Deltas and valley networks on Mars
Deltaic depositional systems are the most prominent evidence suggesting the existence of long-lasting standing bodies of water on Mars. Due to this genetic link, Martian deltas are fundamental for the reconstruction of the paleohydrological cycles and paleoclimates of Mars, testifying to the occurrence of past climatic conditions that created hydrological settings quite different from those of modern Mars. Ancient terrains of Mars are covered with additional signatures of past water. Branching channel systems on Mars, known as “valley networks,” have long been viewed as some of the best evidence that water flowed across the surface. Recent data, including topography and high-resolution imagery, have shed new light on valley network formation. It is now believed that most valley networks were formed by precipitation and surface runoff instead of groundwater processes. Two main planet-wide equipotential surfaces that encompass complete topographic enclosures within and along the margins of the northern lowlands. The inferred levels could represent two paleoshorelines of a past ocean covering the northern hemisphere of Mars during its early history. These boundaries are generally consistent with the “Arabia shoreline” previously suggested from geomorphologic and topographic observations and also with the global distribution and age of valley networks. Reconstructed scenario implies that climatic conditions allowed the occurrence of a hydrosphere presenting similarities with that of Earth, and integrating valleys, deltas, lakes, an ocean encircling the planet, and possibly a number of suitable niches for life to exist.