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hree hundred years ago, Isaac Newton used the motions of the ~ Galilean satellites (the four moons of Jupiter discovered by Galileo) to determine fupiter's mass. A century later, William Herschel deduced that fupiter's density was anomalously low. In the 20th century it became clear that Jupiter was composed primarily of the lightest elements, hydrogen and heli- um. Further studies of Jupiter, com- bined with analyses of the spectrum of light reflecting off the planet, gave rise to the so-called solar composition model of the giant planets. That is, as far as their overall elemental composi- tions are concerned, Jupiter and also Saturn appear to be pieces of the Sun cooled down to planetary temperatures. Unfortunately, the solar composi- tion model does not work for Uranus and Neptune, which are twice as dense A composite image of the four Galilean satellites and fupiter's Great Red Spot. as Saturn. Their densities indicate that they formed from material that was rich in water, ammonia, and methane ices and more deficient in the light gases than Jupiter, Saturn, or the Sun. Since oxygen and carbon are the third and fourth most abundant elements in the Sun after hydrogen and helium, the modified solar composition model was proposed to explain the creation of all of the planets. This model starts with a young Sun surrounded by a disk fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~ of leftover material, a mix of elements similar in overall composition to that of itself. During an early active phase, which many young stars undergo, the Sun ejected a wind of high-speed elec- trons, protons, and heavier particles that swept the hydrogen, helium, and other gases out of the disk. The mix- ture that remains has a composition similar to that of the Sun, except for the missing gaseous component. Close to the Sun, where it is hot, the ices too are lost, and only the rocks and metals remain. This interpretation fits our solar system, with small rocky planets in the inner solar system and the gaseous giant planets further out. In this theory, timing is critical. The giant planets had to have formed before the gases were swept out of the solar system. Timing might explain the compositional difference between the ice giants, Uranus and Neptune, and the gas giants, Jupiter and Saturn. According to theory, giant planets could form faster at the orbits of Jupiter and Saturn where the density of material was higher and collisions more frequent. Perhaps Uranus and Neptune were just starting to accumu- late gases when the Sun blew the lighter gases out of the solar system. The time that it takes to produce a fupiter-size object depends on the method of formation, and here there are two possabilities. The slow way is to first form a rock-ice core about 10 times the mass of Earth the resulting dense, solid object is able to attract gas and grow in mass once it reaches this size. The fast way assumes that Jupiter formed much the way the Sun did the gas in one region of the solar nebula became sufficiently dense that its col- lective gravity caused it to collapse in a spherically symmetric manner. If creat- ed this way, Jupiter would resemble an object known as a brown dwarf a star with insufficient mass to sustain nuclear fusion reactions in its core. Distinguishing between these hypothe- ses required determining if the giant planets have rock-ice cores. While the evidence indicates that Saturn, Neptune, and Uranus do indeed have cores, the nature of Jupiter's deep interi- or remains unknown. Another mystery about Jupiter con- cerns the distance from the Sun at which it formed. An analysis of Galileo spacecraft data shows that Jupiter has greater amounts of certain heavy ele- ments than does the Sun. One explana- tion for this suggests that Jupiter formed far out in the solar system, where such elements were more prevalent, and then migrated inward toward its present orbit. Another possibility is that Jupiter formed approximately where it is today but was more likely to collect heavier elements than lighter ones. The key to resolving which if either of these ideas is correct is to determine the relative amounts of hydrogen and oxygen in Jupiter's atmosphere. Studies of Jupiter also have the potential to significantly improve our understanding of planetary magneto- spheres and their interactions with the solar wind. fupiter's magnetosphere is sustained in a manner different from Earth's it derives its energy from the rotation of the planet itself. In addi- tion, Jupiter has the strongest magne- tosphere in the solar system. By studying Jupiter's magnetosphere, especially using spacecraft to see regions unobservable from Earth, we could learn answers to questions about a diverse set of objects, ranging from Earth to distant pulsars. Answering these questions requires measurements both inside and above Juipiter's atmosphere. The Jupiter Polar Orbiter with Probes is, in a sense, two missions in one. A carrier spacecraft equipped with three probes is launched toward Jupiter. As the spacecraft nears the planet the probes are released and penetrate Jupiter's thick atmosphere, taking measure- ments and reporting back data on Jupiter's interior. Following the com- pletion of the probe mission, the carri- er enters a low-altitude polar orbit about Jupiter from which vantage point it conducts additional studies for a year or more. The Jupiter Polar Orbiter with Probes mission has five primary objec- tives. The first is to determine if Jupiter has a core. The second is to measure the water abundance below the visible clouds and, hence, determine the

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Jupiler Polar Orbiler with Probes oxygen/hydrogen ratio. Both of these investigations address outstanding questions about the formation of Jupiter and, thereby, the solar system. To address the third objective, the spacecraft's probes will measure the deep winds to a depth of 100 bars while another instrument may be able to give some information about the winds to thousands of bars. (Depth on Jupiter is measured by the atmospheric pressure, not by distance; 1 bar is the atmospheric pressure at sea level on Earth.) The deep winds may be key to the extreme stability of the weather systems observed at cloud top. The fourth objective is addressed by virtue of the spacecraft's cloud-skim- ming orbit, which will permit more pre- cise measurements of the planet's mag- netic field than previously possible. Similarly, the polar nature of the orbit Guiding Themes Addressed Important Planetary Science Questions Addressed Profile Jupiter Polar Orbiter with Probes Mission Type: Orbiter with atmos- pheric probes Cost Class: Medium Priority Measurements: Probe Jupiter's interior with gravity and magnetic field measurements from a polar orbit. Measure condensable gas abun- dances, temperature, wind velocity, and cloud opacity down to the 1 00-bar pressure level. Determine how internally produced plasma is ejected from a rotation- dominated magnetosphere. Artist's concept of the fupiter Polar Orbiter with Probes spacecraft illustrat- ing how the three probes will enter dif- ferent parts of the planet's atmosphere. permits the mission's fifth objective- repeated visits to the hitherto unex- plored polar magnetosphere to be addressed. Taken together, these latter two investigations will allow researchers to map Jupiter's magnetosphere much more accurately, learn more about the magnetic field's origins inside Jupiter, study how these fields interact with Jupiter's moons, and teach us much about Jupiter's magnetic activity.