Mercury is the closest planet to the Sun. You could be forgiven for confusing a picture of Mercury with a picture of the Moon. It is a grey world and is heavily marked by craters from meteor impacts.

Mercury as seen by the Messenger spacecraft. Credit:NASA

As Mercury is closer to the Sun than the Earth, it orbits the Sun quicker than the Earth. It takes 88 days to orbit the Sun, so 1 year on Mercury is 88 days long. The name Mercury comes from Roman mythology where Mercury was the messenger of the gods. Mercury spins slower than Earth and only spins once for every 2 orbits of the Sun. This means that on Mercury, a day lasts the same as 176 Earth days.

Mercury is too small to have any significant atmosphere. There is a very thin atmosphere made of atoms of hydrogen, helium, and oxygen. The temperature on the surface of Mercury ranges from −173°C on the night side, to 427°C on the daytime side. Despite the extremely high temperatures on the day side of the planet, there is strong evidence, collected by various Earth based radio telescopes and the Messenger spacecraft, that water ice exists in very deep craters near the poles. This is possible because the water ice would be permanently in shadow, and therefore remain in freezing cold conditions.

Areas in yellow highlight areas where there is evidence of water ice on Mercury. This image was created using a combination of data from the Arecibo radio observatory in Puerto Rico and the Messenger spacecraft. Credit: NASA

Mercury is very heavily cratered from meteor impacts. As there is no weather on Mercury, craters don’t get eroded the same way they would on Earth. One interesting fact about the craters on Mercury is that under the International Astronomical Union (IAU) rules, named craters on Mercury must be named after an artist, composer, or writer who was famous for more than 50 years, and has been dead for more than three years. For example, there is a crater called Disney, named after Walt Disney.

Observing Mercury with a telescope is difficult from Earth as it never gets too far from the Sun in the sky. It is also difficult to get to Mercury with a spacecraft. As Mercury is closer to the Sun than the Earth, Mercury is traveling faster than the Earth around the Sun. This means any spacecraft travelling to Mercury needs to slow down drastically to be able to enter into an orbit around the planet. So far two spacecraft have successfully made it to Mercury. Mariner 10 was launched by NASA in 1973 and made 3 flybys of Mercury during 1974 and 1975. This was the first time we obtained detailed, up close images of the planet’s surface. One problem was that each time Mariner passed, the same side of the planet was in sunlight, so it was only possible to image part of the surface. Mariner 10 also discovered that Mercury has a magnetic field.

The other spacecraft to reach Mercury is MESSENGER, which is an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It was launched in 2004 by NASA, and entered orbit in 2011. The probe mapped the surface in detail as well as finding evidence of volcanic activity, a liquid iron core, water ice and organic compounds in the dark craters. In October 2018 the European Space Agency (ESA) and the Japanese Space Agency (JAXA) launched the next mission to Mercury, BepiColombo. It will reach Mercury in December 2025.

MESSENGER spacecraft during construction. Credit: NASA

If you fancy a chance to see Mercury for yourself, it is possible but difficult. Mercury can be quite bright, but the problem is that as it is so close to the Sun in the sky, it always appears in twilight when the sky is not fully dark. If you are looking for Mercury, always make sure that the Sun it below the horizon to avoid accidental eye damage. In the month of May 2020 it is visible in the evening sky after sunset. It is close to Venus on the 21st and 22nd of May which makes it easier to find. On the evening of May 24th, the crescent moon, Venus and Mercury are all close, making a lovely sight. Go out and have a look and see if you can join the 1% of the world’s population that can say they have seen Mercury with their own eyes!

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