The International Lunar Network: science goals and landing site selection

The Moon is the only extraterrestrial object in our Solar System that has been studied by a suite of in situ geophysical instruments that included seismometers, heat flow probes, magnetometers, and laser ranging retroreflectors. Combinations of these instruments made up the Apollo Lunar Surface Experiment Packages that were deployed at the six Apollo landing sites. Data from these stations were transmitted to Earth until September 1977, and the three passive Apollo retroreflectors, in concert with two retroreflectors on the Lunokhod 1 and 2 rovers, continue to be used today. As a result of these geophysical data, our knowledge of the Moon’s origin and evolution is greatly superior to that of any other extraterrestrial object. Nevertheless, after decades of scientific analyses, fundamental questions remain concerning the structure and constitution of the Moon’s interior. For example, it was determined that the Moon is highly heterogeneous, but with only a handful of stations on the lunar nearside hemisphere, it was not possible to fully delineate the full extent of lateral and vertical variations in lunar structure and composition. In addition, part of the Moon’s internal seismic activity was found to be correlated with the tides raised on the Moon, but given that the largest tidal periodicity (18.6 years) is much greater than the data collection period of the ALSEP network, the origin and characteristics of these moonquakes remain only poorly known. It is in this context that an international partnership was created with the goal to establish a robotic set of geophysical monitoring stations on the surface of the Moon. The creation of a long-lived geophysical network (comprising of several nodes) is no easy undertaking for any single national space agency, and the International Lunar Network (ILN) was chartered with the goal of maximizing the scientific return obtained from individual landers operating in a network manner. As part of the ILN charter, several working groups were created with the purpose of defining a set of core scientific instruments, communications standards, and site selection criteria. The core instruments working group identified four instruments to be included on any future ILN node: a seismometer, heat flow probe, electromagnetic sounder, and laser retroreflector. The focus of this site selection working group is to identify classes of sites and network geometries that would best address lunar science goals. Though our work is preliminary, we have identified two science goals that place stringent constraints on the selection of the initial landing sites. One goal is to elucidate the asymmetric thermal evolution of the Moon, and three sites that are necessary for the heat flow experiment include the Procellarum KREEP terrain, the Feldspathic highlands terrane, and a region where the crust is thin or absent. A second goal is to determine the size of the lunar core, and the key seismic site selection criteria include the identification of reflected and refracted core phases. This paper will represent an update of the site selection working group activities, and our work to optimize a lunar network for all core scientific instruments.