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Top 10 Things You Should Know About Mercury

Named after the Roman god of commerce, travel and thievery, Mercury is the second smallest planet in the solar system, larger only than Pluto, the most distant planet in our solar system. If Earth were the size of a baseball, Mercury would be the size of a golf ball. Viewed from Mercury, the Sun would look almost three times as large as it does from Earth.

1) Mercury is the closest planet to the Sun

Mercury speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun. The planet completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet.

2) Mercury's existence has been known of since before the third century BC

The Greeks gave it two names, Apollo for when it appeared as a morning star and Hermes when it came as an evening star.

3) Mercury's surface very much resembles Earth's Moon

Mercury is scarred by thousands of impact craters resulting from collisions with meteors. While there are areas of smooth terrain, there are also cliffs, some soaring up to a mile high, formed by ancient impacts.

4) Mercury has a large iron core which is most likely at least partially molten

Mercury's interior is made of a large iron core with a radius of 1,800 to 1,900 km, nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600 km thick.

5) Mercury actually has a very thin atmosphere

Merucry's atmosphere is made up of atoms blasted off its surface. Due to the heat of the planet, these atoms quickly escape into space. Thus unlike the Earth and Venus which have stable atmospheres, Mercury's atmosphere is constantly being replenished.

6) Mercury is a planet of extremes

Temperature variations swing from 90 K to 700 K. It's hotter on Venus, but with less fluctuations. Mercuryalso has a very eccentric orbit; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million.

7) Mercury has no known moons or satellites

While Mercury may resemble our own moon in many ways, it has no moon of its own.

8) Until 1965, scientists thought that the same side always faced the Sun

In 1965, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you'd have to stay up for 176 Earth days.

9) The Caloris Basin is about 1,300 km in diameter

One of the largest features on Mercury, it was the result of an asteroid impact on the planet's surface early in the solar system's history. Over the next 1/2-billion years, Mercury actually shrank in radius from 2 to 4 km as the planet cooled from its formation. The outer crust, called the lithosphere, was compressed and grew strong enough to prevent the planet's magma from reaching the surface, effectively ending the planet's period of geologic activity.

10) Mercury is the least explored of our solar system's inner planets.

Only one spacecraft has ever visited Mercury: Mariner 10 in 1974-75. Mariner 10's discovery that Mercury has a very weak magnetic field, similar to but weaker than Earth's, was a major surprise. NASA is planning a new mission to Mercury called Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER), which will orbit Mercury toward the end of this decade. MESSENGER will investigate key science questions using a set of miniaturized instruments.

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