LANDSCAPE EVOLUTION ON THE MOON: CRATER DEGRADATION RATES IN THE COPERNICAN AND ERATOSTHENIAN PERIODS

Compared to the complexity of the Earth and Mars, landform evolution on the Moon is comparatively straightforward. Since the end of widespread maria emplacement (>3 Ga), impact cratering has dominated lunar topographic evolution. As has been shown in a number of pioneering studies (e.g., by Soderblom, Boyce, Basilevsky, Craddock and Howard), this evolution can be modeled as a diffusive process. Recent high-resolution, high-precision measurements of lunar topography obtained using the Lunar Orbiter Laser Altimeter (LOLA) and derived from stereo image data allow improved quantitative constraints on the rate of topographic diffusion. I approach this problem by modeling the observed degradation state D=1-3 km craters superposed on the lunar maria. Approximately 5% of the surface area of the lunar maria have been measured and analyzed to date (515 craters) although more measurements are being actively pursued.

From this preliminary data, the average diffusion rate on the Moon over the last ~3 Ga is estimated as κ=0.005–0.008 m²/Kyr, which is ~160× less than what is typically measured in the western United States (κ~1 m²/Kyr). This estimate of the diffusivity can be converted into an average erosion rate, ~0.3–0.5 mm/Myr. This is similar to, but somewhat larger than, the characteristic lunar erosion rate reported by Craddock and Howard (0.2 mm/Myr).

It is interesting to consider the implications of this rate for lunar landform evolution. Over 3 Ga, a ~300 m diameter crater would be reduced to 6% of its original depth. Smaller craters would be expected to be effectively removed from the surface in this length of time; clearly, crater removal is important to consider when using small craters to estimate surface ages from crater statistics. Other features of interest such as sinuous rilles are also are likely to be considerably evolved from their initial form, as substantial infilling and wall retreat is expected.

Future measurements may provide a viable pathway for testing models for spatial and temporal variation in the rates of landform evolution on the Moon.