Hydrographs of a Martian flood from the breach of Galilaei Crater

Abstract

Spectacular floods were spawned on Mars when large craters filled to overtopping and catastrophically breached. The wealth of data now available permits quantitative analysis of these ancient floods. Galilaei Crater is examined here, along with the likely sources of its water and the processes that formed Tana Vallis, its 1-km-deep outflow channel. Laser altimetry data, stereo-photogrammetry, and scaled images are used to reconstruct the lake volume and the geometry of the preserved breach. It is theorized that surface flows from Hydaspis Chaos eroded the rim and floor of Galilaei Crater and partly filled the basin, leading to degradation of the underlying cryosphere. Groundwater released from a confined aquifer then slowly filled Galilaei Crater until the enclosed lake overtopped its rim. The resulting flood breached the crater wall, rapidly drained the lake, and eroded Tana Vallis. The drainable lake volume was ≥1.110×1013m3, based on laser altimetry data from Mars Global Surveyor. This volume was 90% of the volume in Lake Superior in the Great Lakes. Using the reconstructed breach and channel geometry, methods used to analyze terrestrial dam breaches were applied to produce hydrographs of lake stage and discharge. Peak flood discharge in Tana Vallis likely ranged from 1.5 to 4.7×107m3s−1. Mean channel incision rates are estimated at 0.333, 0.133, and 0.0667mmin−1 for scenarios in which Tana Vallis formed in 2, 5, and 10days. For the 5-day breach erosion scenario, stream power per unit streambed area at the time of peak discharge was estimated at >3.5×105Wm−2. The volume of material eroded to form the vallis was only ~3% of the lake volume that drained in 5days. Open-channel calculations were performed to compare with the dam breach hydrographs for Tana Vallis. The channel as seen today never flowed bank full or even at three-fourths its maximum depth because the implied flows would have exceeded peak discharges from the crater breach. I also evaluate an alternative model (catastrophic ice lake collapse) for the morphology and breach of Aram Chaos, a large crater near Galilaei Crater. That model has fundamental difficulties because of improbable assumptions and inadequate consideration of the knowledge base of terrestrial basalt hydrology. The model of ice lake collapse fails to explain the breach and outflow of crater lakes like those that existed in Aram Chaos and Galilaei Crater. The present analysis supports a simpler model of groundwater inflow that fills a crater to overtopping. This model harmonizes with the low elevations of Aram Chaos and Galilaei Crater and is more consistent with extensive evidence of fluid breakouts in this region and with theoretical and terrestrial knowledge.