Mercury

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    Mercury is one of four so-called terrestrial planets in our solar system. Mercury is essentially a dead rock orbiting the Sun. It is heavily cratered and does not have the large, flat maria which are common on the Moon's Nearside. It has nothing to speak of in the way of atmosphere, it appears to be geologically inactive, and because of its proximity to the Sun, its surface temperature is almost high enough to melt some metals. Mercury's iron core seems to be rather large for such a small sized planet. This gives the planet a relatively high overall density. The spin-orbit coupling of this planet is interesting. This causes the same side of Mercury to face the Sun on every other orbit.

    It is difficult to observe surface features on Mercury from Earth, but it is possible to use CCD cameras to create images of Mercury even in the daytime sky. The feature called Caloris faces the Sun on every second perihelion passage. All of the close-up images that we do have of Mercury are from the Mariner 10 spacecraft, which passed by the planet thirty years ago. The Messenger spacecraft is currently on its way to Mercury for an extended mission which begins with a Mercury flyby in January of 2008.

Here are some of the essential facts about Mercury:

• Radius 2439 km which is about one third the size of Earth
  
  
• Rotation 58.6 days sidereal 176 days synodic
  
  
• Revolution 87.7 days to orbit the Sun
  
  
• Semi major axis 0.39 A.U. Mercury's orbit is about one third the size of Earth's
  
  
• Atmosphere virtually none, although some helium and sodium atoms have been found in very low concentrations.
  
  
• Surface Features Caloris, the huge impact basin
  Lava plains show evidence of ancient flows
  Thousands of craters which are a few tens of kilometers in diameter
  
  
• Moons or rings To the best of our knowledge, there are no small bodies orbiting Mercury.
  
  
• Space Probes Mercury was visited once before, about thirty years ago, by the Mariner 10 spacecraft. A new spacecraft named Messenger is currently in the midst of its exploration of Mercury.

    Here is a nice site that compares Mercury and Earth.

Try this site for more information on Mercury!

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Updated October 16, 2010