A Future Moon Mission: the Lunar Seismic Network

Introduction: The New Views of the Moon initiative has integrated remotely sensed and sample data in its approach to synthesizing lunar research over the last 30+ years. This integration has clearly demonstrated what we know and, maybe more importantly, what we don t know about the Moon. Most significantly, it has helped to formulate fundamental scientific questions about the Moon that still need to be addressed. In addition, the lessons learned from the study of the Moon provide an invaluable road map for our exploration of the inner planets of the solar system. The results of the New Views initiative highlight in explicit detail just how little we know about the nature of the lunar interior. While studies of the Moon have produced the magma ocean hypothesis [e.g., 1], this cannot be adequately tested until seismic data are obtained from around the Moon and the nature of the lunar interior is evaluated in detail. The existing Apollo seismic experiment data only provide us with clues about the interior of the Moon, primarily because the seismometers were set up in a relatively restricted area on the lunar nearside. Interpretations based on these limited data are ambiguous. For example, the presence of garnet in the lunar mantle has been proposed by several authors to accommodate higher velocities in the middle mantle (>500 km) [e.g., 2-4]. This has been supported by geochemical evidence from some mare samples [5]. However, Nakamura et al. [6] and Nakamura [7] suggested that increasing the proportion of Mg-rich olivine in the lunar mantle could accommodate the higher velocities. What has become apparent is the presence of a seismic discontinuity around 500 km, albeit somewhat heterogeneous in nature [8-10] and this has been interpreted as the maximum depth of LMO melting [7,11-13]. While innovative modeling approaches have refined the original data [e.g., 8-10], comprehensive and definitive interpretations of the lunar mantle remain elusive and fundamental questions regarding lunar origin, evolution, and structure still remain unanswered. Science Drivers: The major questions we would want to answer with a Lunar seismic mission are: What is the structure and thickness of the crust on the lunar near and far sides? Are crustal structure changes gradational or are distinct domains present? Is garnet present in the middle and deep lunar mantle? Are nests producing periodic Moon-quakes present on the far side? Is the ~500 km discontinuity a moon-wide phenomenon (magmasphere vs. magma ocean)? What is the lunar core made of (sulfide, metal, ilmenite) and how extensive is it? Are the core and mantle completely solid or do plastic zones still persist? The LUNAR-A Mission: The Japanese LUNAR-A mission is scheduled for launch in 2003 and will carry 2 penetrators, each containing heat sensors and 2 seismometers (5 times as sensitive as the Apollo seismometers) [14-17]. One will be deployed on the nearside (between the Apollo 12 & 14 landing sites) and one on the farside of the Moon, with data being stored in the penetrator before being transmitted to Earth via an orbiter that passes overhead every 15 days [14-17]. The seismic experiment of LUNAR-A is designed to examine internal structure and core size and will last for one year (battery life in the penetrators). However, because of the fact that there will be only two stations, this experiment will be limited to using only Moonquakes from previously located deep Moonquake nests for interior structure studies. A Possible New Frontiers Mission: In building upon the Japanese LUNAR-A mission, a seismic network is proposed for the Moon. In this mission, it is envisaged that a minimum of 8 seismometers will be deployed around the Moon (an example of a lunar seismic network array is in Fig. 1) to cover the near-