Mercury and Venus
The innermost planet is rarely seen because of the Sun's glare. With less than half Earth's gravity, Mercury retains only a wisp of an atmosphere (presumed to be helium). The lack of a significant atmosphere allows temperatures to fluctuate from 750 degrees Fahrenheit during the day to minus 320 Fahrenheit at night.
Like the other terrestrial planets -- Venus, Earth and Mars -- Mercury is made mostly of rock and metal. This small world is scarred by craters and looks somewhat like our Moon.
Mercurius: roman winged messenger of the gods
Mercury has been known since ancient times. Its elusiveness generated the name Hermes, given by the Greeks, later translated to Mercurius by the Romans.
The second planet from the sun bakes under twice as much solar radiation as Earth and reaches temperatures of 895 degrees Fahrenheit (480 degrees Celsius). Pressure from the dense atmosphere of sulfuric acid gas is about 95 times greater than Earth's and would crush a human. The thick cloud cover around Venus rotates much faster than the planet itself -- once every four days. After the moon, Venus is the brightest object in the sky.
The surface of Venus is mostly a rocky desert (this computer-generated view shows lava flows around Sif Mons). Like Mercury, Earth and Mars, Venus is composed of mostly rock and metal.
Venus: roman goddess of love and beauty
The Greeks believed Venus was two separate objects -- one in the morning skyand another in the evening. Because it is often brighter than any other object in the sky -- except for the sun and moon -- Venus has generated many UFO reports.
While all of the planets orbit in an ellipse, Venus' orbit is the closest to a perfect circle. It is the only planet in the solar system whose day (241 Earth days) is longer than its year (225 Earth days).
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