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Mercury is the smallest principal planet, with a diameter of 4878 km. It is also the closest to the Sun, orbiting a mere 0.387 AU away.
(An AU is the abbreviation for an Astronomical Unit; one AU is the distance from the Earth to the Sun).
Mercury has a thin helium and sodium atmosphere and no moons. Its surface is best described as "moon-like", heavily cratered with some lava flows.
There may be ice deposits in the permanently shaded areas of some polar craters, despite Mercury's proximity to the Sun.
An image of Mercury taken by the Mariner 10 spacecraft appears at right. 
Venus has been called Earth's "sister" planet, as the two planets have similar size and mass.
Venus's diameter is 12,104 km and its surface is very hot, about 750 degrees Kelvin.
It has a dense carbon dioxide atmosphere, with a surface pressure of 90 atmospheres (90 times the Earth's surface pressure).
There are sulfuric acid clouds in the atmosphere, making Venus's surface truly inhospitable.
This planet has no moons and its surface below the clouds shows volcanic and tectonic features and few craters.
At right is an image taken by the Mariner 10 spacecraft on Feb. 5, 1974.
Note that the true surface of the planet is not visible through the thick atmosphere.
One of the most notable features of the planet Earth is its substantial atmosphere, composed of nitrogen, oxygen, and water vapor.
Another is the fact that 71% of the planet's surface is covered with liquid water and it has large polar caps of water ice.
But perhaps the most important and unique characteristic of the Earth is the presence of abundant life, both on the land and in the oceans.
The planet's diameter is 12,1756 km. Earth's surface shows tectonic and volcanic activity, but it has relatively few craters.
Our planet also has one large satellite of unusual origin, which deserves to be discussed seperately.
At right is an image of Earth, showing the South American continent, taken by the Galileo spacecraft Dec. 11, 1990.
The surface of our only natural satellite is heavily cratered by impacts and shows dark regions called maria.
These regions are ancient lava flows that occur mostly on the side of the Moon that faces Earth.
(Note that the Moon orbits with one side permanently facing the Earth; this occurs because of tidal forces.)
This satellite has a diameter of 3476 km and is thought to be geologically dead.
The Moon is slowly spiraling away from the Earth, as the angular momentum of the Earth's rotation is transferred to the angular momentum of the
Moon's revolution about the Earth, via tidal interactions. At right appears a true-color image of the Moon taken by the Galileo spacecraft.
Often termed the "red planet", Mars gets its color from oxidized iron minerals on its surface, ie. rust.
It has a thin carbon dioxide atmosphere and occasional thin clouds of water vapor.
It also has polar caps of carbon dioxide and water ices.
The Martian day has roughly the same length as an Earth day and its diameter is 6787 km.
The surface of Mars is very cold, T ~ 190-240 Kelvin, and it shows craters, volcanoes, and strong evidence for liquid water on the surface in the past.
Mars has two small rocky satellites, Phobos and Deimos. At right is an image of Mars during the 1995 opposition, taken with the Hubble Space Telescope.
The asteroids are small chunks of rocky materials that exist in a belt between the orbits of Mars and Jupiter.
Their sizes range from a few meters in diameter to about 1000 km in diameter and there are about 10,000 asteroids known.
They are not the remains of a disrupted planet, but rather are debris left over from the era of planetary formation.
The material comprising the asteroids could not form up into a large planet because of gravitational resonance with Jupiter.
Recently, the asteroid Dactyl was found to be orbiting the asteroid Ida, the first example of an asteroid with a satellite.
A picture of Ida and Dactyl taken by the Galileo spacecraft appears at right; Dactyl is the small blip to the right of Ida.
Jupiter, with a diameter of 142,800 km, is the largest planet in the Solar System.
It is composed mostly of gases and contains more mass than all the other planets combined.
Thus, it has sometimes been referred to as a "failed star".
It has a dynamic atmosphere, consisting of hydrogen, methane, and ammonia.
It has a faint ring system and 16 or more satellites, which are named for the god Jupiter's lovers from Greek mythology.
Four of these satellites, the Galilean satellites, are large and will be treated seperately as worlds in their own right.
An image of Jupiter taken by the Voyager 1 spacecraft appears at right.
With a diameter of 4840 km, Callisto is about the same size as Mercury. It has a heavily cratered rock and ice surface; thus, it resembles the
Moon and Mercury, but with ices. An image of Callisto taken by the Voyager 2 spacecraft appears at right.
Ganymede is the largest moon in the Solar System, with a diameter of 5280 km making it larger than Mercury.
It has a bright ice surface with some heavily cratered areas.
There also are some heavily grooved fracture areas, perhaps giving evidence of tectonics on this moon.
An image taken by the Voyager 1 spacecraft appears at right.
This moon has a diameter of 3130 km and a grooved surface with few craters. Its icy surface likely covers an ocean of liquid water.
An image taken by the Voyager 1 spacecraft appears at right.
Io is the innermost of the Galilean satellites and has a diameter of 3630 km.
Its most notable feature is the fact that it is the most volcanically active body in the Solar System.
Its volcanism is driven by tidal stresses, as will be discussed later. It produces a torus of ions many Io-diameters across surrounding Jupiter.
Io has a tenuous sulfur dioxide atmosphere, that is possibly variable and non-uniform.
The surface is composed of silicate and sulfurus lavas and sulfur dioxide frosts and has almost no impact craters.
An image taken by the Voyager 1 spacecraft appears at right.
Saturn is the second largest planet in the Solar System, with a diameter of 120,660 km. It is also a gas giant planet,
like Jupiter, but is less dense. In fact, Saturn is less dense than liquid water and would float if put in a sufficiently large
bathtub. It has a beautiful, complex ring system and many satellites. Its moon Titan is the second largest in the Solar System
and deserves additional mention. At right is an image of Saturn taken by the Hubble Space Telescope.
This moon of Saturn has a diameter of 5150 km and is unusual in having a dense atmosphere, with a surface pressure 1.6
times that at the surface of the Earth. The atmosphere is mainly composed of nitrogen and methane.
Titan's atmosphere also contains lots of photochemical smog, not unlike that found over Los Angeles on a hot day.
Thus, Titan's atmosphere is filled with organic compounds present in the smog.
The surface temperature is very low, only around 93 Kelvin; no liquid water can exist on the surface.
However, liquid methane or ethane may be present on the moon's surface.
Due to Titan's dense, smoggy atmosphere, we have yet to observe the true solid surface of the satellite; however, the
Cassini spacecraft is scheduled to drop the Huygens probe onto the surface of Titan on or around Nov. 27, 2004.
At right is an image of Titan taken by the Voyager 1 spacecraft.
Uranus is another of the Solar Systen's gas giant planets, with a diameter of 51,118 km.
It has a predominantly hydrogen and methane atmosphere, which has a strong greenish blue color.
This color is due to Rayleigh scattering of blue light, the same process that makes the Earth's sky blue, and to absorption of red light by methane.
The most unique characteristic of Uranus is the fact that it has an obliquity of about 98 degrees.
This means that, unlike the Earth and all the other planets, it orbits around the Sun on its side, with its rotation axis lying almost in the
plane of its orbit. Uranus has lots of satellites, which are named for characters from the plays of William Shakespear.
One of its satellites, Miranda, appears to have been smashed apart at one time in its history, then reformed.
An image of Uranus from the Voyager spacecraft appears at right.
Neptune is the first planet whose existence was predicted before it was discovered in 1846.
It also a gas giant, with a hydrogen and methane atmosphere similar to Uranus's, but more active.
It has eight satellites, one of which is very interesting and deserves further comment.
An image taken of Neptune by the Voyager 2 spacecraft appears at right.
Triton orbits Neptune in a retrograde orbit, while the planet rotates in the prograde direction.
This means that Neptune rotates in one direction, while Triton moves around it in the opposite direction.
This is has led astronomers to believe that Triton was not formed in the vicinity of Neptune but was formed in the
region of Pluto and gravitationally captured by Neptune. (For info on why people think this, read the web paper I
wrote last year, Planetary Formation and Our Solar System.)
Triton has a thin nitrogen and methane atmosphere. Its surface is bright and icy, with polar caps of nitrogen frost.
It also displays occasional geyser-like eruptions. An image of Triton taken by the Voyager 2 spacecraft appears at right.
The last "planet" in the Solar System is tiny Pluto, which has a diameter of a mere 2300 km.
It is much smaller than a number of satellites and there has been much debate about whether it should really be classified as a planet.
It has one moon, Charon, which has a diameter of 1200 km, making it more than half as big as Pluto itself.
Pluto's orbit is very eccentric (non-circular), causing it to cross the orbit of Neptune.
Its surface is thought to be mainly rock and methane, water, and nitrogen ices, rather like the surface of Triton, but colder.
Pluto does have a thin atmosphere, composed of methane, nitrogen, and carbon monoxide.
No really good pictures of Pluto exist, because it is so small and far away, and we have not visited it with a spacecraft yet.
Such as it is, one of the best images of Pluto and Charon, taken by the Hubble Space Telescope, appears at right.
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