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May 16, 2002
As far as we know, there are nine planets locked in orbit around the Sun. Only one, our own Earth, supports life. But there are countless other suns throughout countless galaxies scattered across the expanse of the universe. We still don't know if life exists on another planet in some other galaxy.
Named for the wing-footed messenger of the Roman gods, Mercury races around its orbit at a dizzying speed of 30 miles (48 kilometers) per second, making the Mercurial year only 88 Earth days long. In contrast, one rotation around its axisor a single daytakes almost 59 Earth days.
Mariner 10 gave us a wealth of information about Mercury when it approached the planet in 1974 and 1975. Because Mercury has no water and barely any atmosphere, no erosion has taken place on its surface. We see Mercury much the way it was soon after it formed. Core
We learned that Mercury has an extremely weak magnetic field, which could indicate a hot metallic core, such as molten iron. Geologists think Mercury may be the most iron-rich planet in the solar system. Crust
Mercury's crust seems to be silicate, like that of Earth.
The planet's surface, viewed for the first time via Mariner's cameras, is covered by craters. The battering occurred during the early period of the solar system when clumps of material were ramming into each other to form the planets.
Mercury doesn't have what we think of as an atmospherethat is, a gaseous envelope that produces clouds and weather or protects the surface of the planet from some of the harmful solar radiation. The weak magnetic field on Mercury captures only the barest perceptible trace of charged particles from the Sun.
Shrouded in the cloak of mystery, Venus, our nearest planetary neighbor, takes the name of the Roman goddess of love. For some unknown reason, Venus rotates on its axis in retrogradethat is, in the reverse direction of its revolution around the Sun.
Geologically, Venus appears to have some similarities to Earth. Its crust is probably granitic, overlying a basaltic mantle and a iron-nickel core. The geologic activity that we are familiar with on Earth seems not to exist on Venus, except for the presence two volcanoes along a fault line.
Venus's veil of mystery is really an impenetrable, heavy layer of sulfuric acid clouds above an atmosphere consisting of about 96 percent carbon dioxide. Greenhouse effect
Sunlight entering Venus's atmosphere is converted to heat radiation and is prevented from escaping by carbon dioxidea phenomenon we call the "greenhouse effect." Surface temperatures reach 800 degrees Fahrenheit (341.33 degrees Celsius), and the atmosphere appears to be constantly flashing with lightning. Weight of atmosphere
For an idea of the weight of Venus's atmosphere, consider that walking on the surface of Venus would be comparable to walking on the ocean floor at a depth of half a mile.
For about 500 million years after its initial formation, the Earth remained at a rather stable 2000 degrees Fahrenheit (874.68 degrees Celsius). Comprised predominantly of iron and silicates, the Earth also contained small amounts of radioactive elements, mostly uranium, thorium, and potassium. As these elements decayed, they produced radiation that gradually heated the Earth, melting the iron and silicates. The iron sank to the center, forcing the lighter silicates to the surface and causing the violent processes that formed the Earth's surface as we know it and that continue to form it even today.
Between the iron core and the solid rock crust lies a mantle of heavy silicate rock that is 1800 miles thick (2880 kilometers). The mantle is neither solid nor liquid, but has a viscous, yielding consistency upon which the crust actually floats. The crust is not a single, solid mass,...