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. The need to comprehend and explain our origins--the world of natural phenomena--cannot be properly viewed as exclusively scientific. Instead, it should be viewed as something generally human. Through enchanting, magical narratives involving super-human heroes and heroines, as well as anthropomorphic gods and goddesses, ancient pre-scientific societies attempted to explain and make some order out of the mysterious complexities of the Cosmos. Earth's Moon has always held a place of special fascination for our species, inspiring our human imagination to escape its troubling limitations and--as we search beyond our Earthbound lives--help us to move towards an understanding of who we are, in all our human complexity. Therefore, ancient gods and goddesses mimic our bewitching Moon's unending, gentle tug on the forces of life. In this sense, it may be detrimental to completely dismiss these ancient myths--ascribing them to an unsophisticated and archaic past. But small moons like Methone are usually geologically inactive and bereft of an atmosphere. Therefore, they are usually unable to smooth away the scars. Dr. Peter Thomas of Cornell University in Ithaca, New York, explained it this way in the May 17, 2013 New Scientist: "When we look at objects less than 200 kilometers in radius, they are all like potatoes. They have lumps, grooves, craters." This makes Methone's smooth surface a mystery. Dr. Thomas is a Cassini team member.