Groundwater Depth, Cryosphere Thickness, and Crustal Heat Flux in the Epoch of Ravi Vallis, Mars

Depth to liquid water on Mars during the Hesperian era may be estimated at specific locations where fluvial erosion disrupted the base of the ancient cryosphere. Landforms, topographic data, and a pressure analysis are used to estimate depth to groundwater, cryosphere thickness, and crustal heat flux at an equatorial location. Ravi Vallis is a 220-km-long outflow channel complex in Xanthe Terra that emerges from an area of collapsed ground called Aromatum Chaos. Prominent linear grooves and ridges on the floor of Ravi Vallis are oriented parallel to the former flow direction, consistent with fluvial erosion. A small unnamed chaos formed early along the shallow, southern branch of Ravi Vallis, and the larger Iamuna and Oxia Chaotes formed later along the northern branch in its deepest reaches. We hypothesize that: (A) Groundwater in Xanthe Terra was confined beneath a thick cryosphere prior to outflow channel formation. (B) The cryosphere at Aromatum Chaos was disrupted by an unknown event during the Hesperian era. The presence of polygonal troughs and pit chains southwest of the chaos indicate probable faults or fractures in the subsurface and the blunt margin of the chaos suggests the disruptive event may have been the result of normal faulting. (C) Cryospheric disruption caused groundwater to discharge from the subsurface, flow downslope, and rapidly erode the land surface to the east-northeast, creating a proto-Ravi Vallis. Catastrophic upwelling of groundwater from Aromatum Chaos would have undermined, eroded, and transported near-surface geologic material. Once initiated, flow continued because re-freezing of the cryosphere was inhibited by the initial high rate of discharge of relatively warm groundwater and physical erosion of the cryosphere. (D) A transient lake upstream in the newly formed Aromatum Chaos was hydraulically connected to a regional cyrospherically confined aquifer during Ravi Vallis formation. Initially, rapid release of water produced broad scabland erosion. A transition to a locally unconfined aquifer occurred at Aromatum Chaos because of the cryospheric disruption, and the newly unconfined aquifer source produced a long recession of flood stage. Protracted flows of reduced discharge concentrated erosion in the deepest parts of Ravi Vallis. The existence of a deeply incised inner channel within Ravi Vallis indicates late-stage flows occurred over an extended period of time. (E) Downstream fluvial erosion during late stage flows deeply incised and thinned the permafrost confining layer, creating overpressure conditions ideal for secondary breakouts of groundwater and genesis of secondary chaos zones, e.g., Iamuna Chaos. Our analysis indicates the thickness of the residual confining layer beneath the floor of Ravi Vallis was less than 260 m when formation of Iamuna Chaos was imminent. We thus interpret that the floor of Ravi Vallis at this location required hydraulic pressures less than 2.5 MPa to rupture the confining layer and induce chaos formation. Present-day topography and results of the pressure analysis performed for Iamuna Chaos indicate the regional cryosphere thickness and depth to groundwater may have been 700-1000 m at this location, which also constrains estimates of crustal heat flux to have likely exceeded 50 mW/m2. The fluvial incision and groundwater breakout model described here can fully account for the inception of Iamuna Chaos, Oxia Chaos, and the smaller unnamed chaos within Ravi Vallis.