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A day on Mercury
Question
While the days are long on Mercury the _________ are relatively short.
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Mercury is the target for two orbital missions. NASA’s MESSENGER , which began orbiting Mercury in March 2011 and the European Space Agency’s BepiColombo , due to arrive in 2022.

BepiColombo has significant investment from the UK Space Agency and will carry an X-ray spectrometer (MIXS) to map the abundances of chemical elements across the planet’s surface. Listen to this recent BBC interview by the Open University’s Dr David Rothery, who is a member of the BepiColombo team.

There is far more to Mercury than just the weird relationship between its day and its year. For one thing, it is the only rocky planet other than the Earth to generate its own magnetic field, which shows that part of its iron core must be molten.

Surprisingly, MESSENGER has revealed that although the planet’s magnetic poles coincide very closely with its geographic poles (defined by the planet’s rotation), the magnetic field is offset northwards relative to Mercury’s equator.

It has virtually no atmosphere, so the craters that were formed when the surface was struck by large meteorites and comets are not eroded or buried by sediment as they would be on Earth. At first sight, Mercury’s surface looks rather like the Moon.

However, casual first impressions can deceive. One of many ways in which Mercury differs from the Moon is the occurrence of clusters of shallow, steep-side, flat-bottomed depressions up to a few km across, and which are younger than almost all the impact craters.

Beyond agreeing that this moth-eaten appearance attests to some mysterious process at work, stripping away Mercury’s outer layer, planetary scientists are mightily puzzled by this. It is one of several lines of evidence suggesting that Mercury is rich in volatile elements – and that’s another puzzle, because a planet so close to the Sun was previously expected to have lost its volatile constituents long ago.
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