The Planet Mercury, newly revealed

Tuesday 19th July, 2016

Our speaker was Prof David Rothery who is a Professor of Planetary Geosciences at the Open University. He has appeared on numerous occasions on the BBC's "Sky at Night" programme and was a science consultant on the second series of "Stargazing Live". He has authored a number of introductory books such as "Moons" and "Planets" as well as a couple of Teach Yourself books on geology. He is currently the UK Lead Scientist on MIXS (Mercury Imaging X-ray Spectrometer), which is the only UK Principal Investigator instrument on BepiColombo, the European Space Agency mission to Mercury to be launched in January 2017.

He began his talk, "Mercury: new views of the Sun's innermost planet", by saying that Mercury can be a tricky planet to observe and is best placed in the Spring. Close up pictures of its cratered surface resemble those of the Moon but it is actually quite different. One major difference is that the Moon has a small iron core whereas Mercury has a surprisingly large central iron core for its size, approaching half of its volume. The latest research suggests that when the Solar System was still in the process of forming the planets and objects we see today a larger body hit Mercury head on. This would explain how most of the material surrounding the core was removed but the small planet was still able to hold on to certain volatile elements such as potassium, sodium, chlorine and sulphur.

Another strange characteristic of Mercury is its day length compared to its year. It rotates on its axis exactly three times for every two orbits around the Sun with a day lasting 59 Earth days and its year 88 Earth days. So this means that an observer on Mercury would see the Sun come to a halt in the sky and then appear to move backwards before resuming its slow sweep across the sky.

The BepiColombo mission is a collaboration between the European Space Agency and the Japanese Space Agency (JAXA). Two orbiting spacecraft (one Japanese and one European) are to be ferried to the planet on a carrier spacecraft known as the Mercury Transfer Module or MTM. This carrier spacecraft will be powered by an ion engine which is a highly efficient low thrust propulsion system that creates thrust by ejecting ions out backwards, effectively pushing the craft forwards.

Assuming a successful launch, the two piggybacked spacecraft will begin orbiting Mercury in December 2024 after one flyby of Earth, two flybys of Venus and then five flybys of Mercury. This rather circuitous route is necessary as it is surprisingly tricky to reach a stable orbit around Mercury where the Sun's strong gravitational pull actually makes it difficult to slow down any spacecraft.

Prof Rothery said that he is looking forward to the returned data solving some of the unanswered questions surrounding the smallest planet in the Solar System. The last spacecraft to visit Mercury from 2011 to 2015 was NASA's Messenger orbiter which showed that Mercury's magnetic field was lop-sided being three times stronger in the northern hemisphere.