No abstract

[1] The Ares Vallis region is surrounded by highland terrain containing both degraded and pristine large impact craters that suggest a change in climate during the Late Noachian-Early Hesperian, from warmer, wetter conditions to colder, dryer conditions. However, the regional occurrence of Hesperian-age crater outlet channels indicates that this period on Mars was characterized by episodic climate fluctuations that caused transient warming, facilitating the stability of liquid water at the surface. An extensive survey of the morphology and topography of 75 impact basins in the region indicates that of the largest degraded craters, 4 were identified with single outlet channels that suggest the former presence of water infill. These basins lack inlets indicating that water influx was likely derived from sapping of groundwater. A comparison of measured crater rim heights to modeled rim heights suggests that the bulk of the depth/diameter reduction on these craters was the result of infilling, possibly by sediments. Crater statistics indicate that crater degradation and infill occurred during a short 200 Ma interval in the Late Noachian, from 3.8 Ga to 3.6 Ga. Craters that formed after 3.6 Ga exhibit a near-pristine morphology. Our results support the hypothesis of rapid climate change at the end of the Noachian period. However, geologic relationships between the crater outlet channels and Ares Vallis indicate that drainage occurred only after the period of intense crater modification, during the Hesperian (3.5-2.9 Ga). This suggests a delay between the time of infill of the craters and the time of drainage.

An automated crater detection algorithm is presented which exploits image data. The algorithm is briefly described and its application demonstrated on a variety of different Martian geomorphological areas and sensors (Viking Orbiter Camera, Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and High Resolution Stereo Camera (HRSC)). We show assessment results through both an intercomparison of automated crater locations with those from the manually-derived Mars Crater Consortium (MCC) catalogue and the manually-derived craters. The detection algorithm attains an accuracy of 70 to 90 percent and a quality factor of 60 to 80 percent depending on target sensor type and geomorphology. We also present crater detection results derived from HRSC images onboard the ESA Mars Express on a comparison between manually-determined Size-Frequency Distributions (SFDs) and those derived fully automatically. The approach described appears to offer great potential for chronological research, geomatic and geological analysis and for other purposes of extra-terrestrial planetary surface mapping.

We describe the evolution of an ~600-m-deep tributary outfl ow channel to Ares Vallis, Mars. High-resolution topography, image analysis, and crater statistics indicate that this tributary canyon developed by the upstream migration of a large, ~300-m-tall cataract during multiple fl ood events that span ~1 b.y. of Mars history (3.7-2.6 Ga). Issuing from Hydapsis Chaos, these fl oods were initiated at a similar time and occurred over a similar time range to fl ooding in Ares Vallis, suggesting a potential regional control on fl ood initiation and chaos formation. In addition, we provide evidence that cataract retreat and signifi cant incision within the tributary canyon occurred only after a series of downcutting events within Ares Vallis. Topography data and crater statistics taken from the fl oor of Ares Vallis indicate an ~300 m base-level drop that coincides temporally with an Early Amazonian (ca. 2.6 Ga) fl ood event and cataract formation within the tributary canyon. The results both confi rm the hypothesis of long-term, multiple fl ood events within martian outfl ow channels and demonstrate the infl uence of base-level change on their incision.

No abstract