[P-PS23_2PM1] Lunar science and exploration

[PPS23-15] Re-examination of Excavation Cavity of the Impact Basins of the Moon based on GRAIL based Crustal Thickness Model

*Yoshiaki ISHIHARA¹, Ryosuke NAKAMURA² (1.JAXA, 2.AIST)

Keywords:Impact Basin, Excavation Cavity, Melting Cavity, Moon

Large impact features, whose diameters are more than hundreds of kilometers, are called impact basins. Large impact basins can provide comparatively clear information of the cratering process and/or constrain the lunar thermal history. The internal or subsurface structures of basins can be assessed through an analysis of their associated gravitational and topographic signatures. The recently Kaguya/SELENE mission has improved the crustal thickness model not only for the nearside but also for the farside based on the first direct farside gravity and global topography mapping. Moreover most recent GRAIL mission vastry improved spatial resolution and overall accuracy of the lunar gravity models and lunar crustal thickness models. The GRAIL crustal thickness model gives us the opportunity to re-analyse excavation depth and diameter of basin forming impact processes anywhere on the Moon with improved accuracy. This study uses the GRAIL crustal thickness model, to reconstruct the excavation cavity geometry of large impact basins on the Moon.Our method of reconstructing the excavation cavity of large impact basins is fairly simple. We assume that the thinned crust and uplifted Moho beneath features is a direct consequence of (1) the amount of crustal material excavated during the cratering process and (2) the subsequent rebound of the crater (basin) floor. We first construct azimuthally averaged profiles for the surface topography, mare thickness and subsurface structure of the Moho for each basin. Next, we restored the uplifted Moho and overlying crust to its pre-impact position. Estimating procedures of pre-impact position is almost the same as previous analysis. After removing mare fill, this process resulted in a roughly parabolic surface depression, that we interpret as being the first-order representation of the basin's excavation cavity.One of the most important values of understanding the large impact basin is the depth-to-diameter ratio of the excavation cavity. We examine the depth versus the diameter of our reconstructed excavation cavities (excluding the Imbrium Basin and the South Pole-Aitken Basin). It seems that up to 400 km cavity diameter, the depth (hex) and diameter (Dex) are linearly related. Further more, the linear relationship (hex/Dex=0.079+/-0.006) is almost consistent with, though slightly smaller than, the value for craters orders of magnitude smaller in size (hex/Dex=0.1), suggesting that proportional scaling is valid for basin scale impact structures except the largest impact structures on the Moon. One of the reasons of smaller depth-to-diameter ratio are probably effects due to the post impact modifications. Impact basins which has excavation cavity diameter larger than 400 km show the different state. The average crustal thickness of GRAIL lunar crustal thickness model is 34 to 43 km. So excavation cavity diameter of 400 km is located the ragime boundary between the excavation/melting cavity within crust ragime and the excavation/melting cavity exeed the Moho interface ragime.