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Mercury's geological history


Very little was known about Mercury's geological history prior to the three flybys of Mariner 10 in 1974 and 1975. Earth-based telescopes could only resolve 300km at best so most of Mercury's smaller surface features could not be discerned. What could be distinguished mainly were faint albedo markings. Polarimetric and photometric observations showed Mercury's texture to be similar to Earth's moon, scarred with impact cratering. The images returned by Mariner 10 gave scientists their first truly useful look at Mercury's surface, though only a fraction of the images were obtained during the closest approaches. Also, the three passes Mariner 10 made were all on just one side of the planet. The entire opposite side remained unimaged until MESSENGER completed the mapping portion of its mission in 2011.

The first photogeological map of Mercury's surface was prepared in 1975 following Mariner 10's third and final pass. From that map, the theoretical model of Mercury's geological history was refined and like the other terrestrial planets, broken down into distinguishable periods based on relative events and their relation to observable Mercurian stratigraphy.

Pre-Tolstojan Period
After the formation of Mercury along with the rest of the solar system 4.6 billion years ago, heavy bombardment by asteroids and comets ensued and Mercury became pock-marked with craters. The last intense bombardment phase, the Late Heavy Bombardment came to an end about 3.8 billion years ago. It was during that phase that most of multi-ring basins were formed and the intercrater plains materials were deposited and redeposited through lava flooding, covering over many of the early craters.

Kuiperian Period
Some regions or massifs, a prominent one being the one that formed the Caloris Basin, were filled by magma eruptions from within the planet. These created smooth intercrater plains similar to the maria found on the Moon. Later, as the planet cooled and contracted, its surface began to crack and form ridges; these surface cracks and ridges can be seen on top of other features, such as the craters and smoother plains – a clear indication that they are more recent.