MARS ATMOSPHERIC WINDS INDICATED BY MOTION OF THE VIKING LANDERS DURING PARACHUTE DESCENT

Winds in the Martian atmosphere at altitudes from 1.5 to 3.5 km have b een determined from parachute descent trajectories of the two Viking l anders. Viking 2 reached equilibrium with nearly steady winds of appro ximately 8 m/s to the southeast below 3.3 km at 1000 LT. Viking 1 wind s at 16 1 5 LT rotated clockwise with decreasing altitude but were gen erally to the northwest at approximately 20 m/s. At 1.5 km, the Viking 1 wind direction was close to that measured by the landed anemometer at the same local time, while at the Viking 2 site, wind direction at 1.5 km was nearly opposite that near the surface. This is consistent w ith the fact that Viking 1 descended within a vigorously convective bo undary layer, while Viking 2 at 1.5 km was above the boundary layer. T urbulent velocities in the Viking 1 boundary layer were approximately 3 m/s. Mean upflow velocity was approximately 1 m/s. The Viking 2 atmo sphere was relatively quiescent, with orderly wind directional variati on possibly suggesting the presence of waves. Comparison of the measur ed winds with a recent global circulation model showed little or no co rrespondence, probably an indication that the winds were locally contr olled. The boundary layer, slope-wind model of Haberle et al. (1993), with ground slope adjusted to give best fit to these and landed meteor ology data, gave roughly the wind direction and the observed magnitude within a factor of approximately 2, but with opposite rotation of the wind vector with altitude. The ground slopes deduced are consistent w ith terrain observed in approach to the Viking 1 landing, and do not c onflict with the less definitive terrain data at the Viking 2 site. Th e high sensitivity of winds at altitudes up to several kilometers to t errain slopes as small as a few meters per kilometer would suggest tha t slope winds may be widely found in the lowest few kilometers of Mars atmosphere.