History of the Solar System

Jupiter Origin

Using the motions of the four Galilean moons, Sir Isaac Newton first determined Jupiter's mass over three hundred years ago. In the past four decades, the use of instrumented spacecraft, earth based observations and theoretical research has significantly advanced our knowledge of the early formation of the planets from the solar nebula some 4.5 billion years ago. However, these findings have also raised many new questions and posed the need for more comprehensive observations to distinguish between the two main theories for solar composition models of the giant planets. The key question remains as to how Jupiter, a gas giant primarily composed of hydrogen and helium, acquired the heavy elements that must be present in its interior, based on its observed physical properties. These heavy elements are also the seeds for the Earth's formation, and likely required for the origins of life itself.

The Juno Mission will distinguish between the differing Jupiter formation theories by answering several key questions, including: How did Jupiter evolve from the planetesimals? How far from the Sun did it form? When did it form, and how long did it take to reach its present state?

Results from Galileo, the last mission to explore Jupiter from an orbiting spacecraft, included an atmospheric probe that returned data from a depth of nearly 22 bars below the cloud tops. These new data showed that the prevailing ideas for the formation of Jupiter were not in agreement. Specifically, the different volatile substances that were found within Jupiter were in greater abundances when compared with the solar composition with about the same amount of enrichment. This indicated that Jupiter formed in a cool region of the Solar System, further from the Sun than its present location (at 5 Astronomical Units) and was perhaps formed in a colder environment. The underlying idea is that the solid material that enriched Jupiter was more abundant in the early stages of the solar system formation. This material is now found in the inner core of Jupiter.

The Origin and Formation of Jupiter

The formation of the planets depended on the physical properties of the ingredients found in the solar nebula and found in the modern day Sun. Close to the Sun where it is much warmer, only rocks and metals can condense while further away from the Sun, where it is cooler, substances such as water, ammonia and methane begin to condense – all of which are rich in hydrogen, the most abundant gas in the universe. The imaginary boundary marking the distance from the Sun where these gases begin to condense and form ice crystals or icesis termed the frost line, as shown in the figure below. The condensed ice is thought to clump together, much like snowballs, which then collide and combine to form bigger snowballs. Once these reach a sufficient size, they acquire enough gravitational attraction to not only hold themselves together, but also to attract and hold an envelope of hydrogen.

Jupiter's Formation

The Galileo Probe results indicated that Argon, Krypton, Xenon, Carbon and Nitrogen were shown to be 2-3 times more abundant on Jupiter than on the Sun, except for Oxygen which was found to be far less prevalent. The enrichment, as compared to the solar composition, can be explained by the addition of heavy elements from other planetesimals in the later stages of Jupiter's formation. But, why is Jupiter depleted in oxygen (water)? Was this an anomaly as the Probe did enter a "hot spot" on Jupiter? Or, does this point to other formative processes for Jupiter?

To explain the abundance of Nitrogen and Argon it is necessary for the planetesimals to have the heavy element enrichment occur at temperatures less than 30 K. This implies that either Jupiter formed far from the Sun and arrived closer to its present day location near the Sun, or that the heavy elements were brought by comets and asteroids that crashed into Jupiter such as the impact of the Shoemaker Levy 9 comet fragments in July, 1994.

Jupiter formation theories vary from heavy elements being brought to Jupiter by asteroids which formed further away from the sun, to Jupiter itself having formed much further away from the sun and then migrating closer to its current location. A more detailed measurement of the heavy elements will provide more detailed abundance information and will help scientists to distinguish between the various enrichment theories. If elements were carried in from the Kuiper belt, scientists expect to observe Oxygen in quantities similar to the other heavy elements. However, if scientists discover that water is more abundant, then we will know that the heavy elements originated in the region where Jupiter currently presides.

Water should be well mixed within the deep atmosphere of Jupiter, so its depletion as measured by Galileo is also a challenge for the formation theories. It is likely that water may be the original "carrier" of the heavy elements into Jupiter, and hence determination of its deep atmosphere abundance on Jupiter is a key observation that Juno will make from its Microwave Radiometer.