Our Solar System and Its Origin

6.1 A Brief Tour of the Solar System Our Goals for Learning What does the solar system look like?

The planets are tiny compared to the distances between them (a million times smaller than shown here), but they exhibit clear patterns of composition and motion. The patterns are far more important and interesting than numbers, names, and other trivia

Recall scale of solar system

Sun Over 99.9% of solar system’s mass Made mostly of H/He gas (plasma) Converts 4 million tons of mass into energy each second

Mercury made of metal and rock; large iron core desolate, cratered; long, tall, steep cliffs very hot and very cold: 425°C (day), –170°C (night)

Venus nearly identical in size to Earth; surface hidden by thick clouds hellish conditions due to an extreme greenhouse effect: even hotter than Mercury: 470°C, both day and night atmospheric pressure equiv. to pressure 1 km deep in oceans no oxygen, no water

Earth An oasis of life The only surface liquid water in the solar system; about 3/4 of surface covered by water One surprisingly large moon Earth and Moon to scale

Mars Looks almost Earth-like, but don’t go without a spacesuit! Giant volcanoes, a huge canyon, polar caps Water flowed in the distant past; could there have been life?

Jupiter Much farther from Sun than inner 4 planets (more than twice Mars distance) Also very different in composition: mostly H/He; no solid surface. Gigantic for a planet: 300  Earth mass; >1,000  Earth volume. Many moons, rings

Moons can be as interesting as the planets themselves, especially Jupiter’s 4 large “Galilean moons” (first seen by Galileo) Io (shown here): active volcanoes Europa: possible subsurface ocean Ganymede: largest moon in solar system — larger than Mercury Callisto: a large, cratered “ice ball” with unexplained surface features

Saturn Giant and gaseous like Jupiter most spectacular rings of the 4 Jovian planets many moons, including cloud- covered Titan currently under study by the Cassini-Huygens spacecraft

Saturn Rings are NOT solid; they are made of countless small chunks of ice and rock, each orbiting like a tiny moon. Artist’s conception

Saturn Cassini probe arrived July 2004 (Launched in 1997) Link Link Enceladus

Uranus much smaller than Jupiter/Saturn, but still much larger than Earth made of H/He gas, hydrogen compounds (H 2 O, NH 3, CH 4 ) extreme axis tilt — nearly tipped on its “side” — makes extreme seasons during its 84-year orbit. moons also tipped in their orbits

Neptune Very similar to Uranus (but much smaller axis tilt) Many moons, including unusual Triton: orbits “backward”; larger than Pluto.

Pluto A “misfit” among the planets: far from Sun like large jovian planets, but much smaller than any terrestrial planet. Comet-like composition (ices, rock) and orbit (eccentric, inclined to ecliptic plane, long years). Its moon Charon is half Pluto’s size in diameter Best current photo above; New Horizons mission launch 2006, arrival 2015

What have we learned? What does the solar system look like? Our solar system consists of the Sun, nine planets and their moons, and vast numbers of asteroids and comets. Each world has its own unique character, but there are many clear patterns among the worlds.