Astronomers had hoped that secondaries could be identified, thereby alleviating the confusion.
Not so; a new paper in indicates that many secondaries are very difficult to distinguish from primaries, because debris lofted up may go into orbit for years, falling down far away from the initial impact (distant secondaries).
For example a surface may be covered by a few hundred meters of mobile sand dunes; the numbers of craters of diameter D and depth d would give a mean characteristic age of topographic features of the scale of D,d.
Smaller craters in mobile dunes would disappear faster and have lower mean ages.
Below you will find a text describing the basic principles behind the Planetary Science Institute system of utilizing crater counts and isochron diagrams in order to estimate crater retention ages of surfaces on Mars.
As discussed by Hartmann (1966) the crater numbers can date the actual formation age of a surface in an ideal case, such as a broad lava flow which forms a one-time eruptive event.
The work, led by geochemist Ken Farley of the California Institute of Technology (Caltech), could not only help in understanding the geologic history of Mars but also aid in the search for evidence of ancient life on the planet.
A surface could be formed and covered by sediments at some later time, degrading any craters on the original surface.
Secondary craters are formed by fallback debris from large impacts (primary craters).
A single large impact can produce a million secondary craters, blurring relationships between crater counts and the age of a surface.
The authors tested dating by counting small craters in a variety of presumed “old” and “young” regions of the moon, and got widely divergent results despite using standard methods and software.
They urged a high degree of caution, therefore, when trying to infer the age of a planetary surface.