The relationship between MOLA northern hemisphere topography and the 6.1-Mbar atmospheric pressure surface of Mars

To assist in targeting of landers and to enable comparison of new elevation data to previous topography models, we have re-determined the position of the 6.1-mbar atmospheric pressure surface on Mars with respect to surface t opography from the Mars Orbiter Laser Altimeter (MOLA). The 6.1-mbar surfac e lies, on average, 1600 m below the geoid-defined zero level of MOLA topog raphy, which occurs at an average pressure of 5.2 mbars at L-S=0 degrees. T he elevation of the 6.1-mbar surface is expected to vary by 1.5-2.5 km over the Martian year due to the seasonal exchange of CO2 between the atmospher e and polar caps. For average Martian atmospheric conditions, the pressure at the Mars Surveyor '98 landing site is expected to be similar to 5.0 mbar s during the lander science phase of the mission assuming an elevation of 1 600 meters above the zero level defined by MOLA topography.