File:MarsAtmosphere T Profile Pathfinder.jpg

English: This figure presents a preliminary evaluation of the atmospheric temperature structure encountered by Mars Pathfinder during its descent through the Martian atmosphere on July 4, 1997. The deceleration of the probe during its entry is directly related to atmospheric density. Pressures and temperatures can be derived from the density using well-established physical principles. The Atmospheric Structure Instrument measured the probe's deceleration using high precision accelerometers. The temperature profile measured by the Viking 1 lander during its descent to the surface of Mars on July 20, 1976 is shown for comparison. A key debate in the Martian atmospheric sciences community has been over whether Martian climate has changed significantly since the era of the Viking missions. The issue has been whether the lower and middle atmosphere of Mars (altitudes less than 50 km) are "cold" relative to Viking or whether they are "warm" as at the time of Viking. The temperature profile in this figure shows the upper atmosphere of Mars to be quite cold relative to Viking. In fact at about 80 km altitude, the temperature is the lowest ever measured on Mars-- a brisk -275 degrees Fahrenheit. The cold temperatures in this region are not surprising since Mars Pathfinder entered the atmosphere at 3 AM Mars Local Time when the upper atmosphere cools due to the lack of solar heating. Below 60 km altitude, the temperatures measured by Pathfinder are quite close to those measured by Viking. Therefore, the measurements by Mars Pathfinder, which represent one slice through the atmosphere at one location and time, show that the atmosphere is "warm" as it was at the time of Viking.