A Mystery in Mare Crisium
The Luna missions were a series of unmanned spacecraft missions sent to explore the Moon from orbit and on the surface, by the Soviet Union between 1959 and 1976.
Three of these missions, Luna 16 (September 1970), Luna 20 (February 1972) and Luna 24 (August 1976), are of interest because they returned soil samples from the surface of the Moon. These missions returned 0.33 kg (about 12 ounces) of lunar soil in total and the descent stages of the spacecraft still remain on the Moon and can be seen in LROC images (see LROC: Lunar Sample Return Missions below).
Luna 16 returned 101 grams from Mare Fecunditatis. A sampling arm was used to drill into the surface to collect soil which was then placed into a sample container on top of the spacecraft which later launched itself off the luna lander and returned to Earth.
Luna 16 spacecraft [National Space Science Data Center]
Luna 20 was the second successful sampling mission and the material returned from the Apollonius Highlands was similar in composition to the samples returned by the Apollo 16 astronauts.
Luna 23 and Luna 24 were both sent to sample Mare Crisium but Luna 23 failed (it may have landed on a slope and fallen over). Luna 24 was successful and sent back a 170 gram sample.
The Soviets originally thought that the two landers were within a few hundred meters of each other but using the images of the landers found on NAC images they appear to be about 2400 meters apart (see LROC: Mare Crisium Failure then Success below).
Luna 24 [NASA/GSFC/Arizona State University]
The sample material returned by Luna 24 was different from what was expected by the scientists. They had made certain assumptions about what the material should be, based on remote sensing data of Mare Crisium and their understanding of what Mare material should consist of. This mystery was only solved when the Luna 24 lander was found on NAC strip M174868307LC and shown to be sitting on impact ejecta from a nearby secondary crater. The sample returned to Earth consisted of material from beneath the surface of the Mare which had been excavated by the secondary impact.
Quote: (author=Jeff Plescia, LROC) The returned Luna 24 sample surprised scientists as it had unexpected characteristics based on the understanding of Mare Crisium geology at the time. Most importantly, the titanium content and the maturity (or the amount of time the sample was exposed to the space environment) of the sample material were different than anticipated. But how could this be? Based on the geologic context of the lander, the reason for the difference may now be understood. With the precise location of the landing site now known, the LROC images show that the mission sampled impact ejecta from a nearby 64-meter diameter crater. That crater has excavated below the surface bringing up material from deeper lava flows that had not been previously exposed to the space environment. Thus, the Luna 24 sample may not represent nearby Mare Crisium surface materials observed using remote sensing techniques, but rather the subsurface which was only exposed to the space environment for the relatively short time. It’s amazing what geologic context can tell you!
The Luna 24 mission may help with another Moon mystery – according to Soviet Moon Lander Discovered Water on the Moon in 1976 the sample returned by Luna 24 contained 0.1 percent water by mass.