Mercury

 Mercury is the planet that is nearest to the sun. As a result, it orbits the sun quicker than any other planet, prompting the Romans to name it after their swift-footed messenger god. Mercury is the second densest planet after Earth, with a massive metallic core of around 2,200 to 2,400 miles in diameter, or about 75% of the planet's diameter. Mercury's outer shell is just 300 to 400 miles (500 to 600 kilometers) thick in contrast. For years, scientists have been perplexed by the combination of its huge core and composition, which contains an abundance of volatile elements.

Mercury is the smallest planet in the solar system, being just slightly bigger than Earth's moon. The planet is riddled with craters because it lacks a strong atmosphere to deflect impacts. An asteroid 60 miles diameter collided with Mercury around 4 billion years ago, leaving a massive impact crater 960 miles wide. This crater, known as the Caloris Basin, could house the whole state of Texas. According to studies published in 2011, another massive impact may have contributed to the planet's unusual rotation.

Mercury, as if it wasn't tiny enough already, has shrunk in the past and is still shrinking now, according to a 2016 analysis. A single continental plate sits above a cooling iron core on the small planet. The planet's volume decreases when the core cools and hardens, causing it to shrink. The process crumpled the surface, resulting in lobe-shaped scarps or cliffs hundreds of miles long and soaring up to a mile high, as well as Mercury's "Great Valley," which is larger than Arizona's famous Grand Canyon and deeper than East Africa's Great Rift Valley at about 620 miles long, 250 miles wide, and 2 miles deep.