The planet Mercury is the first in the Solar System line-up, meaning it is closet to the Sun.  It moves very quickly across the sky and so was named Mercury, which in Roman mythology was the god of commerce, travel, and thievery. 

Mercury is small and rocky and makes its trip around the Sun once every 88 days.  Faster than any other planet, mercury travels at almost 50 km per second.  Its highly elliptical orbit places Mercury between 47 million km and 70 million km from the sun at different positions.  This causes temperatures on the surface to reach as high as 467 degrees Celsius, and as low as -183 degrees Celsius, due to the very small atmosphere.  This highly eccentric orbit confused 19th century astronomers until Einstein’s General Theory of Relativity was found to correctly predict the motions of Mercury.  These accurate predictions were important in the early acceptance of Einstein’s theory.

Mercury is always very near the sun and therefore, difficult to see in the twilight sky, but it is often visible with binoculars and sometimes can be seen with the naked eye.  The illumination of Mercury’s disk varies when viewed with a telescope from Earth because it is closer to the Sun than the Earth. 

The Mariner 10 is the only spacecraft that has visited the planet.  In 1974 and 1975 it flew by only three times and mapped 45% of the surface.  Mercury is too close to the Sun to be safely imaged by HST.  NASA launched the MESSENGER, a new discovery-class mission, in 2004, which is expected to orbit Mercury starting in 2011.  This spacecraft will investigate a set of key scientific questions using miniaturized instruments.

The surface of Mercury has many similarities to the Moon.  It is heavily cratered and has no plate tectonics.  Mercury is very old and is the second densest major body in the solar system after Earth.  The majority of the planet is composed of mostly iron, as is Mercury’s core; therefore it has only a thin silicate mantle and crust.  The core’s radius is 1800 to 1900 km and the mantle and crust is only 500 to 600 km thick.  Some of the enormous escarpments on Mercury’s surface are hundreds of kilometers in length and 3 km high.  Some were formed through compression indicated by the way the rings of the craters are cut through.  Mercury’s surface area actually shrank by almost 0.1% due to this compression.

Mercury’s surface has many smooth areas in addition to the craters.  One of the largest features was the result of an asteroid impact early in the solar system’s history and is called the Caloris Basin.  It is 1300 km in diameter and over the next half a billion years the planet shrank from 2-4 km as it cooled from its formation.  The surface cooled and compressed preventing the planet’s magma from reaching the surface.  This ended the period of geologic activity on Mercury. 

Mercury’s atmosphere is constantly being replenished, as opposed to Earth and Venus whose atmospheres are stable.  The atmosphere of Mercury is very thin and consists of atoms that were blasted off the surface by solar winds.  These atoms quickly escape into space.

Mercury is now the smallest planet in the solar system, now that Pluto is no longer a planet.  If Earth were compared to the size of a baseball, Mercury would be the size of a golf ball.  The sun looks almost three times the size it does from Earth, when viewed from the surface of Mercury. 

Mercury has no known satellites and has a magnetic field approximately 1% of that of Earth.  There is still much to be known about the solar system’s least explored planet and scientists will hopefully continue to learn about its history and future.

 

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