Keith P. Harrison

Groundwater‐controlled valley networks and the decline of surface runoff on early Mars

[1] Fluvial erosion on early Mars was dominated by valley networks created through a combination of groundwater processes and surface runoff. A reduced greenhouse effect due to CO 2 loss, together with a declining geothermal heat flux, promoted the growth of a cryosphere and a Hesperian hydrologic regime dominated by outflow channel formation. We test the hypothesis that the transition from valley network to outflow channel formation was preceded by a more subtle evolution characterized by a weakening of surface runoff, leaving groundwater processes as the dominant, final source of valley network erosion. Our hypothesis, supported by a terrestrial analog in the Atacama desert of Chile, is related to the groundwater sapping reactivation hypothesis for densely dissecting highland valley networks on Mars suggested by Baker and Partridge in 1986 and focuses on the age analysis of large, sparsely dissecting valley networks such as Nanedi Valles, Nirgal Vallis, valleys in fretted terrain, and tributaries of outflow channels and Valles Marineris chasmata. We find that these features are consistently late Noachian to Hesperian in age, younger than Noachian densely dissecting dendritic valley networks in the southern highlands. In the Tharsis region the observation of dense and sparse valley network morphologies on Hesperian terrain suggests that while surface runoff gave way to groundwater processes consistent with our hypothesis, the transition may have occurred later than elsewhere on the planet. The volcanic nature of Tharsis suggests that geothermal heat and volatile production led to episodically higher volumes of surface runoff in this region during the Hesperian.

Controls on Martian Hydrothermal Systems: Application to Valley Network and Magnetic Anomaly Formation

magnetic anomalies. For host rock permeabilities as low as 10 � 17 m 2 and intrusion volumes as low as 50 km 3 , the total discharge due to intrusions building that part of the southern highlands crust associated with magnetic anomalies spans a comparable range as the inferred discharge from the overlying valley networks. INDEX TERMS: 1832 Hydrology: Groundwater transport; 1860 Hydrology: Runoff and streamflow; 1545 Geomagnetism and Paleomagnetism: Spatial variations (all harmonics and anomalies); 5440 Planetology: Solid Surface Planets: Magnetic fields and magnetism; 5114 Physical Properties of Rocks: Permeability and porosity; KEYWORDS: Hydrothermal, groundwater, runoff, crustal magnetism, intrusions

Rheological constraints on martian landslides

We use a dynamic finite-difference model to simulate martian landslides in the Valles Marineris canyon system and Olympus Mons aureole using three different modal rheologies: frictional, Bingham, and power law. The frictional and Bingham modes are applied individually. Fluidized rheology is treated as a combination of frictional and power-law modes; general fluidization can include pore pressure contributions, whereas acoustic fluidization does not. We find that general fluidization most often produces slides that best match landslide geometry in the Valles Marineris. This implies that some amount of supporting liquid or gas was present in the material during failure. The profile of the Olympus Mons aureole is not well matched by any landslide model, suggesting an alternative genesis. In contrast, acoustic fluidization produces the best match for a lunar slide, a result anticipated for dry crust with no overlying atmosphere. The presence of pressurized fluid during Valles Marineris landsliding may be due to liquid water beneath a thin cryosphere (<1–2 km) or flash sublimation of CO2.

6 – Episodic ponding and outburst flooding associated with chaotic terrains in Valles Marineris

The Valles Marineris canyons are among the deepest of topographic depressions on Mars. As such, they may be expected to have served as sinks for a variety of mobile materials, including water, regardless of their particular formation history. The interpretation of ILD formation is not critical to the putative Valles Marineris lake considered here. Considerably more important are chaotic terrains, which may constitute the primary source of water for certain putative ancient lakes in, and near, the Valles Marineris. Chaotic terrains are large regions of fractured and subsided materials found, in several cases, at the termini of Valles Marineris canyons. Some chaotic terrains that are topographically connected to Valles Marineris canyons suggest that groundwater discharge at these terrains flooded both the canyons and the outflow channels. Such floods undoubtedly led to ponding, either locally or at points along the drainage path. Coincidence of the MOLA-derived basin overflow point with channel erosion strongly supports the ponding hypothesis. Although the most likely source of ponded water was groundwater discharge at chaotic terrain in Capri and Eos Chasmata, or at large-scale fractures bounding the Valles Marineris canyons, an alternative supply from a northern plains ocean, which has a putative shoreline at a similar elevation to the Valles Marineris paleolake, is possible. Direct production of channel floods by groundwater discharge remains difficult to explain, and the buffering effect provided by surface ponding may circumvent this problem.