General Astronomy/The Terrestrial Planets/Mercury

The Geology of Mercury
Mercury exhibits some of the most unusual, and unique, geology in the solar system. One of the most prominent features on Mercury is that of the enormous amounts of impact craters. There are a plethora of these smaller craters within larger craters. These features from the Highlands of Mercury. There are smaller areas of Lowlands, which are areas that appear to have been flooded by lava at some point in Mercury's history - however, these are few and far between, and much smaller than that of the Lunar Maria. The prominent examples of Lowlands on Mercury are that of the Borealis Planitia, or "Northern Plain", and the Caloris Planitia, or "Hot Plain". The Caloris Basin is the most prominent impact zone, covering approximately 800 miles (1,300 KM) of Mercury's surface. This collision was likely from some sort of solar debris that was approximately 90 miles across. The collision was so massive that it sent shockwaves across the entirety of Mercury's surface, resulting in "weird terrain" at the antipodal to the impact. This "weird terrain" is defined by depressions, hills and valleys that result from the wrinkling of the crust. The Caloris Impact likely occurred 3.85 billion years ago. Some of the strangest terrain to be found on Mercury is that of the Rupes (or Scarps). These surface features are unique to Mercury, and can cut across entire craters. Where this occurs, the craters typically have shorter diameters perpendicular to the rupes rather than parallel to them. The favored theory for how these formed is that as Mercury's mantle cooled, the planet shrank, resulting in thrust faults to jut parts of the planet up (some up to 2.5 miles). Some rupes extend for hundreds of miles. The fact that the rupes cut through craters and have not been flooded by molten material suggest that they are likely some of the youngest features on Mercury. 70% of Mercury's mass is attributed to its enormous iron core, 25% can be attributed to its mantle, and the other 5% can be attributed to the crust. The core itself is as large as the moon, resulting in an unusually heavy planet. This was likely caused by an impact with another protoplanet, resulting in the ejection of much of the planet's initial mantle. Only half of Mercury has been mapped to date.

The Rarefied Atmosphere
Mercury has a tenuous shell of gases that it either extracts from the Sun or receives from micrometeorite impacts. Other elements may leak from the surface from rocks or ice locked deep in craters. Gases found in the atmosphere include: Oxygen (42%), Sodium (29%), Hydrogen (22%), Helium (6%), and Potassium & other gases (1%). The atmosphere is often blasted apart by solar wind and solar storms.