The Mars Global Surveyor Mars Orbiter Camera has acquired data that establish the present-day impact cratering rate and document new deposits formed by downslope movement of material in mid-latitude gullies on Mars. Twenty impacts created craters 2 to 150 meters in diameter within an area of 21.5 x 10(6) square kilometers between May 1999 and March 2006. The values predicted by models that scale the lunar cratering rate to Mars are close to the observed rate, implying that surfaces devoid of craters are truly young and that as yet unrecognized processes of denudation must be operating. The new gully deposits, formed since August 1999, are light toned and exhibit attributes expected from emplacement aided by a fluid with the properties of liquid water: relatively long, extended, digitate distal and marginal branches, diversion around obstacles, and low relief. The observations suggest that liquid water flowed on the surface of Mars during the past decade.

Abstract:The Ordinary High Water Mark (OHWM) is an approach for identifying the lateral limits of non-wetland waters. However, determining whether any non-wetland water is a jurisdictional "Water of the United States" (WoUS) involves further assessment in accordance with the regulations, case law, and clarifying guidance. In the Arid West region of the U.S., the most problematic Ordinary High Water (OHW) delineations are associated with the ephemeral/intermittent channel forms that dominate the Arid West landscape. This report presents a method for delineating the lateral extent of the non-wetland waters in the Arid West using stream geomorphology and vegetation response to the dominant stream discharge.

We examined the formation and evolution of preferred pathways in fl ow interiors by sequentially extruding different colors of polyethylene glycol wax (PEG) from a point source into a cold sucrose solution. The setup was videotaped from the top and side to show time-lapse views of the developing surface morphology, and from the bottom to capture the interaction of the different PEG colors in the fl ow interior. We conducted 18 experimental runs that clearly show the development of interior fl ow pathways as a function of emplacement variables, which include effusion rate, cooling rate, and time. High effusion rate combined with slow cooling produced extrusions with little or no surface crust, and fl ow interiors that displayed a breakdown of radial fl ow into smaller broad fronts as the experiment proceeded. Where crusts formed near the margins of some of these fl ows late in the run, narrower pathways formed beneath the solidifi ed surface. Lower effusion rates and more rapid cooling produced completely crusted fl ows with a highly complex interior pathway network. Flow surface morphology was affected by interior pathway development. Broad interior pathways capable of applying stress over large areas resulted in widespread fl ow surface disruption and the formation of rifts and levees, whereas narrow fi nger-like pathways capable of applying stress over much smaller areas resulted in many local surface disruptions such as pillows. Pillows typically consisted of fl uid that had been residing in the interior for some time, rather than the freshest, hottest liquid to emanate from the point source. We suggest that the development of Saffman-Taylor fl uid instabilities in lava fl ow interiors leads to viscous fi ngering and the formation of preferred