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.

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.

[1] The origin mechanisms and geologic evolution of chaotic terrain on Mars are poorly constrained. Iani Chaos, located at the head Ares Vallis, is among the most geomorphologically complex of the chaotic terrains. Its morphology is defined by (1) multiple, 1 to 2 km deep basins, (2) flat-topped, fractured plateaus that are remnants of highland terrain, (3) knobby, fractured remnants of highland terrain, (4) plateaus with a knobby surface morphology, (5) interchaos grooved terrain, (6) interior layered deposits (ILDs), and (7) mantling material. Topography, the observed geomorphology, and measured fracture patterns suggest that the interchaos basins formed as a result of subsurface volume loss and collapse of the crust, likely owing to effusion of groundwater to the surface. Regional patterns in fracture orientation indicate that the basins developed along linear zones of preexisting weakness in the highland crust. Multiple overlapping basins and fracture systems point to multiple stages of collapse at Iani Chaos. Furthermore, the total estimated volume loss from the basins (10 4 km 3 ) is insufficient to explain erosion of 10 4 -10 5 km 3 of material from Ares Vallis by a single flood. Comparisons with the chronology of Ares Vallis indicate multiple water effusion events from Iani Chaos that span the Hesperian, with termination of activity in the early Amazonian. Recharge of groundwater through preexisting fracture systems may explain this long-lived, but likely episodic, fluvial activity. Late-stage, early to middle Amazonian aqueous processes may have deposited the ILDs. However, the topography data indicate that the ILDs did not form within lacustrine environments.

On Earth, permafrost thawing is linked to climate warming. Similarly, on Mars, permafrost degradation, described from mid-latitude and equatorial settings, is likely linked to global or regional climate change. Putative thermokarst depressions identifi ed on Mars are widely considered to be the result of sublimation, evaporation, or thawing of an ice-rich substrate. The possibility that the depressions formed by melting of permafrost to create alas-like lakes has been recently proposed, but is controversial, owing to the lack of primary evidence for liquid fi lling the depressions. Here we use high-resolution Mars Reconnaissance Orbiter Context Camera images and derived topographic data to characterize possible thermokarst terrain in Ares Vallis. The terrain comprises subcircular to irregular, fl at-fl oored rimless topographic depressions that occur at varying elevations. We report the discovery of narrow channels connecting thermokarst-like depressions that provide evidence for the previous presence of ponded liquid water. Crater counts on these surfaces indicate resurfacing that is likely related to fl ood deposition of water-saturated sediments in Ares Vallis during the Hesperian (ca. 3.6-3.0 Ga). We infer that thermokarst lakes formed after fl ooding by thawing of ice within the sediments during transient warm periods in the Hesperian, a time previously considered to be too cold to permit ice thaw.