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Downrange Landings; Dust Opacity and Pressure (Updated 5/24/2017)


        A NASA paper challenges its own assumptions about air pressure, although it goes in the opposite direction of what we think is true (however, only at mid altitudes between 20 and 50 km). The 2009 article by Prasun N. Desai is entitled All Recent Mars Landers Have Landed Downrange - Are Mars Atmosphere Models Mis-Predicting Density?77 It notes downrange landings of 27 km (Pathfinder), 13.4 km (Spirit) 14.9 km (Opportunity) and 21 km (Phoenix).  Desai et al. (2008) thought Phoenix encountered a lower density profile ranging from a few percent to a maximum of 8%, but he wrote that “the primary cause of the Phoenix downrange landing was a higher trim angle of attack during the hypersonic phase of entry, which resulted in Phoenix flying a slightly lifting trajectory.”  The cause was unknown. It resulted in parachute deployment occurring 6.4 seconds late.   His work, and reports about Pathfinder, suggest up to 40% less density than expected at 50 km, but about 5% higher density than expected at h = 0.  

       We asked Dr. Desai if Phoenix might have experienced a limited skip effect. If a spacecraft comes in a bit too shallow, the increased buoyancy felt from below might make it take a small skip, not causing it to return to space, but resulting in it landing long. This seemed to line up with what he called a slight lifting trajectory in his article. However, Desai’s overall position was that if the air is denser than expected, the friction will cause the probe to slow faster than expected, and land short of its target (not long, as with Pathfinder, Spirit, Opportunity, and Phoenix).

       Desai was not always consistent about the altitude that was most important with respect to deceleration.    He wrote that,

“Another important aspect to the atmospheric density is in what altitude region is the density lower. The most important altitude band for entry and descent is between 20-50 km, prior to the parachute being deployed. That is where almost all of the deceleration occurs (~90% of the velocity is reduced), and therefore the downrange distance traveled. Above and below this altitude band, the downrange distance traveled is minimally affected by mis-prediction of density... Also, the density just doesn’t disappear in the entire column of air (actually CO2). If the density is lower in this mid-altitude band, then the density is higher at lower altitudes 0-20 km. Basically, more of the column of CO2 moves lower (the CO2 just doesn’t disappear). As such, a little of the effect of the lower density at higher altitude is made up by the higher density at lower altitudes, although far from all.(Desai, personal communication, March 22, 2010)

        The pressure graphs in the Desai (2008) article are reproduced on Figures 32-34.  They show data beginning at 100 km for Spirit, 80 km for Opportunity, and 70 km for Phoenix. Missing in the Desai article was a graph for Pathfinder (which was furthest downrange at 27 km). Desai concludes:

        "Does the fact that every one of these entries encountered a lower atmospheric density profile than predicted indicate a random occurrence or is there a systemic bias in current Mars atmospheric models? As such, a question is posed to the atmospheric community to consider if the Mars modeling assumptions are appropriate or are there underlying modeling issues that need to be reexamined or reevaluated. Additionally, although the entire density profile is necessary for entry, descent, and landing design; nearly all deceleration during entry occurs between 10-50 km. As such, prediction of density within this altitude band is most critical for entry flight dynamics and design.”

        Note the second (published) statement by Dr. Desai refers to a minimum deceleration altitude of concern of 10 km rather than his more recent e-mail of 22 March 2010 that used 20 km.

Figure 32 Reconstructed density for Spirit Landing (redrawn from Desai, 2008)

       For Spirit it looks like all reconstructed densities were lower than what was expected or encountered (see Figure 32). However, as noted earlier, Spirit is the rover that photographed sand filling in its tracks during the 2007 dust storm (see Figure 30). This is not consistent with low air densities at the surface.

Figure 33 - Reconstructed Density for Opportunity Entry (redrawn from Desai, 2008)

        For Opportunity (Figure 33) the densities encountered were lower than expected only below ~32 km (especially so between 10-20 km), but higher than expected above 32 km.  For Phoenix all reconstructed pressures were below what was assumed for landing day (Figure 34). Desai informs us that for successful landers, navigation errors upon Mars arrival were very small and that, as such, entry interface conditions (initial targeting on entry) was not responsible for downrange landings. What about MSL Curiosity?  It landed about 2 miles northeast of its target but the accuracy was not due to better understanding of air pressure. Rather, the lander had thruster rockets that allowed it to make a more controlled landing, with corrections applied as necessary. 

Figure 34 - Reconstructed Density for Phoenix Entry (redrawn from Desai, 2008)

       The moment aerodynamic issues are introduced for entry into an alien atmosphere there are many places for errors to occur. Density is one such area, but not the only issue.  Buoyancy determines overall structure of the atmosphere and what causes air to move around (Read & Lewis, 2004).79 Buoyant forces combine with aerodynamic issues when it comes to getting a landing right. Increasing density of the fluid increases buoyancy forces, even before we consider parachute issues, although, strangely enough, the parachute used for the Phoenix was actually reduced to 39 feet from the 42 feet used for Pathfinder. 

       I asked Dr. Desai about the buoyancy issue.  He replied, “As for buoyancy forces, if you make calculations of its magnitude, it is quite small not only due to the density on Mars being low, but also because the volume of these landers are quite small as well. Hence, for these reasons, it is just a very small effect.” (Desai, Personal Communication, March 22, 2010)

       The answer above is based on assumption that the density of the Martian atmosphere is always low at all altitudes. Yet dust storms can radically alter the density equations in short order. A dust storm at Luke Air Force Base on July 5, 2011 turned day to night in surrounding areas (see Figure 35). While the measured pressure increased by at least 6.6 mbar (more than average pressure at Mars areoid), pressure was only taken once per hour; all the increase was due to dust in a cloud that only rose to somewhere between 5,000 and 8,000 feet. Dust storms also turn day to night on Mars (see Figure 36). The essential question is, “What ambient Martian air density is required to support such a mass of dust?” Finally, Desai only requested help in explaining four spacecraft landing long. It is possible that two other craft listed as lost (Mars Polar Lander and Deep Space 2 on December 3, 1999) actually landed short and crashed as a result of it.

        The Vaisala pressure transducer used for MSL was rated for a maximum pressure of 11.5 mbar. Without considering the 11.49, 11.54, 11.77 and 12 mbar pressures that were off the curve, with no dust storm, the highest revised average daily pressure for MSL Year 1 was 9.25 mbar, and as a daily average, there must have been higher pressure than that sometime during the Sol 172 (at Ls 254) in question. The highest pressure for MSL Year 2 was also 9.25 mbar (Sol 846 at Ls 257). If we add the 6.6 mbar increase in pressure caused by a dust storm at Luke Air Force Base just to the 9.25 mbar pressure, the total reaches 15.85 mbar, far above the maximum 11.5 mbar maximum pressure allowed for the Vaisala transducer. So the pressure range (publically) chosen makes no sense at all, and may be indicative of a less than honest Martian image being put out by NASA. The 11.49, 11.54, 11.77 and 12 mbar pressures reinforces this conclusion. For Sol 370, even if we accept the 8.65 mbar replacement pressure that is likely manufactured, 8.65 + 6.6 mbar still equals 15.25 mbar, which is above the transducer’s capacity. We warned Dr. Vasavada (MSL Project Scientist) about this twice before MSL launched in November 2011, once in August at the Mars Society Convention in Dallas, and again by phone in October.

Figure 35 Arizona Dust Storm of July 5, 2011. Pressure at Luke Air Force Base increased during the dust storm by 6.6 mbar more than average pressure (6.1 mbar) at areoid on Mars.

       Earlier we reported the remark made by one of the Vaisala transducer’s designers, “The fact that we at FMI did not know how our sensor was mounted in the spacecraft and how many filters there were shows that the exchange of information between NASA and the foreign subcontractors did not work optimally in this mission!” (Kahanpää, personal communication, December 15, 2009).  Kahanpää is part of the current REMS Team for the MSL. We see no evidence that the exchange of information between NASA/JPL and the REMS Team or FMI has in any way improved since he wrote that above in 2009. Earlier on Figure 17a we showed the REMS Team weather reports from August 29, 2012 through September 6, 2012. They reported that the pressure suddenly went up from 7.4 hPa (mbar) on August 29 to 742 hPa on September 1. We were not alone in immediately notifying JPL and Ashima Research about this. In fact, for five days we wrote and received e-mails back from JPL’s public relations man, Guy Webster. He in turn indicated that Dr.  Vasavada at JPL was notified. But JPL is in California, and the REMS Team is in Spain. The REMS Team continued to publish Earth-like pressures of 742 to 747 hPa until reverting back to 7.47 hPa on September 6. Unless they deliberately chose to reveal a secret that pressure was two orders of magnitude higher than advertised, they proved that communication (perhaps due to language barriers) was again not working optimally. As for the 1,200 Pa pressure they reported for Sol 1,161 (Ls 66 - see Figure 21A) we doubt that they meant 1,200 hPa/mbar unless they were taking a wild guess. If the real pressure for September 5, 2012 (Sol 30, Ls 166) was 747 mbar, it’s not likely to increase to 1,200 mbar suddenly in 2015 unless some explosive event occurred nearby. It seems unlikely that a dust or sand storm could be the cause because opacity for every day of concern is listed as “sunny.” Perhaps the never changing “sunny” over at least the first 1,583 sols is also disinformation. Whatever the reason, especially because these high pressures are beyond the 11.5 mbar limit of the pressure transducer to measure, we need to hear from Kahanpää himself and/or NASA on the issue.


 Dust storms can greatly alter the opacity (τ) on Mars.  Figure 36 shows visibility for different values of opacity on Mars due to a dust storm at Opportunity between sols 1205 and 1235. All photos were taken between 10:53 and 11:30 local time. The dust in the Martian air over Opportunity blocked 99 percent of direct sunlight.  This fact alone makes it very hard to accept that pressures would be unaffected.

Figure 36 - Opacity changes at Opportunity from sols 1205 to 1235. Redrawn from

       J. D. Parsons (2000)80 addresses the compressibility of dust storms and positive feedback for their formation. Pre-dust storm density values are around 9.4 g/m3. A sample dust storm given in the Parsons paper would have additional densities of 17g/m3 in order to even be created.  This is an order of magnitude greater than terrestrial storms.  It also constitutes an increase of at least several hundred percent over previously accepted values.  In the Sahara, pressures have been observed to increase during dust storms.  Likewise when the huge dust storm hit Luke Air Force on July 5, 2011,  pressure rose by 6.6 mbar (more than accepted average pressure at Mars areoid) between the storm’s arrival at 0255Z 6 July 2011 (pressure 1,004.7 mbar) and 0555Z when the pressure was up to 1,011.3 mbar. Pressure dropped as visibility cleared at 0655Z (personal call to Luke AFB meteorology, July 6, 2011).

       The Parsons (2000)80 paper proposes a gravity current analog for dust storms and mentions that such currents should be constrained to the height of the inversion layer (but dust storms on Mars can still have effects at 160 km). Perhaps most important, increased pressure makes it easier to entrain particles (hence higher pressure may explain dust storms and dust devils). 

       During the Martian year opacity varies greatly.  The clear season is in the northern summer with optical depth τ values of ~0.3 to 0.5. During northern winter τ values of ~2 to 5 or higher were seen during dust storms (see Figure 37).  Black dots are the Year One data, black pluses are the Year Two data, and the red X’s are extrapolations from the pressure data.  This is for Viking 1.  There is a relation between pressure and opacity, however the figure adapted from page 181 in The Martian Climate Revisited by Read and Lewis,79 states that τ is derived from pressure data. This is the same pressure data that might be distorted by clogged pressure filters.  There is a need to quantify how increased density and opacity due to dust storms affect pressure on Mars.

Figure 37: VL1 Pressure and Opacity, redrawn from Figure 7.2 in The Martian Climate Revisited, Read and Lewis (2004), adapted from Martin and Zurek (1993).

This report is continued with Section 10 HERE.