Martian lee waves revisited

Abstract

SEVERAL of the Mariner 9 television frames show standing-wave patterns in the clouds of the Martian atmosphere near surface features of high relief. These lee waves have been interpreted as high level condensation clouds in which the brightness variations are due to a combination of true lateral variations in the local concentration of condensed material and the shadows cast on lower levels by such scattering material. Here I demonstrate that the variation in slope of the mean scattering layer with respect to the local horizontal produces a photoclinometric brightness variation strong enough to explain the data with the invocation of quite modest relief in the cloud deck. Although some lee wave clouds in the later frames of the mission leave no doubt that they are mainly pressure trough condensations, lee wave clouds in the early dust-storm phase of the mission are probably largely the photoclinometric effect of vertical oscillations of approximately 40 m in the lines of flow past land protuberances. One interpretation of this result yields a wind velocity of 92 m s⁻¹.