|Martian impact crater formed between July 2010 and|
May 2012. NASA image ESP-034285_1835.
In discussing this with a colleague, I pointed out that many of the studies of impact ejecta processes date back to the 1960's and 1970's, and were in the context of where to send an astronaut to explore on the Moon. If you wanted to sample material from deep in the crust, it would be too hazardous for an astronaut to climb down the walls of an impact crater (believe me, having scrambled around the walls of Meteor Crater in Arizona many times, you do not want to be wearing a space suit while climbing down into an impact crater!). One thought was that you could sample the ejecta by going to the rays of a crater. For example, from this source:
"Lunar crater rays are those obvious bright streaks of material that we can see extending radially away from many impact craters. Historically, they were once regarded as salt deposits from evaporated water (early 1900s) and volcanic ash or dust streaks (late 1940s). Beginning in the 1960s, with the pioneering work of Eugene Shoemaker, rays were recognized as fragmental material ejected from primary and secondary craters during impact events. Their formation was an important mechanism for moving rocks around the lunar surface and rays were considered when planning the Apollo landing sites. A ray from Copernicus crater crosses the Apollo 12 site in Oceanus Procellarum. Rays of North Ray and South Ray craters cross near the Apollo 16 site in the Descartes Highlands and a ray from Tycho crater can be traced across the Apollo 17 site in the Taurus-Littrow Valley on the eastern edge of Mare Serenitatis. There is still much debate over how much ejecta comes from the primary impact site or by secondary craters that mix local bedrock into ray material."
In a 1971 article, Verne Obereck concluded that the bright rays "only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters:"
Observations of high resolution photographs of part of one of the prominent rays of the lunar crater Copernicus show that there is a concentration of small bright rayed and haloed craters within the ray. These craters contribute to the overall ray brightness; they have been measured and their surface distribution has been mapped. Sixty-two percent of the bright craters can be identified from study of high resolution photographs as concentric impact craters. These craters contain in their ejecta blankets, rocks from the lunar substrate that are brighter than the adjacent mare surface. It is concluded that the brightness of the large ray from the crater Copernicus is due to the composite effect of many small concentric impact craters with rocky ejecta blankets. If this is the dominant mechanism for the production of other rays from Copernicus and other large lunar craters, then rays may not contain significant amounts of ejecta from the central crater or from large secondary craters. They may in fact only reflect local excavation of mare substrate material by myriads of small secondary or tertiary impact craters.
Recently, Valery Shuvalov proposed a ray production mechanism based on a large supercomputer simulation. In this simulation, the hypothesis was that rays result from interaction between the shock wave associated with a developing crater and nonuniformities in the target surface. The results of a simulation of the formation of a crater by a 5-km diameter asteroid on the Moon at an impact velocity of 15 km/s are shown in the adjacent figure. This impact would have produced a crater approximately the size of Tycho, a famous rayed crater on the Moon. The target and projectile material were both assumed to have the mechanical properties of granite.
When the shock wave from the developing primary crater hits a depression (preexisting small crater) a jet of material is spalled off the wall of the small crater proximal to the primary crater (upper left in the simulation sequence shown). In contrast to the effect of a depression on ray formation, a ray-suppressing effect is seen if there is a nearby elevation.