The Galileo mission

So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter in December 7, 1995. It orbited the planet for over seven years and conducted multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event. However, while the information gained about the Jovian system from the Galileo mission was extensive in its own right, its originally-designed capacity was limited by the failed deployment of its high-gain radio transmitting antenna.


Jupiter as seen by the space probe Cassini. This is the most detailed global color portrait of Jupiter ever assembled.An atmospheric probe was released from the spacecraft in July, 1995. The probe entered the planet's atmosphere in December 7, 1995. It parachuted through 150 km of the atmosphere, collecting data for 57.6 minutes, before being crushed by the extreme pressure to which it was subjected. It would have melted and vaporized shortly thereafter. The Galileo orbiter itself experienced a more rapid version of the same fate when it was deliberately steered into the planet on September 21, 2003 at a speed of over 50 km/s, in order to avoid any possibility of it crashing into and possibly contaminating Europa, one of the Jovian moons.

Life on Jupiter

It is considered highly unlikely that there is any life on Jupiter, as there is little to no water in the atmosphere and any possible solid surface deep within Jupiter would be under extraordinary pressures. However, in 1976, before the Voyager missions, Carl Sagan hypothesized (with Edwin E. Salpeter) that ammonia-based life could evolve in Jupiter's upper atmosphere. Sagan and Salpeter based this hypothesis on the ecology of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish. The Jovian equivalents Sagan and Salpeter hypothesized were "sinkers", "floaters", and "hunters". The "floaters" would be giant bags of gas functioning along the lines of hot air balloons, using their own metabolism (feeding off sunlight and free molecules) to keep their gas warm. The "hunters" would be almost squid-like creatures, using jets of gas to propel themselves into "floaters" and consume them. These ideas are only hypotheses and there is currently no way to prove or disprove them.

Jupiter's Atomosphere

Jupiter's atmosphere is composed of ~81% hydrogen and ~18% helium by number of atoms. The atmosphere is ~75%/24% by mass; with ~1% of the mass accounted for by other substances - the interior contains denser materials such that the distribution is ~71%/24%/5%. The atmosphere contains trace amounts of methane, water vapour, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.

This atmospheric composition is very close to the composition of the solar nebula. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium.

Jupiter's upper atmosphere undergoes differential rotation, an effect first noticed by Giovanni Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes, known as tropical regions flow in opposing directions on the prevailing winds. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot, and has persisted through more than three centuries of human observation.

The only spacecraft to have descended into Jupiter's atmosphere to take scientific measurements is the Galileo probe. It sent an atmospheric probe into Jupiter upon arrival in 1995, then itself entered Jupiter's atmosphere and burned up in 2003.