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Atmospheric Structure Instrument / Meteorology Package (ASI/MET)

NSSDCA ID: 1996-068A-02

Mission Name: Mars Pathfinder
Principal Investigator:Dr. Alvin Seiff


The Mars Pathfinder Atmospheric Structure Instrument / Meteorology Package (ASI/MET) was designed to study the atmospheric structure of Mars as the lander descended and the meteorological conditions at the surface after landing. During the entry, the data collected by the instruments from above 100 km altitude down to the surface allowed reconstruction of profiles of the atmospheric density, temperature, and pressure. On the ground, the meteorology package collected temperature, pressure, and wind data for use in the study and characterization of diurnal and longer term variations of the atmosphere.

The ASI/MET consisted of an accelerometer and temperature, pressure and wind sensors. The accelerometer is a redundant set of x-, y-, and z- axis sensors. Several gain states, ranging over 0.016 g, 0.8 g, and 40 g (g equals one Earth gravity) were available to cover the wide dynamic range from atmospheric entry to deceleration and landing. The maximum sensitivity along each axis was 20 micrometers per second squared. The accelerometer data is used to determine the density profile of the martian atmosphere. The pressure sensor was a Tavis magnetic reluctance diaphragm sensor (similar to that used by Viking). This instrument was used to measure pressure during the descent and after landing. The sensor was exposed to the atmosphere via a 1 m inlet tube during parachute descent and after landing. The measurements have a sensitivity of 0.25 microbar. Temperature was measured during the descent by a thin wire thermocouple. The sampling frequency of the sensors during descent is governed by the velocity of the lander.

After landing, a 1.1 meter high mast was deployed. Three more thin wire thermocouples are mounted on the mast at heights of 25, 50 and 100 cm above the surface, to monitor atmospheric temperature at these heights. This information is necessary to understand the near-surface atmospheric temperature gradient and heat flux. The thermocouples had sensitivities of 0.01 degree K and time constants of 1 to 2 seconds. The wind sensor consisted of six hot wire elements distributed uniformly about the top of the mast. Wind speed and direction 1.1 meters above the surface are derived from the temperatures of these elements. The meteorology instruments were connected to an electronics board which controlled operations and data output digitizing.

The instruments recorded data throughout the descent until about 1 minute after impact at 03:00 local solar time (LST) except for the entry pressure measurements, terminated at 0.3 km altitude by the inflation of the airbags, and the entry deceleration measuremnts, terminated at 0.1 km altitude by the descent rocket firing. Regular surface measurements by the MET began at 07:00 LST and the MET mast was deployed at 13:30 LST. Over the 30 day primary mission 51 3-minute measurement sessions were performed each martian day (sol), equally spaced in time. There were also 15 minute and one hour sessions of 1 second sampling and on sol 25 a full-day session of continuous sampling at 4-second intervals was performed. Some sampling was missed during the first 30 sols due to computer resets and problems with the MET software.

The ASI/MET instrument found lower temperatures and densities in the upper atmosphere during descent than the Viking 1 lander, possibly due primarily to the Viking 1 descent being in the local late afternoon as opposed to the early morning for Pathfinder. Density and temperature in the lower atmosphere were similar for the two craft. After Pathfinder landed at 19.33 N, 33.55 E at 3389.7 km from the center of Mars during local mid-summer the measurements showed evidence of 0.2 to 0.3 mbar daily variations indicating strong thermal tides and daily cycles showed two minima and two maxima. The temperatures were consistent from day to day, showing a maximum at the top mast thermocouple of approximately 263 K at 14:15 LST and a minimum of 197 K at 5:15 LST. Wind measurements showed the wind direction on a typical sol would be from the south at night and early morning and rotate slowly through west, north and east over the day. Possible evidence of ground fogs and dust devils was also detected.

Alternate Names

  • ASI
  • MET
  • MarsPathfinder/ASI
  • urn:nasa:pds:context:instrument:asimet.mpfl

Facts in Brief

Mass: 2.04 kg
Power (avg): 3.2 W

Funding Agency

  • National Aeronautics and Space Administration (United States)


  • Planetary Science: Atmospheres

Additional Information

Questions and comments about this experiment can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. Alvin SeiffTeam LeaderNASA Ames Research Center

Selected References

  • Sieff, A., et al., The atmosphere structure and meteorology instrument on the Mars Pathfinder lander, J. Geophys. Res., 102, No. E2, 4045-4056, doi:10.1029/96JE03320, Feb. 1997.
  • Schofield, J. T., et al., The Mars Pathfinder Atmospheric Structure Investigation/Meteorology (ASI/MET) experiment, Science, 278, No. 5344, 1752-1758, doi:10.1126/science.278.5344.1752, Dec. 1997.
  • Magalhaes, J. A., et al., Results of the Mars Pathfinder atmospheric structure investigation, J. Geophys. Res., 104, No. E4, 8943-8955, doi:10.1029/1998JE900041, Apr. 1999.
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