At first glance, the floors of many large impact craters on Mars have areas that appear smooth and flat. But a closer look at some of these flat floors shows a much more rugged landscape, where one small cuplike pit adjoins another so tightly they share a common wall. These pits partly resemble the secondary craters produced by the fall of debris from the original impact — but not quite.
A team of geologists led by Joseph Boyce (University of Hawaii) offers a different explanation. Writing in Icarus, they attribute the pits instead to violent steam jets and blasts: “Our model predicts the explosive degassing of water from this pitted material.”
The water, they explain, came almost entirely from the impact target material. In the immediate aftermath of the impact, this water-rich material (which could also include ice) became violently mixed with broken rocky fragments of the impacting meteorite, pieces of deeper rock layers, and rock that was liquified by the heat and shock of the impact.
This hot mixture (about 750°C or 1400° F) collapsed into the crater and onto its rim, and started to cook. Water turned quickly into steam, creating a myriad of small “vent pipes” in this mixture, through which it blasted and jetted its way to the surface at speeds of 300 meters per second (1,000 feet/sec) or more. On its way, the steam snatched up and carried fragments from the walls of the vent pipes leading upward through the impact debris and flaring at the surface. These pipes opened the way for still-deeper steam to escape.
The whole process wouldn’t have ended until the water was gone and the impact melt had cooled and solidified, which could be decades to a century in the deepest part of the crater. But the main show, they say, was over in few days to a month.
Because the pitted areas are produced by the escape of steam from the mixture of impact debris, the presence of the pits indicates the target material was water/ice-rich.
“If this model is correct,” says the team, “it provides a new method for investigating the distribution of subsurface volatiles at the time of crater formation.”