Chapter 2 - Outflow channels on Mars

Abstract

The surface of Mars displays many landform assemblages associated with liquid flows. Martian outflow channels are one such feature. These long sinuous incisions containing streamlined islands can be thousands of kilometers long and hundreds of kilometers wide, with depths reaching 2.5km. These forms were certainly carved by enormous flows of liquid. Similar long, sinuous incisions occur also on Venus and must have been incised by lava. However, the Martian outflow channels resemble channels on Earth formed by catastrophic glacial meltwater discharges. Moreover, the cross-sectional geometry of Martian outflow channels is more closely aligned with the morphological properties of large rivers on Earth than with lava channels on Venus, Mercury, or Earth’s moon. Contextually, however, the availability of enough water to produce sufficiently large discharges on Mars has been fundamentally queried, while there is evidence that Martian lava flows occurred nearly globally and continually until very recently. Moreover, because the rheology of Martian lavas could have allowed them to flow turbulently, and over greater distances than on Earth, many scientists believe that the Martian outflow channels were lava incised. However, although Mars is at present very dry, its surface displays many ancient landscapes confidently believed to reflect the action of flowing water. Significantly, many outflow channel heads occur in chaotic terrain, thought to be produced by ground disturbance resulting from the release of large quantities of groundwater. This is supported by the presence of hydrated minerals, which required the sustained presence of liquid water to form, in many chaotic terrains and within outflow channels. This chapter, therefore, considers the hypothesis that incision of the Martian outflow channels involved extremely high magnitude discharges of water.