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Photomosaic of Mercury by Mariner 10
Mercury is the innermost planet of the solar system and second smallest after Pluto. It is surpassed in size (but not in mass) by both Ganymede and Titan. In ancient Greece it had two names – Apollo for its appearance as a morning star and Hermes as an evening star – although Greek astronomers knew that a single body was involved. Being named after the fleet-footed god Hermes, or its Roman equivalent Mercury, is especially apt as the planet races around the Sun at an average speed of 48 km/s, completing one circuit in 88 days.

Mercury's day

Until the 1960s it was thought that Mercury's day (the time it takes to spin on its axis) was the same length as its year (the time it takes to complete one orbit) so that it always kept one face to the Sun. However, Doppler radar measurements in 1965 showed that Mercury actually rotates three times in two of its years.

This fact and Mercury's highly elliptical orbit (see below) would lead to strange effects for an astronaut on the surface. At some longitudes the Sun would appear to rise briefly, set, and rise again before traveling westward across the sky. At sunset, the Sun would set, rise again briefly, and then set again. In other places, an observer could watch the Sun come to a standstill and then move backward for a while, before continuing its original motion, having done a complete loop. See Mercury, rotation.


Although Mercury's mean distance from the Sun is 57,910,000km (0.387 AU), its orbit is very elongated. The range from perihelion to aphelion is 46 to 70 million km.

Mercury's perihelion precesses around the Sun at a rate that 19th century astronomers couldn't properly explain. The "advance" of the perihelion was 476 arc-seconds per century – 42.6" more than expected. This discrepancy remained a nagging problem for many decades, and it was even suggested that the answer might lie in the existence of an intra-Mercurian planet, sometimes called Vulcan. In the end, the puzzle was solved in much more dramatic style, by applying a new theory of gravity: Einstein's general theory of relativity.

Interestingly, Mercury is the closest planet to Earth on average. Mercury is, one average, 155.4 million kilometers from Earth, Venus is 170.5 million km away, and Mars is 254.6 million km away.

Mercury, the closest planet to Earth (on average)

Surface features

Mercury Caloris Basin
Ridges and fractures on the floor of Caloris Basin
Despite being fairly close to us, Mercury is one of the least-studied planets in the solar system. Until 2008, it had been visited by only one spacecraft, Mariner 10, which flew by three times in 1974 and 1975, mapping 45% of the surface. Mariner observed a world similar in general respects to the Moon: heavily cratered but with regions of relatively smooth plains, some of which may be the result of ancient volcanic activity, others due to the deposition of ejecta from cratering impacts. Its most distinctive features is the Caloris Basin, a colossal, multiringed basin about about 1,350km (840 miles) in diameter, whose inner floor contains mostly smooth plains, known as Caloris Planitia, but also many ridges and fractures, some of them radial and others arranged in two or three concentric rings. Its name, meaning "basin of heat," stems from the fact that it lies near the subsolar point when Mercury is at perihelion and thus can experience temperatures as high as 700K.

There are also great escarpments, up to 1,500km long and 3km high, some of which slice through the rings of craters and other features in a way that shows they were formed by compression of the crust when Mercury's interior cooled and shrank. Estimates suggest that the planet's surface area decreased by about 0.1% and its radius by about 1km.

Radar images taken by astronomers at the Jet Propulsion Laboratory and the California Institute of Technology in 1991 suggest that the polar regions of Mercury may be covered with patches of water-ice. Although this seems impossible due to the planet's sizzling heat, the polar regions receive very little sunlight and may get as cold as -148°C.


Hypothesized interiors of Earth and Mercury compared
Interiors of Earth and Mercury compared
Mercury is the second densest major body in the solar system, after the Earth, pointing to a relatively enormous iron core with a radius of 1,800 to 1,900km – proportionately much larger than Earth's and making up most of the planet. Above this is a relatively thin silicate mantle and crust, only 500 to 600km in total from top to bottom.

Mercury has a weak magnetic field about 1% as strong as Earth's. Recent research into irregularities in the rate of Mercury's spin suggest that this magnetic field may be due to a molten outer core.


Mercury has an extremely thin atmosphere, about one trillionth the density of Earth's, composed mainly of atoms of argon, neon, and helium that have been blasted off the surface by the solar wind.

Current and future probes

On August 3, 2004, NASA launched a spacecraft called MESSENGER that will go into orbit Mercury in 2011. MESSENGER made its first flyby of the planet on January 14, 2008, sending back images including the first ones of the hemisphere of Mercury that Mariner 10 missed (see below for images of Mercury acquired by MESSENGER during its first flyby). Europe and Japan are also developing a robot mission to Mercury, called BepiColombo, for launch early in the next decade.

Mercury statistics
distance from Sun (mean) 57.9 million km (35.9 million miles, 0.387 AU)
orbital period 87.969 days
orbital eccentricity 0.206
orbital inclination 7.00°
equatorial diameter 4,879 km (3,032 miles), 0.382 × Earth
mass (Earth=1) 0.055
mean density 5.43 g/cm3
axial period 58.65 days
axial inclination ~0.01°
atmospheric composition negligible – traces of sodium, helium, hydrogen, oxygen
max. temperature (day) 427°C (801°F)
min. temperature (night) -183°C (-297°F)
surface gravity (Earth=1) 0.38
escape velocity 4.3 km/s (15,480 km/h, 9,621 mph)
albedo 0.11
number of moons 0

Images of Mercury returned by MESSENGER

This image was taken today by NASA's MESSENGER spacecraft at a distance of approximately 17,000 miles following the spacecraft's closest approach to Mercury. It shows features as small as 6 miles in size. Similar to previously mapped portions of Mercury, this hemisphere appears heavily cratered. It also reveals some unique and distinctive features. On the upper right is the giant Caloris basin, including its western portions never before seen by spacecraft. Formed by the impact of a large asteroid or comet, Caloris is one of the largest, and perhaps one of the youngest basins in the solar system.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury's previously unseen side

MESSENGER acquired this image on Jan. 14, 2008, as part of a mosaic that covers much of the sunlit portion of a hemisphere not view by previous spacecraft. Images such as this can be read in terms of a sequence of geological events and provide insight into the relative timing of processes that have acted on Mercury's surface in the past. The double-ringed crater at upper right of this image appears to be filled with smooth plains material, perhaps volcanic in nature. This crater was subsequently disrupted by the formation of a prominent cliff, the surface expression of a major crustal fault system that runs alongside part of its southern rim. This may have led to the uplift seen across a portion of the crater's floor. A smaller crater in the upper left of the image also has been cut by the cliff, showing that the fault beneath the cliff was active after both of these craters had formed. Credit: NASA/Johns Hopkins University APL/Carnegie Institution double-ringed crater on Mercury

This image obtained by MESSENGER reveals a variety of intriguing surface features, including craters as small as 300 yards across. The image also shows landscapes near Mercury's equator on the side of the planet never before imaged by spacecraft. These highly detailed close-ups enable planetary geologists to study the processes that have shaped Mercury's surface over the past 4 billion years. One of the highest and longest scarps cliffs yet seen on Mercury curves from the top center down across the right side of this image. Great forces in Mercury's crust have thrust the terrain occupying the left two-thirds of the picture up and over the terrain to the right. An impact crater has subsequently destroyed a small part of the scarp near the top of the image. This image was taken from a distance of 3,600 miles from surface and shows a region approximately 100 miles across. Credit: NASA/Johns Hopkins APL/Carnegie Institution closeup of Mercury's surface by MESSENGER

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