Why the next generation of Moon exploration needs a global seismic network C. R. Neal Dept. of Civil Eng. and Geological Sciences, University of Notre Dame, Notre Dame, IN 46556, USA (
[email protected] / Fax: 1-574-631-9236 / Phone: 1-574-631-8328
The 4-station Apollo Passive Seismic Experiment (APSE) network was completed in April 1972, and operated until it was switched off on 30 September 1977. During this time, the network demonstrated that the Moon exhibits seismic activity on a similar scale to that of an intraplate setting on Earth. However, there are significant gaps in our understanding of lunar seismicity and what it tells us about the lunar interior. For example, collection of seismic data that pass through the interior of the Moon are critical for determining the size, state and composition of the lunar core, the nature of the deep lunar mantle, determining the global extent of discontinuities, and mapping the extent of crust/mantle heterogeneities. Additionally, with the advent of a new era of lunar exploration and potential colonization, understanding the nature and extent of lunar seismicity is now required in terms of risk assessment for a permanent lunar habitat. The data needed to address all of these issues must be collected using a network of seismometers that is global in coverage. Lunar Seismic Events. There were four types of lunar seismic event defined by the APSE network. 1) Thermal moonquakes – the smallest magnitude event (associated with stresses induced by diurnal temperature changes at the lunar surface); 2) Deep moonquakes - magnitude ≤2 (>7,000 having been recognized occurring 700-1,200 km within the Moon and associated with Earth’s tidal pull); 3) Meteoroid impacts (>1,700 events representing masses of 0.1 to 1,000 kg were recorded); 4) Shallow moonquakes - strongest type of event, with 7 of the 28 recorded events being magnitude 5 or greater although the exact causes of such quakes are not known (focal depths 50-200 km, but exact depths and locations are unknown as all recorded events were outside the APSE network). Relevance. Apart from a direct impact from a meteorite, shallow moonquakes offer the greatest potential seismic risk to a permanent lunar habitat, but the amount of epicentral ground motion associated with such events is difficult to estimate. Estimates of ground acceleration at the epicenter of a magnitude 5.7 shallow moonquake is estimated to be ∼ 0.75 m s−2 for a focal depth of 25 km and ∼0.22 m s−2 for a focal depth of 100 km. However, the estimates could be meaningless because the calculations were conducted using formulations for earthquakes and there are distinct differences in seismic wave transmission between the Moon and Earth. For example, the maximum signal from a shallow moonquake can last up to 10 minutes with a 1
slow tailing off that can continue for hours, indicating that damping is less efficient in the Moon than it is in the Earth. In other words, seismic energy is more efficiently propagated through the Moon, especially at higher frequencies. This is particularly significant for shallow moonquakes as they contain more energy at high frequencies than earthquakes of comparable total energy. In addition, the scattering properties of the regolith need to be fully evaluated. A Lunar Seismic Network: At this time, it is suspected, but not known, that seismic events could seriously compromise a permanent lunar habitat. In order to fully evaluate this risk, as well as answer fundamental science questions regarding the lunar interior, a long-lived, global lunar seismic network needs to be established. In order to achieve this, technological issues such as deployment and low mass power supplies that can supply consistent power over a period of at least 6 years need to be addressed.
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