The planet Mercury is the smallest and nearest planet to the Sun in the Solar System.
The planet Mercury is the closest planet to the Sun and with a diameter of 4,880 km it is also the smallest – its mass is just 6% of Earth.
It orbits the Sun in a markedly elliptical orbit in just 88 days but it rotates once every 59 days. In other words, it rotates on its axis just three times for every two orbits of the Sun making its days and years very different to those on Earth. Mercury has no moon.
Transit of Mercury 2016
In May 2016 we watched the Transit of Mercury through our Great Equatorial Telescope: see the live stream:
Surface features on the planet Mercury
The first spacecraft to visit the planet was Mariner 10 which left Earth in 1973 and made three flybys of the planet in 1974. Taking over 2,800 photos it managed to photograph around 45% of the planet's surface.
NASA's Messenger mission began orbiting the planet in 2011. In 2015 it was deliberately crashed into the planet after years of study. Almost the entire planet has now been imaged, revealing a surface that has been shaped both by extensive volcanism and impacts.
The surface looks very like that of the Moon with many craters of very different sizes and lava plains called maria.
There are also shallow cliff-like structures, which are not seen on the Moon, believed to result from wrinkling of the surface as the planet cooled and shrank. The largest craters on Mercury are well preserved and as they are probably about 3 or 4 billion years old, indicate that there has been no migration of plates (like we see on the Earth) since then.
Temperature and seasons
As Mercury is so close to the Sun it has very high mid-day temperatures, approaching 450°C. However it has almost no atmosphere to retain the heat, so night-time temperatures dip as low as -180°C.
The planet Mercury has a small magnetic field, which is evidence that it probably has a large nickel-iron core.
Mercury has no seasons, as the Earth and Mars have. Instead it has a seasonal variation with longitude on the planet’s surface. The longitudes near 0° and 180° receive two and a half times as much radiation overall as do those near 90° and 270°.
Water on the planet Mercury?
The planet Mercury’s axis is almost perpendicular to the plane of its orbit around the Sun. This means that the Sun’s rays always strike at a very shallow angle to the surface at the poles and so the floors of the deepest craters are never exposed to sunlight. Earth-based radar has detected an extremely reflective material in these regions, which may be sulphur or even water ice.