Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System
The absolute ages of cratered surfaces in the inner solar system, including Mars, are derived by extrapolation from the impact flux curve for the Moon which has been calibrated on the basis of absolute ages of lunar samples. We reevaluate the lunar flux curve using isotope ages of lunar samples and the latest views on the lunar stratigraphy and the principles of relative and absolute age dating of geologic surface units of the Moon. The geological setting of the Apollo and Luna landing areas are described as far as they are relevant for this reevaluation. We derive the following best estimates for the ages of the multi-ring basins and their related ejecta blankets and present alternative ages for the basin events (in parentheses): 3.92 +/- 0.03 Gyr (or 3.85 +/- 0.05 Gyr) for Nectaris, 3.89 +/- 0.02 Gyr (or 3.84 +/- 0.04 Gyr) for Crisium, 3.89 +/- 0.01 Gyr (or 3.87 +/- 0.03 Gyr) for Serenitatis, and 3.85 +/- 0.02 Gyr (or 3.77 +/- 0.02 Gyr) for Imbrium. Our best estimates for the ages of the mare landing areas are: 3.80 +/- 0.02 Gyr for Apollo 11 (old surface), 3.75 +/- 0.01 Gyr for Apollo 17, 3.58 +/- 0.01 Gyr for Apollo 11 (young surface), 3.41 +/- 0.04 Gyr for Luna 16, 3.30 +/- 0.02 Gyr for Apollo 15, 3.22 +/- 0.02 Gyr for Luna 24, and 3.15 +/- 0.04 Gyr for Apollo 12. The ages of Eratosthenian and Copernican craters remain: ~ 2.1 (?) Gyr (Autolycus), 800 +/- 15 Myr (Copernicus), 109 +/- 4 Myr (Tycho), 50.3 +/- 0.8 (North Ray crater, Apollo 16), and 25.1 +/- 1.2 (Cone crater, Apollo 14). When plotted against the crater densities of the relevant lunar surface units, these data result in a revised lunar impact flux curve which differs from the previously used flux curve in the following respects: (1) The ages of the stratigraphically most critical impact basins are notably younger, (2) the uncertainty of the calibration curve is decreased, especially in the age range from about 4.0 to 3.0 Gyr, (3) any curve for ages older than 3.95 Gyr (upper age limit of the Nectaris ejecta blanket) is abandoned because crater frequencies measured on such surface formations cannot be correlated with absolute ages obtained on lunar samples. Therefore, the impact flux curve for this pre-Nectarian time remains unknown. The new calibration curve for lunar crater retention ages less than about 3.9 Gyr provides an updated standard reference for the inner solar system bodies including Mars.