They compared a series of before and after shots, and discovered 222 new craters had formed.
This led to a new estimate, that says 180 craters of at least ten metres in diameter form each year, according to an accompanying News and Views article in This is partly down to the team recognising surface reflectance zones of the craters that were wider than recorded.
The impact occurs at high speed, and the final crater depth, diameter, and shape are effectively determined by the surface gravity, the mass of the impactor, and the velocity of the impactor.
Almost all impact craters are circles; only impacts at very low angles (less than 10°) will form elliptical craters.
The accuracy of age estimates of geologically young surfaces based on crater counting on Mars has been questioned due to formation of large amounts of secondary craters.
It assumes, however, that impactors arrive at a roughly steady rate and produce one crater per hit.
After compensating for various complicating factors, like atmospheric density, gravity, and geological activity, scientists had been confident of their time charts -- until recently.
In one case, the impact that created Zunil crater produced about a hundred secondary craters, some more than 1000 km from the primary impact.
If similar impacts also produced comparable amounts of secondaries, it would mean a particular crater-free area of Mars had not been "splattered by a large, infrequent primary crater", as opposed to suffering relatively few small primary impacts since its formation.