Mercury facts and information

Mercury facts and information

Mercury is the smallest planet in the solar system, and as such is often obscured by its much larger neighbor, the Sun.

Photo Credit: NASA
By L.E. Terry

As the smallest planet in the solar system, and the one closest to the Sun, Mercury is often obscured by its massive next-door neighbor. In fact, although the planet can be seen from Earth with the naked eye or telescope, it can be difficult to spot without the aid of detailed star charts to point out its exact location.


Humans knew about Mercury as long ago as the third millennium B.C. The Greeks gave the planet two identities: Apollo, when it appeared in the morning, and Hermes, or the messenger of the gods, when it appeared in the evening. They knew, however, that both bodies were in fact the same planet.

Hermes, Mercury’s original identity, was considered the most clever of all the Greek gods. It was believed he invented the alphabet, astronomy, the musical scale and boxing. He was also known as a messenger god, and Mercury probably earned the name because it orbits so quickly through the sky. Later, the planet was known by the Romans as Mercury, their counterpart to Hermes.

Behavior and Movement

Mercury orbits the Sun once every 87.97 days, making it the fastest planet in the solar system. However, it only rotates once every 58.65 days, or three times during two of its years. Because of this, one side stays exposed to the Sun for an extended period of time, reaching a searing 810 degrees Fahrenheit. The other side, left in the dark, drops as low as -290 degrees Fahrenheit. This makes the planet’s temperature variations the most drastic in the solar system.

One of Mercury’s most noteworthy features is its contribution to physics, specifically how it helped validate one of science’s most breakthrough theories. Mercury follows a seemingly erratic orbit, one that brings it as close as 29 million miles to the Sun, and as far away from it as 43 million miles. Its unusual path frustrated early astronomers, who found that the planet’s behavior did not correspond to what they predicted using the astronomical models of the time. They noticed that Mercury’s perihelion, the point at which it comes closest to the Sun, was slightly different each time, something they could not explain using Newtonian mechanics, which dominated scientific thought in that era. The planet’s behavior was a mystery until Albert Einstein formulated his General Theory of Relativity, which for the first time enabled an accurate prediction of Mercury’s movements. Not only did this help scientists better understand the planet, it also lent support to Einstein’s theory, and was a contributing factor in the theory’s early acceptance.

Composition and Surface

Mercury has no known satellites, and virtually no atmosphere, what little there is consisting primarily of oxygen, sodium and helium. The surface pressure is a trillion times less than Earth’s, and because the planet’s surface temperature reaches such extremes, the atmospheric atoms are quickly swept off Mercury’s surface into space by the solar wind and replaced. Because of this, Mercury has a constantly changing atmosphere, unlike the stable atmosphere characteristic of Earth. The force of gravity on Mercury is about one-third of the force of gravity on Earth’s surface, and about twice the force of gravity on Earth’s moon.

The planet’s surface has both heavily cratered regions and smooth plains. Billions of years of meteor bombardment have left behind a surface, like that of Earth’s moon, marked by craters and basins. One of Mercury’s largest features is the Caloris Basin, measuring about 812 miles in diameter and resulting from an asteroid impact early in the solar system’s history. The asteroid impact was probably the source of the unusual surface on the opposite side of the planet as well. Here, an unusually hilly surface cuts across the pre-existing terrain. It is thought that seismic waves generated by the Caloris impact affected this area as well.

Mercury has a high density, and is the second-densest major body in the solar system after Earth. This high density is probably due to a large proportion of the element iron in the planet’s composition. But because the planet’s surface contains relatively little iron, it would seem that the element is concentrated primarily in the planet’s core. Collisions with other bodies may have destroyed the planet’s original crust, leaving behind this iron-rich core, estimated to have a radius between 1,125 miles and 1,187 miles, which is nearly the same size as Earth’s moon. The planet’s outer shell is only about 312 miles to 375 miles thick.

In 1991, observations using powerful radio telescopes revealed massive ice deposits in the planet’s north and south poles. It is believed the ice could have originated either from comet or asteroid impacts, in which water was released and settled in the planet’s craters. Because these craters stay permanently in the shadows and are never exposed to sunlight, the water froze and has never thawed or evaporated.

Visits to Mercury

There has been one space mission to Mercury: the Mariner 10 spacecraft, which flew by the planet three times from 1974 to 1975. The craft only mapped about 45 percent of the planet, but sent back several pictures that revealed a crater-pocked surface similar to that of Earth’s moon. Mariner 10 also discovered that Mercury had a magnetic field similar to Earth’s, only much weaker.

Viewing Mercury

Though Mercury is relatively close to Earth, several factors prevent it from being easily observable. Because it is so small, and so close to the Sun, which is much larger, its presence is often obscured. In addition, because of its rocky and porous surface, it is a very poor reflector of sunlight. Its albedo, the amount of sunlight it reflects, is only about 12 percent, in contrast to Earth’s 37 percent. Because of these things, Mercury can be nearly impossible to detect in the night sky.

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