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Mercury (planet), in the solar system, the planet closest to the sun. Its mean distance from the sun is approximately 58 million km (about 36 million mi); its diameter is 4875 km (3030 mi); its volume and mass are about : that of Earth; and its mean density is approximately equal to that of Earth. Mercury revolves about the sun in a period of 88 days. Radar observations of the planet show that its period of rotation is 58.7 days, or two-thirds of its period of revolution. The planet, therefore, rotates one and a half times during each revolution. Because its surface consists of rough, porous, dark-colored rock, Mercury is a poor reflector of sunlight.
Mariner 10 also detected a magnetic field about 1 percent that of Earth, making Mercury the only other planet with density and composition close to that of Earth to have a magnetic field. Because it has a magnetic field, the outer core of the planet must be a liquid iron compound, which produces a magnetic field as it moves. Scientists believe Mercury's crust acts as an effective insulator to keep the planet's outer core liquid despite the very cold temperatures on the dark side of the planet.
Spectroscopic studies indicate that only an extremely thin atmosphere, containing sodium and potassium, exists on Mercury, its atoms apparently diffusing from the crust of the planet. Collisions with other protoplanets early in the history of the solar system may have stripped away lighter materials, thereby accounting for Mercury's great density. The force of gravity on the planet's surface is about one-third of that on Earth's surface.
The Mariner 10 spacecraft passed Mercury twice in 1974 and once in 1975. It sent back pictures of a moonlike, crater-pocked surface and reported temperatures to be about 430° C (about 810° F) on the sunlit side and about -180° C (about -290° F) on the dark side. Unlike that of Earth's moon, the surface of Mercury is criss-crossed by long escarpments, or cliffs, dating perhaps from the period of contraction the planet experienced as it cooled some time early in its history.
In 1991 powerful radio telescopes on Earth revealed unmistakable signals of vast sheets of ice in Mercury's polar regions, areas that had not been covered by Mariner 10.
The point in Mercury's orbit at which the planet is closest to the sun (the perihelion) moves a tiny amount every orbit, but too much to be accounted for by the force of gravity from other planets. A full explanation of the changes in Mercury's perihelion was one of the first confirmations of the theory of relativity. |
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Venus (planet), one of the planets in the solar system, the second in distance from the sun. Except for the sun and the moon, Venus is the brightest object in the sky. The planet is called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because of the distances of the orbits of Venus and earth from the sun, Venus is never visible more than three hours before sunrise or three hours after sunset.
When viewed through a telescope, the planet exhibits phases like the moon. Full Venus appears the smallest because it is on the far side of the sun from earth. Maximum brilliance (a stellar magnitude of -4.4, or 15 times the brightest star) is seen in the crescent phase. The phases and positions of Venus in the sky repeat with the synodic period of 1.6 years (see Time; Year). Transits across the face of the sun are rare, occurring in pairs at intervals of a little more than a century. The next two will be in 2004 and 2012. |
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Mars (planet) , one of the planets in the solar system, it is the fourth planet from the sun and orbits the sun at a distance of about 228 million km (about 141 million mi). Mars is named for the Roman god of war because it appears fiery red in the earth's night sky.
Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth's mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth's moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth.
The Martian day, or the time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year, or the time it takes to revolve once around the sun, is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny bodies are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one Martian day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west. |
