close to the observed value 29sin(ᐉ ϩ 241°) mas. 23. We thank the Mars Pathfinder project team for their enthusiasm and assistance in acquiring and understanding the tracking measurements; R. Wimberly for recovery of the Viking lander Doppler data; and J. Williams and an anonymous referee for helpful suggestions. The research described in this paper was carried out by the Jet Propulsion Laboratory, Califor- The Mars Pathfinder atmospheric structure investigation/meteorology (ASI/MET ) experiment measured the vertical density, pressure, and temperature structure of the martian atmosphere from the surface to 160 km, and monitored surface meteorology and climate for 83 sols (1 sol ϭ 1 martian day ϭ 24.7 hours). The atmospheric structure and the weather record are similar to those observed by the Viking 1 lander ( VL-1) at the same latitude, altitude, and season 21 years ago, but there are differences related to diurnal effects and the surface properties of the landing site. These include a cold nighttime upper atmosphere; atmospheric temperatures that are 10 to 12 degrees kelvin warmer near the surface; light slope-controlled winds; and dust devils, identified by their pressure, wind, and temperature signatures. The results are consistent with the warm, moderately dusty atmosphere seen by VL-1.The ASI/MET experiment consists of a suite of sensors designed to measure the vertical structure of the atmosphere during entry, descent, and landing (EDL) and to study martian surface meteorology and climate for the duration of the Pathfinder mission (1, 2). In situ vertical structure measurements were made only twice by the Viking entry vehicles (3), both during the daytime. In addition to adding a third profile, ASI/MET provides the first nighttime observation, giving information about the diurnal variation of vertical structure, particularly in the upper atmosphere, which is inaccessible to existing remote-sensing techniques. Both Viking landers obtained records of atmospheric pressure, temperature, and wind velocity at the surface that extended over several Mars years. More recent Earth-based, disk-averaged microwave observations have been interpreted to indicate episodic cooling of the martian lower atmosphere by about 20 K relative to the conditions observed during the Viking missions (4). By continuing the Viking record after 21 years, ASI/MET results are able to determine whether martian meteorology and climate have changed or remained stable in the late northern summer. Improved measurement sensitivity and temporal resolution (2) also reveal phenomena not seen by Viking and, together with temperature measurements at three levels, give better information on the exchange of heat and momentum between the atmosphere and the surface.

The Mars Global Surveyor (MGS) z -axis accelerometer has obtained over 200 vertical structures of thermospheric density, temperature, and pressure, ranging from 110 to 170 kilometers, compared to only three previous such vertical structures. In November 1997, a regional dust storm in the Southern Hemisphere triggered an unexpectedly large thermospheric response at mid-northern latitudes, increasing the altitude of thermospheric pressure surfaces there by as much as 8 kilometers and indicating a strong global thermospheric response to a regional dust storm. Throughout the MGS mission, thermospheric density bulges have been detected on opposite sides of the planet near 90°E and 90°W, in the vicinity of maximum terrain heights. This wave 2 pattern may be caused by topographically-forced planetary waves propagating up from the lower atmosphere.