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 over the 2-week-long night just before lunar dawn, they can fall as low as -180°C. In the permanently shadowed craters at the lunar poles, it can fall as low as -230°C. Studying the detailed temperature of the lunar surface as it is heated and cooled can give important clues to its nature.

Apollo 17's infrared scanning radiometer was an early attempt to measure the Moon's thermal 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. There was no multi-line or multi-pixel imaging sensor to create a 'picture', and images had to be processed from the results of 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 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 closely analyse infrared light and deduce rock types from the results. These were used at Mars, both in orbit and on the surface, to locate rocks that inferred a history of running water on the red planet.

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