Paolo’s low Sun view of the 62 km wide crater Abulfeda beautifully shows the shadow of its rim peaks, and he noticed a narrow rille-like feature on the eastern part of the floor. Wes Higgins’ higher Sun image from 2005 shows only vague hints of a possible rille. What is the feature? Paolo looked at the Lunar Orbiter IV and Clementine images which have higher illumination and realized that they don’t help understand what is on the floor. The orbiting Apollo 16 Command Module also imaged this area (while documenting the nearby Apollo 16 landing site at Descartes) with both high resolution Metric and Panoramic cameras. These images don’t indicate the presence of a rille, but do explain what Paolo imaged. The curved section of the possible rille near the small crater on the northern end of the floor is actually a secondary crater chain. And the apparent straight segment of the rille near the middle of Abulfeda’s floor is probably the line of four separate craters seen on the images of Apollo 16 and Wes. There is no rille on the floor of Abulfeda. But the interpretation of the feature as a weak rille is reasonable from Paolo’s low Sun image, and he did exactly the right thing in checking spacecraft imagery to try to confirm it. Finding the lower Sun Apollo 16 images make it fairly certain that there is no rille; hopefully someday a SMART-1 image will be released and completely resolve this issue. This episode is similar to the discovery of canals on Mars by visual observers more than 100 years ago: Small separate features, not quite resolved, get interpreted by our brains as continuous lines. The excitement comes from pushing imaging to detect features at the limit of resolution, and then pushing further to confirm what is really there.
Lazzarotti image: 13 Oct 2006, Gladio 315 Lazzarotti Opt. telescope (f/25), Lumenera Infinity 2-1M camera, Edmunds Optics R filter; 100 frames stack out of 1800. Higgins image: 25 Aug, 2005. Starmaster 18″ Newtonian + DMK-21FO4 camera.
Yesterday's LPOD: Digging Below the Surface
Tomorrow's LPOD: An Increasingly Complex Tale