Seismic Investigation Of The Moon

Renee Weber
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Seismic Investigation Of The Moon
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The lunar seismic data gathered during the Apollo missions revolutionized our understanding of the Moon’s interior and its formation and subsequent evolution. Nevertheless, many questions remain regarding the amount and distribution of seismicity, as well as the detailed structure of the crust, mantle, and core. The Planetary Decadal Survey recognizes geophysics as a high-priority science objective for a future lunar landed mission [1]. This abstract outlines the requirements levied on a landing site by the deployment of seismic instrumentation such as that currently being proposed for the New Frontiers Lunar Geophysical Network (LGN) mission [2]. Seismometers are stationary instruments that monitor the ground for seismic shaking induced by natural tectonism and meteorite impacts. The instruments themselves require good ground coupling, continuous data collection/transmission, and longevity – the ability to survive for many diurnal cycles, ideally over a period of several years. The data recorded by seismometers can be used to study the transmission of seismic energy through the planet, allowing recovery of structural properties and layering (crust, mantle, core) as well as the amount and distribution of seismicity (ground shaking), which can illuminate tectonic processes as well as quantifying potential threats to future landed assets. Known lunar seismicity consists of four primary types: shallow moonquakes (large but rare “tectonic” events similar to intra-plate quakes on Earth), deep moonquakes (small but frequent events that occur monthly according to the tidal cycle), meteorite impacts, and thermal events (localized cracking of rocks and regolith induced by large day/night temperature swings) [3]. Science return is maximized by recording as many of these events from as wide a geographical area as possible, and most mission implementations assume a networked deployment with N>1 instruments [4]. The Apollo Passive Seismic Network deployed four instruments on the near side of the Moon (Fig. 1), which telemetered data directly back to Earth. The LGN will have a node deployed on the far side to ensure as global coverage as possible. Site selection is dictated primarily by the ability to detect the maximum number of potential sources (which also drives instrument development through sensitivity requirements), the suitability for instrument deployment, and the ability to provide a geographically wide (global) footprint (e.g. [5]). As seismometers are often packaged with other geophysical instruments (e.g. heat flow probes, laser retroreflectors, magnetometers), previous mission proposals included the additional constraints of being well away from terrane boundaries (in order to facilitate unambiguous heat flow data for distinct lunar terranes), and increasing the span of the current network of retroreflectors (Fig. 1). For this reason, we favor sites that are either internal or external to the Procellarum KREEP Terrane (not at boundaries), and/or close to the lunar limbs. A site in the southwest quadrant of the near side maximizes chances for detection of core-transmitted seismic phases from the north-eastern concentration of active deep moonquake clusters (Fig. 2). A far side station would provide the opportunity to measure seismicity, heat flux, and internal structure not obtained by Apollo. [1] Vision and Voyagers for Planetary Science in the Decade 2013-2022 - a report of the National Research Council. [2] Kedar et al. (2017) this meeting. [3] Ewing, M. et al. (1971) Highlights of Astronomy, De Jager (ed.), 155 – 172. [4] Neal, C. R. et al. (2010) Abstract submitted to the Ground-based geophysics on the Moon conference (LPI). [5] Hempel et al. (2012) Icarus 220, 971 – 980. [6] Dickey, J. O. (1994) Science 265, 482 – 490.
R.C. Weber (1), C.R. Neal (2), B. Banerdt (3), S. Kedar (3), N. Schmerr (4), M. Panning (3), M. Siegler (5,6) 1. NASA Marshall Space Flight Center, 2. University of Notre Dame, 3. NASA Jet Propulsion Laboratory, 4. University of Maryland, College Park, 5. Planetary Science Institute, 6. Southern Methodist University