Juno is looking for how Jupiter formed, which would say a lot about how all planetary systems form, and all theories regarding that are on the line. First, determining how much water it has would help determine where and how fast it formed.
Oxygen is the 3rd most abundant element in the universe, including in our solar system, and its most common form is combined with hydrogen in water. Observation to date has indicated a range of possible values for water on Jupiter:
The latest analyses of data from the Voyager spacecraft that flew by
Jupiter in 1979 have suggested a water abundance for the planet of
twice the solar level (based on the Sun's oxygen content).
Observations of the propagation of atmospheric waves across Jupiter's
cloud tops from the Comet Shoemaker-Levy 9 impacts implied that
Jupiter might have a water content of ten times the solar level.
Actual probe measurements, while subject to scientific debate, suggest
a level near that of the Sun. Scientists are left to wonder, "where is
the oxygen?," "where is the water?,"
The Galileo probe detected levels much lower than the solar level of oxygen.
Juno will measure the water in the atmosphere below the troposphere using a microwave radiometer. The clouds are transparent to microwaves, while water absorbs them.
How much water is in this layer will help determine how Jupiter formed. One possible explanation for a low oxygen content is that water ice became depleted in the inner region of the cloud of gas our solar system formed from:
That Jupiter formed in a disk with C/O ∼1 implies that water ice was
heterogeneously distributed over several AU beyond the snow line in
the primordial nebula and that the fraction of water contained in icy
planetesimals was a strong function of their formation location and
time. The Jovian oxygen abundance to be measured by NASA's Juno
mission en route to Jupiter will provide a direct and strict test of
Or put another way on phys.org:
Steve Levin, Juno project scientist from NASA's Jet Propulsion
Laboratory, said water figures are the most important ones that Juno
is going to bring back. "If Jupiter formed far from the sun, where it
is cold, out of blocks of ice... you would get a different amount of
water inside Jupiter than if it formed closer to the sun than it is
But there are other possibilities, as one would expect for something as complex as the formation of a planet. There is a good summary in the final two sections of this article by Katharina Lodders. The two main ideas are first, the core accretion model in which a solid core forms slowly, and starts capturing gasses by its gravity once it is big enough, and second the disk instability model, in which slight differences in matter density in the protoplanetary disk cause matter to clump and quickly densify into a planet. How much water Jupiter has, and how that compares with the exact amounts it has of other elements heavier than hydrogen and helium, will give hints about which of the two it was, and how other mechanisms were involved, such as later capture of other objects by Jupiter.
To put it together, it is also crucial to know how big the core of Jupiter is, and whether it is solid or liquid. Some core accretion models predict a large solid core of metals, like Earth's but many times the mass of Earth. Others propose a small solid core could still accrete a planet like Jupiter. Those proposing that instead disk instability is the source of planet formation say that accretion can't be the explanation because study of other star systems indicates the protoplanetary disk doesn't last long enough for accretion to happen. They say Jupiter might not have a solid core at all, or have a much smaller one that gathered over a long time. It is also possible both mechanisms played a part, and there is also a completely different idea in which accretion happened through the medium of formation around a carbonaceous core, a tar ball. So knowing the size and density of the core won't immediately settle the matter, but has to be known to get any further.
Juno has a magnetometer to measure Jupiter's magnetic field and a nice big antenna so that Goldstone can map its gravity field, together this will tell us about Jupiter's core.