Crater Count Dating Still Unreliable

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’ve 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.

Some of those “other factors” include not knowing the incoming rate (impact flux), saturation criteria, differences in target properties, erosion rates, or what complicated resurfacing histories have occurred.  In addition, human judgment can bias the counts based on what individuals consider significant: “this problem is severe in small crater counts,” the authors noted, although it might be alleviated with automated counting methods in the future.  Even so, someone would have to tell the computer the difference between the small potatoes and the large potatoes.  Non-impact structures (like volcanic vents) can sometimes look like craters.

Crater count dating would be straightforward if impactors of predictable size came in at a predictable rate within predictable rates of speed, were made of predictable materials and impacted a uniform surface material and left marks distinct from those of any other source.  Even correcting for known complicating factors, astronomers thought they could calibrate the “old” and “young” craters with radiometric measurements from Apollo samples.  Unfortunately, complications quickly arise in all those assumptions.  Absolute dating was not a help:

We counted the small crater populations on both young and old lunar surfaces to determine the problems of using small crater counts for age dating. Our counting areas are not smaller than those used in other publications, and the absolute model ages were determined from the widely employed production and chronology functions. Great discrepancies are observed in the small crater counts.

The authors were so pessimistic, they could not even recommend crater counting to assess relative ages.  “Small crater counts are highly unreliable for either relative or absolute age dating on both old and young surfaces,” they said.  In addition, the uncertainties they learned from testing the method on the moon and Mars extends to the whole solar system: “All crater counting ages on other celestial bodies,” they warned, “are based on certain assumptions about the origin and impact rate of the impactors.”  The take-home message from the paper was bleak:

In general, statistics of small craters are affected by numerous factors, e.g., contamination of secondaries and different target properties. Crater counting is a subjective process which causes more uncertainties to the results. Simplistic attempts to date planetary surfaces from small crater counts may be invalidate if they do not take these factors into account.

Update 5/24/2012: Mike Wall, seni0r editor for Space.com, unwittingly illustrated the arbitrariness of crater count dating in an article on Live Science: “One of the Red Planet’s most mysterious landforms is probably 2 billion years older than has been thought,” he began, “suggesting it may have had a volcanic origin, a new crater count finds.”    While not questioning the validity of the method, Wall wrote that different astronomers, all using crater counts, arrived at ages for the Medusae Fossae area on Mars of a few hundred million years (“very young”), then 1.6 billion years, and now, up to 3.8 billion years.  This not only shows that the same method produced results differing by over 250%, but also raises doubts whether the newly-published date is any more reliable.

1. Xiao and Strom, “Problems determining relative and absolute ages using the small crater population,” Icarus, accepted manuscript May 17, 2012.

Some relative dating can be preserved.  If a crater sits inside or on top of another crater, all that can be said is that the inside or top crater happened after the other one.  But without knowing when they formed or how much time had passed between them, how useful is that?  It’s a subjective process with too many uncertainties.

So, toss another dating method into the dustbin, along with all its secondary impacts – the Late Heavy Bombardment, the age of the solar system (A.S.S.), relative events between solar system bodies and all the other mythoids* the moyboys** use to pretend they have gnosis about an unobserved history and can tell us all about it.

*Mythoid: An easily understood, workable falsehood that can be stated succinctly to sound like a fact.  Not to be confused with factoid.

** Moyboy: someone who assumes “millions of years, billions of years” will solve any problem.