The cratered surface of Mercury (Image map courtsey of NASA).
Mercury is the closest planet to Sol. This, coupled to the fact that its atmosphere is extremely thin
means that surface temperatures soar massively during the day, but they also plummet massively
during the planet's long night. Mercury is not as hot as Venus, but experiences greater temperature
Planet type: terrestrial, hot cratered rocky desert planet.
Equatorial Diameter: 2440 km (0.3829 Earth radii).
Mass: 0.055 Earth masses.
Surface gravity: 0.38 g.
Orbit: Mercury orbits Sol at 0.307 - 0.467 astronomical units with a year lasting 87.97 Earth days and
a day length of 58 days and 15.5 hours.
Atmosphere: extremely thin (trace) composition by volume - 42% oxygen (O2), 29% Na, 22%
hydrogen, 6% He, 0.5% K and traces of inert gases.
Surface Temperature: highly variable! Night-time temperatures: -173 to -193 degrees C, daytime
temperatures of up to 427 degrees C; mean surface temperature = -73 degrees C near the equator.
Magnetic Field: weak - 1.1% the strength of earth's and also a magnetic dipole, strong enough to
generate a magnetosphere.
Life: none as yet discovered, deemed extremely unlikely: the planet is probably sterile. The surface is
too dry and blasted by ultraviolet rays.
Key Tourist Attractions
Mercury is a good place to go to observe craters. The craters display several types of
morphology. Smaller meteorites (10 000 to 100 000 tonnes) produce simple circular craters.
Complex craters are between 20 and 150 km diameter and are formed by meteorites ranging
between 1 and 100 billion tonnes. Recoil forms a central peak inside the plain of a complex
crater. These craters are surrounded by a ring of radial debris which has rained down around
the impact site. The very largest craters, basins, have one to several central rings instead
of a central peak on the crater plain, e.g. The Strindberg basin (165 km diameter). In the
formation of these impact basins, the debris rained down to produce radiating chains of
secondary impact craters. Radiating rays of ejected material, several hundred kilometres
long, frequently form around these craters.
Long ridges or 'wrinkles' in the planet's crust extend for hundreds of kilometres and possibly
formed when the core and mantle cooled and contracted after the crust had already solidified.
Above: a model of Mercury's internal structure. Beneath the relatively thick crust (100-300 km) is
thought to occur a 500 to 700 km deep mantle of silicates. Mercury has a density similar to that of the
earth but being much smaller its internal pressures are much lower. One way of accounting for this is
to incorporate a relatively large molten and iron-rich core, making up 42% of the planet's volume and
rich in silicates. This suggests that either mercury was at one time a much larger planet, or that it
failed to accrete lighter materials due to the density of the condensing pre-solar nebula and perhaps
high protostellar winds surrounding the forming Sun. The abundance of volatile materials on the
surface, such as potassium and sodium, suggests that it is unlikely that heat vaporised away the outer
5th Sept 2015
6th sept 2015
Click images to enlarge
Above: the Debussy crater (lower right) is a prominent feature of Mercury with radiating ejecta rays.
The solar winds blacken ejecta rays, causing them to fade over about one billion years. Bright rays is
therefore an indication of geological youth. Crater density can also help reveal the age of a surface.
Older surfaces have statistically more craters, whereas renewal of a surface, such as by volcanism,
obliterates craters so that youthful surfaces are smoother and less cratered.
Mercury's Anomalous Orbit
Although Mercury is itself very round and a near-perfect sphere, its orbit is very elongated and
ellipsoidal (eccentric). Its day-length (rotation period) is also precisely two-thirds as long as its year
(revolution period). This is due to tidal forces with the Sun, prevalent as mercury orbits so close,
slowing down the rotation of mercury, resulting in its long day of 58 earth days and 15.5 hours. The
two-thirds ratio is predicted to be stable (another stable configuration is tidal-locking in which a body
keeps the same face towards its central star or planet, in which case its rotation period equals its
As is typical of elliptical orbits in planetary systems, the pull of the other planets perturbs mercury's
orbit and its orbit slowly precesses (the ellipse rotates around the Sun so that the planet does not
come back to exactly where it started through one complete orbit). Newtonian mechanics fails to
accurately predict the rate of precession, instead Einstein's theory of General Relativity has to be used
to give a much more exact prediction. This is because Mercury is so close to the Sun that the
spacetime it occupies is greatly warped by the energy-density of the Sun. This is one piece of evidence
in favour of the validity of General Relativity.