The changing nature of rainfall during the early history of Mars

Several explanations have been proposed for the temporal differences in geologic processes associated with the modification of martian impact craters, which occurred throughout the Noachian, and the formation of valley networks, which occurred during the Noachian/Hesperian transition. Here we show that it could be a result of the changing nature of rainfall as the primordial atmospheric pressure on Mars waned through time. We calculate the terminal velocity and resulting kinetic energy from raindrops > 0.5 mm in diameter that would impact the surface of Mars in a CO2-rich atmosphere ranging in pressure from 0.5 to 10 bars. Our analyses indicate that the primordial atmosphere of Mars could not have exceeded ∼4.0 bars as raindrop sizes would have been limited to < 3 mm and surface erosion from rain splash and subsequent crater modification would not have occurred. At pressures between ∼3.0 and 4.0 bars, sediment transport from rain splash could occur, but surface runoff would have been limited, which could explain the modification of impact craters. Once atmospheric pressures waned to ∼1.5 bars, rainfall intensity could begin to exceed the infiltration capacity of most soils, which would be necessary to initiate martian valley network formation. Due to the lower gravity, a storm on Mars that occurred in a 1 bar atmosphere could generate raindrops with a maximum diameter of ∼7.3 mm compared to 6.5 mm on the Earth. However, rainfall from such a storm would be only be ∼70% as intense on Mars, primarily due to the lower martian gravity and resulting lower terminal velocities of the rain drops.