History of the Solar System
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.
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.
