| Abstract: | Careful examination of seven giant front face basins on the moon will show that the basins most densely covered by younger craters are the oldest. With increasing age they exhibit lower external rims, not scarp heights. The rims are progressively more subdued with age. This paper proposes that absolute ages for these basins can be obtained by calculating an effective viscosity of the moon's outer layers from 3.85 × 109 y, the date of Imbrium, to the present. Similarly viscosity measures can be determined for the oldest basin. To do this we need the present and the original rim heights. The present values are observed. The original heights are calculated by extrapolating the relationship between diameter and rim height for normal Class I craters. It turns out that as long as the larger basins have proportionately higher original heights than the smaller, the absolute values are of little importance and the ages are definitive. There are many similar families of viscosity changes with age and they yield similar absolute ages. In each case equations relating viscosity changes with age were derived and for each basin there is only one age that will yield the final rim height. Ages, × 109 y, of the basins are: Orientale 3.82, Imbrium 3.85, Crisium 4.00, Nectaris 4.07, Serenitatis 4.14, Humorum 4.23 and an Unnamed basin between Werner and the Altai ring 4.30. |
