Dr. Tiantian Liu

The purpose of the work is to understand the long-term effect of the impact gardening process of impact melt at the near-surface of the Moon. Current computational capabilities make it feasible to simulate accumulation and displacement of impact melt by the Monte Carlo model, and thus to build a picture of the time evolution laterally and longitudinally. 

By tracking the creation and presence of melt, we can understand how the impact melt is mixed into the regolith, and what abundances of melt with different ages could be expected in surface samples. Comparing the simulation results with the age-dating results based on the lunar samples, the knowledge of lunar impact history will be improved. This aspect is very much tied to our previous work, where we studied crater formation rates and concentrated on refining the crater-dating method.

Most recent results:

Lunar megaregolith mixing by impacts: diffusion of the highland materials and the implications on the origin of the nonmare component in the lunar mare samples

Tiantian Liu^1,2, Greg Michael², Kai Wünnemann^2,3, Jürgen Oberst^1,4

To investigate the origin of abundant nonmare component found in the collected lunar mare soil samples, we developed a numerical model to investigate the diffusion of nonmare material on the Moon. It is found that almost the entire mare regions are mixed with nonmare material with the average fraction of ~0.2. In the regions near the mare/highland contact, the nonmare abundance has no strong correlation with the distance with the fraction of ~0.3, but in the regions further than ~100 km away from the boundary, it falls rapidly with the fraction smaller than 0.05. The contact of the mare and highlands regions is still apparent on present, although the boundary of the older mare surface is getting blurred. By comparing with the estimate of the lunar mare samples, we speculate the plausible origin of the collected nonmare component: more than 50% of the nonmare component in the Apollo 15 and 17 samples is caused by the downslope slumping or lateral transport of the nearby massifs; more than 75% of nonmare component in the Apollo 12 mare samples is derived from the mixing of the ejecta of Copernican crater; both the Apollo11 and Luna 16 samples contain the comparable amount of the vertically and laterally transported nonmare component.

Figure (a) Global distribution of nonmare component, where the darker color indicates the less abundance. Red stars show the location of sampling sites. (b) Histogram of the fraction of nonmare component of the mare regions. (c) Predicted fraction of nonmare component at the sampling sites versus distance from the mare/highland boundary, where the grey dashed rectangles are the expected value and the red dots are the results from the lunar soil samples (Rhodes 1977).