Absolute model ages from lunar crater morphology

The degradation state of an impact crater is an indicator of its age. Previous workers have used crater degradation states to estimate ages of surfaces or geomorphological features; one example is the degree of freshness method developed by Pohn and Offield (1970). Here we attempted to produce an empirical calibration that yields absolute model ages based upon the degree of freshness technique for craters ~8–20 km in diameter. To produce the calibration, we first selected 15 craters with degree of freshness values ranging from 2.5 to 6.3. Next, we used the Kaguya Terrain Camera data to measure crater density on the ejecta of these craters, from which absolute model age could be calculated. The resulting absolute model ages ranged from 0.9 to 4.0 Ga. We used two linear regressions to describe the relationship between the absolute model age and degree of freshness of the craters. We fitted each trend with two linear least-squares regressions, where the first regression represents craters with a degree of freshness from 0.0 to 4.9 and the second regression from 5.0 to 7.0. The 95% confidence belt shows that the calibrations are accurate to ±0.5 Ga to ±1.1 Ga for the fresh crater regression (5.0–7.0) and slightly more accurate, to ±0.3 Ga to ±0.1 Ga, for the degraded crater regression (0.0–4.9). However, the degraded crater regression is likely based upon craters with continuous ejecta that are crater saturated, thus implying that craters with a degree of freshness <5.0 is likely to be >3.8 Ga.