The method is based upon the assumptions that a new surface forms with zero impact craters, and that impact craters accumulate at some constant rate.
The method has been calibrated using the ages of samples returned from the Moon.
Planetologists were not unaware of secondary cratering, but until recently, underestimated its significance.
Crater counting is a method for estimating the age of a planet's surface.
The reliability of small crater counts is tested by counting small craters at several young and old lunar surfaces, including Mare Nubium and craters Alphonsus, Tycho and Giordano Bruno.
Dating methods are like human pyramids; they depend ultimately on the support of the bottom layer. If the bottom guy buckles under pressure, the circus act quickly turns into a dogpile.
A single large impact can produce a million secondary craters, blurring relationships between crater counts and the age of a surface.
Astronomers had hoped that secondaries could be identified, thereby alleviating the confusion.
It assumes, however, that impactors arrive at a roughly steady rate and produce one crater per hit.
Or that the surface of the moon Io is younger than 50 years?
Or that the youngest stretches of terrain on our moon’s surface dates back to about 3 billion years ago?
After compensating for various complicating factors, like atmospheric density, gravity, and geological activity, scientists had been confident of their time charts -- until recently.
New thinking about "secondary craters" has thrown this whole foundation of comparative planetary dating into disarray.
Secondary craters are those formed from the debris of an initial impact.