| Abstract |
The first attempts in planetary seismology started 50 years ago, with the launches of the Ranger-3-4-5 US missions to the Moon, which all failed... Success was reached however 40 years with the Apollo missions, which deployed a network of 4 passive seismometers on the Moon, at landing sites 12, 14, 15 and 16. A seismometer was also deployed on Apollo 11 and a gravimeter on Apollo 17. More than 12500 events were cataloged during the operation and although this network stopped its operation in 1977, the analysis of the data is still ongoing, and has led to the determination of many features of the lunar interior, as well properties of the seismic sources.
The first part of the lecture summarizes the state of the art of the lunar interior determination achieved with the Apollo data, as well as the major differences of lunar seismology as compared to Earth seismology, especially in terms of seismic wave propagation and seismicity. We will present the major results achieved in term of lunar structure determination, including the recent discovery of seismic core phases. Special focus will be given to the crustal structure, both in term of thickness and lateral variation, and to the core structure in term of radius, core state, temperature and composition. We also discuss the existence of possible discontinuities in the mantle, proposed by some early seismic models but challenged by others, and interpreted as the possible limit of an early magma ocean. A critical analysis will however be also provided, addressing the limitations of these models, their complementarities with those generated by other source of geophysical data (e.g. gravimetry, electromagnetic sounding, geodesy) as well as the typical uncertainties remaining on the lunar structure.
The second part of the lecture will be devoted to our present knowledge on seismic sources in planetary seismology, which, for might be either related to quakes, impacts and, for planets with atmosphere, e.g. Mars and Venus and possibly Jupiter, continuous excitation from atmospheric turbulences. After a rapid comparative presentation of the expected quake rates on the terrestrial planets, a special focus will be given to the two later types of sources. Concerning impacts, we will focus on two extreme cases: The Shoemaker Levy-9 impact on Jupiter and the impacts detected by the Apollo seismic network. In both case, we will present the main physical properties of the seismic sources (and associated shock wave) associated to an impact on solid surfaces or in atmosphere including magnitude and duration. For the Moon, we will use the calibrated seismic signals generated by artificial impacts (e.g. LEM or SIVB), which craters have been furthermore be characterized by the recent NASA LRO mission. Such calibration enables to estimate the flux of impactors on the Moon as well as the expected continuous micro-seismic noise generated by the continuous impacts on our satellite. The methodology for the extrapolation of these results in the prediction of the rate of seismically detectable impacts on Mars, Venus and asteroids will be finally given. Atmospheric source will, on the other hand, be presented mainly by using the case of Jupiter and Mars, with, in the later case, an estimation of the micro-seismic noise associated to the atmospheric interaction.
The last part of the lecture will be a description of future missions or projects aiming to deploy seismometers or to perform remote sensing seismology, with a special attention on the instrumental constraints. We conclude by presenting the potential challenges, science objectives and discoveries of this future step in the seismic exploration of our solar system. |
