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NASA meatballNational Aeronautics and Space Administration
Ames Research Center
Space Science Division and Educational Programs Office
Educational Brief EB-117
Subject: Planetary Science, Astronomy

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Galileo: Probe Into Jupiter

page 3

Impact of a Comet

In 1994 an unusual circumstance provided scientists with an unexpected opportunity to investigate Jupiter's atmosphere and prepare for the experiments of the Galileo Probe mission. Comet Shoemaker - Levy 9, a string of cometary bodies, was headed for Jupiter. Astronomers calculated the comet 's orbit and found that the comet had been captured by Jupiter. Calculations showed that Jupiter 's strong gravity had broken the comet apart as it made a close approach to Jupiter, about 2000 km (1250 miles) above thegiant planet 's cloud tops, in 1992. Astronomers world - wide became excited about this comet when further calculations showed that the comet would crash into Jupiter at 210,000 km/hr (130,000 mph) in July 1994.

Powerful telescopes, including the Hubble Space Telescope, were aimed at Jupiter, but the comet fragments impacted Jupiter on its nightside where the impact could not be seen directly from Earth although some of the effects were observed. The atmospheric scars of each impact were seen later when Jupiter 's rotation brought the impact sites into view. These scars were large enough to be seen by observers with relatively small telescopes.

a diagram showing the position of the Galileo spacecraft relative to Jupiter and Earth
Viewpoint of Galileo Spacecraft for
theComet Shoemaker-Levy 9 impacts

Fortunately, the Galileo spacecraft, speeding through interplanetary space toward Jupiter, could see the actual impacts from its vantage point in space. Images returned to NASA from the spacecraft showed the effects of the impacts as fire Dark spots produced by comet 's impact balls on the dark hemisphere of Jupiter. Observers on Earth also sawthe spectacular effects of the impacts when hot gases surged into space over the visible edge of Jupiter. The series of impact events started when astronomers witnessed a dull infrared glow as a swarm of small debris plunged into Jupiter 's atmosphere. Soon afterward the first large fragment of the comet plunged into Jupiter. Scientists calculated that it penetrated about 100 km (about 60 miles) into Jupiter 's atmosphere before exploding into a large fireball. The enormous amount of released energy shot an incandescent plume of superheated gases some 3000 km (1850 miles) above the cloud tops. This cosmic explosion repeated as each of the 21 cometary nuclei hurtled into Jupiter. Some fireballs were so bright they momentarily outshone the planet itself. The scars from the impacts spotted the Jovian clouds with extensive dark areas surrounded by concentric circles.

Coupled with the many ground - based observations, data gathered by the Galileo spacecraft gave scientists key information about the impacts. Scientists calculated the duration, size, and temperature ofthe fireballs. They analyzed the light coming from the fireballs and plumes and found evidence of many substances such as methane, sulfur, hydrogen sulfide, and carbon monoxide, and other molecules never before detected on Jupiter. Whether these substances came from Jupiter or from the comet cannot be settled yet. Galileo's Probe is expected to supply answers. One hope of astronomers was that by stirring up and disturbing Jupiter's deep atmosphere, the comet impacts could tell us more about the mysterious region below the visible cloud tops. A preliminary analysis of the dark concentric blemishes as they moved away from the impact sites suggests they are waves like ripples from a rock thrown into a pond. The speed and structure of these waves suggest that the deep atmosphere of Jupiter has a very high abundance of water. Again, the Galileo Probe should help resolve this question.

an image of an impacting comet fragment at Jupiter
Briliant flash of an impacting comet
fragment imaged by Galileo spacecraft

Mysteries of Jupiter

Astronomers have gathered sufficient information to conclude that Jupiter 's composition is similar to that of the Sun. This supports theories about planets accreting from a solar nebula - the vast cloud of gas and dust which billions of years ago condensed to form the Sun and planets. However, scientists are not sure how close Jupiter 's composition is to that of the Sun, By accurately comparing the composition of Jupiter 's atmosphere to the Sun 's, we can learn about how planets formed and the origin of the Solar System.

Images of Jupiter show that the planet is veiled by clouds. But these clouds could not be penetrated even with the high resolution instruments on board the Voyager spacecraft. Scientists think that Jupiter 's visible clouds consist of ammonia, but this composition is not entirely certain. A problem with ammonia is that it is white whereas the Jovian clouds are colorful. These colors are a mystery also. Scientists think that the colors are due to sulfur compounds or organic materials in the atmosphere. Solar abundances of elements would suggest that beneath the clouds there are probably various cloud decks of different compositions. But this would depend onabundances of materials in the original solar nebula at Jupiter 's distance from the Sun. Scientists also speculate that Jupiter might have water clouds like Earth.

an image showing the dark spots on Jupiter impacted by the comets
Dark spots produced by comet's impact

Researchers discovered that Jupiter has multiple east - west jet streams traveling at up to 400 km/hr (250 mph) that are related to the cloud bands on the planet. Several years of observations with the Voyager spacecraft and the Hubble Space Telescope have shown that the jet streams have maintained the same strength and location for at least ten years. In contrast with Earth 's jet streams the Jovian streams are very stable. We do not yet know why. Also, we want to know how far into Jupiter these winds extend.

A major question is the power source driving the winds of Jupiter. On Earth, energy from sunlight heats the atmosphere more at the equator than at the poles. Resulting temperature and pressure differences combined with the Earth 's rotation produce the major wind patterns on our planet. At Jupiter, sunlight is much weaker and is probably not sufficient to drive the Jovian wind systems. For Jupiter a source other than or in addition to sunlight is necessary to power the winds. Most likely the internal heat makes major contributions. Also there may be energy derived from the latent heat of water - rising water vapor, cooling to where water condenses into droplets, releases heat - or from latent heat of other substances. This could contribute to the driving energy of Jupiter 's wind systems. By the Probe 's measurements of where sunlight is de - posited in the cloudy atmosphere, and the depth of the winds, important clues about the origin of Jupiter 's winds can be had.

A prominent feature of Jupiter 's cloud system is a Great Red Spot that has been observed for almost 400 years. The spot is an extensive vortex storm, somewhat like a hurricane on Earth. Its diameter is about twice Earth 's diameter. The intensity and size of the spot has, however, varied over the many years that it has been observed. Big questions awaiting answers are why is it red and why is it so permanent? Earth 's storms last for days or weeks only. Also,why is there only one red spot? Increased knowledge about Jupiter 's atmosphere is expected to solve these mysteries.

a diagram of the composition of Jupiter
Structure of Jupiter based on data
gathered prior to Project Galileo

Twice as much heat leaves Jupiter as that arriving from solar radiation. This heat could be escaping from the interior of Jupiter by large scale convection like a boiling pot of water, or by narrow plumes like those in thunderstorms. Rising columns of air produce thunderstorm - like features. Spacecraft have recorded brilliant lightning flashes in Jupiter 's atmosphere. These are far brighter than typical lightning occurring on Earth. On Jupiter, lightning seems to occur in a few favored locations on the planet, unlike Earth where it is distributed fairly randomly over land and sea. The mystery is why? Another question that may be answered by Galileo is whether or not these lightning bolts originate in water clouds as lightning does on Earth.

Scientists think that the internal heat of Jupiter comes from the gravitational contraction of this huge fluid world together with heat left over from the formation of the planet. Soon after its formation Jupiter was most likely much hotter than it is now, almost approaching a second star in our Solar System. The Galilean satellites were heated and atmospheres driven from them. However, Jupiter did not possess sufficient mass for nuclear reactions to start converting hydrogen into helium, the power source that makes stars shine.

The interior of Jupiter is very different from those of the Solar System 's terrestrial planets. These inner planets - Mercury, Venus, Earth, and Mars - are rocky bodies possessing thin atmospheric shells. By contrast Jupiter is unlikely to have a solid surface. Studies of the density, size, shape, rotation, and gravitational field of Jupiter allowed scientists to approximate what the interior of Jupiter is like. There is a deep atmospheric shell of mainly gaseous hydrogen and helium below which pressure and temperature are high enough for a liquid hydrogen shell. Deeper still the hydrogen is converted into a shell of metallic hydrogen in which currents give rise to Jupiter 's intense magnetic field - the strongest of all planets of the Solar System. At the center of the metallic hydrogen region there may be a small rocky core consisting of rocks that because of the extremely high temperature and pressure must be unlike any rocks we know on Earth.

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This page was created by Tobin A. Snell and Josh Parker.

 
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