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Potential Pressure on Mars, Methane, and Sky Color (Updated 5/17/2017)


      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, and, as will be indicated below in conjunction with Figure 44, the true pressure at Hellas Basin might actually be higher than what is found at sea level on Earth.

12.1 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? Figure 40 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 40 – 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 44 and the stratus clouds seen 16 km above Mars Pathfinder.

12.2 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 41.

       Where did initial the methane seen by SAM during its initial check out come from? SAM had miniature pumps (Wide Range Pumps -see Figure 41A). 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 41A below – Sample Analysis at Mars (SAM)

Figure 41B below - Methane spikes seen by MSL at Gale Crater, Mars.

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."

Figures 42-A to 42-I plus Plates 5 and 6 ( illustrate the controversy over Martian sky color ever since Viking 1 touched down. 42A 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. 42B shows true sky color in accordance with colors of the U.S. flag. 42C 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. 42D shows the Martian sky near sunset. 42E shows sky as seen from MSL with a dust cap over the camera lens. 42F shows what has often been portrayed as the Martian sky color as seen from MER Opportunity. Figure 42G shows the same area as 42F, but with “false color applied.” 42-H and 42-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.

The Figure below is found in the PDF version of this report as Figure 62.