Higher than Advertised Martian Air Pressure - Part 1

1. INTRODUCTION TO PART 1.

Mars has long fascinated humanity and often been seen as a possible safe harbor for life.  In July, 1964 that hope was dealt a crushing blow by Mariner 4.  Images taken and data obtained from no closer than 9,846 km showed a heavily cratered, cold, and dead world. Air pressures are posted on a NASA site as estimated at 4.1 to 7 mbar, (http://nssdc.gsfc.nasa.gov/planetary/mars/mariner.html) although A. J. Kliore (1974) of JPL listed the Mariner 4-derived pressure range as 4.5 to 9 mbar. The different figures indicated by Government sources in my initial search of essential information were often a problem. Mariner 4 saw daytime temperatures of -100^o C, with no magnetic field.  Mariners 6, 7 and 9 got closer but still did not give us a picture that was much friendlier.  Mariner estimates for pressure, based on radio occultation, spanned a range of 1 or 2.8 to 10.3 mbar.  All pressure estimates were close to a vacuum when compared to average pressure on (1,013.25 mbar). However from a distance of 1,650 km, after a dust storm that obscured everything upon its arrival in orbit, Mariner 9 could see evidence of wind and water erosion, fog, and weather fronts.  When Vikings 1 and 2 landed, we learned that there is a high frequency of dust devils on Mars too.  Phoenix witnessed snow falling. The HIRISE and MER Spirit showed unexpected bedform (sand dune and ripple) movement (Bridges et al., 2011).

All landers agreed that pressure at their respective locations was somewhere between 6.5 and 10.72 mbar (Viking 2 sol 277.34 at Ls 279.93), but these low pressures make it very hard to explain the weather plainly seen.  This is particularly true of dust devils and blowing sand.

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1. INTRODUCTION

Mars has long fascinated humanity and often been seen as a possible safe harbor for life.  In July 1964 that hope was dealt a crushing blow by Mariner 4.  Images taken and data obtained from no closer than 9,846 km showed a heavily cratered, cold, and dead world. Air pressures are posted on a NASA site as estimated at 4.1 to 7 mbar, (http://nssdc.gsfc.nasa.gov/planetary/mars/mariner.html) although A. J. Kliore (1974) of JPL listed the Mariner 4-derived pressure range as 4.5 to 9 mbar. The different figures indicated by Government sources in my initial search of essential information were often a problem. Mariner 4 saw daytime temperatures of -100^o C, with no magnetic field.  Mariners 6, 7 and 9 got closer but still did not give us a picture that was much friendlier.  Mariner estimates for pressure, based on radio occultation, spanned a range of 1 or 2.8 to 10.3 mbar.  The important differences in minimal pressure will be elaborated on further in Section 5.  However all pressure estimates were close to a vacuum when compared to what we see on Earth (average pressure 1,013.25 mbar). Even from a distance of 1,650 km, after a dust storm that obscured everything upon its arrival in orbit, Mariner 9 could see evidence of wind and water erosion, fog, and weather fronts.  When Vikings 1 and 2 landed, we learned that there is a high frequency of dust devils on Mars too.  Moreover, Phoenix witnessed snow falling. 

All landers agreed that pressure at their respective locations was somewhere between 6.5 and 10.72 mbar (Viking 2 sol 277.34 at Ls 279.93), but these low pressures make it very hard to explain the weather plainly seen.  This is particularly true of dust devils.

1.1. Comparison of Martian and terrestrial dust devils 

Dust devils on Earth and Mars are similar with respect to geographic formation regions, seasonal occurrences, electrical properties, size, shape, diurnal formation rate, lifetime and frequency of occurrence, wind speed, core temperature excursions, and dust particle size.  The only significant differences lie in measured absolute and   relative pressure excursions in the cores of Martian and terrestrial dust devils. Clogged dust filters/pressure equalization ports on landers may have diminished accuracy of dust devil pressure excursion measurements (see sections 2.1 through 2.4 below).

1.1.1 Geographic Occurrences and the Greenhouse and Thermophoresis Effect 

Thousands of dust devils per week occur in the Peruvian Andes near the Subancaya volcano (Metzger, 2001) which is 5,900 meters high. Dust devils are also seen in abundance on a Martian volcano, Arsia Mons.  But the base altitude of some dust devils there has been about 17,000 meters.  Such an altitude on Mars supposedly would have about 1.2 mbar pressure, compared to about 478 mbar at Subancaya on Earth.  Reis et al. (2009) state that 28 active dust devils were reported in their study region for Arsia Mons, with 11 of them at altitudes greater than 16 km, and most inside the caldera (see Figure 1).  They don't fully understand how particles that are a few microns in size can be lifted there, and state that 1 mbar “requires wind speeds 2-3 times higher than at the Mars mean elevation for particle entanglement.”

Reis et al. (2009) suggest a greenhouse-thermophoretic (GT) effect that they believe explains ~1 mbar dust lifting at Arsia Mons. Their article states that “Laboratory and microgravity experiments show that the light flux needed for lift to occur is in the same range as that of solar insolation available on Mars.” They concede that high altitude dust devils do not follow the season of maximum insolation, but indicate that the GT-effect would be strongest around pressures of 1 mbar. However, if anything we would expect such dust lifted at high altitude to just drift away.  The GT effect simply does not explain the structure of these events at high altitude, or why the dust rotates in columns that precisely match dust devils produced at lower altitudes.  Further, Figure 1 shows that dust devils form at successively lower levels (i.e., higher pressures) as altitudes decline from 17 km to about 7 km, so there is nothing unique about reaching the ~1 mbar-level at the top of Arsia Mons. 

1.1.2. Seasonal occurrences.  Dust devils usually occur in the regional summer on Earth.  On Mars their tracks are most often spotted during regional spring and summer (Balme et al., 2003).

1.1.3. Electrical Properties 

There are indications that there may be high voltage electric fields associated with Martian dust devils. Such fields would mirror terrestrial dust devils, where estimates are as large as 0.8 MV for one such event (Farrell et al., 2004).

1.1.4. Size and Shape

About 8% of terrestrial dust devils exceed 300 m in height.  Bell (1967) reports some seen from the air that are 1,000-2,500 m high. Mars orbiters have shown dust devils there often are a few kilometers high and hundreds of meters in diameter, outdoing the larger terrestrial events in height. Martian dust devils can be up to 50 times as wide and 10 times as high as terrestrial dust devils (Smith & Nilton, 2001). But there are many smaller Martian dust devils. A NASA Spirit press release (8/19/2005) stated, “Martian and terrestrial dust devils are similar in morphology and can be extremely common.”  

1.1.5. Diurnal Formation Rate and Lifetime

About 80 convective vortices were recorded by Pathfinder.  Most occurred between 1200 and 1300 Local True Solar Time (Murphy & Nellis, 2002). On Earth noon is about the peak time. While dust devils may deposit the bulk of the dust responsible for the color of the Martian sky, it is also claimed that they might cause up to two thirds of all windblown dust for particles under 25 microns in the U.S., and (in the southwest) could be a major cause of poor air quality (Gillette and Sinclair, 1990).

1.1.6 Wind Speed of Tornadoes and Dust Devils 

The most powerful dust devil recorded by Mars Express Orbiter between January 2004 and July 2006 had a speed of 59 m/s (132 mph).  This is comparable in speed to an F2 tornado.  If the Martian atmospheric pressures are as low as advertised, these storms would not be so worrisome.  However, NASA must be certain of the pressures involved before risking lives of human explorers there.

Stanzel et al. (2008) assert that dust devil velocities were directly measured from orbit, and range from (only) 1 meter per second to 59 m/s.  These velocities convert to about 2.2 miles per hour on the low end to 132 miles per hour on the high end.  Even on the high end, we do not see the 70 m/s required to lift dust by a NASA Ames apparatus discussed below in section 1.2.

1.1.7 Core Temperature Excursions. 

Balme and Greeley (2006) state, “Positive temperature excursions in vortices measured by Viking and MPF landers had maximum values of 5-6 K. These values are similar to terrestrial measurements.”  However they note low sampling rates on Mars, and state, “Temperature excursions <10ºC are found consistently, but measurements with an order of magnitude higher sampling rate show temperature excursions as great as 20ºC.”  Ellehoj et al. (2009)  indicate that core excursions for Martian dust devils can be up to 10 K (ºC).

1.1.8 Dust Particle Size - The Problem of Martian Dust <2 Microns and Wind Speeds

Balme and Greeley (2006) also state, “The Martian atmosphere is thinner than Earth’s… so much higher wind speeds are required to pick up sand or dust on Mars.  Wind tunnel studies have shown that, like Earth, particles with diameter 80-100 μm (fine sand) are the easiest to move, having the lowest static threshold friction velocity, and that larger and smaller particles require stronger winds to entrain them into the flow.  However, much of Mars’ atmospheric dust load is very small, and the boundary layer wind speeds required to entrain such fine material are in excess of those measured at the surface (Magalhaes et al., 1999). Nevertheless, fine dust is somehow being injected into the atmosphere to support the observed haze and to supply local… and global… dust storms.” 

The problem of dust particle size is more serious than indicated above.  Optimum particle size for direct lifting by the wind (with the lowest threshold velocity) is around 90 μm. This requires a wind at 5 meters altitude to be around 30-40 m/s. For smaller particles like the 1 μm size dust typically suspended in the air over Mars, the threshold velocity is extremely high, requiring enormous wind speeds (>500 m/s) at 5 m altitude which would never occur.  As 500 m/s = 1,118 miles per hour, it is argued that saltation must be crucial to the lifting of very small particles into the air (Read and Lewis, 2004, 190).

What is saltation?  Bagnold (1954) indicated that large particles that are temporarily lifted into the air by surface winds would quickly fall out by sedimentation. On impact with the surface, they may dislodge smaller particles and lift them into the air.  As for what velocity fine about sand (~ 100 μm) would have on impact, it is only about 50 to 80 cm per second which is about 1.12 to 1.79 miles per hour (Read and Lewis, 2004, 197).

1.1.9. Core Pressure Excursions 

Roy E. Wyatt (1954) of the Weather Bureau Regional Office in Salt Lake City, Utah reported that a small, approximately 50-foot high, 50 to 60 foot wide dust devil had its center pass within 8-10 feet of a microbarograph on August 12, 1953 in St. George, Utah (Figure 2) at an altitude of 2,951 feet above sea level. A drop from 26.98 inches of mercury (913.644 mbar) to 26.94 inches of mercury (912.289 mbar) was recorded.  This 1.355 mbar drop in pressure equals 0.148%.

Balme and Greeley (2006) report that Pathfinder “identified 79 possible convective vortices from pressure data.” Recorded pressure drops were from ~0.075% to ~0.75%.   Figure 3 shows dust devils events for Pathfinder and Phoenix. If we examine the pressure drop seen by Phoenix from 8.425 to 8.422 mbar, that 0.003 mbar pressure drop is only about 0.036%. The Pathfinder event shows a drop in pressure from about 6.735 to 6.705 mbar (0.03 mbar). That is about a 0.445% drop. While the percent pressure drop is larger on the Pathfinder event than the Utah event, it was smaller for the Phoenix event. So absolute and percent pressure drops on Mars are producing almost the exact same storms, indeed often bigger storms, than we see on Earth. It might be argued that pressure is smaller on Mars; but so too is kinetic energy. Clearly, as we approach a vacuum, if we are going to see weather events based on pressure differences, there should be at least the same size percent pressure drops to drive them, not smaller ones.  However, most telling is that while the percent drops on Martian dust devils appear to overlap their terrestrial cousins, Viking 1 and 2 almost always saw much larger pressure increases each sol about 7:30 AM local time with increases up to 0.62 mbar from the previous hour at that time.

1.1.3. Electrical Properties 

There are indications that there may be high voltage electric fields associated with Martian dust devils. Such fields would mirror terrestrial dust devils, where estimates are as large as 0.8 MV for one such event (Farrell et al., 2004).

1.1.4. Size and Shape

About 8% of terrestrial dust devils exceed 300 m in height.  Bell (1967) reports some seen from the air that are 1,000-2,500 m high. Mars orbiters have shown dust devils there often are a few kilometers high and hundreds of meters in diameter, outdoing the larger terrestrial events in height. Martian dust devils can be up to 50 times as wide and 10 times as high as terrestrial dust devils (Smith & Nilton, 2001). But there are many smaller Martian dust devils. A NASA Spirit press release (8/19/2005) stated, “Martian and terrestrial dust devils are similar in morphology and can be extremely common.”  

1.1.5. Diurnal Formation Rate and Lifetime

About 80 convective vortices were recorded by Pathfinder.  Most occurred between 1200 and 1300 Local True Solar Time (Murphy & Nellis, 2002). On Earth noon is about the peak time. While dust devils may deposit the bulk of the dust responsible for the color of the Martian sky, it is also claimed that they might cause up to two thirds of all windblown dust for particles under 25 microns in the U.S., and (in the southwest) could be a major cause of poor air quality (Gillette and Sinclair, 1990).

1.1.6 Wind Speed of Tornadoes and Dust Devils 

The most powerful dust devil recorded by Mars Express Orbiter between January 2004 and July 2006 had a speed of 59 m/s (132 mph).  This is comparable in speed to an F2 tornado.  If the Martian atmospheric pressures are as low as advertised, these storms would not be so worrisome.  However, NASA must be certain of the pressures involved before risking lives of human explorers there.

Stanzel et al. (2008) assert that dust devil velocities were directly measured from orbit, and range from (only) 1 meter per second to 59 m/s.  These velocities convert to about 2.2 miles per hour on the low end to 132 miles per hour on the high end.  Even on the high end, we do not see the 70 m/s required to lift dust by a NASA Ames apparatus discussed below in section 1.2.

1.1.7 Core Temperature Excursions. 

Balme and Greeley (2006) state, “Positive temperature excursions in vortices measured by Viking and MPF landers had maximum values of 5-6 K. These values are similar to terrestrial measurements.”  However they note low sampling rates on Mars, and state, “Temperature excursions <10ºC are found consistently, but measurements with an order of magnitude higher sampling rate show temperature excursions as great as 20ºC.”  Ellehoj et al. (2009)  indicate that core excursions for Martian dust devils can be up to 10 K (ºC).

1.1.8 Dust Particle Size - The Problem of Martian Dust <2 Microns and Wind Speeds

Balme and Greeley (2006) also state, “The Martian atmosphere is thinner than Earth’s… so much higher wind speeds are required to pick up sand or dust on Mars.  Wind tunnel studies have shown that, like Earth, particles with diameter 80-100 μm (fine sand) are the easiest to move, having the lowest static threshold friction velocity, and that larger and smaller particles require stronger winds to entrain them into the flow.  However, much of Mars’ atmospheric dust load is very small, and the boundary layer wind speeds required to entrain such fine material are in excess of those measured at the surface (Magalhaes et al., 1999). Nevertheless, fine dust is somehow being injected into the atmosphere to support the observed haze and to supply local… and global… dust storms.” 

The problem of dust particle size is more serious than indicated above.  Optimum particle size for direct lifting by the wind (with the lowest threshold velocity) is around 90 μm. This requires a wind at 5 meters altitude to be around 30-40 m/s. For smaller particles like the 1 μm size dust typically suspended in the air over Mars, the threshold velocity is extremely high, requiring enormous wind speeds (>500 m/s) at 5 m altitude which would never occur.  As 500 m/s = 1,118 miles per hour, it is argued that saltation must be crucial to the lifting of very small particles into the air (Read and Lewis, 2004, 190).

What is saltation?  Bagnold (1954) indicated that large particles that are temporarily lifted into the air by surface winds would quickly fall out by sedimentation. On impact with the surface, they may dislodge smaller particles and lift them into the air.  As for what velocity fine about sand (~ 100 μm) would have on impact, it is only about 50 to 80 cm per second which is about 1.12 to 1.79 miles per hour (Read and Lewis, 2004, 197).

1.1.9. Core Pressure Excursions 

Roy E. Wyatt (1954) of the Weather Bureau Regional Office in Salt Lake City, Utah reported that a small, approximately 50-foot high, 50 to 60 foot wide dust devil had its center pass within 8-10 feet of a microbarograph on August 12, 1953 in St. George, Utah (Figure 2 above) at an altitude of 2,951 feet above sea level. A drop from 26.98 inches of mercury (913.644 mbar) to 26.94 inches of mercury (912.289 mbar) was recorded.  This 1.355 mbar drop in pressure equals 0.148%.

Balme and Greeley (2006) report that Pathfinder “identified 79 possible convective vortices from pressure data.” Recorded pressure drops were from ~0.075% to ~0.75%.   Figure 3 shows dust devils events for Pathfinder and Phoenix. If we examine the pressure drop seen by Phoenix from 8.425 to 8.422 mbar, that 0.003 mbar pressure drop is only about 0.036%. The Pathfinder event shows a drop in pressure from about 6.735 to 6.705 mbar (0.03 mbar). That is about a 0.445% drop. While the percent pressure drop is larger on the Pathfinder event than the Utah event, it was smaller for the Phoenix event. So absolute and percent pressure drops on Mars are producing almost the exact same storms, indeed often bigger storms, than we see on Earth. It might be argued that pressure is smaller on Mars; but so too is kinetic energy. Clearly, as we approach a vacuum, if we are going to see weather events based on pressure differences, there should be at least the same size percent pressure drops to drive them, not smaller ones.  However, most telling is that while the percent drops on Martian dust devils appear to overlap their terrestrial cousins, Viking 1 and 2 almost always saw much larger pressure increases each sol about 7:30 AM local time with increases up to 0.62 mbar from the previous hour at that time.

1.2. NASA Ames Test of Martian Pressures and Dust Devils