Linear ridge networks are found in various places on Mars in and around craters. These features have also been called "polygonal ridge networks," "boxwork ridges", and "reticulate ridges." Ridges often appear as mostly straight segments that intersect in a lattice-like manner. They are hundreds of meters long, tens of meters high, and several meters wide. It is thought that impacts created fractures in the surface, these fractures later acted as channels for fluids. Fluids cemented the structures. With the passage of time, surrounding material was eroded away, thereby leaving hard ridges behind. It is reasonable to think that on Mars impacts broke the ground with cracks since faults are often formed in impact craters on Earth. One could guess that these ridge networks were dikes, but dikes would go more or less in the same direction, as compared to these ridges that have a large variety of orientations. Since the ridges occur in locations with clay, these formations could serve as a marker for clay which requires water for its formation. Water here could have supported past life in these locations. Clay may also preserve fossils or other traces of past life. These ridges could be formed by large impacts that produced fractures, faults, or dikes made up of melted rock and/or crushed rock. One formation mechanism proposed by Quinn and Ehlmann in 2017 was that sediment was deposited and eventually the sediment underwent diagenesis which caused a loss of volume and fractures. After erosion exposed the fractures, they were filled with minerals possibly by acid-sulfate fluids. More erosion removed softer materials and left the more resistant ridges behind. If the impact-caused dike is made of purely melted rock from the heat of the impact, it is called a pseudotachylite. Also, hydrothermalism may have been involved due to the heat generated during impacts. Strong evidence for hydrothermalism was reported by a team of researchers studyingAuki Crater. This crater contains ridges that may have been produced after fractures formed with an impact. Using instruments on the Mars Reconnaissance Orbiter they found the minerals smectite, silica, zeolite, serpentine, carbonate, and chlorite that are common in impact-induced hydrothermal systems on Earth. Other evidence of post-impact hydrothermal systems on Mars from other scientists who studied other Martian craters. Because ridges seem to be found in older crust only, it is believed that they occurred early in the history of Mars when there were more and larger asteroids striking the planet. These early impacts may have caused the early crust to be full of interconnected channels. These networks have been found many regions of Mars including in Arabia Terra, northern Meridiani Planum, Solis Planum, Noachis Terra, Atlantis Chaos, and Nepenthes Mensa. A somewhat different ridge formation has been discovered in the Eastern Medusae Fossae Formation; these dark ridges can be 50 meters in height and erode into dark boulders. It has been suggested that there are from lava filling fractures in the Medusae Fossae Formation which is surrounded by lava flows.
