Ames Probe Workshop Abstract Paul Withers Center for Space Physics, Boston University, 725 Commonwealth Avenue, Boston MA 02215, USA withers@bu.edu Should we believe atmospheric temperatures measured by entry accelerometers travelling at "slow" near-sonic speeds? The accuracy of atmospheric structure reconstructions from entry probes is heavily dependent on the accuracy of the available aerodynamic database. These databases are primarily derived from numerical modelling and are typically stated to have uncertainties of 5 per cent or so. The Mars Pathfinder atmospheric structure reconstruction, performed by several groups, identified "at 16 km an unexpected strong temperature inversion ... in which temperature decreases from 199K at this altitude down to 183K at 9.9km." It has been suggested that this inversion "arises from radiative cooling by a water ice cloud layer." However, temperature profiles in the same region at the same Mars season of the following year measured by the Mars Global Surveyor Radio Science occultations do not show any significant inversions. Assuming that the martian climate is somewhat repeatable from year to year, how can this discrepancy be reconciled? According to the nominal aerodynamic database, Pathfinder's drag coefficient is changing significantly over this altitude region. It seems possible that the difference between the nominal and the actual drag coefficients is also changing significantly over this altitude region, which would cause the derived temperature profile to deviate from the actual temperature profile. I will conduct numerical simulations of Pathfinder's entry conducted with modified aerodynamic databases to quantify this idea and determine whether errors in the nominal aerodynamic database can cause such large discrepancies in atmospheric temperatures. I will also discuss different ways of handling the errors due to inaccurate aerodynamic databases and whether they explain why the published Pathfinder results have uncertainties much smaller than the size of the suspicious temperature inversion. This work will have an impact on the atmospheric structures derived from the Spirit and Opportunity atmospheric entries, and possibly on results from other missions. If data from Spirit and Opportunity are publicly available at the time of the meeting, then their results will also be used to discuss this issue.