Mercury-the Weird Planet

Planet Mercury is named after a god from Roman mythology.Mercury was the messenger god who, with his winged helmet and winged feet, used to deliver messages to humans on behalf of all the other gods.

Mercury the messenger god delivers messages to humans

Planet Mercury has the fastest orbit of any planet in the Solar System; it reaches a maximum orbital velocity of 105,947 mph (170,505 kmh).

By comparison Earth’s orbital velocity is a much slower 70,000 mph. (108,000 kmh)

mercury closest planet to the sun

Mercury has a diameter of 3,032 miles (4,880 km); compare this with Earth’s diameter of 7,917 miles (12,742 km)

mercury and earth compared

Ganymede (a moon of Jupiter) and Titan (a moon of Saturn) both have larger diameters than Mercury.

mercury ganymede titan and moon compared in diameter

Mercury was formed 4.6 billion years ago in the hot, inner region of the early Solar System. It has a massive iron core.

Planet Mercury cross section showing core, mantle and crust

Mercury’s iron core occupies 42% of its total volume; contrast this with Earth’s core which occupies only 17% of its total volume.

solid inner cores, liquid outer cores and mantles of Mercury and Earth compared

It would take the mass of 18 Mercurys to make one ‘Earth mass.’

Mercury Size; it would take the mass of 18 mercurys to make one 'earth mass'.

Mercury is one of the four so called ‘terrestrial’ (rocky) planets in the Solar System. Like all other terrestrial planets Mercury has a central metallic core with a surrounding silicate mantle.

four terrestrial planets, mercury, venus, earth and mars compared in size

The formation of Mercury

Two hypotheses exist to explain how Mercury was formed.

The giant impact hypothesis suggests that the early Mercury used to be much larger than it is now.

In the early Solar System a large planetesimal collided with Mercury, striping it of much of outer mass including much of its mantle; this collision reduced Mercury to the size you see today.

Mercury collides with a planetesimal

Another hypothesis suggests that, at the time Solar System was formed, Mercury was twice the size it is now.

The high temperatures emitted by the proto Sun fractured and vaporised a large portion of Mercury’s surface, reducing it in size.

early formation of Mercury

Mercury in orbit

Mercury rotates around its axis in an almost ‘upright’ position. It has an axial tilt (also called axial ‘obliquity’) of only 0.1°. Compare this with Earth’s axial tilt of 23°.

axial tilt of mercury earth and mars

Mercury orbits in an elliptical shape round the Sun. At perihelion Mercury is 28,5 million miles from the Sun; at aphelion 43.3 million miles.

orbit of mercury at aphelion and perihelion
This elliptical orbit means that, for the hypothetical observer standing on the surface of Mercury, the Sun would appear to change size depending on whether the planet is at perihelion or aphelion.

size of sun from mercury at aphelion and perihelion compared to earth

Mercury’s orbital velocity around the Sun increases at perihelion and decreases at aphelion.

orbital velocity of mercury perihelion and aphelion

Mercury orbits the Sun so close and so fast that it only takes 88 earth days for Mercury to orbit the Sun once. So we can say that a mercurian year lasts all of 88 earth days.

Day light on Mercury

If a mercurian year is very short, the length of a mercurian day is incredibly long. A single 360° rotation around its axis, a so called sidereal day, takes the equivalent of 58 earth days.

You can see one ‘sidereal day’ represented in the diagram below.

The sun rises on Mercury on earth day 1. By earth day 58 it will be early afternoon; by the end of one sidereal day the planet will have only completed only 65% of one full orbit around the Sun.

mercury rotates around polar axis

Night time on mercury

The Sun finally sets on earth day 88; one mercurian period of daylight, from sunrise to sunset, takes the equivalent of 88 earth days.

one Mercury year takes 88 days

Apparent motion of the Sun

At perihelion Mercury’s orbital velocity around the Sun is greater than the rotational speed around its axis.

As a result the Sun appears to move across the mercurian sky in an entirely unique way; over the course of eight earth days (fours days before perihelion to four days after perihelion), the Sun appears to stop, reverse its course  and then double back to resume its normal path.

Mercury at perihelion sun appears to move backwards

When the Sun rises at perihelion, it will rise not once but twice!

The Sun will partially rise above the eastern horizon, pause and then drop back below the horizon, before rising again a second time! You can watch what happens in this animation.

Night time on Mercury

The long mercurian night starts on earth day 89 and continues until earth day 176. During the course of one earth year Mercury will have just two sunrises and two sunsets.

Night time on Mercury

Mercury’s magnetic field

Mercury has a magnetic field generated by the planet’s liquid outer iron core. Mercury’s magnetic field is similar in shape to Earth’s but only 1% as strong.

Despite its weakness, the mercurian magnetic field is still strong enough to deflect solar winds away from the planet.

The solar winds travel towards Mercury at incredibly fast speeds of more than 1,000,000 mph. As they approach the magnetic field, the solar winds abruptly slow down at the boundary known as the ‘bow shock’.

solar winds and mercury's magnetosphere

In this image you can see more clearly how the magnetic field deflects the particles in the solar winds away from the planet’s surface.

Mercury magnetosphere repels solar winds

However some particles can still hit the surface of the planet at the polar regions where there are no magnetic field lines to deflect any solar wind.

magnetic field lines north and south poles of Mercury

However when the Sun sends out massive bursts of energy during a coronal mass ejection (CME)….

during coronal mass ejection lethal electro magnetic radiation hits planet mercury

the planet’s magnetic field is pushed right back to the surface of the planet.

In these circumstances the planet’s surface is bombarded with massive amounts of electro magnetic radiation.

magnetosphere pushed back on Mercury during a coronal mass ejection

Temperatures on Mercury

Mercury is completely dwarfed by its stellar neighbor, as you can see from this image showing a transit of Mercury across the face of the Sun.

transit of Mercury across face of sun in 2006

Orbiting so close to the Sun and rotating on its axis so slowly means that Mercury has an extreme range of temperatures.

As soon as the Sun begins to rise the surface of the planet begins to warm up. The temperature continues to rise and often reach a sizzling 426 °C (800 °F) by the early afternoon. Temperatures decrease in the late afternoon and plunge  once the Sun sets.

During the long mercurian night the lack of any significant atmosphere surrounding the planet means that infrared radiation (heat) is very quickly lost to the darkness of space; temperatures often plunge to as low as  -184°C (-300 °F)

On the terminator line, that continuously moving line where night meets day, temperatures are around -100° C, (-148 °F)

terminator line on planet mercury

Mercury’s atmosphere

Mercury has an atmosphere which a trillion times smaller than Earth’s. As a result of having such an insignificant atmosphere the mercurian sky always appears pitch black.

Sunrise On Mercury© Don Dixon at

Mercury has such an insignificant atmosphere because the planet is unable to retain any molecules of gas which build up around the planet.

Any gas molecules present in the atmosphere  become superheated during the day; after becoming superheated they whizz around at a velocity which allows them to escape Mercury’s force of gravitational attraction. They then head off into deep space.

Mercury’s small mass (5.5%of Earth’s) means that the planet has a relatively low surface gravity.

As a consequence the ‘escape velocity’, the speed which molecules must reach in order to escape Mercury’s force of gravitational attraction, is also relatively low.

Mercury showing how superheated gases in atmosphere escape as a result of solar radiation

Another factor is the close proximity of the Sun; powerful solar winds often bombard the planet’s surface and strip away molecules of gas  in the atmosphere.

Features of the surface

The surface of Mercury looks much like the surface of the moon; it is pitted with impact craters.

Craters on Mercury from meteorite impacts

There are small impact craters…

small impact crater on Mercury

….. and larger  impact craters. The circular mountains (so called peak rings ) in the middle of the 257 km wide Raditladi Crater are thought to be made of material forced up from under the planet’s surface by the original meteor impact.

Raditladi impact basin of Mercury showing mountains

Some areas of the planet characterised by a high density of impact craters  have been infilled by volcanic lava flows. These areas, the intercrater plains, are among the oldest regions of the planet.

Mercury inter crater plains

Wide, flat valleys were formed after rivers of lava flowed away from volcanic lava plains.

Vallis of mercury formed by lava flows

Mercury is home to the Solar System’s largest impact crater called  The Caloris Basin. It was formed by the impact of a massive asteroid and is 960 miles (1,550km) in diameter.

caloris impact basin is one of largest in solar system

It is surrounded by mountains up to 1.2 miles (2 km) deep; the floor of the crater has been infilled with volcanic lava plains.

The force of the impact sent shock waves right through the center….

Formation of weird terrain on MercuryImage source:

leading to the formation of weird terrain the far side of the planet.  The Weird Terrain is made of unusual hilly terrain with long shallow trenches or furrows.

Mercury weird terrain is antipodal to Caloris Impact basin on Mercury

This is an artistic impression of the Weird Terrain:

Mercury weird terrain by Ron Guyatt is at antipodal point on planet at far side of Caloris Impact Basin© Ron Guyatt 

Material ejected during the Caloris Basin impact event has infilled many craters outside the basin for a distance of 1,000 km. (620 mi). You can see how the process of infilling takes place in this video:

Mercury has several  long thrust scarps. A thrust scarp ( also called a lobate scarp ) is a continuous cliff, or escarpment many miles long.

Rupes of mercury

Discovery thrust scarp is about 550 kilometers (350 miles) long and  1.5 kilometers (1 mile) high.

Notice how the walls and floors of two impact craters have been deformed by the thrust scarp that runs straight through them.

Discovery rupes lobate scarp or thrust scarp on Mercury

Thrust scarps were formed as a result of the interior of the planet cooling down. A fall  in temperature in Mercury’s interior led to the contraction (shrinking) of its core and mantle.

As a result some sections of crust were thrust up and over other sections of crust to form escarpments.

thrust or lobate scarp on Mercury

Another amazing feature of the planetary surface is the presence of water ice at the polar regions.

As a result of the fact that the axial tilt of Mercury is almost zero, there are areas at the poles which never see sunlight. In these areas of permanent cold temperatures and darkness are deep craters containing water ice; water ice which has existed for billions of years.

Mercury's north pole showing water ice and permanently shaded areas

Planetary change

Mercury is geologically inactive; there has never been any erosion by wind or rain; neither has there been any volcanic activity for billions of years.

The only ways in which the surface is now sculpted is through impact cratering by meteorites and erosion by solar winds.

These hollows in the Raditladi Crater are thought to have been formed by the solar winds vaporising minerals in rocks; the rocks are then weakened and eventually disintegrate, forming these depressions.

Hollows inside Raditladi impact Basin

Mercury Fun Stuff

  • Explore Mercury’s orbit around the Sun  here.
  • Watch an animation of a Mercury day here
  • Take a guided tour of Mercury in Google Earth at

Original Science Projects

  • Investigate the different features of Mercury. A good starting point is  Different types of features  are named after different things; for instance  craters are named after famous writers and artists.
  • Explain the orbit and rotation of Mercury.
Post a Comment

3 Responses to “Mercury-the Weird Planet”

  1. Saahil Sharma says:

    Great order of information! I’m really lucky to find this website and it really helped in my Physics assignment on Mercury!

    Thanks a LOT!

  2. TD says:

    this helped a lot!
    thanks 🙂

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