Colleen Milbury

The fractured Moon: Production and saturation of porosity in the lunar highlands from impact cratering

We have analyzed the Bouguer anomaly (BA) of ~1200 complex craters in the lunar highlands from Gravity Recovery and Interior Laboratory observations. The BA of these craters is generally negative, though positive BA values are observed, particularly for smaller craters. Crater BA values scale inversely with crater diameter, quantifying how larger impacts produce more extensive fracturing and dilatant bulking. The Bouguer anomaly of craters larger than 93 þ47 � 19 km in diameter is independent of crater size, indicating that there is a limiting depth to impact-generated porosity, presumably from pore collapse associated with either overburden pressure or viscous flow. Impact-generated porosity of the bulk lunar crust is likely in a state of equilibrium for craters smaller than ~30km in diameter, consistent with an ~8km thick lunar megaregolith, whereas the gravity signature of larger craters is still preserved and provides new insight into the cratering record of even the oldest lunar surfaces.

Preimpact porosity controls the gravity signature of lunar craters

We model the formation of lunar complex craters and investigate the effect of preimpact porosity on their gravity signatures. We find that while preimpact target porosities less than ~7% produce negative residual Bouguer anomalies (BAs), porosities greater than ~7% produce positive anomalies whose magnitude is greater for impacted surfaces with higher initial porosity. Negative anomalies result from pore space creation due to fracturing and dilatant bulking, and positive anomalies result from destruction of pore space due to shock wave compression. The central BA of craters larger than ~215 km in diameter, however, are invariably positive because of an underlying central mantle uplift. We conclude that the striking differences between the gravity signatures of craters on the Earth and Moon are the result of the higher average porosity and variable porosity of the lunar crust.

Interactions between complex craters and the lunar crust: Analysis using GRAIL data

A high-resolution gravity map over the entire lunar surface has been derived from data acquired by the Gravity Recovery and Interior Laboratory (GRAIL) mission. Soderblom et al. (2015) showed that crater Bouguer gravity anomalies scale with crater diameter and porosity for craters in the lunar highlands. Here we extend this study globally, examining complex craters in each of the three lunar terranes: highlands, maria, and the South Pole-Aitken basin. We find that craters within South Pole-Aitken basin and in the lunar maria have statistically different Bouguer anomalies from those in the lunar highlands. These differences are best explained by differences in crustal porosity among the three terranes. Though there is still much unresolved scatter in the data, we find that no other lunar material properties (crustal thickness, density gradient, etc.) are able to improve our model fit to the data.

Evidence of large empty lava tubes on the Moon using GRAIL gravity

NASAs GRAIL mission employed twin spacecraft in polar orbits around the Moon to measure the lunar gravity field at unprecedentedly high accuracy and resolution. The low spacecraft altitude in the extended mission enables the detection of small-scale surface or subsurface features. We analyzed these data for evidence of empty lava tubes beneath the lunar maria. We developed two methods, gradiometry and cross-correlation, to isolate the target signal of long, narrow, sinuous mass deficits from a host of other features present in the GRAIL data. Here, we report the discovery of several strong candidates that are either extensions of known lunar rilles, collocated with the recently discovered “skylight” caverns, or underlying otherwise unremarkable surfaces. Owing to the spacecraft polar orbits, our techniques are most sensitive to east-west trending near-surface structures and empty lava tubes with minimum widths of several kilometers, heights of hundreds of meters, and lengths of tens of kilometers.