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The Planet Mercury

Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury is the closest planet to our Sun, the smallest of the eight planets, and one of the most extreme worlds in our Solar Systems. Named for the Roman messenger of the Gods, the planet is one of a handful that can be viewed without the aid of telescopes. As such, it has played an active role in the mythological and astrological systems of many cultures.

In spite of that, Mercury is one of the least understood planets in our Solar System. Much like Venus, its orbit between Earth and the Sun means that it can be seen at both morning and evening (but never in the middle of the night). And like Venus and the Moon, it also goes through phases, a characteristic which originally confounded astronomers, but eventually helped them to realize the true nature of the Solar System.

Size, Mass and Orbit:
With a mean radius of 3.3022×1023 kg and a mass of 3.3022×1023 kg, Mercury is the smallest planet in our Solar System – equivalent to 0.38 Earths. And while it is smaller than the largest natural satellites in our system – such as Ganymede and Titan – it is more massive. In fact, Mercury’s density (at of 5.427 g/cm3) is the second highest in the Solar System, only slightly less than Earth’s density of 5.515 g/cm3.

Mercury has the most eccentric orbit of any planet in the Solar System. At 0.205, its distance from the Sun ranges from 46,000,000 to 70,000,000 km (29,000,000 – 43,000,000 mi), and it takes 87.969 Earth days to complete an orbit. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees compared to Jupiter’s 3.1 degrees which has the second smallest axial tilt of all the planets.

Mercury and Earth, size comparison. Credit: NASA / APL (from MESSENGER)

Mercury and Earth, size comparison. Credit: NASA / APL (from MESSENGER)

But with an average orbital speed of 47.362 km/s, Mercury takes 115.88 days to complete a single rotation. Between its slow rate of rotation and its orbit, a single day on Mercury lasts 176 Earth days. Another consequence of this slow rotation is Mercury’s spin-orbit resonance, which is 3:2. This means that the planet makes three completes rotations for every two orbits it makes around the Sun.

Composition and Surface Features:
As one of the four terrestrial planets of the Solar System, Mercury is composed of approximately 70% metallic and 30% silicate material. Based on its density and size, a number of inferences can be made about its internal structure. For example, geologists estimate that Mercury’s core occupies about 42% of its volume, compared to Earth’s 17%.

The interior is believed to be composed of a molten iron which is surrounded by a 500 – 700 km mantle of silicate material. At the outermost layer is Mercury’s crust, which is believed to be 100 – 300 km thick. The surface is also marked by numerous narrow ridges that extend up to hundreds of kilometers in length. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

Mercury’s core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury was once a larger planet which was then struck by a planetesimals measuring several thousand km in diameter. This impact could have then stripped away much of the original crust and mantle, leaving behind the core as a major component.

Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL

Internal structure of Mercury, consisting of the crust (100–300 km thick), mantle (600 km thick) and core (1,800 km radius). Credit: MASA/JPL

Another theory is that Mercury may have formed from the solar nebula before the Sun’s energy output had stabilized. It would have originally been twice its present mass, but as the protosun contracted, Mercury would have been subjected to temperatures of 2,500 – 3,500 K, or even as high as 10,000 K. This process would have vaporized much of Mercury’s surface rock, leaving behind on oversized iron core.

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost and not gathered by Mercury. However, further analysis is needed before any of these theories can be confirmed or ruled out.

At a glance, Mercury looks similar to the Earth’s moon. It has a dry landscape pockmarked by asteroid impact craters and ancient lava flows. Combined with extensive plains, these indicate that the planet has been geologically inactive for billions of years. However, unlike the Moon and Mars, which have significant stretches of similar geology, Mercury’s surface appears much more jumbled. Other common features include dorsa (aka. “wrinkle-ridges, Moon-like highlands, montes (mountains), planitiae (plains), rupes (escarpments) and valles (valleys).

Names for these features come from a variety of sources. Craters are named for artists, musicians, painters, and authors; ridges are named for scientists; depressions are named for works of architecture; mountains are named for the word “hot” in different languages; planes are named for Mercury in various languages; escarpments are named for ships of scientific expeditions and valleys are named after radio telescope facilities.

Enhanced-color image of Munch, Sander and Poe craters amid volcanic plains (orange) near Caloris Basin NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Enhanced-color image of Munch, Sander and Poe craters amid volcanic plains (orange) near Caloris Basin. Credit: NASA/Johns Hopkins University/Carnegie Institution

During and following its formation 4.6 billion years ago, Mercury was heavily bombarded by comets and asteroids, and perhaps again during the Late Heavy Bombardment period. During this period of intense crater formation, the planet received impacts over its entire surface, thanks in part to the lack of any atmosphere to slow impactors down. During this time the planet was volcanically active, and release magma would have produced smooth plains.

Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins hundreds of kilometers across. The largest known crater is Caloris Basin, with a diameter of 1,550 km. The impact that created it was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. Overall, about 15 impact basins have been identified on the imaged part of Mercury.

Atmosphere and Temperature:
Mercury is too hot and too small to retain an atmosphere. However, it does a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10?14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.

Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. Because of this, and its high eccentricity, the planet experiences considerable variations in temperature. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C), while the side in shadow dips down to 100 K (-173° C).

Images of Mercury's northern polar region, provided by MESSENGER. Credit: NASA/JPL

Images of Mercury’s north pole, provided by MESSENGER. Red indicates shaded regions while yellow indicates the presence of ice. Credit: NASA/JPL

Despite these highs in temperature, the existence of water ice has been confirmed on Mercury’s surface. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below the average. These icy regions are believed to contain about 1014–1015 kg of frozen water, and may be covered by a layer of regolith that inhibits sublimation. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.

Historical Observations:
Much like the other planets that are visible to the naked eye, Mercury has a long history of being observed by human astronomers. The earliest recorded observations of Mercury are believed to be in the Mul.Apin, a Babylonian compendium of Babylonian astronomy and astrology.

The observations, which were most likely made during the 14th century BCE, refer to the planet as “the jumping planet”. Other Babylonian records, which refer to the planet as “Nabu” (after the messenger to the gods in Babylonian mythology) date back to the first millennium BCE.

To the ancient Greeks, Mercury was known as variously as “Stilbon” (a name which means “the gleaming”), Hermaon, and Hermes. As with the Babylonians, this latter name came from the messenger of the Greek pantheon as well. The Romans continued this tradition, naming the planet Mercurius after the swiftfooted messenger of the gods, which they equated with the Greek Hermes. The reason for this has to do with Mercury being the fastest-moving planet across the sky.

Ibn al-Shatir's model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant. Credit: Wikipedia Commons

Ibn al-Shatir’s model for the appearances of Mercury, showing the multiplication of epicycles using the Tusi couple, thus eliminating the Ptolemaic eccentrics and equant. Credit: Wikipedia Commons

In his book Planetary Hypotheses, Greco-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun. For Mercury, he suggested that no transits had been observed either because the planet was either too small to see or because the transits are too infrequent.

To the ancient Chinese,  Mercury was known as Chen Xing (“the Hour Star), and was associated with the direction of north and the element of water – based on the Five Elements. Similarly, modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the “water star”. In, Hindu mythology, the name Budha was used for Mercury, the god that was thought to preside over Wednesday.

The same is true of the Germanic tribes, who associated the god Odin (or Woden) with the planet Mercury and Wednesday. The Maya may have represented Mercury as an owl – or possibly four owls, two for the morning aspect and two for the evening – that served as a messenger to the underworld.

In medieval Islamic astronomy, the Andalusian astronomer Abu Ishaq Ibrahim al-Zarqali in the 11th century described Mercury’s geocentric orbit as being oval, although this insight did not influence his astronomical theory or his astronomical calculations. In the 12th century, Ibn Bajjah observed “two planets as black spots on the face of the Sun”, which was later suggested as the transit of Mercury and/or Venus.

In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric planetary model in which Mercury orbits the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the 16th century.

The first observations using a telescope took place in the early 17th century by Galileo Galilei. Although he had observed phases when looking at Venus, his telescope was not powerful enough to see Mercury going through similar phases. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler.

In 1639, Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. These observations demonstrated conclusively that Mercury orbited around the Sun, which helped to definitively prove that the Copernican Heliocentric model of the universe was correct.


The first spacecraft to reach Mercury was NASA’s Mariner 10, which flew past the planet in 1974. It was only able to photograph about half of the planet over the course of a few flybys. And then 2004 NASA launched its latest MESSENGER spacecraft mission to Mercury. At the time of writing, the spacecraft had completed several flybys of the planet, is scheduled to go into orbit in 2011, where it will be able to study the planet in great detail.

Even though you can see Mercury without a telescope, it’s hard to find it because it’s lost within the glare of the Sun most of the time. When Mercury is visible, you’ll be able to see it to the west just after sunset, or to the east just before sunrise.

Here are some articles on Mercury that we hope you find interesting, illuminating, and fun to read:

Location and Movement of Mercury

Structure of Mercury

Conditions on Mercury

History of Mercury

Other Mercury Articles


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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