Results of the Mars Pathfinder atmospheric structure investigation


  • Julio A. Magalhães,

  • John T. Schofield,

  • Alvin Seiff


We report on a thorough analysis of the Mars Pathfinder atmospheric structure investigation (ASI) accelerometer data spanning the altitude range 161–8.9 km. Entry, descent, and landing occurred within 850 km of the Viking 1 lander and somewhat later in northern summer. The early morning entry (0300 hours) provided the first opportunity to study Mars' nighttime atmospheric structure; the close proximity to the Viking 1 site has permitted a search for changes in atmospheric structure during the 21 years between the landings. Our results confirm and refine the major features of the atmospheric structure discovered in the initial analysis of the ASI results reported by Schofield et al. [1997]. The current analysis has yielded a much better definition of the temperature structure above 90 km altitude. Pathfinder measured a thermospheric peak temperature of 153°K at 134 km, which is 30°K colder than the value found by Viking 1. Between 110 and 88 km, Pathfinder found a broad, nearly isothermal plateau at 127°K, which correlates very well with a similar structure evident in Viking 1 data at the same pressures, although the Pathfinder layer is 25°K colder. At levels above 88 km, wavelike oscillations with a wavelength ∼5 km and amplitudes of 2–3°K are evident, possibly reflecting gravity or planetary wave activity. Between 90 and 60 km the average temperature is 20–30°K cooler than Viking 1 values. Large-amplitude (10–20°K) long-wavelength (20–40 km) oscillations, which show a remarkable correspondence to equivalent structures found in the Viking 1 profile at the same pressures, occur in this region and are likely due to a diurnal tidal mode. Between 85 and 77 km a minimum of one oscillation results in temperatures up to 7°K below the saturation temperature of CO2, well outside the uncertainties in the measurements and the vapor pressure curve and comparable to observed supersaturations of water in Earth's mesosphere. Between 55 and 16 km, temperatures are close to or warmer than Viking 1 values; superposed long-wavelength oscillations correspond well with similar structures observed by Viking 1. Between 16 and 9.9 km the temperature profile shows an unexpected strong thermal inversion, which may reflect radiative cooling due to a water cloud. At the base of the inversion, temperatures begin to increase abruptly until the last measurement at 8.9 km. The Viking-like temperatures in the lower atmosphere are consistent with the Viking-like dust optical depths observed by the Mars Pathfinder imager. The cool temperatures in the middle and upper atmosphere may be indicative of significant nighttime cooling at these levels, although other interpretations are possible.