Apollo 11 landing site, with lower reflectance blast zone (LR-BZ) and higher reflectance blast zone (HR-BZ) outlined. Credit: NASA/LROC/ASU
Images of Surveyor landing sites with blast zones highlighted. Credit: NASA/LROC/ASU
Newly noted research in the pages of the scientific journal, Icarus, a team of scientists have spotlighted the impact of past rocket exhaust upon the lunar landscape.
The paper, quite literally, kicks up some dust!
Thanks to NASA’s Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC), high-resolution images of the U.S. robotic Surveyor, manned Apollo, and robotic Soviet Luna landing sites have been taken.
Using special processing techniques, the imagery show regions around the American and Soviet landers where reflectivity of the surface was modified.
Rocket exhaust
The researchers interpret those changes in reflectance properties in those regions mainly as disturbance of the regolith (topside materials on the Moon) by rocket exhaust during descent of spacecraft.
The team refers to these areas as “blast zones” or BZs for short.
The BZs consist of an area of lower reflectance (LR-BZ) compared to the surroundings that extends up to a few meters out from the landers, as well as a broader halo of higher reflectance (HR-BZ) that extends tens to hundreds of meters away from the landers.
Blast zone size scales with lander thrust.
Still visible anomalies
The LR-BZs are affected by macroscopic disruption of the surface and, in the case of the Apollo landing sites, astronaut activity on the lunar surface.
In detailing their work, the researchers found that the descent engine exhaust plumes of the Surveyor, Luna, and Apollo spacecraft significantly affected the regolith surrounding their landing sites.
Furthermore, owing to the lack of rapid weathering processes on the Moon, these surface alterations are still visible as photometric anomalies in LROC and NAC images.
Apollo blast zones
Most of the Apollo blast zones are elliptical in shape, but some are irregular. Apollo 12’s HR-BZ was significantly expanded by Surveyor Crater, and Apollo 17 has a lobe that may indicate the path of descent of the two-person lunar lander.
On only two missions (Apollo 14 and Apollo 17) did the astronauts report that the blowing dust did not hamper their ability to land safely.
Apollo 11 moved sideways to avoid landing in a crater, and both Apollo 12 and Apollo 16 hovered over the surface while looking for safe landing spots.
The largest measured blast zone is that of Apollo 12.
The largest blast zones of Apollo landers were measured to be roughly 492 feet (150 meters) to 850 feet (260 meters) in diameter. However, modeling and analysis of dust particle velocities and trajectories indicate that most particles probably traveled kilometers.
What’s more, some of those particles may even have reached the escape velocity of the Moon.
Fairy-castle structure
Another research finding is the possibility that Apollo lander exhaust plumes destroyed “fairy-castle structure” at their respective touchdown sites.
Fairy-castle structure, according to the Icarus paper, refers to the stacking of grains when particles are small enough that adhesive forces overcome gravitational forces, allowing a small particle to be supported by one contact rather than the three points required for a large particle.
This stacking configuration creates a very cohesive and highly porous structure that when viewed with a stereoscopic microscope appears to consist of towers leaning at random angles and connected by bridges – hence the term “fairy castle,” the research paper notes.
Future steps
The researchers are continuing their studies.
The work to date has been led by Ryan Clegg of Washington University in St. Louis, Department of Earth and Planetary Sciences. Also taking part in the investigation is Bradley Jolliff of Washington University; Mark Robinson of the School of Earth and Space Exploration at Arizona State University, Tempe, Arizona; Bruce Hapke of the Department of Geology and Planetary Science, University of Pittsburgh; and Jeffrey Plescia at the John Hopkins University Applied Physics Lab in Laurel, Maryland.
“These future steps will help to better understand what process(es) occurred at the landing sites to create the observed blast zone characteristics, as well as provide valuable information about the behavior of potentially damaging lunar soil during powered descents for future human and robotic missions to the lunar surface,” they conclude in their accepted paper.
By Leonard David