Mercury

The Winged Messenger

In Roman mythology Mercury was considered the god of commerce, travel and thievery. Mercury was the winged messenger of the Greek gods. The people of antiquity, had already noticed that Mercury moved across the sky with the speed of all the planets. Mercury probably got its name due to that.

Mercury has been known since the time of the Sumerians, as early as 3000 BC. Originally it was the Greeks gave it two name: it was called Apollo in the morning, and Hermes in the evening.

Greek astronomers knew that those two names referred to the same object. The Greek philosopher, Heraclitus (540?- 475? BC), believed that Mercury and Venus orbited the Sun, but not the Earth.

Mercury is the first planet from the sun and the second smallest:

             Diameter : 4878 km (3030 miles)
             Mass, Earth = 1 : 0.055
             Orbit : 57.9 million km (36 million miles) 0.38 AU from Sun

The first and only spacecraft to visit Mercury was Mariner 10, in 1974, from which most of our detailed knowledge has been obtained. It mapped only 45% of the surface. Unfortunately, it is too close to the sun to be photographed by the Hubble Space Telescope.

Mercury´s orbit is very peculiar. At aphelion it is about 70 million km away from the Sun. At perihelion it is only 46 million km from the sun. This peculiar orbit could not be adequately explained by using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades, some astronomers guessed that Mercury was pulled out by another planet called "Vulcan", that might exist in an orbit near Mercury´s account for the discrepancy. The real answer was very dramatic. Einstein´s, General Theory of Relativity. Its correct predictions of motions of Mercury was an important link in the early acceptance of the theory.

Until the year 1962 it was thought that one Mercury day, was the same length as its year. But this was shown to be false in 1965 Doppler radar observations. It is now known that Mercury rotates three times while completing two revolutions around the sun...

Because Mercury is so near the sun, is extremely warm. Temperature variations on Mercury are the most extreme in the solar system ranging from -183°C  to 430°C. The average temperature is about 170°C. Although Venus is warmer it is thermally very stable.

In many ways, Mercury is similar to the Moon. For example, its surface is heavily cratered and very old, it has no plate tectonics. But Mercury is much denser than the Moon (5.43 gm/cm3vs 3.34), actually Mercury is the second densest body in the solar system after mother Earth.

Mercury´s interior is dominated by a large iron core, whose radius is about 1850 km. The silicate outer shell, like the Earth´s mantel and crust, is only 550 km thick. Some of the core is probably molten.

The atmosphere of Mercury is very thin, and consists of atoms blasted off its surface, by the solar winds, making Auroras in the process. These atoms escape quickly into space because of Mercury´s heat. Thus, in contrast to the Earth and Venus, whose atmosphere are stable, Mercury´s atmosphere is constantly being replenished.

One of Mercury´s largest surface feature is the Caloris Basin. It is about 1300 km in diameter, and is similar to the Imbrium Basin on the Moon. It was probably caused by a titanic impact early in the history of the planet. The impact probably also caused the odd terrain on the exact opposite side of the planet.

Mercury has also many regions of smooth planes. It may be the results of ancient volcanic activity.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury, but  more data is needed to support that.

Mercury has a small magnetic field whose strength is about 1% of Earth´s magnetic field.

Mercury, like its neighbour Venus, has no known moons.

Mercury´s mean magnitude at greatest elongation is 0.0.

Mercury is visible with binoculars or even with the naked eye. It is always near the Sun, which means that it´s hard to see in the twilight sky.

Planetary Data - Mercury

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