Juno will give us the first close-up view of Jupiter's poles.

Modern exploration of Jupiter began with the Voyager Mission. Two flybys gave us our first encounters with this giant planet, preparing the way for more detailed investigations. After Voyager's extraordinary encounters, curiosity about the planet and its intriguing atmosphere deepened-- we knew we had to spend more time studying Jupiter than a flyby could provide. So, the Galileo Mission, with its orbiter and probe, began its detailed investigations of not only Jupiter, but also its mysterious moons.

The Galileo Mission was a mission of firsts: On its way to Jupiter it made the first discovery of an asteroid with a moon. It then became the first spacecraft to orbit the mysterious and powerful gas giant, Jupiter and was the first to launch a probe into Jupiter's atmosphere. But more firsts were needed that Galileo could not provide.

The tremendous harvest of new knowledge obtained by Galileo still left some mysteries still unsolved. This archetypical giant planet holds some extraordinary secrets concealed beneath its dense layer of clouds: fundamental processes underlying the formation of our solar system.

A New Mission: Exploration Continues

In 2011, the Juno mission to Jupiter will embark on a journey to study the planet's atmosphere, interior and magnetosphere, from a new and unprecedented perspective. Juno will significantly improve our understanding of the formation, evolution and structure of Jupiter, answer critical science questions and test present theories about the formation of our solar system.

The insights Juno will give us will also have significant implications for understanding planetary formation in other stellar systems across the Universe.

In 2016 the Juno spacecraft will arrive at Jupiter, entering the first polar orbit around the planet. With its suite of nine science instruments Juno will make the first measurement of the amount of water on Jupiter. It will also investigate the interior structure of the planet, including the first search for the existence of a high-density planetary core. Juno will make the first detection of ammonia clouds in the deep atmosphere, and will map Jupiter's intense magnetic field, observing the planet's auroras from positions above the planet's poles for the first time.

A Hubble Space Telescope imge of Jupiter and its largest moon, Ganymede.

Jupiter's Origins and Interior

Most scientists believe that Jupiter originated early in the formation of the solar system-primarily because it consists mostly of hydrogen and helium like the Sun. Young Jupiter captured most of the material left over from the formation of the infant Sun when a giant interstellar cloud of dust and gas collapsed to form the pancake-shaped solar nebula. Our biggest planet was able to keep its original composition (the composition of the solar nebula), because of the gravity generated by its enormous mass. The Earth was much too small to do this. Thus giant Jupiter harbors hidden secrets from the earliest whispers of the planetary system we now call home. But how did all this happen? Did Jupiter start from a massive solid core whose gravity pulled in all of that gas or did an unstable region collapse inside the nebula, triggering the planet's formation? , How did icy planetesimals or small proto-planets form in Jupiter's vicinity, bringing the planet, water, ammonia and compounds of every element we know? How did Jupiter's formation shape the origin of Earth and the other planets?

One of the most important clues required to answer these questions is presently hidden far below the depths the Galileo probe could ever have reached. It is the abundance of water on Jupiter. If Jupiter is very wet it must have captured more of the icy planetesimals than a simple formation theory would predict. A very dry Jupiter would also challenge existing theories.

A separate scenario suggests that with its huge gravitational influence and sheer size, Jupiter may have deflected damaging intruders such as comets and asteroids away from the inner solar system where Earth and other terrestrial planets orbit. Planetesimals that escaped these deflections brought some of the water and other essential compounds for the origin of life. Unraveling this history requires better knowledge of the evolution of the planet's gravitational field, which requires improved models of Jupiter's interior, for which the planetary abundance of water is essential.

Jupiter may have been critical for the evolution of Earth-like planets and even life itself. If so, giant planets in other systems will have done the same thing. Such planets may therefore be essential to the emergence of life elsewhere in the universe.

All of these possibilities depend on water, whose abundance on Jupiter that we do not know.

The Juno spacecraft will measure the amount of water in Jupiter for the first time by detecting microwave radiation from the atmosphere at great depths. It will also reveal the structure of the planet's interior in far more detail than ever before. It will also determine if the planet has a dense core. These measurements will allow scientists to determine which of the competing theories about the planet's origin is correct. And by mapping Jupiter's gravitational and magnetic fields far better than has been possible before, Juno will be able to reveal the planet's interior structure for the first time.

Jupiter's Red Spot and colorful clouds as seen by the Hubble Space Telescope's Wide-Field Planetary Camera.

Atmosphere

Jupiter's colorful zones, belts, and other cloud phenomena such as its famous Great Red Spot are some of the most fascinating features of the giant planet. How deep do they extend into the planet's atmosphere? For the first time, Juno will determine the global structure and motions of the planet's atmosphere below the cloud tops, mapping variations in the atmosphere's composition, temperature, clouds and patterns of movement down to unprecedented depths.

This is a spectacular NASA Hubble Space Telescope close-up view of an electric-blue aurora that is eerily glowing one half billion miles away on the giant planet Jupiter.

Magnetosphere

Under great pressure at huge depths in Jupiter's atmosphere, hydrogen gas is squeezed into a fluid known as liquid metallic hydrogen. At these enormous pressures, the hydrogen acts like an electrically conducting metal, and this region is believed to be the source of the planet's intense magnetic field. This powerful magnetic environment creates the brightest auroras in our solar system, as charged particles precipitate down into the planet's atmosphere. Juno will directly sample the charged particles and magnetic fields near Jupiter's poles for the first time, while simultaneously observing the auroras in ultraviolet and infrared light produced by the extraordinary amounts of energy crashing into the polar regions.

Juno's investigations will greatly improve our understanding of these remarkable phenomena and teach us how they may appear in similar magnetic objects, like young stars with their own planetary systems.

In short, understanding the formation of Jupiter is essential to understanding the processes that led to the development of the rest of our solar system and what the initial conditions were that led to Earth and humankind. With its intense yearlong investigation of Jupiter, Juno will allow us to take a giant step forward in our understanding on how giant planets form and the role that process plays in creating a solar system.

Last Updated: 24 January 2011