Nice sites:
• Missions To Jupiter and Saturn: Cassini (Saturn), Juno (Jupiter), and Galileo (Jupiter). Juno is the only currently active mission.

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Galileo to Jupiter and its Moons Galileo was launched on October 18, 1989 entering the Jupiter system December 7, 1995. The Galileo mission ended when Galileo plunged into Jupiter's crushing atmosphere on Sept 21, 2003, deliberately destroyed to protect one of its own discoveries - the possible ocean beneath the icy crust of Europa. Galileo was the first probe to fly past an asteroid and to then discover a moon around an asteroid. It made direct observations of a comet colliding with a planet. Galileo was the first to measure Jupiter's atmosphere with a descent probe. The probe made the most difficult planetary atmospheric entry ever attempted, entering Jupiter's atmosphere on 7 December 7 1995, it survived entry speeds of over 106,000 mph, temperatures twice those on the surface of the Sun and forces up to 230 times greater than the strength of gravity on Earth, surviving for a remarkable 57 minutes as it descended into Jupiter.

Juno to Jupiter and its Moons Juno was launched on August 5, 2011 arriving at Jupiter in July 2016. It has been in continuous operation since then with its mission extended beyond its original end date for its prime mission of July 2021. Juno was tasked with improving our understanding of the origin and evolution of Jupiter through study of fundamental processes and conditions that took place in our solar system during its formation. Jupiter, as the principal Jovian planet, provides critical knowledge needed for understanding planetary systems around other stars. The extension of Juno, tasks it with becoming an explorer of the full Jovian system, Jupiter's rings and moons.

Hubble Views of Jupiter and Comet Shoemaker-Levy 9 In 1993, Comet Shoemaker-Levy 9 was found orbitng Jupiter and was to impact the planet in July 1994. To the left and below are images taken by the Hubble Space Telescope (HST) of Shoemmaker-Levy 9.The photos show that the comet fragmented before it reached Jupiter (it passed within the Roche limit of Jupiter) leading to a stream of objects, the largest of which was around two kilometers in size which eventually impacted on Jupiter.

The impacts occurred at night and Galileo was the only obesrvatory to observe the direct impact of Comet Shoemaker-Levy 9 on Jupiter. Because of Jupiter's rapid rotation, the impact sites were quickly brought into view (within a few minutes). To the right is shown the impact of Fragment W.

Cassini to Saturn and its Moons Cassini, launched in 1997 arrivied at Saturn in 2004, was scheduled for decommission in September 2010, but was reprieved in the 2011 NASA budget proposal which allocated $60 million per year for seven years to keep the probe pumping out science from the ringed planet (hooray!). The end came for Cassini when it made its final approach to Saturn on September 15, 2017 and accomplished its last Saturn mission as it plunged into Saturn's atmosphere with last contact on September 15 at 7:55:46 a.m. EDT.

The above shows the small moon Tethys slipping behind the large moon Titan. Cassini always showed great interest in Titan, the giant, haze-enshrouded moon of Saturn. Cassini studied the structure and complex organic chemistry of Titan's smog-filled atmosphere. Cassini revealed vast methane lakes and widespread stretches of wind-driven hydrocarbon sand dunes on Titan's surface. Cassini researchers also deduced the presence of an internal, liquid water-ammonia ocean. Cassini searched for signs of seasonal changes such as storms, flooding, or changes in lake levels, as well as evidence of volcanic activity on Titan. The lower left image shows the plumes on Enceladus. The middle image shows the chemical make-up of the plumes and the far right image shows the region near the south pole of Enceladus, in particular, the large fissures known as the Tiger Stripes, the cracks in the ice shell of Enceladus through which the plumes erupted.

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• Jupiter and Saturn are the two most massive planets in the Solar System;
  • Jupiter is 318 times as massive as the Earth with a rocky/icy core that may be as massive as 10 to 20 times the mass of the Earth. The core material is, however, distributed and mixed with overlying material over perhaps the inner 50 % of Jupiter, the core is diffuse not a compact rock/ice region.
  • Saturn is 94 times as massive as the Earth with a core that appears to be a sludgy mixture of rock, ice, hydrogen, and helium 55 times the mass of the Earth. The rock/ice portion of the core may be up to 17 times the mass of the Earth.
  • Jupiter although only about 0.1 % the mass of the Sun is around 2.5 times the mass of all of the other planets combined. So, as already emphasized, our Solar System is pretty much Jupiter and the Sun or Jupiter/Saturn and the Sun.

• Jupiter and Saturn both have low density, roughly the density of water (as is the Sun) which implies they are composed of simple elements such as hydrogen and helium. Jupiter and Saturn both have chemical make-ups similar to the Sun, more like stars than earth-like (Terrestrial) planets.

• Jupiter and Saturn are both gaseous with no solid surfaces. When the Galileo and Cassini probes were retired, their final acts were to plunge into Jupiter and Saturn, respectively. Both simply sank into the planets (see Galileo homepage and Cassini homepage) probing the atmospheres until they were no longer able to get signals out.

• Jupiter and Saturn both have short spin (rotation) periods, on the order of 10 hours. Translated into the speed of an observer standing on the equator of Jupiter, this corresponds to a speed of ~25,000 miles per hour! Recall that for the Earth, an observer would move with a speed of ~ 1,100 miles per hour. This rapid spin has profound effects on the appearance of Jupiter and Saturn and in fact tells much about the interior structure of Jupiter and Saturn.

In the following lectures, we will look at the properties of Saturn and Jupiter in more detail highlighting results obtained from the Galileo, Juno, and Cassini missions mentioned above. In partcular, we consider results that pertained to our models for the formation of our Solar System and planetary systems in general and to our searches for extra-Terrestrial life in our Solar System on moons of Jupiter and Saturn.

I. Interiors of Jupiter and Saturn

II. Atmospheres of Jupiter and Saturn

III. Moons of Jupiter and Saturn