Mercury, a planet full of surprises

The Messenger probe marks a long-awaited return to Mercury and the most recent flight over the planet to date has changed our view of it.

 

When the NASA Mariner 10 probe flew over Mercury in March 1974 fascinated scientists discovered the first photos of its surface... and the existence of a magnetic field that was totally unexpected! But it was the photos that caught everyone’s attention. The closest planet to the Sun appeared to look like the Moon and as a result lost a lot of interest on the part of politicians responsible for setting the budgets...

A geology similar to Mars
It will therefore be necessary to wait just over 30 years for a probe to once again fly over this somewhat neglected planet in terms of space exploration. Launched on 3 August 2004, NASA’s Messenger (MErcury Surface Space ENvironment GEochemistry and Ranging) embarked on a long journey which will culminate in it orbiting Mercury on 18 March 2011. However in the meantime, 3 flybys are scheduled including two already completed on 14 January 2008, 6 October 2008 plus 29 September 2009. On 30 April last year, the Messenger mission scientists presented their analysis of data collated during the October 2008 flyby revealing a planet henceforth definitively different from the Moon.

Left: general view of Mercury from the Messenger probe during the October 2008 flyby. Mercury orbits between 46 and 68.9 million km from the Sun. It has a diameter of 4 878 km. Right: Rembrandt impact crater, 715 km wide. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington

In spite of a curious resemblance, the geology of Mercury is actually radically different from that of our natural satellite. While the latter is characterised by a marked difference between the face turned away from the Earth, known as the dark side (and features many more impact craters), the closest planet to the Sun appears to be much more uniform. Approximately 40% of its surface is constituted of plains of volcanic origin (lava covered vast areas and subsequently solidified). In this respect, the verdict of Brett Denevi of the University of Arizona, member of the scientific team of the Messenger mission, is undisputed: «much of Mercury's crust may have formed through repeated volcanic eruptions in a manner more similar to the crust of Mars than to that of the Moon». Mercury’s geological history is therefore much more dynamic and complex than anticipated as confirmed by the analysis of certain craters. During a meteorite impact, the shock creates a crater in the surface and expulses materials buried at depth in the surrounding areas. On Mercury, the Messenger data indicates several different layers which are superimposed (see diagram below).

Top: Crater on Mercury (centre) 68 km in diameter. The picture on the right is in artificial colours and combines data in the visible and infrared spectra in order to highlight surface composition variation. The dark blue indicates a geological layer richer in iron and titanium present in the sub-strata. During a meteorite impact, this normally buried layer is exposed in the centre of the crater and expulsed around the edges (black arrows). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington

Still in terms of geology, in two flybys the NASA probe has enabled a giant leap forward in terms of surface imaging. While after Mariner 10 less than half of Mercury had been photographed, coverage has now reached 90 %! And when Messenger sent back images of previously unknown areas during its October 2008 flyby scientists discovered an enormous impact basin measuring 715 km in diameter formed 3.9 billion years ago, at the end of the intense meteor bombardment period typical of the start of the solar system. Named Rembrandt, this region is a true vestige of this period and its study shows that it has been deformed by tectonic movements, supplementary proof of historic high geological activity.

A magnetic field & an «atmosphere»
Mercury’s magnetic field discovered by Mariner 10 thirty years ago was brilliantly confirmed by Messenger in particular during the October 2008 flyby. The magnetosphere is indeed 100 times weaker than that of the Earth, but it remains astonishingly intense for a small body rotating so slowly - one day on Mercury lasts the equivalent of 58.6 earth days (rotation contributes to a «dynamo» effect which feeds the magnetic field). As a result, this has lent weight to the hypothesis that the planet still has a liquid core (convection movements of this core could result in such a magnetic field).
However, this magnetosphere cannot totally protect the small planet from the imposing solar flux. This is true to an even greater extent when it experiences cyclical weaknesses. And when they hit the ground solar winds send surface elements into space forming an extremely thin «atmosphere» (more precisely an atmosphere which is near negligible) mainly constituted of magnesium, calcium and sodium. Mercury’s gravity is insufficient and therefore the planet cannot retain this exosphere which persists nevertheless since it is permanently renewed by the effect of solar winds or by dust ejected at altitude during meteor impacts.

Renewed exploration of Mercury: Messenger (left) and Bepi-Colombo (right) probes. Credit: NASA / ESA-D. Ducros

Long considered to be a dead planet devoid of interest, Mercury displays historic or current phenomena of the utmost interest to scientists. In the short and medium term its study in terms of space resources will depend on the Messenger probe, set to fly over it once again on 29 September 2009 before positioning itself on orbit in 2011 as indicated above. In the longer term, Europe and Japan will take over in 2013, launching a joint mission named Bepi-Colombo which will reach its destination 6 years later in 2019.

Published on 2 June 2009