MARS LANDING SITES

ATTEMPTED MARTIAN LANDINGS*AND MARTIAN WEATHER DATA

 * Source: http://en.wikipedia.org/wiki/Exploration_of_Mars#Timeline_of_Mars_exploration
   For Mars Pathfinder, pressure was measured by a Tavis magnetic reluctance diaphragm sensor similar to that used by Viking, both during descent and after landing. 

     The Phoenix lander in 2008 also carried a meteorological station.  However, it carried a 26-gram instrument developed by the Finish Meteorological Institute.  The device is based on the Vaisala Oy sensor technology and components for the instrument which was delivered by the Micro Analog Systems Oy and Selmic Oy. 

The Vikings were designed to only measure pressure up about 13.79 mbar.  The Pathfinder was designed to only measure pressure up to 12 mbar.  The Phoenix was limited to 5 to 12 mbar.  None of these landers had equipment that could function right on Earth below an altitude of at least 68,000 feet - and that's figuring that the Tavis sensor would work past its designed 13.79 mbar level right up to 36 mbar.  If they would not work right past 13.79 mbar, then the figure would be 81,000 feet above the Earth.  The Tavis magnetic reluctance transducer works on the basis of converting pressure to voltage, with 0 volts at 0 mbar, 5 volts at 0.2 psia (about 13.79 mbar), which was its rating, and 15 volts maximum possible voltage allowed.  After that its a real question of what would happen.  The instrument was apparently designed to work well only with temperatures above -50 degrees Celsius, but it was regularly exposed to -78 C. Pathfinder was also exposed to -170 degrees Celsius at 80 km on its entry.

Smith figures from Smith, D. E., et al. (2001), Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars, J. Geophys. Res., 106, 23,689–23,722, doi:10.1029/2000JE001364.http://www-geodyn.mit.edu/mola.summary.pdf

ALTITIUDES FOR LANDERS WITH METEOROLOGICAL INSTRUMENTS:

On Earth there is a loss of about 1 inch of mercury (about 33.86 mbar) for each thousand feet ascended.  Lander altitude and latitude can have an impact on pressure readings, and must be compensated for when comparing mission pressure readings. The source for the information below is a draft copy of a paper by Peter A. Taylor, et. al. (October 20, 2009) entitled On Pressure Measurement and Seasonal Pressure Variations during the Phoenix Mission.  Because Pathfinder is at a higher altitude, its minimum pressure was adjusted from 670 Pa (6.7 mbar) to 698 Pa.  They state that Viking 1 amplitudes are increased by a factor of 1.0455 to compare them with the Phoenix values (Viking 1 was higher, so it feels less air pressure than Phoenix) while Viking 2 values should be reduced by a factor of 0.967 to compare with Phoenix (Viking 2 was lower than Phoenix, so it should feel more pressure than Phoenix).  The paper listed the mean (adjusted) pressure at sites as follows: Viking 1 year A = 7.936 mbar, Viking 1 Year B = 7.942 mbar and Phoenix 8.326 mbar.  Again, the above Pathfinder pressure was listed not as a mean, but rather as a minimum.  

Figure 1 below: MOLA map of Mars with major topographic features, landing sites, and locations of methane plumes identified by Krasnopolsky et al. (2004).