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

#### Max Phalange

##### Member
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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#### Alexandria Nick

##### 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.

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

##### 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

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/

#### 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:

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

And dust devils:

[Picture from Curiosity rover]

#### Mick West

Staff member
The claim seems to center around NASA accidentally using hPa instead of Pa (1 hPa = 100 Pa)

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.

#### Amber Robot

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

#### Easy Muffin

##### 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.

#### Max Phalange

##### Member
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

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

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.

#### Hevach

##### Senior Member.
The ESA lander mentioned wasn't motionless. There were four factors being tracked to determine landing status: an atmospheric pressure altimeter, a radar altimeter (important to note that these both agreed, which would NOT have happened if the pressure was 100x higher than claimed), a timer, and an inertial guidance system.

Three of those systems told the computer the craft was still in flight. The inertial guidance system was returning garbage data - it became saturated by spin when the parachutes deployed, and there is no way to perform a desaturation maneuver at that point. Inertial guidance was supposed to have lowest "trust" in disagreements between the four sensors (radar being the most trusted).

A software bug handled the saturated sensor incorrectly and cut the parachutes at altitude, ignoring the other three sensors which said the craft was not landed.

This is a particularly bonehead software bug because the inertial sensor was maxed out - it wasn't saying the craft was motionless, it was saying the craft was moving at impossible speeds in every axis. Even if it was trusted over altimetry it shouldn't have triggered the parachute release.

Full explanation at around 6:00 in this video:
Source: https://youtu.be/Xsqe3utT6rs

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#### nathanmariels

##### New Member
The ESA lander mentioned wasn't motionless. There were four factors being tracked to determine landing status: an atmospheric pressure altimeter, a radar altimeter (important to note that these both agreed, which would NOT have happened if the pressure was 100x higher than claimed), a timer, and an inertial guidance system.

Three of those systems told the computer the craft was still in flight. The inertial guidance system was returning garbage data - it became saturated by spin when the parachutes deployed, and there is no way to perform a desaturation maneuver at that point. Inertial guidance was supposed to have lowest "trust" in disagreements between the four sensors (radar being the most trusted).

A software bug handled the saturated sensor incorrectly and cut the parachutes at altitude, ignoring the other three sensors which said the craft was not landed.

This is a particularly bonehead software bug because the inertial sensor was maxed out - it wasn't saying the craft was motionless, it was saying the craft was moving at impossible speeds in every axis. Even if it was trusted over altimetry it shouldn't have triggered the parachute release.

Full explanation at around 6:00 in this video:
Source: https://youtu.be/Xsqe3utT6rs

The ESA lander used the same Barocap pressure sensor as MSL. I have been unable to find any raw telemetry data for this decent, so I can't be sure, but if they were calculating altitude based on pressure readings from previous spacecraft, they would match, as both would be off by the same factor of 100.

The lander likely went into a high speed spin as a result of the parachute forces being substantially higher than expected.

Root Cause of failure: "Insufficient uncertainty and configuration management in the modeling of the parachute dynamics"
-EXOMARS 2016 - Schiaparelli Anomaly Inquiry

#### Hevach

##### Senior Member.
The parachute dynamics referred to the cables. In deployment the vehicle spun on the parachute lines and saturated the inertial guidance system. The public part of the report referenced above was a lot more than the one line copied and makes clear what happened, and it was ultimately an issue the ESA has had some embarrassing history with - a low priority sensor gave garbage data and software design mistakes caused it to be used in place of reliable data from higher priority sensors. See also: Arianne V launch failure.

I want to go back to propulsive deceleration, because that'll need an explanation, as well.

http://www.aerospaceweb.org/question/propulsion/q0220.shtml

Specifically:
http://www.aerospaceweb.org/design/aerospike/figures/fig11a.jpg

Ideal rocket thrust is achieved when nozzle exit pressure is equal to ambient. Propulsive engines used for landing on Mars are calibrated to .09 psi. Higher ambient pressure will cause lower thrust.

In small degrees this makes "mach diamonds" often seen on spacecraft launches with hydrogen or methane engines, because they are calibrated for some medium altitude. However in extreme cases, where ambient pressure is more than 4-5 times nozzle exit pressure, it causes separation of the exhaust from the nozzle and incursion of ambient atmosphere inside the nozzle. This is a loss-of-misson failure, bigger engines can disintegrate when this happens, smaller ones will simply lose nearly all thrust.

The most overexpanded rocket engine viably used was the RS25, which had an output pressure of around 13% of sea level, and had to be specially designed to make that happen (visible in the bell as a bend where the curve abruptly steepens). This was necessary because on the STS the RS25 engines fired from launch all the way to orbit.

If Mars pressure were 9 psi, then that would mean rockets designed for it's atmosphere would be on the order of 100x overexpanded. The Curiosity and Perseverance skycrane engines would be running below 20% thrust, assuming the nozzles didn't break. Some older retro thrusters used on Mars had chamber exit pressures that low (compressed gas thrusters rather than actual hypergolic liquid engines like the skycranes) and would have actually been unable to even fire.

#### nathanmariels

##### New Member
The parachute dynamics referred to the cables. In deployment the vehicle spun on the parachute lines and saturated the inertial guidance system. The public part of the report referenced above was a lot more than the one line copied and makes clear what happened, and it was ultimately an issue the ESA has had some embarrassing history with - a low priority sensor gave garbage data and software design mistakes caused it to be used in place of reliable data from higher priority sensors. See also: Arianne V launch failure.

I want to go back to propulsive deceleration, because that'll need an explanation, as well.

http://www.aerospaceweb.org/question/propulsion/q0220.shtml

Specifically:
http://www.aerospaceweb.org/design/aerospike/figures/fig11a.jpg

Ideal rocket thrust is achieved when nozzle exit pressure is equal to ambient. Propulsive engines used for landing on Mars are calibrated to .09 psi. Higher ambient pressure will cause lower thrust.

In small degrees this makes "mach diamonds" often seen on spacecraft launches with hydrogen or methane engines, because they are calibrated for some medium altitude. However in extreme cases, where ambient pressure is more than 4-5 times nozzle exit pressure, it causes separation of the exhaust from the nozzle and incursion of ambient atmosphere inside the nozzle. This is a loss-of-misson failure, bigger engines can disintegrate when this happens, smaller ones will simply lose nearly all thrust.

The most overexpanded rocket engine viably used was the RS25, which had an output pressure of around 13% of sea level, and had to be specially designed to make that happen (visible in the bell as a bend where the curve abruptly steepens). This was necessary because on the STS the RS25 engines fired from launch all the way to orbit.

If Mars pressure were 9 psi, then that would mean rockets designed for it's atmosphere would be on the order of 100x overexpanded. The Curiosity and Perseverance skycrane engines would be running below 20% thrust, assuming the nozzles didn't break. Some older retro thrusters used on Mars had chamber exit pressures that low (compressed gas thrusters rather than actual hypergolic liquid engines like the skycranes) and would have actually been unable to even fire.

The MR-80B engines used on MSL, which did successfully land, could output ~800 lbs thrust each. There were 8 of them, and the lander was ~2000 lbs. The engines have a 1:100 min to max thrust ratio, so they can output a total force from 64 lbs. to 6400 lbs.

Even if you assume a 20% thrust reduction from 9 PSI atmospheric pressure, there is still a large margin of available thrust for landing.

#### nathanmariels

##### New Member
There is also the fact that Mars has about one third the gravity, so that gives the sky crane substantially more thrust than required to land, even when you add the weight of the landing platform in, it's still about a 4 to 1 thrust to weight ratio.

Would a thinker atmosphere be a possible cause of the ESA landers violent, unexpected movements, when the parachute opened? In any case, these are events that let me to go in search of the cause of all the discrepancies with Mars, not direct proof of the atmospheric surface pressure being 9 PSI.

My main evidence for this is:

Specific errors in the 1976 Viking log data showing incorrect conversion factors

Specific errors showing discrepancies between hPa and Pa on MSL, Perseverance, and ESA lander using the Barocap pressure sensor

Specific discrepancies between hPa and Pa when decoding MSL telemetry data. (I wrote my own software to decode as a check)

Pressure sensor on backshell of Perseverance rover reading max value (>4.2PSI) (NASA's assumption this was because the sensor failed)

Diffuse light is not explained by dust, as there is not direct evidence of the dust, nor any proposed way it stays evenly dispersed and unaffected by wind, dust devils, lander rockets, etc...

All the other other information generally points in the direction of 9 PSI. It explains far more features of Mars than 0.09 PSI does.

#### Amber Robot

##### Active Member
All the other other information generally points in the direction of 9 PSI. It explains far more features of Mars than 0.09 PSI does.
Including remote sensing techniques like limb sounding of the atmosphere?

#### Hevach

##### Senior Member.
The MR-80B engines used on MSL, which did successfully land, could output ~800 lbs thrust each. There were 8 of them, and the lander was ~2000 lbs. The engines have a 1:100 min to max thrust ratio, so they can output a total force from 64 lbs. to 6400 lbs.

Even if you assume a 20% thrust reduction from 9 PSI atmospheric pressure, there is still a large margin of available thrust for landing.
Thrust is not the issue, exhaust pressure is. A vacuum optimized engine has more thrust than a sea level optimized one, but fire it at sea level and it will have far less.

The skycrane hovered with four engines firing at low throttle, chamber pressure of under 50 psi and an expansion ratio of 27.2 for a nozzle pressure of under 1.8 psi, 20% of claimed ambient. In other words, the engine is very overexpanded, 20% is generally on the edge of grossly overexpanded (a nozzle extension can be specifically shaped to allow greater overexpansion than this, but the MR-80b doesn't have that), where the exhaust "unsticks" from the curve of the nozzle and ambient atmosphere intrudes into the bell, causing the exhaust stream to whip around and produce erratic thrust. This would also be far more than a 20% loss of thrust, though actually calculating how much would require modeling beyond my available tools and information about the MR-80b that Rocketdyne doesn't make publicly available.

If not grossly overexpanded, it is still substantially overexpanded, and would produce a compressed exhaust stream.

From the engineering camera views, we can see the thrust isn't just stable, but the plume spreads outward, it does not compress. Which tell sus the engine nozzle is underexpanded, not over. Top image, not bottom.

9 psi would be on the line between the bottom two - with the difference being loss of vehicle (bottom case, grossly overexpanded) and viable but with visible effects (overexpanded, second from the bottom). However, we can clearly see the top case, which puts a ceiling on the ambient pressure, it has to be lower than the nozzle pressure of the engines.

#### nathanmariels

##### New Member
At sea level, 14.7 PSI, the MR-80B engines have > 80% of vacuum thrust. These engines can run with a combustion chamber pressure from 2 PSI to 282 PSI, 1:100 ratio.

The lander has 8 of these engines, for 6400 lbs total thrust, lifting an effective lander weight of < 1000 lbs (30% earth gravity).

MR-80B engines can handle 460 PSI maximum back pressure and were extensively tested at 14.7 PSI as well as 0.25 PSI.

I don't see any issues here with the rocket engine working at 9 PSI.

I do, however, find many issues related to excessive forces during parachute decent and aerobraking, when assuming 0.09 PSI, that match models nicely at 9 PSI.

#### Amber Robot

##### Active Member
I do, however, find many issues related to excessive forces during parachute decent and aerobraking, when assuming 0.09 PSI, that match models nicely at 9 PSI.

here is a link to a paper describing determination of surface pressure from remote sensing, utilizing a spectrometer in the Mars Express spacecraft:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JE002871

They describe their process quite thoroughly and address sources of error. I would be very surprised if their results were two orders of magnitude off.

I do not find 9 PSI to be a reasonable estimate of the surface pressure of Mars.

#### nathanmariels

##### New Member
here is a link to a paper describing determination of surface pressure from remote sensing, utilizing a spectrometer in the Mars Express spacecraft:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2006JE002871

They describe their process quite thoroughly and address sources of error. I would be very surprised if their results were two orders of magnitude off.

I do not find 9 PSI to be a reasonable estimate of the surface pressure of Mars.

That method likely returns the correct amount of C02 present in the atmosphere. Based on the assumed pressure, this is how they come to the conclusion that the atmosphere is 95% CO2. I think that it is 0.95% CO2. The pressure being 9 PSI with ~1% CO2 matches the observed CO2 ice condensation and sublimation rates observed at the poles of Mars. 0.09 PSI is not enough pressure at the measured temperatures for CO2 to condense to CO2 ice (dry ice).

Almost every research paper I read about the martian atmosphere, hundreds of them, describes how the atmosphere of Mars is not well understood. Almost every single one of these issues fit existing models very well at 9 PSI.

#### Amber Robot

##### Active Member
That method likely returns the correct amount of C02 present in the atmosphere. Based on the assumed pressure, this is how they come to the conclusion that the atmosphere is 95% CO2. I think that it is 0.95% CO2. The pressure being 9 PSI with ~1% CO2 matches the observed CO2 ice condensation and sublimation rates observed at the poles of Mars. 0.09 PSI is not enough pressure at the measured temperatures for CO2 to condense to CO2 ice (dry ice).

Almost every research paper I read about the martian atmosphere, hundreds of them, describes how the atmosphere of Mars is not well understood. Almost every single one of these issues fit existing models very well at 9 PSI.
What is the other 99% of the atmosphere made of? What is the spectral signature of it?

#### nathanmariels

##### New Member
That is a really good question. There are absorption lines for O2, CO2, N2, Ar, and H20, but the ratios are all based on atmospheric pressure. If the pressure is not as expected, its rather difficult to figure out from the available data. The index of refraction and diffusion seem to imply N2, H20 and O2, but with lower O2 and H20 than Earth. Even if the atmospheric pressure is 9 PSI at the surface, I don't know that humans would want to breath it, but it would definitely make surviving there much easier in any case.

Without any specific data for the ratios, my guess would be 13% O2, 1% CO2, 1% AR, 85% N2, but I don't have enough information to have confidence in these numbers.

I had thought the atmospheric pressure of Mars to be around 10 PSI for years until I found the math/software errors that all pointed to 9 PSI.

(This is why my page of notes changed from mars10psi.com to mars9psi.com a few years ago.)

#### nathanmariels

##### New Member
I wish I could put a mechanical pressure gauge and a prism in view of a camera on the next Mars mission...

#### Easy Muffin

##### Member
Do you have an explanation for the roughly 30% seasonal pressure variation observed by the Mars landers? General consensus is that it is caused by the freezing out and subsequent re-release of atmospheric CO2. This data set from the Phoenix lander for example shows the pressure changing from ~850 Pa to ~720 Pa over a course of around 140 sols: https://data.mendeley.com/datasets/3yn4rhzp3m/1. That's a difference of 130 Pa. If we are to assume that these pressure readings are off by a factor of 100, what could account for a pressure variation of 13,000 Pa? Are you saying that the observed shrinking and expanding of the ice caps is actually releasing and capturing 100 times more CO2? Are the pressure readings fundamentally wrong? Do you propose another, as of yet unknown, mechanism?

#### nathanmariels

##### New Member

I have not looked at this dataset before, but this looks like sensor drift to me. The Barocap sensors are very sensitive to noise and temperature. This dataset has also been "corrected" twice to make it match what they are expecting. I know that on MSL, they have changed the telemetry data format for the pressure data at least seven times, as I wrote seven decoders for the different packet formats. Many of those also has new sensor calibration data, which makes longer term trends more difficult to understand.

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