ESA Science & Technology - Background Science

Mercury, the closest planet to the Sun

Mercury is also difficult to visit. Only one spacecraft has made the journey so far, making it one of the least explored planets in the Solar System. NASA's Mariner 10 flew by three times in 1974-5 and sent back close-up images, but of one side of the planet only. The other side has still never been seen.

Mercury's surface, heavily scarred with craters and plains of frozen lava, looks similar to the Moon. The most striking feature is the Caloris Basin, a huge impact crater that is matched on the opposite side of the planet by hilly, fractured terrain. Scientists suspect that the shock wave from the impact may have travelled through the planet to cause fracturing of the crust into giant blocks on the other side. On Earth, this would be similar to an impact occurring in Europe reaching through to the South Pacific and leaving its mark there.

Mariner 10 also revealed some other surprises, in particular that Mercury has a magnetic field. Although it is one hundred times weaker than Earth's magnetic field, the field's very existence is a surprise and could challenge ideas about planetary evolution. If it is generated by an internal dynamo, as is the case with Earth, at least part of the planet's iron core must still be molten. According to accepted theory, however, the core of a planet the size of Mercury should have cooled down and solidified long ago.

Mercury is surprisingly heavy for its size, which has been known since well before Mariner 10. As a general rule, the larger a rocky terrestrial planet, the higher its density. This applies to Mars, Earth, Venus and the Moon, but not to Mercury. The smallest of the terrestrial planets, it has the highest density of all. Scientists think this is because it has a surprisingly large iron core.

Planet formation

The Solar System is thought to have condensed from a nebula of gas and dust. The Sun formed at the swirling centre with planets composed of dense, high-melting point material forming near the Sun, and planets made of lower melting point materials (gases) forming further away.

According to this theory, the terrestrial planets have undergone a similar evolution. However, Mercury shows evidence of lying outside of this evolutionary trend. Finding out why will be one of BepiColombo's chief tasks. The answer will lead to a better understanding not only of the evolution of the four terrestrial planets, but also of the conditions that gave rise to life on at least one of them.

The instruments on the BepiColombo Mercury Planetary Orbiter (MPO) will consist of two cameras for high-resolution imaging and a suite of spectrometers for observing the radiation emitted by the surface of the planet in the infrared, ultraviolet, X-rays, gamma-rays and neutrons. The spectra recorded will reveal the minerals and elements in the surface, the presence of water ice beneath the surface, and the constituents of Mercury's thin atmosphere. The MPO will also carry a laser altimeter for measuring the morphology of Mercury's surface, and two radio science experiments for measuring the distribution of mass within the planet. The Mercury Magnetospheric Orbiter (MMO) will carry detectors to observe Mercury's magnetic field and its interactions with the solar wind.

Mercury's orbit

Mercury travels quickly through the sky, which is probably why the Romans named it after their swift-footed messenger of the gods. It orbits the Sun in a highly elliptical orbit at about one third of the Sun-Earth distance. Only the orbit of Pluto, the outermost planet, is more elliptical. At perihelion (closest approach to the Sun), it is only 46 million kilometres from our star, but at aphelion (furthest approach) it is 70 million kilometres away.

In 1845, Urbain-Jean-Joseph Le Verrier, a French astronomer, noticed that the point of perihelion was moving around the Sun faster than predicted by Newton's theory of gravity. The motion of Mercury remained a mystery until Albert Einstein overhauled the theory of gravity in 1915. BepiColombo will measure Mercury's motion more accurately than ever before and thus provide one of the most rigorous tests ever of Einstein's theory.

Mercury takes only 88 Earth days to complete one orbit, but it rotates slowly about its own axis, only once every 59 Earth days. This means that it rotates precisely three times for every two orbits, known as a 3:2 orbital resonance. Once every two Hermean years therefore, the same side of the planet faces the Sun. Mercury is the only body in the Solar System known to have such a resonance with its parent body.

The axis of rotation is almost perpendicular to the orbital plane, so Mercury does not have opposite seasons in each hemisphere, as Earth has. Instead, the temperature at the equator varies as the planet's distance from the Sun changes during its orbital travels. The maximum temperature is 700K (427^oC), but the minimum only 90K (-183^oC). Mercury has very little atmosphere so the surface cools down rapidly on the night side. By contrast, Venus, with its thick atmosphere, maintains a fairly stable temperature which is slightly higher than the maximum on Mercury.

Mercury and Earth - comparing the two planets