Impact Cratering Dating Nathan Marsh Relative Dating Simple

Impact Cratering Dating Nathan Marsh

Relative Dating • • • Simple but not as informative Measures the crater densities (craters per square kilometer) Generally a less cratered area is younger than a more cratered area

• Lab analysis of Apollo samples of the Mare provided data on absolute dates when lava solidified • Provided data on the relationship between age and number of craters

Reference points in dating • • The Apollo and Luna missions analyzed specific dates on certain areas of the moon Late Heavy Bombardment craters ~3. 9 Gy Light-toned Terrae (highlands) – plagioclase feldspar ~4. 5 Ga Dark-toned Mare – volcanic basalts Maria have ~200 times fewer craters Mare ages 3. 1 -3. 8 Ga Terrae basin ages all 3. 8 -4. 0 Ga

Absolute Dating Production Function • • A more exact way to date surfaces This is the ratio of smaller craters created for every large one Plotted in log-log space Vertical position is related to age

Absolute Dating • • This can be estimated if the rate of formation of craters is known The crater size/frequency distribution is displayed on a “relative” (R) plot D- the geometric mean diameter of the size N- Number of crates in the size A is area counted Bu is the upper limit of the size Bl is the lower limit

• • Absolute ages allow comparison of events on different planets The impact crater size distributions for Mercury and other inner solar system objects seem to be consistent with an early cataclysmic heavy bombardment. • The comparison of the R plots on the highland cratering records on Mercury, the Moon and Mars describes the nature of the objects that were impacting during the period of heavy bombardment within the inner solar system

Crater Saturation • • • Eventually a point is reached when the craters are so crowded that new impacts destroy craters as fast as they make new ones The surface reaches a phase known as steady state cratering, or crater saturation During this period, no new information can be deciphered about the age other than the surface is at least ~4 billion years because only during an early intense bombardment was a surface able to acquire enough hits to become saturated

The Late Heavy Bombardment • Crater dating is used to estimate the relative age of a planetary object based on crater statistics • The manned Apollo missions returned iron rich lunar rocks that dated back to ~3. 9 billion years • Same time as the Late Heavy Bombardment

• • • Thought that the bombardment came from the asteroid belt and effected all inner solar system objects including Mercury – May have been ejected from the asteroid belt by the combined gravitational fluctuations of Jupiter – Or a a giant collision in the asteroid belt Late heavy bombardment is used as a reference point Not many large craters (>100 km) are made, mostly smaller impacts

Crater Retention Ages • • Erosion, dust movement and lava flows can completely bury craters Smaller craters may only last 1 Myr while larger ones will last 100 Myrs on Mars • Lunar craters are eroded away at a much slower rate due to smaller impacts and the walls slump into the cavity and fill it up

Conclusion • Relative and Absolute dating – Crater count can date a surface – Crater ages are linked relative to position on other craters – Apollo missions convert the relative data to absolute ages • Crater Saturation – Craters become saturated to a point beyond ability to date past ~4 Ga • Late Heavy Bombardment – May have come from the asteroid belt – Caused by collision of gravity of Jupiter • Dating Difficulty – Craters are “eroded” by other impacts and lava flows