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P. Withers (University of Arizona), S.W. Bougher (University of Michigan), G.M. Keating (George Washington University at NASA Langley)
Winds are challenging to measure. We present a new technique for deriving winds from atmospheric density measurements and apply it to the martian upper atmosphere using MGS accelerometer data. This technique could be applied to Cassini observations of Titan's atmosphere to measure Titan's winds.
We have used the MGS accelerometer's upper atmospheric density profiles from aerobraking, together with the assumptions of hydrostatic equilibrium and negligible winds, to derive atmospheric pressure profiles along the spacecraft trajectory around periapsis. Unlike entry probes, atmospheric profiles from aerobraking are not vertical. Those from MGS traverse about one degree of latitude for each kilometre of altitude. Two independent estimates of atmospheric pressure at periapsis are derived, one from the inbound leg of the atmospheric pass, the other from the outbound leg. Observed discrepancies between these two estimates of periapsis pressure indicate an error in the assumptions. We make order-of-magnitude estimates of the neglected terms in the Navier-Stokes, or momentum conservation, equation and conclude that the largest neglected term is due to the zonal wind. The simplest possible extension to our initial assumptions is to include a constant and uniform zonal wind. We solve for this new, unknown parameter by requiring the two estimates of periapsis pressure to match. Self-consistent pressure and temperature profiles are also obtained.
Using this technique, each aerobraking pass can provide self-consistent density, pressure, and temperature profiles and a measurement of the zonal wind speed from the original density profile. There are many research questions on Mars and other planets that can be addressed with this technique.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.