Basics: Planet Mercury
Main image: Source: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie, Published: December 21, 2009
Mercury is named after the roman god of commerce, who was also termed, the messenger of the Gods. It is the closest planet to our sun. It is also the smallest true planet in the solar system. It is, however, twice the size of the Dwarf planet Pluto.
Not only does Mercury have no moons at all, it is also comparable in size to many of the moons of the solar system. Two known moons in the solar system are larger than Mercury. These are:
Jupiter’s largest moon: Ganymede
Saturn’s largest moon: Titan
Mercury is just slightly larger than Jupiter’s moon Callisto, but mercury is much denser. It has literally about 3 times more mass. So although they look similar in appearance in photo, they are really quite different. Mercury is pretty dense in comparison to ALL the above mentioned moons.
Rough Mass Comparison (All are “×10^23 kg”):
Mercury = 3.3
Combined: Ganymede + Titan = 2.8
So apparently size is not everything!
Mercury is one of the 4 terrestrial planets in the solar system, along with Venus, Earth and Mars. It could be landed on (unlike the gas/ ice giants). It has actually been landed on (Messenger), but it was more of a crash. No pictures were taken once Messenger had reached the surface (in a similar way that a laptop would probably not function after throwing it from the top of a sky scraper).
Although mercury does not have an atmosphere, it does have an “exosphere”. When put into the context of Earth: the outer edge of Earth’s atmosphere is the exosphere. So, basically, Mercury has this type of outer edge only, while lacking all the rest of what could be called atmosphere. It is made up of mostly of oxygen, sodium, hydrogen, helium, and potassium.
The earth’s moon does not look that much different to Mercury. The moon however could be said to have more interesting shapes and features. This is due to the various moon “seas”, as they are often called (although “mare” is preferable).
The craters of each object look very similar though. Closer inspection may indicate that Mercury is more interesting than it seems. See what you think with the NASA app (make sure you type the name of the object into the search):
Composition and Structure
As already mentioned, Mercury is very dense. The large metallic core takes up about 85% of the planet’s radius. Put another way: 55% of the volume (whichever you find easier to understand). NASA believes it to be partly molten based on evidence.
All following images from: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Image Addition Date: April 2015