Lunar Megaregolith Structure Revealed by GRAIL Gravity Data

We use gravity data from NASA's GRAIL mission to characterize the porosity structure of the upper lunar crust. We analyze the gravitational anomalies produced by the porosity of craters with diameters D between 10 and 30 km. We find that the gravitational signature of craters changes significanficantly at D = 16.4(-0.6)(+1.4) km, which is related to a discrete change in porosity at a depth similar to 3-5 km. We propose that this discrete porosity change reveals the location of the boundary between large-scale basin ejecta and the deeper less porous portion of the megaregolith, known as the structurally disturbed crust. The ejecta thickness can help constrain models of material transport and mixing on the Moon and, because the ejecta layer acts as an insulating blanket, models of heat flow and magmatism. Plain Language Summary The amount of porosity, in a planet or Moon's crust controls how magma and heat are transferred through it. We use gravity data from NASA's GRAIL mission to determine the vertical distribution of porosity in the upper lunar crust, shedding light on the thermal and magmatic history of the Moon. We analyzed the gravitational signature of lunar craters because their gravity signature depends on the density of the target material, which depends on the crustal composition and porosity. We consider lunar craters 10-30 km in diameter as these craters are most sensitive to the shallow depths that we are investigating. We find that the gravitational signature of craters changes significantly at diameters of around 16 km. Relating the size of the craters to the depth of the porosity induced by the impact, we find a porosity boundary at a depth of 3-5 km in the lunar crust. This boundary is most likely the predicted boundary between two of the most broken-up layers in the upper lunar crust. This is the first time this boundary has been observed with global gravity data helping us understand the origin and evolution of porosity in the lunar crust.