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A new hope for a microphone on Mars: Enhancing Mars 2020 science with sound

Posted by Bruce Betts and Emily Lakdawalla

15-02-2016 10:19 CST

Topics: Mars 2020, Planetary Society Projects

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
Acoustic signal
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.” 

Microphone signal
Microphone signal
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:

Mars Microphone
Mars Microphone
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.

See other posts from February 2016


Or read more blog entries about: Mars 2020, Planetary Society Projects


Ralph Lorenz: 02/16/2016 06:17 CST

This is great! We planned to carry a microphone on the Titan Mare Explorer (TiME) mission for education/outreach. Almost certainly one can learn a lot of interesting things from a microphone in a planetary environment, but it is essentially impossible to contrive a scientific question that one can be certain of answering with a microphone, so they usually fail 'science traceability' and get descoped. This ablation mass estimation looks like an excellent and useful application with which to justify the inclusion of an acoustic experiment.

David M.: 02/16/2016 07:05 CST

Yes, many interesting things ahead ! We also plan to have a strong student involvement.

Ocean McIntyre: 02/16/2016 12:58 CST

In addition to listening to the general sounds on Mars and that the rover and its instruments make, will they be generating specific sounds (tones) to see how those sounds are impacted by the Martian atmosphere? It seems that this could be an interesting experiment in and of itself, especially in anticipation of putting people on Mars. Seems that understanding how sound is effected could be something that the astronauts are trained to recognize (sound is one of the key sensory responses that humans use to protect themselves and understand their environment).

Paul_Wi11iams: 04/11/2016 11:41 CDT

A bit of a late comment here, but never mind: Sending a microphone looks like a low-investment and maybe high-return decision. Installing cameras on Viking would not have needed to be justified by testing a specific hypothesis. The same goes for sound detection. Both sight and sound are detected on an un-targeted "what's there ?" search. Cameras on Spirit and Oppy were not *expecting* dust devils or blueberries. So we can expect the unexpected which is justification enough. Then people will be regretting not having sent microphones on all previous missions.

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