Relative dating using cratering distribution

Boris Ivanov used the SFD of lunar mare craters to estimate the size–frequency distribution of projectiles that have cratered inner solar-system bodies (Ivanov 2003).

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Absolute age dating determines the "calendar" time at which a rock, surface, or feature formed; relative age dating determines the order-but not the time-of formation. If the rocks have remained as closed isotopic systems, it is possible to calculate their age by measuring the amount of radiogenic isotopes relative to the amount of stable isotopes now present.

In practice, this procedure requires an accurate assessment of the initial abundances of the isotopes produced in the radioactive decay.

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.

Because Hartmann has shown that the structure in the mare crater SFD may be a signature of a collisionally evolved impactor population, and because asteroids are probably more collisionally evolved than comets, the implication is that asteroids are the main component of the family of impactors striking the terrestrial planets, and have been for much of the past 4 Ga.

After mare rocks had been radiometrically dated, these calibrated surfaces were the key to using crater counting to date other planetary bodies.

Crater count isochrons have been constructed for every body in the inner solar system and, with more caution, for outer solar-system bodies as well.

So now, when we look at the surface of Mars and see evidence for the action of water or volcanism, we can say with confidence that those processes have been acting in the geologically recent past – the last 10 Ma – and may still be going on today.

The problem becomes intricate if more than one event that affected the radiogenic isotope systems has occurred during the evolution of the rock.

Many rocks have complex histories, and the challenge in isotopic age determination is to unravel and date not one, but each of the events that affected their evolution.] To date, only terrestrial, lunar, and meteoritic samples have been dated by isotopic methods.

Shoemaker (1965) used photos of the lunar surface to show that debris thrown out of a classic planetary impact crater has a steep SFD slope.