The Mars Pathfinder Atmospheric Structure Investigation/Meteorology (ASI/MET) Experiment

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Science  05 Dec 1997:
Vol. 278, Issue 5344, pp. 1752-1758
DOI: 10.1126/science.278.5344.1752


  • Figure 1

    (left). The atmospheric density profile derived from ASI/MET. The solid lines give the mean atmospheric density profile derived from the accelerometer data and profiles reflecting ±2σ uncertainties in density based on uncertainties in the entry velocity and the finite digital resolution of the instrument. Errors in aerodynamic characteristics are not included but are not expected to change the error envelopes substantially. Further work with the accelerometer and pressure data will allow us to extend the density profile down to the surface, where the MET observations of pressure and temperature indicate an atmospheric density of 1.76 × 10 2kg/m3, marked by an oval on the x axis of the figure. Results from the VL-1 atmospheric structure instrument (ASI) (2, 3) and the Viking 1 upper atmosphere mass spectrometer (UAMS) are also plotted for comparison (2,3).

  • Figure 2

    (right). The atmospheric temperature profile derived from the Pathfinder atmospheric density profile. Temperature profiles corresponding to the nominal and ±2σ density profiles of Fig. 1 are represented by solid lines. The hydrostatic equation is integrated to derive a pressure profile from the density profile, and temperature is calculated from density and pressure with the use of the ideal gas law. To begin the integration, an upper boundary temperature is determined from the density scale height at the upper boundary. Uncertainties in this temperature affect the derived profiles significantly only above 125 km. In order to derive temperatures, we have constructed a molecular weight versus atmospheric density model based on the results of the Viking UAMS (29). At present, we have no way of quantifying the accuracy of this model, which influences temperature above 120 km. At lower altitudes, the martian atmosphere is well mixed, with a constant molecular weight of 43.49. Temperature profiles from the VL-1 ASI and UAMS experiments (2, 3), the CO2 condensation temperature profile, and the surface temperature measured by the Pathfinder MET instrument (circle) are also shown for comparison.

  • Figure 3

    (A) Time-averaged surface pressures measured by the MET instrument over the first 30 sols of the Pathfinder landed mission. The averages are primarily over the 3-min default measurement sessions, of which there are nominally 51 per sol; and the resulting points have been connected with straight lines, except for sols 12 through 15, where cubic spline interpolation has been used to fill data gaps of about 8 hours in length. MET operation was restricted to nighttime observations during this period to prevent spacecraft resets associated with MET data collection. The major gaps in the data set at sols 1, 8, 11, and 17 are caused by various spacecraft software reset and downlink problems. After sol 17, the reset problems associated with MET were corrected, and continuous sampling was resumed. The long-term trend in pressure is represented by a third-order polynomial fit to the data (solid curve). (B) Diurnal pressure cycles for sols 9 (solid line) and 19 (dashed line), illustrating the observed day-to-day changes in the diurnal pressure cycle and allowing details of the daily pressure variation to be seen more clearly.

  • Figure 4

    (top). The diurnal variation of atmospheric temperature measured by the top (red), middle (black), and bottom (blue) mast thermocouples, from 06:00 LST on sol 25 to 06:00 LST on sol 26. These thermocouples are respectively 100, 50, and 25 cm above the plane of the lander solar panels. Temperatures are sampled continuously at 4-s intervals throughout this period, but the plots use 30-point (2-min) running means for clarity (this smoothing reduces the amplitude and frequency of the fluctuations that are present in the raw data).

  • Figure 5

    (bottom). The data of Fig. 4 plotted as temperature deviations from the mean of all three thermocouples. Sampling times and data smoothing are identical to those of Fig.4.

  • Figure 6

    Time-averaged wind direction measured by the MET instrument over the first 30 sols of the Pathfinder landed mission, plotted as a function of LST. Each point represents an average over a 3-min default measurement session. Wind direction is defined as follows: 0° and 360° (northerly), 90° (easterly), 180° (southerly), and 270° (westerly).

  • Figure 7

    (A) The amplitude of the diurnal (black diamonds) and semidiurnal (open diamonds) surface pressure tides for the first 30 sols of the Pathfinder landed mission. Amplitudes are normalized relative to the mean pressure for each day. (B) The phase, in LST, of the diurnal (black diamonds) and semidiurnal (open diamonds) surface pressure tides. Enough pressure data were collected to characterize the thermal tide on 20 sols. Pressure measurements from each of these sols were fitted with a cubic spline, from which 51 equally spaced intervals were sampled to define the tide.

  • Figure 8

    Pressure, wind, and temperature changes associated with a small-scale vortex, or dust devil, passing through the Pathfinder landing site. The measurements were taken at 4-s intervals.

  • Figure 9

    An average power spectrumS(f) for the martian atmosphere (dashed line) is compared to a similar spectrum from Earth obtained under weakly unstable conditions (bold line). The average spectrum is derived from night- and daytime power spectra for 10-min martian atmosphere temperature time series measured by the bottom mast thermocouple. The spectra are normalized by the turbulence temperature scaleT * derived from the temperature profiles, assuming a logarithmic profile ofT(z) =T */κLn(z/z0), where κ is Von Karman's constant and z is altitude, and plotted versus the frequency f. The difference between the Earth and Mars spectra at high frequencies is mainly due to sensor time constants. The line marked 2/3 represents the ideal slope of the inertial subrange power law.