2. The new, close-up view from space

    • Close examination of the planets and moons began when spacecraft flew past them, providing an initial reconnaissance. Orbiting spacecraft that mapped out the global terrain of the Earthís Moon, Venus and Mars, as well as the realms of Jupiter and Saturn followed this. Probes have been parachuted down into the atmospheres of Jupiter, Titan and Venus, and landers and rovers have been sent to the surfaces of the Moon and Mars.

    • The space-age investigation of the solar system began in a cold war competition between the Soviet Union, which launched the first artificial satellite, and the United States, which won the race to the Moon.

    • The Voyager 1 and 2 flyby spacecraft transformed our understanding of the four giant planets, Jupiter, Saturn, Uranus and Neptune, and revealed fascinating, unexpected aspects of their moons and rings.

    • The Giotto spacecraft was the first to provide a close-up view of a comet, showing that its nucleus is a black, city-sized chunk of water ice and dust that emits sunward jets of water when passing near the Sun.

    • Orbiting spacecraft have greatly increased the time for study of the planets and moons, revealing ancient water flow on Mars, vast outpourings of lava on Venus, Jupiterís volcanic moon Io and an ice covered ocean on its satellite Europa, and Saturnís marvelous rings, water-spewing satellite Enceladus, and haze-shrouded moon Titan.

    • Three rovers have explored the surface of Mars and provided evidence for water flow across its surface roughly 4.0 billion years ago.

    • The Huygens Probe and radar from the orbiting Cassini spacecraft have discovered rain, rivers and lakes of liquid methane on Saturnís moon Titan.

    • The planets and moons gathered together as the result of the collisions of smaller bodies beginning about 4.6 billion years ago.

    • Every solid planet or satellite contains impact craters, but in different amounts that depend on the ages of their surfaces.

    • Impact craters on the Moon, Mercury, and Jupiterís icy moon Callisto all record an ancient, intense rain of meteorites, which occurred about 4.0 billion years ago, and a continued cosmic bombardment at lower rates since then.

    • Ancient craters on the relatively young surfaces of Earth, Venus and Io have been erased by geologic and volcanic activity.

    • The round craters on the Moon were formed by the explosive impact of large meteorites that came from interplanetary space, releasing enormous energy, melting rock and excavating circular craters with raised rims on impact.

    • Massive impacts of exceptionally big objects gouged out large impact basins on the Earthís Moon and the planet Mercury. The impact basins can be surrounded by concentric, ring-like rims as tall as mountains, and the basins have been subsequently filled with lava,

    • Giant impacts in the early history of the solar system may account for the origin of the Earthís Moon, the removal of Mercuryís low-density mantle, the backwards rotation direction of Venus, and the crustal dichotomy between the low-lying northern plains and southern highlands of Mars.

    • Upon impact with the surface of Mars, ground water ice can be melted, lubricating the ejected material that flows like mud.

    • The material ejected from craters on Venus has been shaped by the planetís hot, thick atmosphere into asymmetric, lobate forms. Small impacting projectiles have been burnt up in the thick atmosphere, so there are no small craters on Venus.

    • Internal heat can be produced by radioactive decay of rocks inside a terrestrial planet, or within a satellite as the result of varying gravitational interaction with its planet. The giant planets still retain the heat of their formation.

    • Molten rock, or magma, that is localized in underground chambers of a planet can rise to the surface and cause two types of basaltic volcanism – tall shield volcanoes and smooth volcanic flows known as plains.

    • Earth has unique underwater volcanoes found in mid-ocean ridges that supply a spreading sea floor, as well as chains of hot-spot volcanoes, such as the Hawaiian Islands, and volcanoes arising from the downward plunge of moving plates.

    • Upwelling of internal magma is cracking part of Africa open, in a great rift valley.

    • Extensive lava flows filled large impact basins on the Moon, creating the dark lunar maria between 3.9 and 3.2 billion years ago.

    • Ancient, smooth volcanic flows on Mercury have obliterated small craters, filled the interiors of large impact basins, and spread out between large craters producing about 40 percent of the planetís surface. Most of this volcanic activity occurred after the heavy bombardment about 4.0 billion years ago, but before smaller craters were formed on the smooth plains.

    • Extensive volcanic activity on Venus resurfaced the planet about 750 million years ago.

    • Mars has the tallest volcanoes in the solar system, and most of its northern hemisphere is covered with volcanic flows of lava.

    • The volcanoes on Jupiter's satellite Io have turned the satellite inside out; it is heated inside by the tidal flexing action of nearby massive Jupiter.

    • Liquid water flows out of cracks in the icy surface of Jupiterís moon Europa, and erupts as jets of water ice and water vapor from Saturnís moon Enceladus.

    • Volcanoes of ice may have created some of the features now frozen into the bright smooth surface of Neptune's largest moon, Triton; dark geyser-like plumes have been observed in the process of eruption on the satellite.

    • Seventy one percent of the Earth's surface is covered with liquid water, and our bodies are largely composed of water.

    • Dark, permanently shadowed regions inside craters in the Moonís polar regions could contain water. The Clementine, Lunar Prospector, Chandrayaan-1 and LCROSS spacecraft have provided evidence very small amounts of water on the Moon.

    • Strong radar echoes from the highly reflective polar regions of Mercury suggest that thick deposits of water ice reside in the permanently shadowed interiors of craters near the planetís poles.

    • Although Venus is now dried out, it may have once contained a small ocean.

    • Small amounts of water vapor are found in the atmosphere of Mars, together with clouds and fogs of water ice.

    • Vast amounts of frozen water now exist in the polar caps of Mars and beneath the surface of the polar, mid-latitude, and equatorial regions of the red planet.

    • It cannot now rain on Mars, and liquid water cannot now exist for any length of time on the planetís surface.

    • Catastrophic floods and deep rivers once carved channels on Mars, and an ancient ocean may have once covered the planet's northern lowlands.

    • Saturnís rings consist of billions of particles of water ice.

    • Jupiter's satellite Europa is covered with bright, smooth water ice, which has cracked due to the contorting tidal effects of Jupiter's strong gravity. The warmth generated by tidal heating may have been sufficient to form an ocean of liquid water below Europa's icy covering.

    • Magnetic measurements provide indirect evidence for an ocean of salty, liquid water below the icy crust of Jupiter's satellite Europa.

    • Jupiterís satellite Ganymede also probably contains an ocean of liquid water under its ice-covered surface.

    • Saturnís satellite Enceladus has a frozen covering of water ice, and the moon emits icy jets, feeding the E ring that encircles Saturn.

Copyright 2010, Professor Kenneth R. Lang, Tufts University