Debunked: Atmospheric pressure on Mars is 9 PSI, not 0.09 PSI as claimed by NASA

Article:
Before the Viking spacecraft landed on Mars in 1976, it was thought that the atmospheric pressure of Mars was somewhere between 0.4 PSI and 4.4 PSI. When the Viking spacecraft landed, the pressure sensor appeared to indicate that the atmospheric pressure on the surface of Mars was 0.09 PSI.


There was a software error in the conversion of pressure sensor data, where Pa and hPa were not considered as different units, although they differ by a factor of 100. This implies that Mars has an atmospheric pressure of 9 PSI. This has rather large implications for our understanding of physics, and may be an explanation why most spacecraft attempting to land on Mars fail, and the ones that do land are many miles from the intended landing location.



The question I asked myself a few years ago and still can not find a better answer is:

The sky is not black when viewed from Mars rovers.

At 100,000 feet on Earth, the pressure is similar to the currently accepted pressure on Mars, and the sky is black.

If the diffuse light on Mars is from the dust, what is holding up the dust?


Why does it matter if the pressure on the surface of Mars is 60% of the pressure on the surface of Earth?

We could use aircraft on Mars.

We could roam the planet of Mars without spacesuits using just warm clothing and rebreathers.

The mathematical equations of physics fit together in a nice way that currently can not be done.


A strange claim, but there's some work there which superficially seems to back it up. Maybe someone with a bit of relevant knowledge can refute? I guess whether or not the Perseverance helicopter works will be a good litmus test - aerodynamic surfaces would perform quite differently with an atmosphere 100 times more dense than NASA expects.
 
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Just off the top of my head, none of the drag chutes and parachutes would have functioned as designed, if the atmosphere was denser than what NASA says it is.

This:

may be an explanation why most spacecraft attempting to land on Mars fail, and the ones that do land are many miles from the intended landing location.

is a faulty premise. Many of the failed landings were because the lift vehicle or orbiter failed. Of the ones that physically arrived at Mars, the failures are largely mechanical problems, not because they smashed into the surface. Schiaparelli EDM had some sort of fault that triggered certain aspects of the landing sequence too soon. Beagle 2 landed and appears to have been completely operational, but a solar panel didn't deploy and blocked comms. Mars Polar Lander shut the engines down too early, as the system may have interpreted the vibration from the landing legs deploying as touchdown vibration.

There have been eight rover missions. One is en route, five are operational or successfully operated until end of mission, one was lost after deployment due to LOS, and one crashed. That's 1/7. Hardly an auspicious start to "most fail." Plus, if we're honest with ourselves, most of the losses were either early on in the space game or are USSR/Russian, who seem to have some difficulty with Mars.

They're also not "many miles" from the intended landing zone. They're inside their planned landing ellipse with few exceptions. We can point out the same problem with returning vehicles here on Earth, where we have a pretty solid grasp of our atmospheric properties.
 

Easy Muffin

New Member
The sky is not black when viewed from Mars rovers.

At 100,000 feet on Earth, the pressure is similar to the currently accepted pressure on Mars, and the sky is black.

If the diffuse light on Mars is from the dust, what is holding up the dust?
This sounds like it's based on the faulty assumption that the Martian atmosphere is like Earth's, only thinner.
 

Amber Robot

Active Member
Direct measurement form the Viking lander isn’t the only datum for determining the surface pressure for Mars. Why ignore all the other data, including remote sensing of the atmosphere?
 

Mick West

Administrator
Staff member
Just off the top of my head, none of the drag chutes and parachutes would have functioned as designed, if the atmosphere was denser than what NASA says it is.
Exactly, that's really all you need to know to understand that NASA knows exactly what the atmospheric pressure is on Mars.

Earth's air pressure on the surface is around 14.7 PSI, about 100,000 Pa.

NASA has weather stations on Mars, and has for years, in the form of the rovers. They are constantly measuring the air pressure.
https://mars.nasa.gov/msl/weather/

2021-03-02_09-52-32.jpg
 

Mauro

Member
I'm no expert on the matter, but for what I know the atmosphere of Mars is colored by dust (and this explains why Mars sky is red). Mars dust is very small and even the little atmosphere Mars has is enough to float it around.

Mars even has global, planet-wide dust storms:

1614708732137.png
[On the right side, Mars surface is obscured by the storm]

And dust devils:

1614708856747.png
[Picture from Curiosity rover]
 

Mick West

Administrator
Staff member
The claim seems to center around NASA accidentally using hPa instead of Pa (1 hPa = 100 Pa)
2021-03-02_10-29-53.jpg

They thought that NASA just used a stock Vaisala Barocap and misread the data sheet. But actually they used a custom version adjusted for the low temperature, as a Vaisala scientist exlains here:
Source: https://youtu.be/K7dvAWLbAh0?t=170

 

FatPhil

Active Member
This sounds like it's based on the faulty assumption that the Martian atmosphere is like Earth's, only thinner.

Yeah, it's thinner (density), but also much thicker (height), and of course the latter increases the diffusion of light which makes sky visible. Its troposphere is about the same height as earth's stratosphere, for example. We have photos of sunsets, there's no reason to be speculating about the changes in lightness and darkness of the sky there: https://www.universetoday.com/wp-content/uploads/2015/05/Mars-sunset-Curiosity.gif

Something tells me that the effect of increased depth of atmosphere is super-linear (exponential, perhaps?), whereas the effect of density is only linear, so the huge density difference may not be as important as the smaller height difference. However, I can't support that with laws and equations (but it seems like the attenuation of a signal, and noise compounds) so I'll offer it as an idea that someone else could possibly investigate.
 

Easy Muffin

New Member
The main difference I suppose is that there's a lot of dust hanging around in Mars' atmosphere (due to the planet essentially being one large dust bowl, the gravity being only about 1/3 that of Earth's and no weather that could wash the particles out) and so the arriving sunlight gets scattered quite effectively before it hits the surface, resutling in a reddish sky (Mie scattering I believe it's called). The air in our own atmosphere on the other hand is much clearer, with not enough particles to cause this effect as strongly, and the main driver behind our sky's appearance is Rayleigh scattering, which depends on atmospheric thickness much more. We sometimes get to see Mie scattering effects too, an example would be the impressive red sunsets after large volcanic eruptions that carried ash particles high into the atmosphere.

One more thing, if the pressure really was 100x higher, wouldn't that facilitate liquid water on the surface? We don't see anything like that though, from all we know the water / CO2 ice sublimates directly from the solid phase to the gas phase, as one would expect with the generally accepted pressures.
 

FatPhil

Active Member
The scale height of the atmosphere on Mars is about 11km, whereas on Earth it is about 8km.

Plenty of martian atmosphere above 11km:

-- https://atmos.nmsu.edu/data_and_services/atmospheres_data/MARS/viking/logs/VL1_entry_profile.txt

-- https://atmos.nmsu.edu/data_and_services/atmospheres_data/MARS/viking/logs/VL2_entry_profile.txt
 

Amber Robot

Active Member
Mie scattering is generally gray. I believe the red sky I’m on Mars comes from the reddish color of the dust itself.
 

Amber Robot

Active Member
Plenty of martian atmosphere above 11km:
P = P_0*exp(-z/H) where H is the aforementioned scale height and equals kT/mg. Due mostly to the lower gravity on Mars, the scale height is larger, thus the pressure drops off more slowly than it does on Earth.
 

Hevach

Senior Member.
https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/rktvrecv.html

The graphic is of course for a different scenario, but the drag force equation for a parachute is the same on Mars, the difference is the r variable. If the air density was 100 times what NASA thought, the force on those hypersonic parachutes used by Curiosity, Perseverance, and other landers would be 50 times higher than expected. In turn, the force on the cables connecting them to the payload and the fastening points at each end would be 50 times higher - for a complex lander system like Perseverance this also carries down through the pyro bolts connecting the aeroshell/parachute assembly to the skycrane, and cables connecting the rover to the skycrane, the forces through the entire system would be 50 times higher than expected.

Short version: Something's going to break.

But, wait. It wouldn't even get to that point. Mars has enough atmosphere to actually decelerate incoming probes from interplanetary transfer orbit directly to a landing trajectory, rather than expending delta-V establishing an orbit first, but also thin enough that doing this isn't instant suicide (try the same thing with Earth and you're going to have a bad time). Not all do this, for example China's current mission includes a lander but is entering a low circular orbit before deploying it from the main orbiter, but it's been the NASA SOP for landers and rovers throughout the successful era on the "Mars scorecard."

https://apps.dtic.mil/dtic/tr/fulltext/u2/a231552.pdf

From page 3, the atmospheric entry heating also uses a (1/2)*pressure term, meaning that upon entry, atmospheric heat would be fifty times higher than expected, and the craft would have burned up very quickly. If Mars' atmosphere were actually as thick as claimed here, landers would be forced to reduce velocity from interplanetary transfer before attempting a landing.



Edit: And one other point on dust: The dust on Mars is incredibly fine. It's very dry and there is none of the complex organics that helps bind earth soil into larger particles.
https://en.wikipedia.org/wiki/Martian_soil#Atmospheric_dust
Average grain size of airborne dust on Mars is 3 micrometers. For comparison, a fine talcum powder will have an average grain size of around 25 micrometers:
https://pubmed.ncbi.nlm.nih.gov/29261577/
 
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FatPhil

Active Member
P = P_0*exp(-z/H) where H is the aforementioned scale height and equals kT/mg. Due mostly to the lower gravity on Mars, the scale height is larger, thus the pressure drops off more slowly than it does on Earth.

Does diffusion depend more on pressure or density? I'd have thought the latter, as it's matter that matters, not how fast it's moving. If so, then T will need to be accounted for.
 
The air in our own atmosphere on the other hand is much clearer, with not enough particles to cause this effect as strongly, and the main driver behind our sky's appearance is Rayleigh scattering, which depends on atmospheric thickness much more. We sometimes get to see Mie scattering effects too, an example would be the impressive red sunsets after large volcanic eruptions that carried ash particles high into the atmosphere.
I think I remember reading somewhere that Mars's sky is pinkish during the day, and becomes bluish during twilight - the opposite of Earth - due to the difference between Mie and Rayleigh scattering.

Thanks all for the wide ranging collection of counter evidence in this thread. What a strange hill the owner of mars9psi.com has chosen to die on.
 

Amber Robot

Active Member
Does diffusion depend more on pressure or density? I'd have thought the latter, as it's matter that matters, not how fast it's moving. If so, then T will need to be accounted for.
From what I’ve seen of the temperature profiles of Mars’ atmosphere it doesn’t change terribly much, so the density drop off is going to be exponential like the pressure is.
 

FatPhil

Active Member
From what I’ve seen of the temperature profiles of Mars’ atmosphere it doesn’t change terribly much, so the density drop off is going to be exponential like the pressure is.

There's about a 2:1 temperature change from 10km to 100km. However, a question about the properties of diffusion cannot be answered with a statement about the properties of density.
 

Trailblazer

Moderator
Staff member
Plenty of martian atmosphere above 11km

For those not familiar with the term, the "scale height" of an atmosphere doesn't refer to the actual depth of the atmosphere (which can't really be defined, since pressure reduces gradually towards zero with height so you can't define a "top edge").

The scale height is the height over which the pressure drops by a factor of e (that is, approx 2.718, the base of natural logarithms).

So if the pressure is 900 Pa at the surface and the scale height is 11km, then 11km up the pressure will be 900/2.718 = 331 Pa. Another 11km above that (ie 22km above the surface), it will be 331/2.718 = 122 Pa, and so on.
 

Amber Robot

Active Member
There's about a 2:1 temperature change from 10km to 100km. However, a question about the properties of diffusion cannot be answered with a statement about the properties of density
You posed the question if diffusion depended more on pressure or density. I pointed out that both scaled essentially together. A 2:1 change is small compared to an exponential drop-off. Comparably the pressure would fall off by a factor of exp(-10/11)/exp(-100/11), which is about 3600, from 10km to 100km.

The atmospheric diffusion equation does use density and not pressure, though. And there’s a height at which the atmosphere becomes no longer well mixed, called the homopause or turbopause, above which the “m” in the scale height equation I noted earlier will be for each species and not the average molecular mass. This is significantly high up, after density has dropped considerably. On Earth this is near the mesopause, at the base of the thermosphere.
 

Hevach

Senior Member.
Another addition to my post above: There's more things that wouldn't work if the pressure was wrong.

Rocket nozzles are optimized for ambient pressure - the maximum thrust happens when the exhaust leaving the nozzle is at ambient pressure, so the ratio of chamber outlet to nozzle opening is based on the ratio of chamber pressure to ambient pressure. This is why if you look at a Falcon rocket the engine on the upper stage looks larger than all nine on the first stage combined, despite being the same engine - to be vacuum optimized it has a far larger nozzle. If the atmospheric pressure were 100 times higher, the rocket nozzles on the skycrane would be far too large and would produce very poor thrust.


It's all literally rocket science, if you go into it with wrong assumptions you're just throwing time and money in the furnace.
 

Amber Robot

Active Member
For those not familiar with the term, the "scale height" of an atmosphere doesn't refer to the actual depth of the atmosphere (which can't really be defined, since pressure reduces gradually towards zero with height so you can't define a "top edge").

The scale height is the height over which the pressure drops by a factor of e (that is, approx 2.718, the base of natural logarithms).

So if the pressure is 900 Pa at the surface and the scale height is 11km, then 11km up the pressure will be 900/2.718 = 331 Pa. Another 11km above that (ie 22km above the surface), it will be 331/2.718 = 122 Pa, and so on.
By definition, almost 2/3 of the atmosphere (1-1/e = 0.632...) is within the first scale height.
 

nathanmariels

New Member
A few comments

1) The pressure sensor on MSL and Perseverance are both made by Vaisala. It is a MEMs sensor etched on a silicon wafer. They only make two types. One for measuring barometric pressure and one for higher pressures used in industrial control systems. The sensor is a pressure to capacitance device, and the raw telemetry data from MSL has the capacitance values returned by the sensor. These values match a pressure of around 9 PSI, or 700 hPa. I have tested the sensors myself in a vacuum chamber and they match the datasheet. The issue is that all of the NASA documents list the units as Pa, while the Vaisala documents all use hPa. In the test reports, hPa and Pa are used interchangeable at random it seems.

The Perseverance rover had a pressure sensor in the backshell that could read up to 4.2 PSI. It returned 4.2 PSI when landing, but they are assuming the sensor failed.

2) All of the dust particle size estimates are based on what the dust particle size would have to be to account for the discrepancies between light diffusion and assumed atmospheric pressure.

3) Before I found the units error, I had though the pressure was around 10 PSI. This was calculated from telemetry data for several landers during decent. MSL and Perseverance both use a dimensionally exact copy of the 1976 Viking landers parachute design. It would function at a 9 PSI surface pressure, just that it would slow it down at a higher altitude. The current parachute dynamics are not well understood, and they assume that the CO2 atmosphere behaves drastically different than air to account for the discrepancies.

4) The ESA lander failed when descending by parachute because it was moving so slowly that the software assumed it had landed, and cut the parachute off and fell several miles up.

5) We'll know more in a couple days when the Mars Helicopter attempts to fly. It should be able to fly at 0.09 PSI or 9 PSI, but will have substantially longer flight time if it is 9 PSI. The rotor RPM vs. motor current should be a good way to estimate the pressure.
 

Mick West

Administrator
Staff member
5) We'll know more in a couple days when the Mars Helicopter attempts to fly. It should be able to fly at 0.09 PSI or 9 PSI, but will have substantially longer flight time if it is 9 PSI. The rotor RPM vs. motor current should be a good way to estimate the pressure.
Do you have some specific numbers there? Like what values will indicate higher pressure. Actual numbers.
 
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