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Potential Pressure on Mars, Methane, and Sky Color (Updated 3/11/2021).


       Read and Lewis (2004, pp. 269-270)79 note potential reserves of CO2-H2O clathrate in regolith that could raise surface pressure to 200 hPa (mbar) during periods of high-obliquity when, at some point in the future, Mars would have its axis inclined at a greater angle than it has today.  If more clathrate is locked up under deeper polar deposits underground, pressure could go as high as 850 hPa (Jakosky et al., 1995).85 But if the soil became rich in water ice through precipitation and adsorption into the porous regolith, Read and Lewis state the value might be limited to 15-30 mbar.

      If the increase of density seen during aerobraking operations by MRO (30 to 350%) was correct, and could be applied to the Hellas Basin, then pressures there would reach 16.37 to 44 mbar. However, the 350% figure was only for operations over the Martian South Pole. As will be indicated below in conjunction with Figure 57 due to stratus clouds seen 12,318 meters above aeroid, the true pressure at Hellas Basin might actually be higher than what is found at sea level on Earth.

13.1 Did NASA Ever Publically Back 20 Mbar on Mars?  In a work entitled SP-4212 On Mars: Exploration of the Red Planet 1958-1978 in Chapter 8, second paragraph (page 243)86 we read:

       Mariner 69’s occultation experiment indicated that the atmospheric pressure at the surface of Mars ranged from 4 to 20 millibars, rather than 80 millibars as estimated earlier. This information had a definite impact on the aerodynamic shape of the Mars entry vehicle being designed, since weight and diameter would influence the craft’s braking ability. Langley engineers had determined that aerodynamic braking was the only practical method for slowing down a lander as large as Viking for a soft touchdown. The entry vehicle would have a diameter of 3.5 meters, an acceptable ballistic coefficient that would help ensure Viking’s safe landing on Mars. 

       It appears that by Mariner 69’s, the article is referring to the Mariner 6 and 7 flyby spacecraft that had their closest approaches to Mars on July 31, 1969 and August 5, 1969.  But their NASA-advertised radio occultation pressures for Mars were only 3.8 to 7.0 mbar. The 20 mbar figure is almost 3 times higher. And what are we to make about the 80 mbar figure that is refuted with the 20 mbar estimate?  Mariner 4 had flown by Mars on July 14, 1965. Its estimate of pressure on Mars was pegged at 4.1 to 7 mbar on their website located at, though as mentioned earlier in Section 5, Kliore had it pegged at 4.5 to 9. 

       If NASA had the 20 mbar figure, and was publishing it too, the question must be asked, why in the world would it select pressure transducers for the Vikings that could only measure up to 18 mbar and why was a transducer that maxed out at 11.50 mbar chosen for MSL and Perseverance? Figure 47 shows there were pressure estimates of 20 mbar in 1965 (Evans), but after Mariner 6 and 7 the issue was supposed to be settled with a maximum pressure at 9 mbar (less than the 10.72 mbar measured by Viking 2). 

Figure 47 – History of Beliefs about Martian Atmospheric Pressure

       Why was a detailed NASA document written in 1978 still putting forward the 20 mbar figure?  Perhaps someone realized what is abundantly apparent in this study. The Viking pressure data is fatally flawed.  Further, without a fix for dust ingestion by Pathfinder, Phoenix and MSL, they were also fatally flawed.  We must at least plan on the pressures seen by studies in 1965 or earlier, but that really should not be the limit. We need a sensor that can measure Earth-like pressures as will be discussed later in conjunction with Figure 57 and the stratus clouds seen 16 km above Mars Pathfinder.

13.2 Biology, Methane, and a Possible Hint of the Real Martian Air Pressure

       Given the discovery of methane plumes (identified back on Figure 25) that have a probable biological origin (Krasnopolsky87 et al., 2004) it was natural that MSL had instruments designed to detect methane. Of particular interest would be methane producing or consuming bacteria that might be attached to dust particles. Bloom of such organisms, with a means of encapsulating or producing methane (lighter than the ambient CO2) might explain the lifting process seen in dust storms and/or dust devils. When MSL landed there was brief, but temporarily unwarranted excitement when methane was detected by the Sample Analysis at Mars (SAM) shown in Figure 48.

       Where did initial the methane seen by SAM during its initial check out come from? SAM had miniature pumps (Wide Range Pumps –see Figure 48). In a JPL press conference held on August 27, 2012 (see , Mahaffy stated, 

The really nice thing about these pumps is they exhaust naturally right at Mars pressure, 10 millibar, 7 millibar. Um, and it turns out there is a very slow leak, uh, into the Tunable Laser Spectrometer and so there was just a little bit of a residual atmosphere” (that is, from the Earth). He went on to say,

“and so the tens of millibars that we had in there, I think we had 51 millibar and we had assumed that the pump would be fine evacuating that, we routinely evacuate Mars ambient out of the cell but it was just high enough the current sensor on the pump said, nah this is a little bit too high I‘m gonna turn myself off and it did but SAM continued merrily along its measuring path assuming that we had not turned off and so we measured that gas with both the mass spectrometer and the Tunable Laser Spectrometer. It really led to some excitement. The TLS (Tunable Laser Spectrometer) Team, Chris and Greg, their eyes were wide open. They saw all this methane, and it turns out it’s terrestrial methane, but it was kind of a good test….      

Figure 48 – Sample Analysis at Mars (SAM)

       We considered that the 51 mbar mentioned by Dr. Mahaffy might be the first real clue about how high Martian pressure really is. On Earth that pressure would equate to an altitude of about 63,057 feet or 19,220 meters. But based on Figure 57 we think the pressure is higher, closer to 511 at areoid.

       On December 16, 2014 JPL announced that it had found methane spikes of 5.5, 7, 7 and 9 ppbv (parts per billion volume), about 10 times higher than the background methane measured earlier (0.7 +/- 0.2 ppbv (see Figure 49). Other organic chemicals found in the Cumberland sample at Gale Crater included chloromethane, dichloromethane, trichloromethane, dichloroethane, 1,2 – dichloropropane, 1,2 – dichlorobutane and  chlorobenzene. This is quite a change from NASA’s Viking stance of no organic chemistry on detected on Mars. We believe Dr. Levin is owed a Nobel Prize for his work which we discuss further at There appears to be ample reason to revisit NASA’s dismissal of positive results about detection of life by the Labeled Release (LR) life detection experiment on both Vikings (Levin, 1997).88 The new finding reinforces the position of Dr. Christopher McKay of NASA Ames on January 4, 201189 when he found that NASA’s 30-year rejection of organic chemicals was wrong.

       Previously, the 1997 Levin paper mentions what looked like lichens seen on Mars (at least until a technician under the order of NASA administrator Dr. James Fletcher went through the JPL control room and manually turned the color knobs on the monitors to make everything look red (see Figures 50A and 50B). If Levin were right about lichens living on Mars now, could we extrapolate an air pressure based on maximum altitude where lichens are found on Earth? While one article described lichens (Cordyceps sinensis) living at Dolpa in the Himalayan mountains of Nepal at 5,177 m (16,984 feet) where pressure would be about 527 mbar, Sancho et al. (2007)90 described an ESA astrobiology experiment on the Foton-M2 mission aboard a Soyuz rocket launched on May 31, 2005. They state that,

“It returned to Earth after 16 days in space. Most lichenized fungal and algal cells survived in space after full exposure to massive UV and cosmic radiation, conditions proven to be lethal to bacteria and other microorganisms… Moreover, after extreme dehydration induced by high vacuum, the lichens proved to be able to recover, in full, their metabolic activity within 24 hours.”

Figure 49 – Methane spikes seen by MSL at Gale Crater.

        Thus it must be determined at what altitude (and minimum pressure) the lichens would go into a protective mode. Aware of all this controversy the MSL SAM had, as one of its purposes, an assignment to revisit the question of organic chemistry on Mars.  Mahaffy stated at the August 27, 2012 press conference that,

The SAM is a key tool in Curiosity’s search for signs of life, past or present, and is more sensitive and sophisticated than the sensors on the Viking lander which came up negative for organics. The system is designed, for example, to examine a wider range of organic compounds and can therefore check a recent hypothesis that perchlorate – a reactive chemical discovered by the Phoenix Mars Mission – may have masked organics in soil samples taken by Viking.”

       In 2018 we learned that there were more organic chemicals found at Gale Crater. Eigenbrode et. al reported the in situ detection of organic matter preserved in lacustrine mudstones at the base of the ~3.5-billion-year-old Murray formation at Pahrump Hills, Gale crater, by the Sample Analysis at Mars instrument suite onboard the Curiosity rover. Diverse pyrolysis products, including thiophenic, aromatic, and aliphatic compounds released at high temperatures (500° to 820°C), were directly detected by evolved gas analysis. Thiophenes (C4H4S) were also observed by gas chromatography–mass spectrometry. Their presence suggests that sulfurization aided organic matter preservation. At least 50 nanomoles of organic carbon persists, probably as macromolecules containing 5% carbon as organic sulfur molecules.120

       The profiles were consistent with the presence of thiophene (C4H4S), 2- and 3-methylthiophenes (C5H6S), methanethiol (CH4S), and dimethylsulfide (C2H6S). The presence of benzothiophene (C8H6S), a bicyclic thiophene that usually co-occurs with thiophenes, is also suggested by a weak peak in both Mojave release of organic sulfur compounds and related volatiles was observed for Confidence Hills.  


Figures 50-A to 50-I plus Plates 5 and 6 ( illustrate the controversy over Martian sky color ever since Viking 1 touched down. 50A shows what NASA released in 1976 after Dr. James Fletcher ordered manual adjustments on monitors that destroyed blue sky color and hid green on rocks. 50B shows true sky color in accordance with colors of the U.S. flag. 50C shows the Martian sky near sunset. 50D shows that for Earth once pressures drop to 11.3 mbar the sky is a dark blue, not bright as seen in day time photos from Mars. 50E shows sky as seen from MSL with a dust cap over the camera lens. 50F shows what has often been portrayed as the Martian sky color as seen from MER Opportunity. Figure 50G shows the same area as 50F, but with “false color applied.” 50-H and 50-I show what MSL sees without a cover over its camera lens. Variations on sky color may be due to amount of dust in the air, which varies seasonally. Blue appears to be the correct color when dust loads are low.