Temperature
Martian air temperature, in
Kelvin ( 200 Kelvin = -100 Fahrenheit; 260 Kelvin = +8 Fahrenheit), measured by the three
thermocouples (electronic temperature measuring devices) located at different heights on
the Mars Pathfinder ASI/MET mast. The top mast thermocouple is located one meter above the
outer edge of one of Pathfinders' three solar panels. The top of the solar panel is 0.4
meters (16 inches) above the martian surface, which puts the top mast thermocouple 1.4
meters (55 inches) above the surface. The middle mast thermocouple is located 0.5 meters
above the solar panel; the bottom thermocouple at 0.25 meters. Since air temperature
generally decreases with increasing distance from the sun-warmed ground during daylight
hours, the bottom mast thermocouple measures the warmest temperature at those times of
day. During the night, the ground rapidly cools and air temperatures are coldest nearest
the ground. Thus, at night, the top mast thermocouple measures the warmest temperature.
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.
The Mars
Pathfinder Lander measures air temperature with sensors located on the Meteorological mast
at 10, 20, and 40 inches, (0.25, 0.50, 1.0 meters) above the solar panel. Air temperatures
measured during the first three days of the mission show an afternoon high near +14
degrees Fahrenheit (264 Kelvin). [The 270 degree Kelvin temperatures measured at noon on
Sol 1 were obtained while the mast was still lying down just above a warm solar panel].
During the day, the air temperatures measured by the lowest of the 3 sensors are 9 to 10
degrees Fahrenheit (5 to 6 Kelvin) warmer than those measured by the top sensor because
the atmosphere is heated by the sunlit surface below it. At night, the ground cools more
quickly than the atmosphere, and the warmest air temperatures are measured by the
uppermost temperature sensor. These temperature differences provide information about the
exchange of heat between the surface and the atmosphere. The Viking landers could not
determine this quantity directly because they measured the air temperature at only one
height (64 inches or 1.6 meters). The air temperatures measured by Pathfinder are slightly
warmer than those seen 21 years ago at the nearby Viking Lander 1 site at the same Martian
season. This may result because the somewhat darker surface at the Pathfinder landing site
absorbs more sunlight than the Viking 1 Landing site.
Air
temperatures measured by the Mars Pathfinder Lander during the first two days of the
mission show an afternoon high near +9 degrees Fahrenheit (260 Kelvin). [The 270 degree
Kelvin temperatures measured at noon on Sol 1 were obtained while the mast was still lying
down just above a warm solar panel]. During the day, the air temperatures measured by the
lowest of the 3 sensors on the meterology mast are 9 to 10 degrees Fahrenheit (5 to 6
Kelvin) warmer than those measured by the top sensor because the atmosphere is heated by
the sunlit surface below it. These temperature differences provide information about the
exchange of heat between the surface and the atmosphere. The Viking landers could not
determine this quantity directly because they measured the air temperature at only one
height. The air temperatures measured by Pathfinder are slightly warmer than those seen at
the nearby Viking Lander 1 site at the same season. This may result because the somewhat
darker soil at the Pathfinder landing site may absorb more sunlight than the Viking 1
landing site. We hope to obtain morning minimum temperature measurements in the early
hours of Sol 3.
This figure compares preliminary atmospheric temperatures
recorded by the Pathfinder MET experiment with temperatures acquired at the same season
and time of day by the Viking 1 lander. Temperature is given in Kelvin and time is local
time of day at the lander site.