October 20, 2019
A Radially Expanding Atmosphere?
Originally published June 8, 2010
north to left image by Nick Smith Oxford, UK
Two days ago a tiny, long and tapering crater chain was a topic of discussion because it was fresh but not radial to any obvious source crater. Nick's excellent mosaic of Copernicus - trimmed unmercifully here - provides another example of a tapering crater chain that isn't radial to its obvious source crater, Copernicus. At top center is another chain of 8 or 9 larger craters that are clearly Copernican secondaries but also not radial to Copernicus. Perhaps there are good published explanations for these non-radial chains, but they do seem non-intuitive. The larger chain may have been formed by ejection of a mass of rock that was fractured into 8-9 pieces that traveled side by side. I don't know why that would have happened when nearly all other ejected masses tended to stretch out in a line along their trajectory. The tapered Copernican chain seems to have resulted from another fractured mass of ejecta, each piece of which had the same velocity but travelled different distances depending on its mass and size. But as Galileo demonstrated with two spheres of different masses, all the particles should move at the same speed and travel the same distance, unless they moved through an atmosphere and were sorted by aerodynamic drag. Or the gases carried the lighter particles further. Either of these interpretations would imply that the ejecta were embedded in gases, rather than simply traveling through the vacuum of space; a new idea?
May 22, 2010, ~ 20.40 UT in twilight. C14 + 1.8x Televue barlow + red filter + Lumenera Infinity 2-1M.
Rükl plates 20 and 32
Yesterday's LPOD: Imbrium Rediscovered
Tomorrow's LPOD: Selenometrics