白石 浩章

Seismic data obtained over a broad frequency range are very useful in investigation of the internal structures of the Earth and other planetary bodies. However, planetary seismic data acquired through the NASA Apollo and Viking programs were obtained only over a very limited frequency range. To obtain effective seismic data over a broader frequency range on planetary surfaces, broadband seismometers suitable for planetary seismology must be developed. In this study, we have designed a new broadband seismometer based on a short-period seismometer whose resonant frequency is 1 Hz for future geophysical missions. The seismometer is of an electromagnetic type, light weight, small size and has good shock-durability, making it suitable for being loaded onto a penetrator, which is a small, hard-landing probe developed in the LUNAR-A Project, a previous canceled mission.We modified the short-period seismometer so as to have a flat frequency response above about 0.1 Hz and the detection limit could be lowered to cover frequencies below the frequency. This enlargement of the frequency band will allow us to investigate moonquakes for lower frequency components in which waveforms are less distorted because strong scattering due to fractured structures near the lunar surface is likely to be suppressed. The modification was achieved simply by connecting a feedback circuit to the seismometer, without making any mechanical changes to the short-period sensor. We have confirmed that the broadband seismometer exhibits the frequency response as designed and allows us to observe long-period components of small ground motions. Methods to improve the performance of the broadband seismometer from the current design are also discussed. These developments should promise to increase the opportunity for application of this small and tough seismometer in various planetary seismological missions. (C) 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.

Information on the lunar central core; size, current state and composition; are key parameters to understand the origin and evolution of the Moon. Recent studies have indicated that possible seismic energies of core-reflected phases can be identified from past Apollo seismic data, and core sizes are determined, but we have still uncertainties to establish the lunar core parameters. We, therefore, plan to detect seismic phases that pass through the interior of the core and/or those reflected from the core-mantle boundary to ensure the parameters using new seismometers for future lunar soft-landing missions such as SELENE-2 and Farside Explorer projects.As the new seismometers, we can apply two types of sensors already developed; they are the Very Broad Band (VBB) seismometer and Short Period (SP) seismometer. We first demonstrate through waveform simulations that the new seismometers are able to record the lunar seismic events with S/N much better than Apollo seismometers. Then, expected detection numbers of core-phases on the entire lunar surface for the two types of seismometers are evaluated for two models of seismic moment distributions of deep moonquakes using the recent interior model (VPREMOON).The evaluation indicates that the VBB has performance to detect reflected S phases (ScS) from the core-mantle boundary mainly on the lunar near-side, and the P phases (PKP) passing through the interior of the core on some areas of the lunar far-side. Then, the SP can also detect PKP phases as first arrival seismic phase on limited regions on the lunar far-side. If appropriate positions of the seismic stations are selected, core-phases can be detected, allowing us to constrain the origin and evolution of the Moon with future lunar soft-landing missions. (C) 2013 Elsevier Ltd. All rights reserved.

We developed a seismometer system for a hard landing "penetrator" probe in the course of the former Japanese LUNAR-A project to deploy new seismic stations on the Moon. The penetrator seismometer system (PSS) consists of two short-period sensor components, a two-axis gimbal mechanism for orientation, and measurement electronics. To carry out seismic observations on the Moon using the penetrator, the seismometer system has to function properly in a lunar environment after a hard landing (impact acceleration of about 8000 G), and requires a signal-to-noise ratio to detect lunar seismic events. We evaluated whether the PSS could satisfactorily observe seismic events on the Moon by investigating the frequency response, noise level, and response to ground motion of our instrument in a simulated lunar environment after a simulated impact test. Our results indicate that the newly developed seismometer system can function properly after impact and is sensitive enough to detect seismic events on the Moon. Using this PSS, new seismic data from the Moon can be obtained during future lunar missions. © 2008 Elsevier Ltd. All rights reserved.

Further study for the planning of the post SELENE mission has been discussed by a dedicated working group. As the extension of the SELENE-B study [Okada, T., Sasaki, S., Sugihara, T., et al. Lander and rover exploration on the lunar surface: a study for SELENE-B mission. Adv. Space Res. 37, 88-92, 2006] which proposed in-situ geological investigations using a robotic rover and a static lander, this report newly proposes a revised configuration which enhances the scientific field of view. The spacecraft of this mission, "SELENE-II", is designed as a full payload of H-II launch vehicle, while the former study was designed as a half payload of the same vehicle. This expansion of capacity enabled us to increase the payload mass of the lander to deploy geophysical instruments and to land on a wider region on the Moon including polar regions. We also gained the opportunity to deploy two penetrators in order to make a wide network for geophysical observations. In the new configuration, this mission can install three stations of a global seismic network, which will be able to refine the deeper structure of the Moon. In this study also, a new type of deployment system, whose mechanical interface is much simpler than that of the LUNA R-A mission, is preliminarily designed.The selection of the landing site is still undergoing discussion, but the lander is required to operate as long as about one year and more for the geophysical observations, especially for seismology. In order to realize this, one possible idea is to land in permanently sunlit regions. Polar regions also have a benefit from the geological point of view; the north polar region is a typical high land area and the south one is a part of or adjacent to the South Pole Aitken (SPA), where the deeper part of the crust or the mantle material are expected to be collected.In addition to the lander scientific instruments designed previously (Okada et al., 2006.) for the geological survey, a broad band seismometer is considered to be deployed prior to other geophysical instruments and we expect it to provide us with information about the bulk layered structure with only one station if free oscillations are successfully detected. Even if the free oscillations cannot be detected, the dispersion of surface waves not affected by scattering of the regolith or megaregolith layer brings information to understand the crustal and upper mantle structures.Several landing missions are planned by NASA, CNSA, ISRO, and ESA by 2010-2015 during which the operational period is possible to be overlapped by the different missions. This must be a great opportunity to make larger network observations in the future. It must be a great opportunity to start international collaboration in various ways for the upcoming lunar exploration era. (c) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.