Mercury

Introduction

A mosaic of images of Mercury taken by the Mariner 10 mission. Image courtesy NASA/JPL-Caltech Mercury is the closest planet to the Sun and travels around it at a distance of just 58 million kilometres or 0.39 astronomical units.

It is only 4900 km in diameter so has a weak gravitational field and does not retain an appreciable atmosphere. Small amounts of helium, sodium and hydrogen outgas from rocks under the fierce heat of the Sun.

Mercury completes each orbit in 88 Earth days and has a sidereal day lasting 58 Earth days. As a result it rotates three times with respect to the stars in every two revolutions. However, the slow rotation and relatively fast movement around the Sun combine to make each solar day last 176 Earth days.

The proximity to our nearest star, lack of atmosphere and slow rotation period mean the surface experiences huge changes in temperature between day and night from a torrid +450°C down to a bitter -180°C.

Observations of Mercury

A Mariner 10 image of part of the floor of the Caloris Basin. Image courtesy NASA/JPL-Caltech Mercury is difficult to observe from Earth as it is always close to the Sun in the sky. Along with Venus it is either a morning or evening object but is nowhere near as bright. Together with the Moon and Venus it shows a set of phases as it moves relative to the Earth and Sun but even through a large telescope it is hard to see much else.

Almost all of our knowledge about Mercury dates from the only mission to the planet to date.

Mariner 10 flew by in 1973 and sent back pictures of a world resembling our Moon, with an ancient and heavily cratered surface. Flooded lava plains like the lunar maria are also present, along with the huge Caloris Basin, a crater around 1300 km across.

The spacecraft also found a weak planet-wide magnetic field indicating that Mercury has a large iron core.

More recently Mercury has been mapped using Earth-based radar. The reflected pulses of radio waves suggested the tentative presence of water ice at the poles. Craters in this region are in permanent shadow and hence extremely cold so any ice could survive for a long time.

Future missions

Artist's impression of the Messenger spacecraft In the coming decade two new missions will reach Mercury. Firstly, the NASA Messenger spacecraft will arrive in late 2009 and map the whole planet for a year including the 55% of the surface that Mariner missed. It will also seek to confirm the detection of polar ice.

Artist's impression of the ESA Bepi Colombo probe Secondly ESA's Bepi Colombo mission will reach Mercury in 2010. On arrival the spacecraft will separate into three components consisting of two orbiters and the first-ever landing craft. The lander should operate for about a week.

Questions to think about

• What is so special about the Mercurian day?