Much of the Moon is blanketed by a thick layer of dust, built up from the rocky surface over billions of years by the impacts of small meteorites. Hidden beneath the dust is evidence of ancient geologic activity – great volcanic eruptions, tectonic shifts in the crust, and vast deposits of once-molten material hurled outward during the formation of the giant impact basins.
Smithsonian scientists, using the world’s two largest radio telescopes, bounce radar signals off the Moon to map these geologic features below the surface. Radio signals have a much longer wavelength than visible light, so they penetrate far into the dry lunar dust and reflect back from boulders and rugged surfaces beneath. The radar maps are created by measuring with great precision the round-trip time of the echoes and the shifts in frequency created by the slow spin of the Moon.
A radar image of the Moon collected using the Arecibo Observatory and Green Bank Telescope in 2015. The dark ring is caused by the shape of the radar beam. The north pole of the Moon is at the bottom center of this image. Younger craters with rugged floors and rims are very bright. Bruce Campbell, Smithsonian Institution
Radar images look like photographs, but the illumination is provided entirely by the transmitted signal so the position of the Sun (the phase of the Moon) does not matter. Sometimes the lighting provided by the radar, especially when it falls on terrain near the visible edge of the Moon, can lead to long shadows behind mountains and ridges.
A radar view of Bel’kovich-A crater (58 km in diameter) near Mare Humboldtianum on the Moon.
Radar can also “see” into craters near the Moon’s poles that are never lit by the Sun, which are of great interest as traps for icy material delivered to the surface by comets. For these polar mapping observations, the slow wobble (or libration) of the Moon becomes important, as it alternately brings the two poles into better view from Earth. Radar images collected at different times during this cycle can look very much like photos taken at different times of day, but with the shadows aligned with the direction of the incident radar signal rather than with the Sun.
Radar views of an area near the Moon’s north pole collected on different days. In the left image, the Moon’s wobble has brought more of the polar region into view, so the shadows of the large mountain at center are shorter. Bruce Campbell, Smithsonian Institution
A more detailed explanation of the radar mapping and image products may be found on the Museum's website.