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
- 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
- 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
- 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
- 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