Phreatomagmatic explosive origin of Hrad Vallis, Mars

Hrad Vallis is a ∼370 m deep, ∼800 km long depression located at 34°N, 218°W, in Elysium Planitia, Mars. A distinctive deposit, present on both sides of the source area of Hrad Vallis, has a maximum width of ∼70 km along the strike of the depression, a lower unit extends for nearly 100 km away from the depression on either side, and an upper unit extends for ∼50 km. Viking and Mars Orbiter Camera (MOC) images show lobate deposit boundaries, rheomorphic deformation, and numerous craters 270–580 m in diameter within the deposit. The deposit coincides with a broad topographic rise ∼30 km wide and ∼100–150 m high extending along the sides of the valley for at least 100 km along strike. We propose that these features formed when a dike underlying the axis of the valley generated a shallow sill, which in turn is partly responsible for the rise topography. The remainder of the rise consists of a mud‐like deposit of ejecta from a large‐scale phreatomagmatic explosion due to violent mechanical and thermal mixing between the central part of the sill and ice‐rich rock layers in the upper few hundred meters of the crust in a fuel‐coolant‐type interaction. The observed range and thickness of ejecta and the depth of the Hrad depression imply explosion steam pressures up to 360 MPa, ejecta speeds of ∼400 m s−1, and a sill at least 150 m thick intruded at a depth of ∼350–400 m below the original surface.