Worries about the crater count dating method, widely relied upon to infer ages of planetary surfaces, began emerging in 2005.  Those worries have not subsided; they have only grown worse.  Crater numbers may have nothing to do with age.

We’re kept track of the crater count crisis since 2005, when the problem of secondary craters was brought to light (10/20/2005, 6/08/2006, 9/25/2007, 3/25/2008, 7/25/2010).  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.

Astronomers had hoped that secondaries could be identified, thereby alleviating the confusion.  Not so; a new paper in Icarus by Xiao and Strom1 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).

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.  The abstract states:

The small crater populations (diameter smaller than 1 km) are widely used to date planetary surfaces. 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. Based on high-resolution images from both the Lunar Reconnaissance Orbiter Camera and Kaguya Terrain Camera, small craters in two different diameter ranges are counted for each counting area. Large discrepancies exist in both the cumulative (absolute model ages) and relative plots for the two different size ranges of the same counting areas. The results indicate that dating planetary surfaces using small crater populations is highly unreliable because the contamination of secondaries may invalidate the results of small crater counts. A comparison of the size-frequency distributions of the small crater populations and impact ejected boulders around fresh lunar craters shows the same upturn as typical Martian secondaries, which supports the argument that secondaries dominate the small crater populations on the Moon and Mars. Also, the size-frequency distributions of small rayed lunar and Martian craters of probable primary origin are similar to that of the Population 2 craters on the inner solar system bodies post-dating Late Heavy Bombardment. Dating planetary surfaces using the small crater populations requires the separation of primaries from secondaries which is extremely difficult. The results also show that other factors, such as different target properties and the subjective identification of impact craters by different crater counters, may also affect crater counting results. We suggest that dating planetary surfaces using small crater populations should be with highly cautious….

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