Mercury is a planet of extremes. It's the fastest and smallest planet in the Solar System - and it's also the closest planet to our Sun.
On average, Mercury is just one-third of the distance from the Sun than the Earth is. It's only slightly larger than Earth's moon, but it's incredibly dense - something that has baffled scientists until a recent discovery was made.
To shoehorn in a quick Lego scale example, if Ironman is the same size as the Earth, his helmet would be Mercury:
What's the surface of Mercury like?
Mercury has a rocky surface and, much like our Moon's surface, it has a lot of impact craters because there's no atmosphere to burn up any space debris that gets in its way.
This lack of atmosphere also means Mercury sees extremes of temperature.
On the side facing the Sun, temperatures reach a scorching 800 degrees Fahrenheit and, on the other side, temperatures drop to -300 degrees Fahrenheit.
Have we ever visited Mercury?
Because Mercury is so close to the Sun, it's difficult to study from the Earth. Although no one has ever stepped on the surface of Mercury, it has had quite a few visits from various spacecraft.
The most recent is NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) probe, which orbited Mercury for four years. Some significant findings include evidence of water ice at the planet's poles and that the planet's core makes up 85% of its radius. To compare, Earth's core only makes up 50% of its radius. This massive core explains why Mercury is so dense compared to other planets.
MESSENGER concluded its mission in April 2015 when the probe dramatically slammed into the planet's surface, gouging a new crater in the planet's heavily dimpled surface.
Extra reading and watching
NASA gives a great overview of Mercury here, including information on how the planet got its name, and you can find out more about the MESSENGER mission here.
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