Higher than Advertised Martian Air Pressure: Part 6

6. SPECTROSCOPY PRESSURE READINGS BY MARS EXPRESS ORBITER.

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 21A. This section compares the data so derived with that of the Viking 1 lander shown on Figure 21B.  

Is it reasonable to base projected pressures for Figure 21A 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 21A greatly resembles both annual pressure curves shown back on Figure 18, but 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 21A 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 (http://www-mars.lmd.jussieu.fr/mars/time/martian_time.html). 

          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, Nancy, 2006).  And yet, in discussing the limitations of the Mars Express spectroscopy operations, the Spiga et al. (2007) article makes clear that water ice clouds and frosts can distort the critical CO₂ absorption band at 2 µm and may falsify the pressure retrieval.  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.”  That, of course, rules out the South Pole where the aerobraking problem was encountered.  

Although it may be just coincidence, 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.  It is not known whether or not the supposed air pressure below was a factor in the loss.  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.

7.  MARTIAN WIND PROBLEMS

Until Phoenix landed in 2008, the only landers carrying dedicated meteorology instruments were Vikings 1, 2 and Pathfinder. There was little wind speed data for Mars after the Vikings due to calibration problems with the wind sensors for Pathfinder (Schofield et al., 1997).  Winds were too light (largely <5 m/s), but wrong assumptions about air pressure on Mars might have also caused calibration problems as wind speed u is related to pressure through Equation 1 from a NASA article about the Mars Pathfinder Windsock:

EQUATION 1: u = sqrt{[2 R(1) M g tan (theta)]/ [R(2) A(d) rho]}

In Equation1 R(1) = distance between pivot and center of mass, M =  non-counter-balanced mass, g = acceleration of gravity, R(2) =  distance between pivot and center of aerodynamic pressure,  A(d) =  effective aerodynamic cross-section, and rho = atmospheric  density (a function of pressure, temperature, and molecular weight).

          An MPF hot-wire anemometer also had calibration problems. Such technology is sensitive to pressure, gas composition, air temperature, and their own overheating which may induce systematic errors (Pedrero, Jaime, 2010).

Schofield et al. (1997) indicate that while Pathfinder was operational from July 4 to September 27, 1997, it had no pressure data for the most crucial sol – its first operational day on Mars. The reason given by the above reference is there were “various spacecraft software reset and downlink problems.” If the problems only occurred after the first day and if the first day’s pressure data was consistent with the Vikings, then Pathfinder’s data could be used to refute the claims made herein. However, that is not the case. We are still dealing with a Tavis transducer with no way to keep the dust out of its pressure tube on or in the seconds before landing, and no way to change a clogged dust filter. The critical time is in the final landing process. So when the spacecraft has to reset the software and correct downlink problems then, the issue of exactly what is entailed in these corrections becomes one of extreme importance.

7.1. Anemometer/Telltale Wind Speed Issues.

        Understanding Martian wind is crucial in preparing for future manned missions to Mars.  As such, one of the first instruments chosen for Phoenix should have been an anemometer, but Taylor et al. (2008) refer to the failure to do so.  Their paper states, “We had hoped to include an anemometer in the MET package.  Faced with a lack of resources to achieve this, and the real desire to have some wind information we decided to make use of the SSI camera and have a novel Telltale to achieve this.”  See Figure 22.

How does the above required 80 mph compare with winds observed on Mars? The set of graphs on Figure 23 below show how wind speed varied at Viking 1 between its sols 1 and 350 (with the exception of sols 116 to 133 because data was missing then). Every sol (Martian day) was divided into 25 time bins, with wind readings provided for each one. During sols 1 to 199 the maximum wind recorded was 36.7 mph. Between sols 200 and 350 there was one incident where winds reached 57.9 mph, but at no measured point over 8,331 measurements, did the wind ever reach 80 mph. Average winds for Viking 1 were about 6.12 mph during sols 1 to 199, and 11.86 mph during its sols 200 to 350. All wind data was obtained from the Viking Project Group headed by Professor James Tillman.

       For Viking 2 during sols 1 to 199 the maximum wind recorded was 22.1 mph. From sols 200 and 399 it was a good bit windier, but the maximum winds at 51.9 mph – were still short of the 80 mph figure required to move the sands. Average wind for Viking 2 was about 7.54 mph from sols 1 to 199; and 13.33 mph from sols 200 to 399. 

7.2.1 Issues Raised by the paper on Planet-wide sand motion on Mars by Nathan T. Bridges (et al., 2012).

The Bridges et al. paper states that, “prior to Mars Reconnaissance Orbiter data, images of Mars showed no direct evidence for dune and ripple motion. This was consistent with climate models and lander measurements indicating that winds of sufficient intensity to mobilize sand were rare in the low-density atmosphere.” It then reveals new findings that show that many sand ripples and dunes across Mars exhibit movement of as much as a few meters per year, demonstrating that Martian sand migrates under current conditions in diverse areas of the planet. However, in an effort to explain it, they speculate that “most motion is probably driven by wind gusts that are not resolved in global circulation models.”

A response to the resolution suggestion is that, as is noted before in conjunction with the 8,331 wind velocity measurements recorded at Viking 1 and Viking 2, in no case was a gust ever caught that hit 80 mph. The windiest day seen was with Viking 1 with a 57.9 mph gust during its sol 214.78 when the planet was at Ls 210.872 (Martian fall in the northern hemisphere).  Did this gust come out of a sudden event like a dust devil? No, obviously it was a storm of some sort, because the winds began to rise in the morning that day at sol fragment 214.38, then the fell off toward Martian midnight. Based on data from Professor Tillman's Viking Project Site, the incident is shown growing and subsiding on Table 9.

Table 9 – Profile of the windiest Viking day on Mars with the greatest wind gust recorded at VL-1 sol 214.78.

Bridges et al. note that dunes and ripples (collectively termed bedforms) are abundant and widespread on Mars, with concentrations surrounding the north polar layered deposits, within craters and other depressions that trap sediment, and as isolated patches on the plains. The area surrounding the north polar layered deposits includes some of the lowest elevations on Mars. Low elevation implies higher pressure, which means that it becomes easier for the winds to move sand, but the assumed increase in pressure at the altitudes in question are still insufficient to move the sands on a widespread basis. Even at Lyot (7.036 km below areoid), the lowest point in the northern hemisphere, we would only expect pressure to peak at about 11.7 mbar if there is 6.1 mbar at areoid (See Table 1 earlier in this report).

The Bridges et al. study notes that comparing the movement map to predictions of the Ames Global Circulation Model (GCM) (Haberle et al., 2003) shows no correlation to the high wind frequency regions. They believe this demonstrates that the models do not resolve small-scale topographic, katabatic winds (as occur in the north polar region; Ewing et al., 2010), and general boundary layer turbulence that may cause gusts above threshold (Fenton and Michaels, 2010). However, the GCMs are based on the assumption that the average pressure at Mars areoid is only 6.1 mbar. If the movement maps do not resemble the GCM predictions, then this again may support our contention that the ultralow pressure is incorrect. The gusts above the 80 mph threshold were not seen in the 8,331 measurements that we checked from Vikings 1 and 2.The Bridges et al. study notes that comparing the movement map to predictions of the Ames Global Circulation Model (GCM) (Haberle et al., 2003) shows no correlation to the high wind frequency regions. They believe this demonstrates that the models do not resolve small-scale topographic, katabatic winds (as occur in the north polar region; Ewing et al., 2010), and general boundary layer turbulence that may cause gusts above threshold (Fenton and Michaels, 2010). However, the GCMs are based on the assumption that the average pressure at Mars areoid is only 6.1 mbar. If the movement maps do not resemble the GCM predictions, then this again may support our contention that the ultralow pressure is incorrect. The gusts above the 80 mph threshold were not seen in the 8,331 measurements that we checked from Vikings 1 and 2. 

       Bridges et al. state, “Below the resolution of HiRISE as seen by the MER rovers, the evidence for motion of fine sand is compelling, with indications of sand blowing out of Victoria Crater that erases rover tracks (Geissler et al., 2010), craters superposed on the ripples being filled with sand (Golombek et al., 2010), ripples from winds funneled along the troughs, and one observation of small sand ripple migration (Sullivan et al., 2008).”

An example of tracks being erased is shown in Figure 25 where Spirit’s tracks vanished during the 2007 global dust storm. Spirit landed at about 1.9 km below areoid. If the average pressure at areoid is about 6.1 mbar, with a scale height of 10.8 km, the average pressure at -1.9 km should only be about 7.27 mbar – quite low if wind is expected to move the sand.  Unfortunately the rover carried no meteorological instruments. This means that it could not measure pressure or wind. However we can compare the time that it felt the dust storm to the time that Viking 1 experienced its two global dust storms in 1977 (see Figure 26). We could also look at what happened to Viking 2 then, but both MER Spirit and Viking 1 were in the Martian tropics while Viking 2 was at almost 48° North. As such, it is appropriate to examine the winds experienced by Viking 1 during dust storm 1977a, which began at Ls ~205, and dust storm 1977b which started at ~Ls 275 (see Figure 26). Note – both Vikings landed at about at an altitude about 3.5 km below the areoid. Identical winds at Spirit, about 1.6 km higher, would be less able to move sand.

We reviewed the hourly winds for 20 sols after each of these Ls (Solar Longitude) positions in the Martian orbit, where Ls 0 = the start of spring (in the northern hemisphere where Viking 1 landed), Ls 90 = the start of summer, Ls 180 = the start of fall, and Ls 270 = the start of winter. In skimming through the data it appears that in the 20 sols that began at Ls 205, the maximum wind at Viking 1 was 25.9 m/s (57.93 mph - see Figure 23 above), but this velocity did not occur until Ls 210.872.  For the second dust storm the maximum wind was 18.3 m/s (40.9 mph). Note: For Global Dust Storm 1977a the first hourly wind for Viking 1, Ls 205 was reached by coincidence at its Sol 205. The initial hourly wind examined was at Ls 205.017 at Sol 205.38. Hourly winds were then tracked through its Sol 224.98. This occurred at Ls 217.301. For Global Dust Storm 1977b the first hourly wind examined for Viking 1 was at Ls 275.005 at its Sol 314.14. Hourly winds were then tracked through its Sol 333.98. This occurred at Ls 287.385.

Bridges et al. offer guidance about the relationship between pressure and threshold speeds. In a discussion about obliquities (planetary axis tilt) greater than the present 25° to 50°, they mention that at pressures of 10–15 mbar compared to the current ~6 mbar because the threshold friction speed is approximately inversely proportional to the square root of atmospheric density, such pressure increases will reduce threshold friction speeds by 30%-60%.

During Viking 1's sols 1 to 350 the maximum wind velocity recorded was 57.9 mph. For Viking 2 between its sols 1 to 399 its maximum wind was 51.9 mph. If the surface pressure is actually 10 to 15 mbar, and threshold speeds are reduced from 80 mph to 30% or 60% less, then these speeds become something between 56 mph (with a 30% reduction) and 24 mph (with a 60% reduction. The 24 mph speed is entirely consistent with velocities plotted on Figures 24 and 25 above. The highest wind recorded for Viking 1 also exceeds the 56 mph requirement. Therefore, the winds seen at Vikings 1 and 2 are consistent with moving sand at pressures of at least 10 and 15 mbar.  The frequently shifting sands could, of course, also be consistent with higher pressure. The 8,331 wind measurements are not at all consistent with a pressure of 6.1 mbar.  

          Bridges et al. conclude that "...these results show that winds in the present low-density atmosphere of Mars are sufficient to move dunes and ripples in many areas of the planet. A major climatic change with a thicker atmosphere is not required." We think that the last sentence needs to be lengthened a bit. The full sentence should read, "A major climatic change with a thicker atmosphere is not required because the thicker atmosphere already exists now."