Mercury

The planet Mercury

The Winged Messenger

astrological symbol

Mercury is the closest planet to the Sun and the eighth largest. Mercury is slightly smaller in diameter than the moons Ganymede and Titan but more than twice as massive.

        orbit:    57,910,000 km (0.38 AU) from Sun
        diameter: 4,880 km
        mass:     3.30e23 kg

In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.

Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was sometimes given separate names for its apparitions as a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbit the Sun, not the Earth.

Since it is closer to the Sun than the Earth, the illumination of Mercury's disk varies when viewed with a telescope from our perspective. Galileo's telescope was too small to see Mercury's phases but he did see the phases of Venus.

Mercury has been now been visited by two spacecraft, Mariner 10 and MESSENGER. Marriner 10 flew by three times in 1974 and 1975. Only 45% of the surface was mapped (and, unfortunately, it is too close to the Sun to be safely imaged by HST). MESSENGER was launched by NASA in 2004 and has been in orbit Mercury since 2011. Its first flyby in Jan 2008 provided new high quality images of some of the terrain not seen by Mariner 10. Since then Messenger has taken over 250,000 photographs coving the entire planet. Global Mosaics.

The mission has provided support for the hypothesis that water ice and other volatiles do exist in the polar regions in permanent shadow.

The hypothesis is supported by three independent lines of evidence:

1. MESSENGER’s Neutron Spectrometer made the first measurements of excess hydrogen at the planet’s north pole.
2. Messenger’s Mercury Laser Altimeter (MLA) measured the reflectance of Mercury’s polar deposits, and
3. The MLA has measured the topography of the polar regions enabling the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions utilizing the actual topography

Mercury's orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at aphelion it is 70 million. The position of the perihelion precesses around the Sun at a very slow rate. 19th century astronomers made very careful observations of Mercury's orbital parameters but could not adequately explain them using Newtonian mechanics. The tiny differences between the observed and predicted values were a minor but nagging problem for many decades. It was thought that another planet (sometimes called Vulcan) slightly closer to the Sun than Mercury might account for the discrepancy. But despite much effort, no such planet was found. The real answer turned out to be much more dramatic: Einstein's General Theory of Relativity! Its correct prediction of the motions of Mercury was an important factor in the early acceptance of the theory.

Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by doppler radar observations. It is now known that Mercury rotates three times in two of its years. Mercury is the only body in the solar system known to have an orbital/rotational resonance with a ratio other than 1:1 (though many have no resonances at all).

This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. All the while the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K. The temperature on Venus is slightly hotter but very stable.

Mercury craters Mercury craters
Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no plate tectonics. On the other hand, Mercury is much denser than the Moon (5.43 gm/cm3 vs 3.34). Mercury is the second densest major body in the solar system, after Earth. Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's, probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate mantle and crust.

Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the core is probably molten. Measurements from the Messenger spacecraft show Mercury’s magnetic field is approximately three times stronger in the northern hemisphere than the southern hemisphere and has led to breakthrough research. Modeling by Hao Cao, a UCLA postdoctoral scholar working in the lab of Christopher Russell after considering many factors, including how fast Mercury rotates and the chemistry and complex motion of fluid inside the planet show the magnetic field of Mercury works differently than it does on Earth.

Inside Earth's core, iron turns from a liquid to a solid at the inner boundary of the planet's liquid outer core and the solid inner core is growing, and this growth provides the energy that generates Earth's magnetic field. Inside Mercury’s core, iron turns from a liquid to a solid at the outer boundary and lacks a solid central core. Christopher Russell describes the mechanism: "It's like a snow storm in which the snow formed at the top of the cloud and middle of the cloud and the bottom of the cloud too". "Our study of Mercury's magnetic field indicates iron is snowing throughout this fluid that is powering Mercury's magnetic field."

According to Hao the cores of both Mercury and Earth contain light elements such as sulfur, in addition to iron; the presence of these light elements keeps the cores from being completely solid and "powers the active magnetic field-generation processes. The research currently appears online in the journal Geophysical Research Letters and will be published in an upcoming print edition.

Mercury actually has a very thin atmosphere consisting of atoms blasted off its surface by the solar wind. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly being replenished.

wide angle view Southwest Mercury
The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length and as much as three kilometers high. Some cut thru the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's radius).

Caloris Basin on Mercury Caloris Basin
One of the largest features on Mercury's surface is the Caloris Basin (right); it is about 1300 km in diameter. It is thought to be similar to the large basins (maria) on the Moon. Like the lunar basins, it was probably caused by a very large impact early in the history of the solar system.
wierd Mercury terrain Weird terrain opposite Caloris Basin
That impact was probably also responsible for the odd terrain on the exact opposite side of the planet (left).

In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains. Some may be the result of ancient volcanic activity but some may be the result of the deposition of ejecta from cratering impacts.

A reanalysis of the Mariner data provides some preliminary evidence of recent volcanism on Mercury. But more data will be needed for confirmation.

Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show evidence of water ice in the protected shadows of some craters.

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

Mercury has no known satellites.

Mercury is often visible with binoculars or even the unaided eye, but it is always very near the Sun and difficult to see in the twilight sky. There are several Web sites that show the current position of Mercury (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program.

More about Mercury

Open Issues

  • Mercury's density (5.43 gm/cm3) is nearly as high as Earth's. Yet in most other respects it more closely resembles the Moon. Did it lose its light rocks in some early catastrophic impact?
  • No trace of iron has been seen in spectroscopic studies of Mercury's surface. Given its presumably large iron core this is very odd. Is Mercury much more completely differentiated than the other terrestrial planets?
  • What processes produced Mercury's smooth plains?
  • Are there any surprises on the other half of the surface we've not seen? Low resolution radar images obtained from Earth show no surprises, but you never know.
  • ESA may also build a Mercury orbiter called BepiColombo but it will launch no sooner than 2012.

Page last updated August 5th 2014


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