Major impacts on the Martian surface include the ancient giant Borealis basin (top of globe), Hellas (bottom right), and Argyre (bottom left).There appears to have been a 400-million-year lull in impacts between the formation of Borealis and the younger basins. University of Arizona/LPL/Southwest Research Institute

It’s magnitude, and infrequency that counts in explaining how asteroid impacts shaped Mars, with new research dramatically revising down the number of giant asteroids that crashed into the Red Planet to just one-tenth of some previous estimates.

The analysis by planetary scientists Wiilliam Bottke, of the Southwest Research Institute, in Colorado, and Jeff Andrews-Hanna, of the University of Arizona’s Lunar and Planetary Laboration, suggests a lull of 400 million years between two periods of intense asteroid numbers and collisions. The first led to the most significant asteroid impact on Mars 4.5 million years ago, while the second to four more giant impacts between 4.1 and 3.8 million years ago.

In their paper published in Nature Geoscience, Bottke and Andrews-Hanna argue on the basis of topographical, gravitational and geochemical analyses against there being any gradual decline in impact events.

Rather, the surface of Mars bears the signature of two distinct periods of intense asteroid activity within the inner Solar System; the earlier period of asteroid impacts associated with the formation of the inner planets; and the later period with the Late Heavy Bombardment, the cause of which a number of explanations have been proposed including the migration of the giant planets.

The most striking aspect of the topography of Mars is the contrast between the remarkably flat lowlands of it northern hemisphere known as the Borealis basin, covering about 40% of the planet’s surface, and the hilly highlands of the southern hemisphere. The calculations by Bottke and Andrews-Hanna concur with previous estimates the northern polar basin – was formed by the impact of an asteroid between 1,100 and 2,300 kilometres wide.

Only one subsequent major asteroid impact, creating the basin known as the Isidis Planitia, has impinged upon the Borealis crater, the researchers argue.

“This sets strong statistical limits on the number of giant basins that could have formed on Mars after Borealis”, says Bottke, who is also a principal investigator with NASA’s Solar System Exploration Research Virtual Institute (SSERVI). “The number and timing of such giant impacts on early Mars has been debated, with estimates ranging from four to 30 giant basins formed in the time since Borealis. Our work shows that the lower values are more likely.”

The similar preservation states of the between most visible impact structures on Mars – the Borealis basin and the Hellas, Isidis and Argyre craters formed more than 400,000 years later, also points to the lull which Bottke and Andrew-Hanna call “the doldrums”, as any impact basins formed in the interim should have been similarly preserved.