Speed of sound on Mars

[GregLocock]

The Mars idiosyncrasy. Due to the unique properties of
the carbon dioxide molecules at low pressure, Mars is
the only terrestrial-planet atmosphere in the Solar
System experiencing a change in speed of sound right in
the middle of the audible bandwidth (20 Hz – 20 kHz)
[10]. For an acoustic wave with a frequency higher
than ~240 Hz (relaxation frequency), CO2 vibrational
modes activated through collisions do not have time to
relax their energy [11]. In that case, it is as if the
medium has only 5 degrees of freedom compared to 7 at
lower frequencies (3 translational modes, 2 rotational
modes and 1 doubly-degenerate vibrational mode).
Therefore the adiabatic ratio at high frequency is
reduced to γ∞ = 7/5 compared to γ0 = 9/7 at low frequencies.
It turns out that, on Mars, frequencies above 240
Hz travel more than 10 m/s faster than low frequencies.
It may induce a unique listening experience on Mars
with an early arrival of high-pitched sounds compared
to bass.

https://www.hou.usra.edu/meetings/lpsc2022/pdf/1357.pdf

Cheers

Greg Locock


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[btrueblood]

Sounds similar to the doppler effect, but happening all the time.

 

[electricpete]

It's interesting that they apply vibration mode analysis of molecules to be able to figure out stuff like that.

I sort of followed the quote up until adiabatic ratio... I don't know what that is or why it would affect the speed of sound.



=====================================
(2B)+(2B)' ?

 

[3DDave]

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/adiab.html

 

[TugboatEng]

The rovers sent to Mars have had microphones. Have any experienced these phenomena? I do know a recording of Martian winds has been released recently. This seems like it would be straightforward to simulate on Earth.

 

[3DDave]

The way it was noticed was when using a laser to ablate some rock at a distance which gives the measurement system an exact time and distance for the noise generating event. Unless someone on Mars stepped on a Lego brick near one of the rovers it seems unlikely that a similar point-in-time sound source existed.

Possibly more amazing is that it seems like they were initially listening for the sound of the ablation as a higher-resolution method of determining the temperature of the Martian atmosphere which might have eddies that a typical thermometer is not fast enough to detect. Can't have thermal lag if it's the air taking its own temperature. This phenomenon would have caused them great pain as they would get two separate speeds and therefore two separate temperatures as two noise spikes came in for each event.

 

[GregLocock]

electricpete, the speed of sound in an ideal gas is defined by a=sqrt(gamma*R*T) where T is the absolute temperature, R is the gas constant for that gas, and gamma is the ratio of the specific heats. As I understand it they are saying that there is a resonance in the molecule under those atmospheric conditions at about 400 Hz, and above that resonance gamma changes.

Like you I don't know the hows whys and wherefores.

Cheers

Greg Locock


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