Mercury 101 | National Geographic

• [Narrator] The planet Mercury is named after the messenger of the Roman gods, because even the ancients could see how swift and fleeting it is in the sky.

But it wasn't until recently that scientists began unraveling Mercury's many mysteries. Mercury is the smallest planet in our solar system. It's diameter currently measures just over 3,000 miles, about the size of the continental United States.

Like Earth, Mercury is a terrestrial planet with three main layers: a core, a mantle, and a crust. Only Mercury's crust has no tectonic plates. Also, its iron core is enormous by comparison, making up 85% of its radius, while Earth's inner and outer core account for just 55%.

Because of the core's exceptional size, it's had a surprising influence on Mercury's overall size by causing it to shrink. The hot iron core has slowly cooled and contracted over the planet's 4.5 billion years. In doing so, it pulled Mercury's surface inward and has caused the planet to shrink radially by more than four miles.

This shrinking planet is also the planet closest to the sun, orbiting our solar system's star at an average distance of roughly 36 million miles. Such proximity affects Mercury's atmosphere, or rather, the lack of one. It only has a very thin exosphere, which is traditionally the outermost layer of a planet's atmosphere.

This exosphere is made of oxygen, sodium, hydrogen, helium, and potassium, all whipped up from the planet's surface by solar winds. The lack of atmosphere and close proximity to the sun also makes Mercury a planet of extremes. The surface temperature can climb to 800 degrees Fahrenheit during the daytime, and fall to 290 degrees below zero at night.

Mercury's proximity to the sun is also the reason behind its ageless reputation of being swift and fleeting. The sun's gravity pulls harder on Mercury than any other planet, and like all planets, Mercury travels in an elliptical orbit, slowing down when it's farther away from the sun, and accelerating as it draws closer.

Clocking in at an average speed of over 100,000 miles per hour, Mercury slings around the sun in just 88 days. From Earth, Mercury is difficult to observe because it's fleeting and so close to the sun. And so far, it's only been visited by two spacecraft, NASA's Mariner 10 and Messenger.

Those missions gave us much of what we know today, but future ventures are in the works with high hopes of revealing more of Mercury's secrets.