P53B-2116
The Effect of Pre-Impact Porosity and Vertical Density Gradients on the Gravity Signature of Lunar Craters as Seen by GRAIL
Friday, 18 December 2015
Poster Hall (Moscone South)
Colleen Milbury1, Brandon C Johnson2, Jay Melosh IV1, Gareth S Collins3, David M Blair4, Jason M Soderblom5, Francis Nimmo6, Carver Jay Bierson7, Roger J Phillips8 and Maria T Zuber9, (1)Purdue University, West Lafayette, IN, United States, (2)Massachusetts Institute of Technology, Cambridge, MA, United States, (3)Imperial College London, London, SW7, United Kingdom, (4)MIT Haystack Observatory, Westford, MA, United States, (5)Massachusetts Institute of Technology, Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (6)University of California-Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States, (7)University of California Santa Cruz, Santa Cruz, CA, United States, (8)Southwest Research Institute, Boulder, CO, United States, (9)Massachusetts Inst Tech, Cambridge, MA, United States
Abstract:
As a result of NASA’s dual spacecraft Gravity Recovery And Interior Laboratory (GRAIL) mission [Zuber et al., 2013; doi:10.1126/science.1231507], we now know that the lunar crust is highly porous and that the porosity varies laterally [Wieczorek et al., 2013; doi:10.1126/science.1231530] and vertically [Besserer et al., 2014; doi:10.1002/2014GL060240]. Analysis of complex craters located within the lunar highlands reveals that: 1) craters larger than diameter D~210 have positive Bouguer Anomalies (BAs), 2) craters with D ≲ 100 km have both positive and negative BAs that vary about the (near 0) mean by approximately ± 25 mGal, and, 3) D and BA are anticorrelated for craters with D ≲ 100 km [Soderblom et al., 2015; submitted]. Numerical modeling by Milbury et al. [2015, LPSC] shows that pre-impact porosity is the dominant influence on the gravity signature of complex craters with D ≲ 100 km, and mantle uplift dominates the gravity for those with D > 140 km. Phillips et al. [2015, LPSC] showed that complex craters located in the South Pole-Aitken (SPA) basin tend to have more-negative BAs than similar craters in the highlands. By including (pre-impact) vertical porosity/density gradients in our impact simulations, we reproduce the observed anticorrelation between BA and D for D ≲ 100 km, and the observed difference between the BAs of SPA and highland craters. We use the iSALE hydrocode including pore space compaction [Wünnemann et al., 2006; doi:10.1016/j.icarus.2005.10.013] and dilatant bulking [Collins, 2014; doi:10.1002/2014JE004708] to understand how the gravity signature of impact craters develop. In this study we vary density/porosity with depth. We find that simulations that have constant porosity with depth have a lower BA for a given crater diameter than those with varying porosity. We used two different mean porosities (7% and 14%) and found that the BA increases with increasing porosity, similar to simulations with constant porosity. Larger impacts affect relatively deeper portions of the crust, and therefore “see” lower overall porosity, so the BA increases with D less strongly than the constant porosity simulations. Our results support the observation that SPA has lower overall porosity, but higher vertical gradients, giving craters within SPA more-negative BAs than those within the highlands crust.
