Mercury: the planet, the missions and the transit – by Prof Dave Rothery
March 2, 2016

Prof David Rothery

In the run-up to the Transit of Mercury on 9th May, the Europlanet Outreach website is featuring a series of guest articles by European scientists that study the innermost planet or whose research relates to transits.  

Our first post is by David Rothery, Professor of Planetary Geosciences at the Open University. 


Introduction to the Transit of Mercury
Mercury will pass across the face of the Sun on 9 May 2016 and again on 11 November 2019, both at excellent times of day for viewing the phenomenon from Europe. I’m involved in the European Space Agency’s upcoming mission to Mercury, BepiColombo, so I’m keen to see this event for myself, to share it with others, and to use the opportunity explain what a fascinating planet Mercury is.

If you plan to watch the transit, Mercury will appear as a black dot about 1/150th of the Sun’s diameter. This is too small to see without magnification. See this video for advice on how to watch safely, or look for webstreamed live images via this ESA website.

Missions to Mercury

Only two spacecraft have visited Mercury so far: NASA’s Mariner 10 made three flybys in 1974-5, and NASA’s MESSENGER orbited Mercury from March 2011 until April 2015. BepiColombo, a mission led by ESA in collaboration with the Japanese Space Agency (JAXA), is due to launch in in 2018 and to arrive in 2024. During its journey to the inner Solar System, BepiColombo will be propelled by an ion-drive housed in the Mercury Transfer Module (MTM). On arrival, the JAXA-led Mercury Magnetospheric Orbiter (MMO) will be placed in an eccentric orbit, and the sunshield that protected  it during the cruise phase will be jettisoned. The ESA-led Mercury Planetary Orbiter (MPO) will be nudged into a lower, relatively circular orbit.

ESA's BepiColombo mission
Artist’s impression of BepiColombo (shown during its cruise to Mercury). The inset (lower left) shows an exploded view with extendable components stowed.  Credit: ESA

The planet

Mercury is the smallest terrestrial planet, and the closest to the Sun. As Mercury is at one end of the planetary chain, we need to determine its nature and origin if we wish to understand our Solar System. It has a more eccentric (non-circular) orbit than any other planet, being 46 million kilometres from the Sun at the closest point in its orbit (perihelion) but 70 million kilometres away at its furthest point (aphelion). The planet rotates slowly, exactly three time for every two orbits round the Sun. To see how this results in Mercury’s day being twice as long as its year (which lasts 88 Earth-days), see this animation.

This slow rotation results in surface temperatures rising in excess of 400 °C in the daytime and dropping below -170 °C before dawn. However, because Mercury spins vertically on its axis, with no tilt relative to its orbit, the floors of some craters near the poles are in permanent shadow. This means that they are permanently cold and can shelter ice derived from comets impacting the surface.

Mercury is a dense planet, with an iron core that occupies a greater portion of its interior than in the case of the Earth. We can tell that the outer part of this core must be molten like the Earth’s (and unlike the solid cores of Venus, Mars and the Moon), because motion of this electrically-conducting liquid generates a magnetic field. This was discovered by the first space mission to visit Mercury, Mariner 10. More recently, the MESSENGER orbiter found that Mercury’s magnetic poles align with its rotational poles, but that its magnetic equator is mysteriously displaced 480 km north of the geographic equator.


Interior of Mercury
Under its rocky crust and mantle, Mercury has an iron-dominated outer and inner core, and possibly an ‘anticrust’ of iron sulfide. Convection in the liquid outer core generates the magnetic field. (Credit: D. Rothery).

Because its core is so big, the rocky part of Mercury (its mantle and crust) is relatively thin. This may result from a giant collision that stripped off most of the original crust and mantle. However, one of the biggest surprises revealed by the MESSENGER mission is that Mercury’s surface is rich in elements such as sulfur, chlorine, sodium and potassium that we would expect to be easily lost in a hot or violent event. There are also patches of Mercury’s surface (called ‘hollows’) where the top ten metres or more of material has simply vanished, relatively recently. Maybe those patches have evaporated (more properly speaking, ‘sublimed’) away to space. We do not know the composition of this volatile and easily-lost material, and one of BepiColombo’s main goals is to find out.

Hollows on Mercury. Credit NASA-JHUAPL-CIW
18 km detail from a MESSENGER image (exaggerated colour) showing hollows on Mercury. Sunlight is coming from the left. (Image: NASA/JHUAPL/CIW)

Mercury’s diameter appears to have shrunk by about 14 km during the past 3 billion years or so, which is most simply explained by thermal contraction as the planet cooled. We see the evidence of contraction mostly in the form of giant thrust faults (breaks in Mercury’s crust that have been pushed upwards), known as lobate scarps, that cross the surface. Mapping of Mercury’s lobate scarps reveals that most examples in mid- and low-latitudes are orientated north-south, and these overlaps imply that most of the shrinking of the crust has been in the east-west direction around the planet’s equator (i.e. Mercury has got thinner around its middle).  This may be evidence of the slowing down of Mercury’s spin leading to the collapse of a former equatorial bulge that had previously been sustained as a result of more rapid spin.

This image taken by MESSENGER shows Unity Rupes, a lobate scarp (named after the ship that took Edmond Halley to St Helena to observe a transit of Mercury in 1677). The scarp cuts through, and so must be younger than the 30 km impact crater near its southwestern end. (Image: NASA/JHUAPL/CIW)
This image taken by MESSENGER shows a lobate scarp on the surface of Mercury called Unity Rupes (named after the ship that took Edmond Halley to the island of St Helena to observe a transit of Mercury in 1677). The scarp cuts through, and so must be younger than the 30 km impact crater near its southwestern end. (Image: NASA/JHUAPL/CIW)

Mercury’s surface was formed almost entirely through volcanic processes. Most of its crust was formed by lava flows more than 3 billion years ago, and there are many volcanic vents where explosive volcanic eruptions have occurred. At least one of these is probably less than a billion years old.

MESSENGER image of volcanic vent on Mercury. Credit: NASA/JHUAPL/CIW
The irregular-shaped pit near the centre of this MESSENGER image is nearly 2 km deep. It appears to be an ancient volcanic vent, the source of explosive eruptions that discoloured the surrounding surface. Below are two high-resolution views of the volcanic vent seen under different illuminations. (Images: NASA/JHUAPL/CIW)

Prof Rothery is funded by the UK Space Agency and the Science & Technology Facilities Council for research on Mercury and his work on the BepiColombo mission. His book, Planet Mercury: from Pale Pink Dot to Dynamic World, was published by Springer in 2015.