Results of the Mars Pathfinder atmospheric structure investigation

We report on a thorough analysis of the Mars Pathfinder atmospheric structu re investigation (ASI) accelerometer data spanning the altitude range 161-8 .9 km. Entry, descent, and landing occurred within 850 km of the Viking 1 l ander 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 landin gs. Our results confirm and refine the major features of the atmospheric st ructure discovered in the initial analysis of the ASI results reported by S chofield et al. [1997]. The current analysis has yielded a much better defi nition of the temperature structure above 90 km altitude. Pathfinder measur ed a thermospheric peak temperature of 153 degrees K at 134 km, which is 30 degrees K colder than the value found by Viking 1. Between 110 and 88 km, Pathfinder found a broad, nearly isothermal plateau at 127 degrees K, which correlates very well with a similar structure evident in Viking 1 data at the same pressures, although the Pathfinder layer is 25 degrees K colder. A t levels above 88 km, wavelike oscillations with a wavelength similar to 5 km and amplitudes of 2-3 degrees K are evident, possibly reflecting gravity or planetary wave activity. Between 90 and 60 km the average temperature i s 20-30 degrees K cooler than Viking 1 values. Large-amplitude (10-20 degre es K) long-wavelength (20-40 km) oscillations, which show a remarkable corr espondence to equivalent structures found in the Viking 1 profile at the sa me pressures, occur in this region and are likely due to a diurnal tidal mo de. Between 85 and 77 km a minimum of one oscillation results in temperatur es up to 7 degrees K below the saturation temperature of CO2, well outside the uncertainties in the measurements and the vapor pressure curve and comp arable to observed supersaturations of water in Earth's mesosphere. Between 55 and 16 km, temperatures are close to or warmer than Viking 1 values; su perposed long-wavelength oscillations correspond well with similar structur es observed by Viking 1. Between 16 and 9.9 km the temperature profile show s; an unexpected strong thermal inversion, which may reflect radiative cool ing 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 du st optical depths observed by the Mars Pathfinder imager. The cool temperat ures in the middle and upper atmosphere may be indicative of significant ni ghttime cooling at these levels, although other interpretations are possibl e.