Relative dating using cratering distribution

10-May-2016 21:40 by 2 Comments

Relative dating using cratering distribution

Beginning with his landmark work (Hartmann 1965) in which he concluded that the lunar mare were 3.6 Ga old – subsequent dating of mare basalts showed this to be astonishingly accurate – he went on to define the methodology in detail.

These calibrated surfaces allowed crater counting to be used as a chronometric tool with greater confidence, firstly to date the rest of the (unsampled) lunar surface.

It became apparent that surfaces older than ∼4 Ga had so many impacts that new craters just obliterated old ones; 4 billion years represents an upper age limit for the method.

There are interesting features in the lunar cratering record.

Gerhard Neukum has shown that although the rate of impacts varied, the overall shape of the crater size–frequency distribution has not changed over the past 4 Ga – the cumulative distributions for pre-mare craters look very like those for post-mare craters (Neukum and Ivanov 1994).

Deducing the age of solar-system surfaces cannot depend on radiometric techniques: we do not yet have representative samples of other planets and moons.

Instead, we assess the relative ages of surfaces from the density of impact cratering and gain absolute ages based on Apollo samples.

Over recent years, planetary science conferences have been enlivened by high-resolution images of solar system bodies from Mars to Europa, that have revealed extremely young surfaces.

The reason we can estimate an age for the surface of objects whose rocks have never been sampled and dated radiometrically is thanks to a technique that was developed in the 1960s: crater counting.

The number of craters on a planetary surface is determined by the age of the surface and the average crater production rate.

Know one of these variables and you can constrain the other.

Ernst Öik used crater counts and impact rates to estimate that the Moon's Mare Imbrium was 4.5 Ga old (Öpik 1960).

The technique was also pioneered by Gene Shoemaker (Shoemaker 1962) and Robert Baldwin (Baldwin 1964), but it was Bill Hartmann who refined it and made crater counting a vital part of the planetary scientist's toolbox.

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