The sounds of a helicopter on Mars
R. D. Lorenz, S. Maurice, Baptiste Chide, D. Mimoun, Alexander Stott, Naomi Murdoch, Martin Giller, Xavier Jacob, R. C. Wiens, Franck Montmessin, Håvard Fjær Grip, Theodore Tzanetos, Bob Balaram, N. R. Williams, Matt Keennon, Sara Langberg, Jeremy Tyler, Tanguy Bertrand, A. J. Brown, Nicolas Randazzo, Benjamin Pipenberg
Abstract
The sounds of the Ingenuity Helicopter flying in the Martian atmosphere are among the most notable recordings of the microphone on the SuperCam instrument on the Mars 2020 Perseverance Rover. Distinct acoustic signatures of the helicopter were recorded on the 4th, 5th, 6th, and 8th flights: prior to this, simultaneous microphone and helicopter operations had not been verified in the testbed, and generally since these early flights the helicopter has been too far away for its emissions to be detectable given CO2 absorption in the Mars atmosphere. The detected signatures are around 84 Hz and (occasionally) at 168 Hz, at the blade crossing frequency and its first harmonic. Several higher harmonics were prominent in hover tests in short-range recordings in a test chamber on Earth; these are attenuated by CO2 absorption at the 50m-plus ranges on Mars. Doppler shift of the 84 Hz signal can be measured and is consistent with the trajectory measured with Ingenuity's navigation camera and inertial navigation unit, and documented by Perseverance's cameras. A striking feature of the sound recordings is an unanticipated deep modulation of the signals with nulls spaced by around 15–20s, superposed on the simple and expected decline in amplitude with distance. We have evaluated and rejected models of multipath sound interference as requiring implausibly strong near-surface temperature gradients. We find instead that the modulation appears to be the signature of a slight asynchrony between the rotation rates of the two coaxial rotors, such that the blade-crossing azimuth rotates slowly during flight, resulting in a ‘lighthouse’ sweeping of the radiated sound pattern. Analysis of blade orientations seen in the shadow of the helicopter observed in down-looking navigation images supports this model.