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Formation of the Giant Planets hree hundred years ago, Isaac of leftover material, a mix of elements Another mystery about Jupiter con- T Newton used the motions of the Galilean satellites (the four moons of Jupiter discovered by Galileo) to similar in overall composition to that of itself. During an early active phase, which many young stars undergo, the cerns the distance from the Sun at which it formed. An analysis of Galileo spacecraft data shows that Jupiter has determine Jupiterâs mass. A century Sun ejected a wind of high-speed elec- greater amounts of certain heavy ele- later, William Herschel deduced that trons, protons, and heavier particles ments than does the Sun. One explana- Jupiterâs density was anomalously low. that swept the hydrogen, helium, and tion for this suggests that Jupiter formed In the 20th century it became clear that other gases out of the disk. The mix- far out in the solar system, where such Jupiter was composed primarily of the ture that remains has a composition elements were more prevalent, and then lightest elements, hydrogen and heli- similar to that of the Sun, except for migrated inward toward its present um. Further studies of Jupiter, com- the missing gaseous component. Close orbit. Another possibility is that Jupiter bined with analyses of the spectrum of to the Sun, where it is hot, the ices too formed approximately where it is today light reflecting off the planet, gave rise are lost, and only the rocks and metals but was more likely to collect heavier to the so-called solar composition remain. This interpretation fits our elements than lighter ones. The key to model of the giant planets. That is, as solar system, with small rocky planets resolving which if either of these ideas far as their overall elemental composi- in the inner solar system and the is correct is to determine the relative tions are concerned, Jupiter and also gaseous giant planets further out. amounts of hydrogen and oxygen in Saturn appear to be pieces of the Sun In this theory, timing is critical. Jupiterâs atmosphere. cooled down to planetary temperatures. The giant planets had to have formed Studies of Jupiter also have the Unfortunately, the solar composi- before the gases were swept out of the potential to significantly improve our tion model does not work for Uranus solar system. Timing might explain understanding of planetary magneto- and Neptune, which are twice as dense the compositional difference between spheres and their interactions with the the ice giants, Uranus and Neptune, solar wind. Jupiterâs magnetosphere is and the gas giants, Jupiter and Saturn. sustained in a manner different from According to theory, giant planets Earthâsâit derives its energy from the could form faster at the orbits of rotation of the planet itself. In addi- Jupiter and Saturn where the density tion, Jupiter has the strongest magne- of material was higher and collisions tosphere in the solar system. By more frequent. Perhaps Uranus and studying Jupiterâs magnetosphere, Neptune were just starting to accumu- especially using spacecraft to see late gases when the Sun blew the regions unobservable from Earth, we lighter gases out of the solar system. could learn answers to questions The time that it takes to produce a about a diverse set of objects, ranging Jupiter-size object depends on the from Earth to distant pulsars. method of formation, and here there Answering these questions requires are two possabilities. The slow way is measurements both inside and above to first form a rock-ice core about 10 Juipiterâs atmosphere. The Jupiter times the mass of Earthâthe resulting Polar Orbiter with Probes is, in a dense, solid object is able to attract gas sense, two missions in one. A carrier and grow in mass once it reaches this spacecraft equipped with three probes size. The fast way assumes that Jupiter is launched toward Jupiter. As the formed much the way the Sun didâthe spacecraft nears the planet the probes A composite image of the four Galilean gas in one region of the solar nebula are released and penetrate Jupiterâs satellites and Jupiterâs Great Red Spot. became sufficiently dense that its col- thick atmosphere, taking measure- lective gravity caused it to collapse in a ments and reporting back data on as Saturn. Their densities indicate that spherically symmetric manner. If creat- Jupiterâs interior. Following the com- they formed from material that was ed this way, Jupiter would resemble an pletion of the probe mission, the carri- rich in water, ammonia, and methane object known as a brown dwarfâa star er enters a low-altitude polar orbit ices and more deficient in the light with insufficient mass to sustain about Jupiter from which vantage gases than Jupiter, Saturn, or the Sun. nuclear fusion reactions in its core. point it conducts additional studies Since oxygen and carbon are the third Distinguishing between these hypothe- for a year or more. and fourth most abundant elements in ses required determining if the giant The Jupiter Polar Orbiter with the Sun after hydrogen and helium, planets have rock-ice cores. While the Probes mission has five primary objec- the modified solar composition model evidence indicates that Saturn, tives. The first is to determine if Jupiter was proposed to explain the creation of Neptune, and Uranus do indeed have has a core. The second is to measure all of the planets. This model starts cores, the nature of Jupiterâs deep interi- the water abundance below the visible with a young Sun surrounded by a disk or remains unknown. clouds and, hence, determine the 16 New Frontiers in Solar System Exploration
Jupiter Polar Orbiter with Probes Profile Jupiter Polar Orbiter with Probes Mission Type: Orbiter with atmos- pheric probes Cost Class: Medium Priority Measurements: â¢ Probe Jupiterâs interior with gravity and magnetic field measurements from a polar orbit. â¢ Measure condensable gas abun- dances, temperature, wind velocity, and cloud opacity down to the 100-bar pressure level. â¢ Determine how internally produced plasma is ejected from a rotation- dominated magnetosphere. Artistâs concept of the Jupiter Polar Orbiter with Probes spacecraft illustrat- ing how the three probes will enter dif- ferent parts of the planetâs atmosphere. oxygen/hydrogen ratio. Both of these pressure, not by distance; 1 bar is the permits the missionâs fifth objectiveâ investigations address outstanding atmospheric pressure at sea level on repeated visits to the hitherto unex- questions about the formation of Earth.) The deep winds may be key to plored polar magnetosphereâto be Jupiter and, thereby, the solar system. the extreme stability of the weather addressed. Taken together, these latter To address the third objective, the systems observed at cloud top. two investigations will allow researchers spacecraftâs probes will measure the The fourth objective is addressed by to map Jupiterâs magnetosphere much deep winds to a depth of 100 bars virtue of the spacecraftâs cloud-skim- more accurately, learn more about the while another instrument may be able ming orbit, which will permit more pre- magnetic fieldâs origins inside Jupiter, to give some information about the cise measurements of the planetâs mag- study how these fields interact with winds to thousands of bars. (Depth on netic field than previously possible. Jupiterâs moons, and teach us much Jupiter is measured by the atmospheric Similarly, the polar nature of the orbit about Jupiterâs magnetic activity. Guiding Themes Addressed Important Planetary Science Questions Addressed The First Billion How long did it take the gas giant Jupiter to form? Years of Solar How was the formation of Jupiter and its gas-giant sibling, Saturn, different from that of the ice giants, System History Uranus and Neptune? How have the orbits of the giant planets changed throughout history? Volatiles and What is the history of volatile compounds, especially water, across the solar system? Organics What is Jupiterâs core made of, and what is the composition of its lower atmosphere? The Stuff of Life Processes How do the processes that shape the contemporary character of planetary bodies operate and interact? How Planetary How does Jupiterâs magnetosphere interact with the Galilean satellites? Systems Work What does the solar system tell us about the development and evolution of extrasolar planetary systems, and vice versa? 17