A new hope for a microphone on Mars: Enhancing Mars 2020 science with sound
When the Mars 2020 rover lands, we may finally hear the first audio recordings from the Martian surface. The Planetary Society has been working for decades to land a Mars Microphone, something that would add a second human sense to the amazing imagery we currently get, and would be very engaging and exciting for not only scientists, but also for the general public. Unfortunately, the only two such instruments to have launched suffered sad fates. The first Planetary Society Mars Microphone crashed with Mars Polar Lander. The second microphone to fly to Mars, on Phoenix, was never turned on because of the potential for an electronic problem. The payload of the ExoMars 2018 rover may include infrasound and pressure sensors that could produce sound-like recordings. In an abstract submitted to the 2016 Lunar and Planetary Science Conference (PDF), members of the Mars 2020 SuperCam team explain how including a microphone on their instrument could support their science -- and record sounds on Mars.
Sylvestre Maurice and his coauthors explain in the abstract that the microphone would be useful both for science and engineering. As with various past proposed and flight Mars microphones, sound in principle could serve as an independent constraint on wind speed, and could help identify the passing of dust devils. The microphone would also record all the various noises made by the rover: the whirr of the actuators, the crunch of the wheels across the ground, the pumps that keep the rover's Freon circulating. And the wind itself would create its own sound, whistling past the rover's various protuberances.
These would all be cool to hear and great for the public, but why include it as part of the SuperCam instrument, the core of which is Laser Induced Breakdown Spectroscopy (LIBS)—vaporizing rock with a laser and analyzing the spectral result to determine composition of the rock? An instrument like SuperCam could use the microphone to enhance their science, since testing on Earth indicates that analysis of the volume of the sound can be used to study the mass of material vaporized by a laser shot. Maurice explains:
When interacting with the target, the LIBS beam – typically 5 nanoseconds in duration, at a wavelength of 1064 nanometers, and irradiance above 1 gigawatt per square centimeter – generates a very sharp pressure wave which is proportional to the mass of ablated material. As the plasma expands, the pressure wave accelerates supersonically for a few hundred nanoseconds. Scientists usually refer to the “LIBS shock wave”. Because the pressure wave is so sharp (microseconds), the acoustic wave is broad-band and contains no effective spectral information. Its energy is proportional to that of the pressure wave and therefore, all things being equal, to the mass of the ablated material.
Acoustic signal (volume) versus ablated mass showing the correlation between the two.
The team has tested the system in Earth and simulated Mars conditions. As expected, volume drops faster with distance in the thin atmosphere, but, as stated in the abstract, “It is still loud on Mars, more than expected since the larger volume of the expanding plasma in the thin atmosphere compensates for the lower atmospheric pressure.”
Measurements of 10 laser shots under Earth conditions (red) and Mars conditions (blue), showing the reduced, but still significant, volume under Mars conditions.
In addition, the sound of the laser ablation is most intense when the instrument is at its optimal focus, so a microphone could augment the instrument's autofocus capability. Here's what the microphone could look like on Mars 2020's mast head:
Possible accommodation of the Mars Microphone on the Mars 2020 mast head. It would be mounted on a tiny tube that protrudes from the warm electronics box, on the bracket that holds the window for the SuperCam instrument.
SuperCam is a selected instrument for Mars 2020 that will fly. The microphone is a proposed addition, so time will tell if it actually flies. The abstract discusses more generally the value of a microphone on a LIBS-type instrument. Hopefully, the scientific rationale will convince the mission and we’ll finally hear the sounds of Mars.
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