The Solar System formed 4. 6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system’s mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The "Dagwood Sandwich" Moon. Earlier models of Ganymede's oceans were based on the assumption that the existence of salt didn't change the nature of liquid very much with pressure. However, Dr. Vance and his colleagues found, through laboratory experiments, that salt does increase the density of liquids under the extreme conditions hidden deep within Ganymede and similar icy moons with subsurface bodies of water. Imagine adding table salt to a glass of water. Instead of increasing in volume, the liquid will actually shrink and become denser. The reason for this is that salt ions lure water molecules. Cassini's successful mission of exploration to the Saturn system is over, but planetary scientists are left with a cornucopia filled with important new information that Cassini/Huygens sent back to Earth before its mission ended. A collaborative NASA/European Space Agency/Italian Space Agency mission, the robotic spacecraft was made up of two components. The first was the European Space Agency's (ESA's) Huygens Probe, that had been named in honor of the Dutch mathematician and astronomer Christiaan Huygens (1629-1695), who discovered Titan. The Huygens Probe also closely observed Saturn's lovely system of gossamer rings. The second component, the NASA-designed Cassini Orbiter, was named after the Italian-French astronomer Giovanni Dominico Cassini (1625-1712), who discovered four of Saturn's other intriguing, numerous, and icy moons. A Distant, Dusty Moon. Titan experiences changing seasons--just like Earth. In particular, Titan's seasons change around the equinox, when our Sun passes Titan's equator. At this time, huge clouds can form in tropical areas, resulting in violent methane storms. Cassini observed these ferocious methane storms during several of its flybys over Titan.