This picture shows spectra of Mars acquired by the TES instrument
during orbit P6. The three spectra show the
effects of varied amounts of solar illumination. The black curves are
the energy that you would see if there were no materials present to
absorb energy (e.g., atmospheric gases, surface rocks). The curves
exhibit different amounts of total energy (or radiance, on the vertical
axis) at different temperatures.
The lowermost curve is very cold because it is an observation of
Mars' nightside. The large absorption at 15 microns (in all three
spectra) is due to the CO2 atmosphere of Mars, and it rises above the
majority of the curve in this spectrum because the atmosphere is
actually warmer than the surface underneath. The center of the
atmospheric band drops back down in radiance because the top of the
atmosphere gets cold again as it becomes thinner and is nearer to
space.
The center spectrum is taken at a place where local time is sunset,
or on the terminator. The terminator is the dividing line between day
and night. As you might expect, this place is warmer than the location
on the nightside, and the atmosphere is colder than the surface.
The upper curve represents a location that is on the dayside of
Mars, where temperatures are the warmest. This spectrum was taken at
an angle looking obliquely through the atmosphere of the planet, not
downward at the surface (nadir). Shallow, broad absorptions in the
8-12 micron region are the result of atmospheric dust and water ice
clouds.
