Astronomy — Planet Mercury

The Planet Mercury

General Information

The planet Mercury is the member of the Solar System closest to the Sun. It is also the smallest of the planets with the exception of Pluto.

The name Mercury comes from Roman Mythology. It is the God of Commerce (the Winged Messenger). It is likely the planet was named as such due to its swift movement across the night sky.

Exploration

There has been little exploration of Mercury due to it’s proximity to the Sun and extremely slow rotation speed.

Mariner 10 is the only spacecraft to travel to Mercury. In 1974-75 it flew by Mercury three times mapping about 45% of the planet’s surface. Due to the extremely slow rotation speed, the same side was visible for all three approaches.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched August 3, 2004 and will make three flybys of Mercury in 2008 and 2009 before settling into an orbit of the planet in 2011. This spacecraft will study Mercury’s composition, structure and atmosphere.

BepiColumbo is a future mission that will be a collaboration of Japan and the European Space Agency (ESA). It will consist of two probes, one to study Mercury’s surface and the other to study it’s Magnetosphere. The launch is planned for 2011.

Surface, Inner Composition & Atmosphere

The surface of Mercury is heavily cratered, much like our own Moon. The shapes of the craters are somewhat different than those on our Moon due to Mercury’s higher gravity. An unusual feature of the planet’s surface is compression folds that cross the plains. The cooling and contracting of Mercury’s interior likely formed the folds.

The interior core of Mercury is almost half of it’s diameter (42%). The iron core is 70% metallic and 30% silicate. The mantle that surrounds the core is only 600KM. It is thought that a major impact in Mercury’s past may have stripped the planet of much of its original mantle.

Mercury has the greatest temperature variation of any planet in the Solar System. On the light side, the temperature can reach 750 degrees Fahrenheit while the dark side drops to –330 degrees Fahrenheit.

The atmosphere of Mercury is extremely thin. It is so thin scientists call it an exosphere. There are six known elements in the exosphere: hydrogen, helium, oxygen, sodium, potassium and calcium; however, the abundance and origin of these elements are unknown.

Rotation and Orbit

Mercury rotates three times for every two times around the Sun. Until 1965, it was thought that Mercury had no rotation (keeping the same face to the sun).

The orbit of Mercury is extremely elliptical, with its distance from the Sun ranging from 46 million to 60 million kilometers. Due to its rotation and elliptical orbit, the Sun appears to rise, set briefly and then rise again. At the opposition, the Sun appears to set, rise briefly and set again. Pluto is the only other planet with a more eccentric orbit.

Random Facts

Distance from the Sun: 157,910,000 KM / 0.39 AU (AU = distance between Earth and Sun)
Diameter: 4,880 KM
Mass (Earth=1): 0.055
Temperature variation: 750F/-330F
Moons: none
Revolution Period: 88 Days
Rotation Period: 58.7 Days
Orbital/Rotational Resonance: 3:2

Sources

Book: Skywatching by David Levy
Nine Planets — http://www.nineplanets.org
NASA — http://www.nasa.gov, http://messenger.jhuapl.edu
WikiPedia — http://en.wikipedia.org
The Planetary Society — http://www.planetary.org