Understanding how scientists determine the relative age of geologic units on the Moon is straightforward, most of the time. One simply follows the law of superposition; what is on top is younger, what is below is older. In some cases superposition relations are not clear, so scientists then compare crater densities. That is the number of impact craters on a common size of ground. Since impacts occur randomly both in time and on the Moon's surface, any piece of ground has an equal chance of being hit. Over time the number craters in a given area increases. Simply stated, the older an area the more craters you will find.
How do scientists determine the absolute age of a geologic unit? Somehow the crater counts (# of craters per area) have to be "connected" to absolute age dates. The rocks brought back by the Apollo astronauts provide that connection. For example, radiometric age dates of Apollo 11 and 17 basalts are 3.6 to 3.8 billion years (by) old, while those collected at the Apollo 12 are "only" 3.2 by old. Crater counts were taken of these three areas allowing absolute ages to be estimated for other places on the Moon that have crater densities between those of Apollo 12 and Apollo 11. What about absolute age dates for the other Apollo sites and the three Soviet Luna robotic samples? Can the range of time be extended beyond the range 3.2 by and 3.8 by? Unfortunately none of the young large mare basalts were sampled by any of these other missions. Look at the absolute age plot (below).
There is a very large gap in time (x-axis on the plot) for units less than 3.2 by old! Both the Copernicus and Tycho age dates are inferred. That is, scientists hypothesized that the Apollo 12 astronauts sampled part of a ray of Copernicus and Apollo 17 astronauts collected pieces of Tycho ejecta (from a distance of >1000 km from the crater itself). For thirty years the Copernicus date did not quite fit the chronology curve defined by all of the other samples. Perhaps the absolute age curve was in error, or perhaps Copernicus ray material was not actually sampled at the Apollo 12 landing site? Or perhaps the crater density value for Copernicus was not as accurate as it could be? New crater counts acquired from LROC WAC and NAC images and Kaguya images reveal a different crater density for Copernicus! Mystery solved, well perhaps. Scientists still have to live with the sparsity of absolute age points that define the chronology curve, for now. To really nail down the connection between crater density counts and absolute ages we need samples of middle aged and young mare (3 by to 1.5 by years). And samples are needed for very young impact materials (<100 million years), such as the craters Tycho, Aristarchus, and Giordano Bruno. Such samples will not only help scientists better define absolute ages all around the Moon, but also for Mercury, Mars and asteroids.
Explore one of the LROC NAC images used to improve the crater density statistics at Copernicus.
The full story on improvements to the absolute lunar chronology can be found in:
How old are young lunar craters? Hiesinger, H., C. H. van der Bogert, J. H. Pasckert, L. Funcke, L. Giacomini, L. R. Ostrach, and M. S. Robinson Journal of Geophysical Reseach, 117, E00H10, doi:10.1029/2011JE003935
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