9:00 AM - 9:15 AM
[MIS12-01] Structures and formation age of lunar linear gravity anomalies constrained by impact simulations
Keywords:moon, gravity anomaly, impact
High-resolution lunar gravity data were acquired by NASA's GRAIL gravity mission, and positive linear gravity anomalies sometimes extending over 100 km were discovered. These gravity anomalies are interpreted as intrusions denser than the crust and indicate that the lunar surface was subject to tensile stress in the past. Since such a stress field is likely to occur during the time of lunar expansion and to affect the mechanism of intrusion formation, estimation of the formation age and shapes of the intrusions possibly constrain the ancient thermal history of the Moon. Although the depths of the intrusions have been estimated from the inversion of gravity anomalies, those depths are possibly overestimated in the conventional calculation method. On the other hand, the formation ages have been determined from the crosscutting relationship with giant basins. However, a qualitative discussion has been made only for the linear gravity anomalies near Crisium basin, and other linear gravity anomalies remain without quantitative discussion. So, we investigate two examples, Roche and Rowland craters, in this study.
In this study, we estimate the effect of crater formation directly above the underlying linear gravity anomaly using the iSALE hydrocode and attempt to constrain the depth and age of the intrusive body. First, we assume the depth and width of the intrusions that match the Bouguer gravity anomalies for various densities. Then we calculate the meteoroid collision numerically with iSALE and trace the movement of the subsurface intrusion. By comparing the simulated gravity anomalies after the crater formation with the observation, we can constrain the depth and age.
Roche crater is an example of craters above linear gravity anomalies. The linear gravity anomalies cut Roche crater in a northwest-southeast direction, and the Bouguer gravity anomalies are about 40 mGal lower inside of the crater than outside. This decrease of gravity anomalies is likely due to impact excavation, indicating that the top depth of intrusive rock is shallower than 10 km below the surface. This depth is shallower than the previous estimate of 20 km. Also, because Roche crater modified the structure of preexisting intrusion, the age of the gravity anomaly is older than the formation of Roche crater and is before Nectarian. The second example is Rowland crater. The linear gravity anomaly does not extend into the crater but stops at the rim. Our numerical simulation shows that the excavation depth of Rowland crater is too shallow to erase the linear gravity anomaly completely. Thus, this gravity anomaly is likely to have formed after Rowland crater and is younger than Nectarian. Based on the comparison between the numerical calculation and the Bouguer gravity anomaly data, we will discuss the formation age and possible structure of the linear gravity anomaly in this presentation.
In this study, we estimate the effect of crater formation directly above the underlying linear gravity anomaly using the iSALE hydrocode and attempt to constrain the depth and age of the intrusive body. First, we assume the depth and width of the intrusions that match the Bouguer gravity anomalies for various densities. Then we calculate the meteoroid collision numerically with iSALE and trace the movement of the subsurface intrusion. By comparing the simulated gravity anomalies after the crater formation with the observation, we can constrain the depth and age.
Roche crater is an example of craters above linear gravity anomalies. The linear gravity anomalies cut Roche crater in a northwest-southeast direction, and the Bouguer gravity anomalies are about 40 mGal lower inside of the crater than outside. This decrease of gravity anomalies is likely due to impact excavation, indicating that the top depth of intrusive rock is shallower than 10 km below the surface. This depth is shallower than the previous estimate of 20 km. Also, because Roche crater modified the structure of preexisting intrusion, the age of the gravity anomaly is older than the formation of Roche crater and is before Nectarian. The second example is Rowland crater. The linear gravity anomaly does not extend into the crater but stops at the rim. Our numerical simulation shows that the excavation depth of Rowland crater is too shallow to erase the linear gravity anomaly completely. Thus, this gravity anomaly is likely to have formed after Rowland crater and is younger than Nectarian. Based on the comparison between the numerical calculation and the Bouguer gravity anomaly data, we will discuss the formation age and possible structure of the linear gravity anomaly in this presentation.