Mars collapse. In 2012, it was proposed that Valles Marineris is not just a graben, but a plate boundary where of transverse motion has occurred, making Mars a planet with possibly a two-tectonic plate arrangement. Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons. The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters". Cave entrances measure from wide and they are estimated to be at least deep. Because light does not reach the floor of most
Mars Global Surveyor shoot pictures of other orbiting bodies, such as other spacecraft and the moons of Mars. In 1998 it imaged what was later called the Phobos monolith, found in MOC Image 55103. After analyzing hundreds of high-resolution pictures of the Martian surface taken by the orbiting Mars Surveyor spacecraft, a team of researchers found that weathering and winds on the planet create landforms, especially sand dunes, remarkably similar to those in some deserts on Earth. Results from the "Mars Global Surveyor" primary mission (1996–2001) were published in the Journal of Geophysical Research by M. Malin and K. Edgett. Some of these