Introduction

Mercury is the closest planet to the sun.  It is the smallest of the true planets.  It has a mean radius of 2,440 kilometres which is two fifths of the radius of the Earth.  

The proximity to the sun makes it difficult to observe Mercury using a terrestrial telescope.

Our knowledge of the planet has been augmented by two space missions.

Before the missions set out scientists knew that Mercury was heavily cratered like the moon. They knew that the planet generated a magnetic field and inferred, from its density, that it must contain a large iron core. The planetary atmosphere is very thin.  Surface temperatures range from about 427 °C (801 °F) at points directly facing the sun to temperatures of −183 °C ( −297 °F) in the shade at the bottom of craters close to the poles. On the dark side of the planet, temperatures average 110 K

Inter Planetary Expeditions

Mariner 10 mapped 45% of the planetary surface between 1974 and 1975.  The MESSANGER mission was launched on 3 August 2004.  This spacecraft mapped a further 30% of the planet during a flyby on 14 January 2008.  Shortly, in September 2009 Messenger will another flypast.  During 2011 the spacecraft will enter orbit around Mercury and complete the mapping of the planet.

Source Wikipedia Image of Mercury produced by the Mariner 10 mission

The Impact Craters

 The heavily cratered surface suggests that Mercury has been geologically inactive for billions of years. The craters suggest that the planet was bombarded with comets and asteroids shortly after its formation 4.6 billion years ago.  At the time the planet was volcanically active forming large smooth basins similar to the “Mares” on the moon.

The Metal Core

The internal structure of the planet is far more interesting. Scientists believe that because Mercury is a very dense planet it mush have a substantial iron core. Their theory is reinforced by the fact that the planet has a magnetic field.

The Objectives of the Current Mission

The current MESSANGER mission has a number of objectives

1. To test a theory that the core is cooling and shrinking.

Numerous narrow ridges, some extending to several hundred kilometres, were found during the Mariner 10 mission.  These were interpreted as signs of surface buckling caused by the contracting core.  Messenger is gathering information from the side of the planet which is not easily observable from Earth.

MESSENGER’s first image of the side of Mercury which was never seen by Mariner 10, from a distance of about 17,000 miles (27,000 km). Source: Wikipedia 

2. To test rival theories regarding the formation of the iron core.

The fact that Mercury has such a large core in relation to the overal size of the planet is puzzling. There are three rival theories:.

• Sometime during early history Mercury might have been hit by an object with a radius of several hundred kilometres. The impact might have stripped the planet of its crust and mantle.
• Mercury might have forming when the sun was far more energetic than today. The surface rocks might have evaporated away,
• During the formation of the planet the lighter particles could have been swept away by the solar wind

3. To establish whether there is water ice on Mercury.In the early 1990s scientists at an earth bound observatory in the early 1990s recieved radar reflections that implied the presence of ice.  Ice might be present in deep polar craters where temperatures are permanently below freezing.   

Scientists hope to solve some of these puzzles during the flyby in the coming month.