Catastrophic Karst-Like Subsidence of Hebes Chasma, Mars

Hebes Chasma, part of the Valles Marineris complex, lacks an outlet channel: by what mechanism was the 104,500 km³ of missing material eroded and removed, and where are the sediments now? A detailed geologic study indicates that Hebes formed by karst-like collapse, and that the material was removed as a fluid by draining into a sub-floor aquifer; hence, the absence of surface outflow channels. The eastern end of the chasma shows an early stage of collapse by sagging of the plains. Advanced stages entailed inward tilting of slabs of cratered plains and flow of wall material itself into closed depressions 8 km below the chasma rim. Viscous flows of wall material up to 50 km long have slopes < 5 degrees and cannot have formed by mobilization of stacks of dry, hard basalt. Instead, the materials probably consisted of mixtures of tephra, ejecta, salts and ice. We simulated collapse of such materials using physical models of Hebes comprising granular materials and viscous silicone polymer. In the models, when material is withdrawn from underneath the closed depressions, the simulated plains surface sags, faults, and tilts inward; then the underlying ductile material flows inward from the failing chasma walls. We propose that collapse of Hebes was initiated by thermal dehydration of salts and melting of ice. Water released into pressurized aquifers would have concentrated the collapse along subterranean fractures that localized the proto-chasma. Heat may have been supplied by volcanic sources, or by local accumulations in insulated parts of the early mega-regolith. Water and clastic material would have been removed through the widening fissures to emerge as distant fountains, perhaps in neighboring Echus Chasma. Similar landforms and geologic relationships elsewhere in the Valles Marineris system suggest a comparable history.