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Radio Occultation and Spectroscopy (Updated 4/3/2018)


       In trying to understand what was seen by radio occultation experiments conducted by the Mariner spacecraft, a problem was encountered when (for too long) we put our faith in a NASA website about the Mariner Mars Missions.57 Later we found important discrepancies between its Figures and those published in 1974 by A. J. Kliore of the Jet Propulsion Laboratory.58 These differences are highlighted on Table 11.

       Initially we thought that a distant flyby might miss pressures at the top of the huge mountains on Mars, but an orbiter should not.  In fact, when Mariner 9 arrived at Mars, a global dust storm obscured everything except the top of Olympus Mons.  However, seeing Olympus Mons does not equate with measuring pressure there by radio occultation.

       Collectively, Mariners 4, 6 and 7 only attempted to make six pressure measurements on Mars. Each of these three spacecraft could only offer a pressure for the point on Mars tangent to the line that ran from the spacecraft to Earth as the craft first passed behind Mars (an occultation entry point) and again when they reestablished line of sight with Earth after emerging from the occultation status (the exit pressure). The dynamic range in geo-potential topography on Mars is huge, from 21,287.4 m on Olympus Mons down to -8,180 m at the bottom of the Hellas Basin. The total change in elevation is 29,467.4 m. At 29,467.4 m above sea level on Earth pressure would fall from 1,013.25 mbar to about 12.75 mbar (about the previously presumed pressure in the Hellas Basin on Mars). Mariners 4, 6 and 7 missed these extremes.

  Table 11 - Comparison of Published Martian Pressures via Radio Occultation 


CPA in km

Max P mbar

Min P mbar

Pressure range in mbar

Mariner 4 (flyby) 7/14/65      Source:





Mariner 4 (flyby) 7/14/65      Source: (Kliore. A.J., 1974)


8 to 9

4.5 to 5


Mariners 6 & 7 (both flyby)     7/31/69 and 8/5/69              Source:





Mariner 6  7/30/69               Source: (Kliore. A.J., 1974)





Mariner 7                            Source: (Kliore. A.J., 1974)





Mariner 9/Orbiter/11/13/71    Source:





Mariner 9






       Did any of the above Mariners ever measure pressure on Olympus Mons? No. Olympus Mons is nowhere near the points on Table 12, which sums up entry and exit points provided by Kliore et al. (1974).58

       The 260 Mariner 9 occultation experiments also failed to see either the highest or the lowest places on Mars (see Figure 24 which includes the Tharsis area). Most of the entry and exit occultation points for Mariner 9 are shown on Figure 24. 

       With respect to Olympus Mons, a literature search shows a remarkable variation of elevations cited with 27 km often at the upper range (Zubrin, 2008).59

       We asked Dr. Shane Byrne at LPL about it. He stated, “The older (higher) elevation number is based on much less reliable stereo topography data and should be discarded” (personal communication, September 2, 2010).  He later referred me to an article by David E. Smith et al. (2001).60 Those Figures are now adapted as standard for this report. For Olympus Mons, this means that its height above areoid is 21.2874 km, although in rising from a basal point 378 m below areoid, its total relief is 21.6654 km.

       Kliore et al. claimed to have measured pressure on Pavonis Mons.58 Table 1 in this report shows its altitude at 14.057 km above areoid, with a pressure of about 1.66 mbar if the pressure at areoid is 6.1 mbar. The Kliore assertion about Pavonis Mons led to a much better understanding of radio occultation deficiencies. Kliore wrote:

“By coincidence, the location of measurement 434 entry fell very close to the top of the volcanic feature known as Middle Spot (Pavonis Mons), which was one of the four prominent features first discovered in Mariner IX television pictures during the Martian dust storm (Masursky et al., 1972).61

“Although the location of the occultation tangency point did not fall within the caldera of the (Pavonis Mons) volcano, the geometry was such that the line of sight practically bisected the entire shield volcanic structure, thus making it virtually certain that the beam was actually intercepted by the highest feature along the track, which is likely to have been the summit area.  The radius that was measured here was 3417.4 km which is about 13.6 to 13.8 km above adjacent occultation measurements. On the basis of pressure altitudes, the height of Middle Spot was 12.5 km, and the pressure at the top was about 1 mb.”

       The last sentence sounds like what was actually measured was only the height of the mountain, but the 12.5 km height specified then does not match the 14.057 km MOLA specified height that is accepted now. Further, the phrase “On the basis of pressure altitudes” seems to imply that once an altitude was determined, a simple scale height calculation was employed to derive pressure. This is quite different from an occultation experiment that directly derives air pressure. If the altitudes asserted as a result of Mariner 9 radio occultation are not being upheld today, it follows that extreme caution should be exercised before accepting pressures based on legitimate attempts to derive pressure by radio-occultation.

5.1 Shifting Standards - The Relationship of the MOLA Topography of Mars to the Mean Atmospheric Pressure.  

       Smith et al. (2001)60 point out that,

“The average atmospheric pressure on Mars is ~6.1 mbars, which is close to the triple point of water. Early topographic models of Mars [e.g., Wu, 1991] were referenced to this atmospheric pressure surface. The use of a pressure surface as a reference introduced considerable error into estimates of elevation because of temporal variability in the height of the pressure surface due to seasonal variations in CO2 content and dynamical motions of the atmosphere…  

       To relate surface topography to atmospheric pressure, it is necessary to first compare planetary radii obtained from spacecraft occultations to those derived from MOLA. The occultations yield a measure of both planetary radius and atmospheric pressure and thereby provide a unique linkage between these quantities… MOLA radii, which are considerably more accurate than radii obtained by occultations, can then be related to occultation-derived surface pressures. By comparing MOLA radii to Viking and Mariner 9 occultations, Smith and Zuber [1998] showed that the zero point of MOLA topography corresponds to an atmospheric pressure of ~5.2 mbars at Ls=0°. (Ls=0° corresponds to the vernal equinox in the northern hemisphere.) The 6.1-mbar pressure level occurs at approximately    -1600 m relative to the zero reference of MOLA topography for Ls=0°. However, the height of the 6.1-mbar surface needs to be adjusted, depending on the date.  Seasonal variations in atmospheric pressure associated with the exchange of CO2 between the atmosphere and polar caps are expected to produce vertical variations in the height of the 6.1 mbar surface of 1.5 to 2.5 km over the course of the Martian year [Zuber and Smith, 1998].”

       The Achilles Heel of the above Smith and Zuber argument is the pervasive need by almost all traditional researchers to relate their findings to the pressure chart represented earlier by Figure 9A. But those Figures match what would be expected in accordance with Gay-Lussac’s Law for a gas trapped behind a dust clot in the air access tubes for the pressure transducers.     

Flyby and Date/Ls/

Season (N.


Entry Position and Pressure

Exit Position and Pressure

Mariner 4


Ls 142.6


50.5° South latitude in the Mare Chronium region.

4.5 to 5 mbar

60° North latitude in Mare Acidalium.

8 to 9 mbar

Mariner 6


Ls 199.5


4° North

Meridani Sinus.

5 mbar

80° N


6.9 mbar

Mariner 7


Ls 202.5



68.2° South near Hellaspontus.

4.2 mbar.

38.1° N in the Arcadia- Amazonis area.

7.3 mbar

Table 12 – The only six attempts conducted by Mariners 4, 6 and 7 to measure pressure on Mars by radio occultation.


Figure 24 Radio Occultation experiments on Mariner 9 missed Olympus Mons and Arsia Mons, but not Pavonis Mons (434N). Mountain locations are: Olympus Mons 17.3495N, 226.31E; Pavonis Mons 0.0626096S, 246.674E; Arsia Mons 9.12736S, 238.261E.

Figure 25 above: MOLA map of Mars with major topographic features, landing sites, and locations of methane plumes identified by Krasnopolsky et al. (2004). 

       One of the most memorable demands ever heard on film was made by the Wizard of Oz for Dorothy to pay no attention to the man behind the curtain (exposed by her dog, Toto). She didn’t believe him. Likewise, it makes no sense to ignore plainly visible Martian weather, be it dust devils, spiral clouds with 10-km wide eye walls over Arsia Mons (shown earlier as Figure 20), sand blowing around without sufficient threshold winds to explain the movement (see Section 7.2 and Figures 27 to 28 below), or global dust storms that reduced visibility at Opportunity - blocking out over 99 percent of direct sunlight received there (see Figure 36 later).


       An attempt to measure surface pressures was made by Mars Express Orbiter.  Results for the nine pressures obtained over a Martian year are shown on Figure 26A. This section compares the data so derived with that of the Viking 1 lander shown on Figure 26B.  

       Is it reasonable to base projected pressures for Figure 26A on Martian year 24 (from July 15, 1998 to May 31, 2000)?  There were two regional dust storms that year - but no global dust storms.  The first regional storm began at Ls 224 in Chryse and lasted until Ls 232 in month 8.  The second storm began in Amazonis at Ls 228 and lasted until Ls 243 in month 9. The curve of pressure changes shown on Figure 26A greatly resembles the annual pressure curves shown back on Figure 21B. Indeed, it is almost an exact match for VL-1 pressures shown on Figure 21B almost two decades earlier despite the fact that the Vikings encountered three global dust storms.  

Figure 26A Mars Express OMEGA spectroscopy-derive surface pressures (redrawn from Spiga et al. 2007).

Figure 26B 4 years of in situ pressures at Viking 1 lander site (redrawn from Tillman, 1985, 1988 and 1997).

       Figure 26A is a bit deceptive. There was no lander on Mars capable of measuring in situ pressure for Martian year 24 (Pathfinder terminated its 2.5 months of operations on September 27, 1997; and Phoenix operations ran from May 25, 2008 to November 10, 2008 ( 

        There are other concerns about spectroscopy.  Pressure may vary radically at times across the planet, and (as will be discussed further below in section 10.2) there are serious questions about why Mars Reconnaissance Orbiter (MRO) encountered atmospheric density that was 350% higher than predicted by the Mars-GRAM (Global Reference Atmospheric Model) during aerobraking operations over the south pole (Atkinson, 2006).62  And yet, in discussing the limitations of the Mars Express spectroscopy operations, the Spiga et al. (2007) make clear that water ice clouds and frosts can distort the critical CO2 absorption band at 2 µm and may falsify the pressure retrieval. 63 They conclude by stating “the spectral signature of water ice is thus not included in our model, thus we simply avoid the regions with clouds and frosts.”  This, of course, rules out the South Pole where the aerobraking problem was encountered. That water distorts pressure readings by spectroscopy for Mars is enormously important because on September 26, 2013 NASA announced that,

“A key finding is that water molecules are bound to fine-grained soil particles, accounting for about 2 percent of the particles' weight at Gale Crater where Curiosity landed. This result has global implications, because these materials are likely distributed around the Red Planet.”64

      As lead author Laurie Leshin, of Rensselaer Polytechnic Institute in Troy, N.Y. put it, “that means astronaut pioneers could extract roughly 2 pints (0.946353 liters) of water out of every cubic foot (0.028317m³) of Martian dirt they dig up.”

       As will be discussed later in conjunction with Figures 45 and 46 in Section 13 of this report, relative humidity at Gale Crater varied from less than 10% to about 60%. Further, in 2011, we learned that, “It seems that previous models have greatly underestimated the quantities of water vapor at heights of 20–50 km, with as much as 10 to 100 times more water than expected at this altitude.”65 

       With an apparent timely reading of pressure by OMEGA in hand from Mars Express, the Beagle-2 which detached from it to land then on December 25, 2003, was immediately lost, however the lander was found largely intact on January 17, 2015. At, we discuss discrepancies between original and revised landing coordinates and target ellipse size with ellipse size varying from 50km*8 km to 500 km*100km. In the end the claim was that Beagle 2 was only 5 km off target, but if that is true it should not have taken 11 years to find it. Between January 17 and 18, 2015 we saw major revisions in Wikipedia about the actual target. Further, where the question of air pressure is greatest around the South Pole of Mars, the attempt by Mars Polar Lander to set down there in 1999 was also a failure – although supposedly due to improper hardware testing.