Mercury

Statistics:

• Planet Diameter: 4878 km, 0.382 Earth diameters
• Planetary Mass: 3.3x1023 kg, 0.055 Earth masses
• Average Distance from the Sun: 58 million km, 0.387 AU
• Length of Day: 58.6 Earth days
• Length of Year: 88 Earth days
• Atmosphere: minimal, composed mostly of sodium
• Day/Night Surface Temperature: 500/150 degrees Celsius

Mercury is the closest planet to the sun. Though smaller in size than Jupiter's moons Ganymede and Titan, it has a greater mass. Mercury is very hard for us to look at due to its proximity to the Sun.

At times when Mercury is visible to us, the light reflected from it must pass through a great deal of our atmosphere; this is because Mercury appears low on our horizon.  The light from objects that are directly overhead (at the Zenith) passes through less atmosphere than the light from objects that are near the horizon.  Therefore, the consequence of this is that any light received from Mercury must pass through a great deal of turbulent and filtering atmosphere.  Images taken of Mercury from terrestrial observations have been fuzzy at best, yielding no real details.

So how is it possible to learn any more information about this mysterious planet?  The answer was to send a planetary space probe to the planet; most of the information that we've obtained from the planets within our solar system has been gathered by an armada of robotic probes.

Mariner 10

In 1974, a probe by the name of Mariner 10 approached the planet Mercury after a brief trip from the Earth.  During the flybys of Mercury, Mariner 10 transmitted back more information to the Earth than humanity had been able to collect on Mercury for centuries!  Among other things, Mariner 10 beamed back roughly 1800 pictures of Mercury's surface, while approaching to within 750 km of the surface.

One of the greatest surprises yielded by the Mariner 10 data was that the surface of Mercury was very much like that of our own Moon!  That is, it is a relatively airless (well, there does exist a barely detectable atmosphere on Mercury) and heavily cratered world.  Other interesting surface features include cliff-lines hundreds of kilometers long called scarps.

Mariner 10 discovered that Mercury has an atmosphere.  It is very thin, so much so that it is almost undetectable, and it is composed mainly of sodium, hydrogen, helium, oxygen, and argon.  Lastly, it was also found that Mercury has a slight magnetic field which is about 0.01 times as strong as Earth's magnetic field.  Planetary scientists believes that this evidence points to Mercury having a molten sulfurous-iron shell surrounding a frozen inner planetary core.

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