The sounds of a helicopter on Mars

• Published in: Planetary and space science Vol. 230; p. 105684
• Main Authors: Lorenz, Ralph D., Maurice, Sylvestre, Chide, Baptiste, Mimoun, David, Stott, Alexander, Murdoch, Naomi, Giller, Martin, Jacob, Xavier, Wiens, Roger C., Montmessin, Franck, Grip, Håvard, Tzanetos, Theodore, Balaram, Bob, Williams, Nathan, Keennon, Matt, Langberg, Sara, Tyler, Jeremy, Bertrand, Tanguy, Brown, Adrian, Randazzo, Nicolas, Pipenberg, Benjamin
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.06.2023; Elsevier
• Subjects: Sciences of the Universe
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/j.pss.2023.105684

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. •Documents sound recordings of Ingenuity helicopter flights on Mars.•Demonstrates acoustic data on rover are diagnostic of flights e.g. via Doppler effect.•Unexpected drops in loudness with 10–20s period may relate to slightly mismatched rotor speeds.•Helicopter shadow images show loudness drops correspond to blade-crossings aimed at rover.