Planetary Seismology

International audienceIn-situ extra-terrestrial seismic measurements have, to date, been limited to data collected during the lunar Apollo missions. These data have provided significant contributions to our understanding of the interior structure of the Moon, and by inference, permissible thermal and mineralogical models. In addition, the seismic activity recorded by the Apollo stations provides information on both internal (moonquakes) and external (meteoroid) sources. The seismic activity of other bodies, notably Mars and Venus, has never been measured. It can however be estimated, subject to approximations regarding the thermal structure of the lithospheres of these bodies, strain rates, and plausible present-day tectonic activity. For both Mars and Venus, events detectable by surface seismometers are likely: Martian estimates are 50(10) quakes annually with moment magnitude M w greater than 3.8(4.5). In addition, impacts large enough for seismology may occur at a rate comparable to the Moon. Venusian estimates are 100(25) quakes annually with M w greater than 5(6). Such seismic activities, much larger than the lunar ones, justify the deployment of seismic monitoring systems on these planets. In the foreseeable future, Mars is the new body most likely to be explored through surface landers, with a first geophysical observatory deployment in 2016 with the InSight mission: current understanding of the interior structure, seismic noise levels and scattering on this body indicate that body wave and regional surface wave investigations could yield enormous insight into the structure and evolution of this planet. Other targets, like asteroids, Europa will also benefit similarly from passive seismic experiments, as well as a return to the Moon with more sensitive seismometers. The deployment of surface landers might be much more difficult on Venus, due to the high surface temperature. Acoustic coupling of a planet's atmosphere with its internal body provides however opportunities for seismic investigations, and can be used either on Venus, a planet with a dense atmosphere, or on Giant planets. On Venus, signals at a given height in the atmosphere for a given quake magnitude can have amplitudes 600x their terrestrial counterparts for a given quake seismic moment. The possibility of detection of Venusian quakes from orbit exists through a combination of the near-source thermal and albedo signatures and ionospheric perturbations associated to Rayleigh waves. Detection of normal modes of Jupiter and other giant planets can on the other hand be forseen with ground or space-based techniques and may provides opportunities for future exploration of the interiors of these bodies