Mars shows more than 20 impact basins with diameters of at least 1,000 kilometers (600 miles), and five of these are 2,500 km wide or larger. Based on crater counts, most of the basins appear to have occurred between in a narrow slice of time, 4.2 to 4.1 billion years ago. This is about when scientists believe the internal dynamo creating a global Martian magnetic field turned off. Is there a connection?
Possibly, argue James Roberts (Johns Hopkins University Applied Physics Laboratory) and Jafar Arkani-Hamed (University of Toronto) in a recent paper in Icarus. Their work involved computer modeling of the thermal effects on the convection in the Martian mantle caused by large impacts.
They write, “We find that the impacts that formed the five largest basins dominate the impact-driven effects on mantle dynamics. A single impact of this size can alter the entire flow field of the mantle. Such an impact promotes the formation of an upwelling beneath the impact site, resulting in long-lived single-plume convection.” A mantle plume is an upwelling of warm, buoyant rock that transports material and heat from the deep mantle to the near-surface.
Also, the researchers note, the basin impacts came too frequently for the mantle to relax and re-establish its normal convection pattern.
“The interval between the largest impacts (about 25 million years) is shorter than the initial recovery time for a single impact (roughly 100 million years),” they explain. “Hence, the change in convective pattern due to each impact sets up a long-term change in the global heat flow.”
Earlier work suggests that the impact heating produces a warm region, or ‘thermal blanket’ in the interior, preventing the deeper layers from cooling and thereby stalling the dynamo activity in the core that generates the magnetic field.