New Data Reveal Mature, Integrated Drainage Systems on Mars Indicative of Past

Introduction: Valley network systems on Mars remain the most unequivocal evidence that water carved the surface of the planet in the distant past and the climate was different than today. Most of these systems date from the Noachian [1], the p eriod of heavy bombardment which is >3.7 Ga [2], although local or regional formation seems to have continued to more recent times [3]. The discovery of valley networks sparked a long-lived debate over the amount and importance of water at or near the martian surface. Both surface runoff and groundwater processes have been attributed to formation of valley networks but the relative importance of each is still unclear. In this study w e analyzed recent Mars Global Surveyor (MGS) data in and around the Arabia Terra region with new techniques and found far more valley networks and mature drainage systems than previously observed. Certainly, both groundwater and surface runoff sculpted the martian landscape, however, our findings argue for a substantial contribution of surface runoff, implying past precipitation. Background: Drainage basin morphometry is often used to infer past climatic conditions and the importance of precipitation [e.g. 4]. Terrestrial categori-zation of basin morphometry incorporates the properties of stream length, order, and density to determine the degree of valley network integration and maturity. Quantitative analysis of stream order was developed by Strahler [5] and is a measure of the number and maturity of valley segments. Drainage density is simply defined as the average length of valley segments per unit area [6] and is a reflection of stream spacing in a drainage basin. The common procedure to calculate this parameter is to determine the area of a drainage basin and then divide by the total length of valley segments. On Earth, mature drainage basins are heavily dissected and typically have a stream order >5 and drainage density >6 × 10-2 km-1 [7]. Valley network systems in this category are believed to reflect a substantial contribution of precipitation and subsequent surface runoff. Methods: Our technique to investigate drainage basin morphometry on Mars utilized the Mars Orbiter Camera (MOC) wide angle global image mosaic and Mars Orbiter Laser Altimeter (MOLA) data. These sets provide a considerable improvement in resolution and clarity of the martian surface over standard products such as the Mars Digital Image Mosaic (MDIM), which was produced with Mariner and Viking data.