Ejecta deposit thickness, heat flow, and a critical ambiguity on the Moon

[1] The Apollo lunar heat flow measurements gave values of 21 and 16 mW m 2 which, after extrapolation based on thorium abundances, yields a global estimate of 18 mWm . A refinement of the assumptions of the subsurface structure and the resulting focusing of heat flux later led to a revision of the global value to 12 mWm . We think that to date none of the models linking the Apollo heat flow measurements has sufficiently highlighted a critical source of ambiguity. Little attention has been paid to the full magnitude of the uncertainty in these measurements caused by near–surface Thorium abundances and the local thickness of the ejecta blanket generated by the Imbrium impact. In a simple study we show that lunar heat flow is contingent upon the thickness of the ejecta blanket of the hypothetical impact. Amodel with an exponential decrease of Th concentration with depth can explain the difference in surface heat flow between the Apollo 15 and the Apollo 17 measurements. A constant Thorium concentration within the ejecta layer amplifies this effect. The variation in local surface Th abundance, if taken as representative of the subsurface Th distribution within the ejecta blanket, amplifies the uncertainty. We conclude that further measurements are essential for making well-founded statements about the subsurface abundance of radioactive elements, mantle heat flux and the thermal state of the Moon. Citation: Hagermann, A., and S. Tanaka (2006), Ejecta deposit thickness, heat flow, and a critical ambiguity on the Moon, Geophys. Res. Lett., 33, L19203, doi:10.1029/2006GL027030.