CONTINUOUS EXCITATION OF PLANETARY FREE OSCILLATIONS BY ATMOSPHERIC DISTURBANCES

Seismology provides a powerful tool for probing planetary interiors(1, 2), but it has been considered inapplicable to tectonically inactive p lanets where earthquakes are absent. Here, however, we show that the a tmospheres of solid planets are capable of exerting dynamic pressure o n their surfaces, thereby exciting free oscillations with amplitudes l arge enough to be detected by modern broad-band seismographs. Order-of -magnitude estimates of these forces give similar amplitudes of a few nanogals for the Earth, Venus and Mars despite widely varying atmosphe ric and ambient conditions. The amplitudes are also predicted to have a weak frequency dependence. Our analysis of seismograms, recorded con tinuously from 1992 to 1993 at 13 globally distributed stations, shows strong evidence for continuously excited fundamental-mode free oscill ations on the Earth. This result, together with other recent studies(3 -5), is consistent with our estimate of atmospheric forcing and we the refore propose that it may be possible to detect atmospheric excitatio n of free oscillations on Venus and Mars as well.