The Solar System Chapter 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Planets, moons and other bodies Solar System Sun 8 planets ~100 moons Thousands of asteroids, millions of icy bodies, comets, … Astronomical unit (AU) Average Earth-Sun distance 1.5x108 km Planet classification: size, density and atmosphere Terrestrial planets Mercury, Venus, Earth and Mars Mostly rocky materials, metallic nickel and iron Giant planets Jupiter, Saturn, Uranus and Neptune Mostly hydrogen, helium and methane Dwarf Planet Pluto Orbits the sun, nearly spherical, has not cleared matter from its orbital zone, and is not a satellite

The order of the planets

Mercury Innermost planet Highly elliptical orbit Average distance ~ 0.4 AU Orbital period ~ 3 months Rotational period ~ 59 days Visible shortly after sunset or before sunrise Highly cratered; no atmosphere No moon

Venus Orbital distance ~ 0.7 AU Morning and evening “star” Exhibits phases, like the Moon Rotational motion opposite orbital motion Venusian “day” longer than Venusian “year” Visited by numerous probes Mostly CO2 atmosphere, high temperature and pressure Surface mostly flat but varied

Earth’s Moon Lunar highlands Maria Craters No water, atmosphere Light colored mountainous regions Breccias - rock fragments compacted from meteorite impact Formed ~4 billion years ago Maria Smooth dark areas formed from floods of lava Basalt - similar to rock formed from cooling lava on earth Formed about 3.1 – 3.8 billion years ago Craters No water, atmosphere Surface 3 meters grey dust containing microscopic glass beads formed by bombardment of meteorites

Stages in Formation of the Moon Origin Stage Moon formed from impact of earth with very large object Molten Surface Stage First 200 million years Lunar surface melted due to rock impact Fewer bombardments, moon surface solidified Craters: result of meteorite impact after formation of crust Molten Interior Stage 3.8 billion years ago Interior melted due to heat generated by radioactivity Cold and Quiet Stage 3.1 billion years ago Last lava solidified Surface change little

Mars Orbital distance ~ 1.5 AU Geologically active regions Inactive volcanoes Canyons Terraced plateaus near poles Flat regions pitted with craters Thin atmosphere, mostly CO2 Strong evidence for liquid water in past Numerous space probes

Spirit and Opportunity Mars Exploration Rovers Found that Mars made of basalt rock and groundwater that is dilute sulfuric acid Confirmed sufficient amounts of water have been present in the past

Jupiter ~ 5 AU from Sun Most massive planet 318 times Earth’s mass Mostly H and He with iron-silicate core Dynamic atmosphere H2, He, ammonia, methane, water, … Great Red Spot 64 satellites

Saturn 9.5 AU from Sun Rings of particles Density = 0.7 that of water Surface similar to Jupiter’s 62 satellites Titan: only moon with substantial atmosphere

Uranus and Neptune Uranus (~19 AU) and Neptune: (~30 AU) Outermost giant planets Similar internal structures

Planet summary

Smaller bodies of the Solar System Comets, asteroids, meteorites Leftover from solar and planetary formation Mass of smaller bodies may be 2/3 of total Solar System mass Bombard larger objects Comet Shoemaker-Levy 9 fragments (bottom)… … and strikes Jupiter (July 1994)

Comet origins Oort cloud Kuiper belt Origin of long-period comets (>200 years) 30 AU to light-year away Kuiper belt Origin of short-period comets (<200 years) Disk-shaped region 30-100 AU from Sun Gravitational nudges deflect objects toward Sun

Comet structure Small, solid objects “Dirty snowball” model Comet head Frozen water, CO2, ammonia, and methane Dusty and rocky bits Comet head Solid nucleus and coma of gas Two types of tails Ionized gases Dust Tail points away from Sun

Asteroids Located in belt between Mars and Jupiter Sizes: up to 1,000 km Varied composition Inner belt: stony Outer belt: dark with carbon Others: iron and nickel Formed from original solar nebula Prevented from clumping by Jupiter nearby

Meteors and meteorites Meteoroids Remnants of comets and asteroids Meteor Meteoroid encountering Earth’s atmosphere Meteor showers: Earth passing through comet’s tail Meteorite Meteoroid surviving to strike Earth’s surface Iron, stony (chondrites and achondrites) or stony-iron

Origin of the Solar System Protoplanet nebular model Stage A Formation of heavy elements in many earlier stars and supernovas Concentration in one region of space as dust, gas and chemical compounds

Origin of the Solar System Stage B Formation of large, rotating nebula Gravitational contraction, spin rate increases Most mass concentrates in central protostar Remaining material forms accretion disk Material in accretion disk begins clumping

Origin of the Solar System Stage C Protosun becomes a star Solar ignition flare-up may have blown away hydrogen and helium atmospheres of inner planets Protoplanets heated, separating heavy and light minerals Larger bodies cooled slower, with heavy materials settling over longer times into central cores