December 1995. It was originally intended to spend two years studying Jupiter, its moons and its magnetic environment. When that original mission ended in December 1997, it was followed by a two-year extended mission, which ended in January 2000. This Io flyby is part of an additional extension, called the Galileo Millennium Mission.

End of mission

Galileo has two close flybys of the frozen Jupiter moon Ganymede planned for later this year. After that it may be commanded to crash into one of Jupiter's moons or even into the atmosphere of Jupiter itself. According to scientists such a dramatic end may yield more scientific returns than just letting it run out of fuel in orbit.

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Jupiter's thunderous rising damp

Thunderstorm on Jupiter with white water clouds

February 9, 2000 - BBC

Astronomers studying the latest images from the Galileo mission believe that the motions of Jupiter's atmosphere are driven by similar processes to those which drive the weather on Earth.

The atmosphere of Jupiter is thick, multi-coloured and layered. Its swirling blanket of gases include hydrogen, helium, nitrogen, methane, ammonia and water vapour.

Jupiter's Red Spot

At the cloud tops are ice crystals of ammonia. Between the jet streams, cyclones and anti-cyclones, there are redder and warmer depths visible.

Swirling clouds of light, the equivalent of the Earth's aurorae, ripple above the clouds while lightning streaks between the cloud layers. Water clouds lurk 50 kilometres below the visible surface but below that it gets too hot for them to survive.

The images on this page come from the Galileo mission, which has been in orbit around Jupiter since 1995. The images are analysed in a report in the journal Nature.

Red Spot

Some aspects of Jupiter's weather system are remarkably stable. The Red Spot, a huge oval storm the size of the Earth, has persisted for at least three centuries. What is more, its atmosphere is divided into stable bands.

Scientists, at Cornell University in the US, looked in detail at two large storms close to the Red Spot, which were accompanied by lightning strikes through the cloud layer.

They suggest that a possible driving force behind these storms is moist convection: the rising of water-rich gases, accompanied by condensation and cloud formation.

Convection is the power behind tropical thunderstorms on Earth. But whereas moist convection on Earth is ultimately powered by the heat of the Sun, on Jupiter there is another power source.

The sunlight falling on Jupiter is only about 4% as strong as it is on Earth. However, Jupiter generates about twice as much heat as it absorbs from the Sun. The energy comes from the giant planet's slow contraction, as well as from the separation of gases within the planet. It is this, scientists believe, which could be powering Jupiter's weather systems.

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