The Terrestrial Planets
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1, Mercury
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Diameter=4,878 km or 38% that of Earth
Mercury, closest planet to the sun, is an airless cratered world, its face still 
showing scars of bombardment by asteroids and comets during the solar system's 
youth. For at least the last three billion years, it has remained essentially 
unaltered. Until recently when spacecraft were sent there, Mercury had been 
extremely difficult to study because its small size and its close proximity to 
the sun made it difficult to see through even the most powerful telescopes. 
Until 1965, it was believed as fact that Mercury did not rotate but that one 
side constantly faced the sun. The truth is that Mercury turns so slowly it is 
difficult to tell that it is turning at all; it orbits the sun twice while it 
rotates on its axis three times. Mercury's day is 58.6 Earth days long and its 
year 87.97, with temperatures on the day side reaching up to 450 degrees C, and 
on the night side dipping to -180 C. 

2,Venus 
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Diameter=12,104  km or 95% that of Earth's
Venus is often referred to as Earth's "sister planet" because of its 
similarities in size, mass, and surface gravity. However, there is little else 
in common. A human standing unprotected on the surface would be simultaneously 
sizzled by the heat, asphyxiated by carbon dioxide, crushed by the density of 
the planet's atmospheric cloak, and scorched by hydrochloric-acid vapours. 
Venus' surface pressure is 90 times that of Earth's and equivalent to the 
pressure at a water depth of 900 meters on Earth. Venus is a hot desert world, 
with a surface temperature of 464 C, topped by an unbroken layer of acid clouds. 
These reflective clouds make it one of the brightest objects in the sky, 
exceeded only by the Sun and Moon. It can be seen for a few hours each night, 
when it rises before dawn or sets after sundown. A day on Venus is longer than 
it's year, taking 243 Earth days to rotate (in the opposite direction to 
Earth's) but only 224.7 days to orbit the Sun. 

3,Earth
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Diameter=12,756 km
Earth is the largest of the terrestrial planets and the only planet to support 
life as we know it. It's surface is a wrinkled layer of solid rock called the 
crust, which is between 10 and 32 km thick; the oceans, comprising about 2/3 of 
the Earth's surface, fill the crust's deepest basins. The areas not covered by 
water are the seven continents of Africa, North and South America, Asia, 
Australia, Europe, and Antarctica. Our atmosphere, composed of 78% nitrogen and 
21% oxygen, is the only one in the solar system which provides the oxygen that 
humans need to breathe. Earth' s day is 23.hours and 56 minutes, its year 365.25 
days. The moon is the Earth's partner in space. About 1/4 the size of Earth, it 
is an airless, waterless world which circles the Earth once every 27 days at an 
average distance of 384,000 km. The moon is Earth's nearest neighbor in space 
and the only celestial body on which humans have landed.

4, Mars 
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Diameter=6,794 km or 53% that of Earth
The last of the terrestrial planets and fourth from the sun, Marsis the second 
closest in similarity to Earth. It's day is almost the same length, at 24 hours 
and 37 minutes, and it has many similar surface features including enormous 
volcanoes, vast canyons, dune fields, and dry channels. The planet's most 
distinguishing feature is the Valley Marineris, a canyon complex that stretches 
5,000 km, far longer than any similar feature on Earth. On a summer day Mars' 
temperature rises no higher than0 degrees C and at night most areas cool to -100 
C. The only sign of water on Mars are the now dry channels, and the currently 
existing northern polar ice cap. Mars is called the Red Planet because of its 
reddish appearance in the night sky, as well as on its surface. Exploration of 
the Martian surface revealed no signs of life, only boulder-strewn fields, sand 
dunes, and a weird pink sky. Mars has two satellites, Phobos and Deimos.
