Infrared scanning radiometer

Had the Moon been a smooth, featureless body with no variations in its composition

or surface structure, then the expected heating and cooling of its surface would be simple to predict. The temperature of any object in space that does not have its own heat source is a balance between the heat it absorbs from the Sun and any other sources, and the heat it radiates into space. These properties are strongly affected by the thermal conductivity of the surface, its structure, its reflectivity, or albedo and the angle of illumination. Under a vertical Sun, surface temperatures can reach well over 100°C while just before lunar dawn after the 2-week-long night they can fall as low as -180°C. In the permanently shadowed craters at the lunar poles, it can be -230°C. Since different materials in their various forms will heat and cool in different ways, important clues to its nature can be gained by studying the detailed temperature of the lunar surface.

Apollo 17’s infrared scanning radiometer was an early attempt to measure the Moon’s thennal profile by having an infrared sensor pass over the landscape, both night and day, as the spacecraft orbited overhead. The concept is similar to the thermal pictures taken of houses in cold climates to show where warmth from the building is being lost, except that Apollo’s sensor was only a single point, like a one – pixel camera. There was no multi-line or multi-pixel imaging sensor to create a ‘picture’, and images had to be processed from the results of the spot scanning a site over multiple passes, one line per orbit.

The instrument showed how varying rock types within craters could strongly affect the temperature profile of a landscape. For example, at night time, the central area of the crater Kepler proved to be over 30°C warmer than the surrounding mare, perhaps due to exposed rocks at the bottom of the bowl absorbing heat from the daytime and slowly radiating it at night while the surrounding dust chilled quickly. It was hoped that hot spots might be found over the night-time hemisphere indicating a source of volcanism, but none was found.

This radiometer was a forerunner of a later generation of instruments that have provided thermal images of many of the solar system’s worlds. Notable among such instruments was the thermal emission spectrograph which could analyse infrared light to deduce rock types. These were used at Mars, both in orbit and on the surface, to locate rocks that implied a history of running water on the red planet. Apollo was paving the way, but with a much cruder technology.