The planet closest to the Sun, Mercury is the smallest and fastest planet in the solar system -- whipping around the Sun every 88 Earth days.

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

Planet Profile

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

History of Mercury

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 processes 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 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.

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 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. 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.

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.

Interest facts about Mercury

  • Mercury is 36 Million miles (58 Kilometres) away from the Sun. This may sound a long way, but it makes it the closest planet to the Sun.
  • The planet is 57 Million Miles (92 Million Kilometres) away from Earth.
  • Mercury was named after the Roman God of messages Mercury who was believed to be the fastest of the Greek Gods. This is due to the planets quick orbit of the Sun, travelling at a very fast speed of 30 miles (48 kilometres) per second.
  • Thanks to its speed and proximity to the Sun a year on Mercury is 88 days. This is the length of time it takes for the planet to orbit the Sun
  • The planet may be orbiting at a fast speed, but it is not so quick to rotate. It takes a total of 59 days to turn on its axis and change from day to night. This means a year on Mercury is only one and a half days in length.
  • For a long time, it was believed that Mercury didn’t rotate at all but always kept one side facing the Sun. In 1965 it was discovered that Mercury did rotate using radar observations.
  • On Mercury, there is a considerable difference in temperature on the side of the planet facing the Sun and the side facing away. The temperature on the side of Mercury facing the Sun reaches a scorching 400 degrees Celcius (750 degrees Fahrenheit) and drops to a rather chilly -200 degrees Celcius (-328 degrees Fahrenheit) on the other side.
  • The atmosphere on Mercury is very thin. This is the reason for this big heat change as there is almost no atmosphere to help protect it from the Sun or retain any of the heat levels reached during the daytime.
  • Mercury is only a little planet in comparison to Earth with a diameter measuring 3100 miles in total (4990 Kilometres)
  • Galileo Galilei was the first person to observe Mercury using a telescope in the 17th century. Although in 1639 it was Giovanni Zupi who observed the planet Mercury orbited the Sun, leading to the discovery that the Universe does not revolve around the Earth.
  • It is the smallest planet in the Solar SystemIt was considered a planet from 1930 to 2006 until it was reclassed as a dwarf planet. It’s only slightly bigger than the Earths moon.
  • The surface of Mercury is mostly deep pits and dry craters. This makes it look a little bit like our moon. Some of the pits are miles deep.
  • Due to Pluto having barely any atmosphere its temperatures vary than any other object in our Solar System. They can range from −173 °C (−280 °F) at night to 427 °C (800 °F) during the day.
  • Mercury does not have any moons or rings because of the lack of atmosphere. Mercury and Venus are the only planets in the Solar System without moons.
  • There are no seasons on Mercury. The axis on Mercury is not tilted and there for the Sun stays the same distance from the planet.
  • Mercury orbits the Sun within the Earth’s orbit. This means we can see it from Earth in the morning and the evening without a telescope – although this is not easy.
  • It was once believed that Mercury was two separate planets. Due to the planet being visible at two separate points during the day, Ancient Greeks believed Mercury was two different planets and named them Mercury and Apollo before realising they were the same one.
  • It’s not known who discovered the planet Mercury. All ancient civilisations observed and recorded Pluto, including; Babylonians, ancient Greeks, ancient China, Rome and ancient Egypt.
  • Two Aircraft have visited Mercury from Earth to learn more about the planet.
  • NASA launched the Mariner 10 in 1973, and the aircraft flew past Mercury three times before the mission came to an end.
  • In 2004 NASA launched MESSENGER which stayed orbiting Mercury for four years making about 4000 rotations of the planet before running out of fuel and crashing into its surface in 2015.
  • Another mission will be launched in 2016 by the European Space Agency and Japan and should reach Mercury in 2024. This will hopefully tell us more about the planet.

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.