MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), a current NASA orbital mission, is collecting the first ever global data set of the planet enabling scientists to study details of the crust, both compositionally and morphologically. One of the regions studied by Marchi and colleagues is the northern heavily cratered terrains, outlined by the black line in the crater areal density map on the left (where crater density is the number of craters at least 25 kilometers in diameter per 100,000 square kilometers). (2013) Global Resurfacing of Mercury 4.0–4.1 Billion Years Ago by Heavy Bombardment and Volcanism, v. Previous studies with older data found the most heavily cratered terrains had fewer craters (300 kilometers in diameter) gives a similar surface age.
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.The 8-km-diameter limit was chosen to exclude the majority of secondary craters and to minimize obliteration effects acting over the age of the planet.The results obtained are average ages valid for units with typical thicknesses between about 1 and 3 km and extending over an area of about 10.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.
The shapes and densities of the crater distribution curves indicate that approximately 60% of the Martian lithologic units in which these craters occur formed during the period of heavy bombardment.
Many of the small volcanoes (those generally less than 200 km diameter), including most in the Tharsis and Elysium regions, date from this period.
Simply stated, the older an area the more craters you will find.
How do scientists determine the absolute age of a geologic unit?
To establish a surface history, it is necessary to determine the sequence of various geologic events and, if possible, their duration.