Mahesh Anand on What the Return of Humans to the Moon Means for Lunar Geology
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We have learned a great deal about the geology of the Moon from remote sensing instruments aboard lunar orbiters, from robot landers, from the Apollo landings, and from samples returned to the Earth by Apollo and robot landings. But in 2025, when NASA plans to land humans on the Moon for the first time since 1972, a new phase of lunar exploration is expected to begin. What will this mean for our understanding of the origin, evolution, and present structure of the Moon? A lot, according to Mahesh Anand. For example, as he explains in the podcast, satellite imagery suggests that volcanism continued for much longer than was previously thought, perhaps until as recently as 100 million years ago. In-situ inspection and sample return should help us explain this surprising finding.
Mahesh Anand is Professor of Planetary Science and Exploration at the Open University, UK.
Podcast Illustrations
Schematic cross-section of the Moon. Like the Earth, the Moon has a core with an inner solid part and a liquid outer part, a mantle, and a crust. The lunar core is relatively smaller (radius of 20% of the whole) compared to that of the Earth (45%).
Landing on the Moon
Landing-site selection for the Apollo missions was mainly governed by practical considerations to ensure safety upon landing and return to Earth. But it turned out that the selected landing sites were geologically atypical, most of them being rich in potassium, rare-earth elements, and phosphorous (KREEP). The Artemis mission landing planned for 2025 will be near the lunar South Pole, which exposes some of the Moon’s oldest surface, estimated to be at least 3.85 billion years old.
Courtesy of NASA
Sites of successful Moon landings. China has also landed one unmanned craft on the far side of the Moon.
Source NASA, BBC
Apollo 17
The image shows the Apollo 17 landing site where a number of scientific measurements were made. The flag marks the site of a geophone, one of four installed to measure seismic waves generated by detonation of an explosive. This yielded a seismic profile down to a depth of 3 km. The mission included a lunar rover, which is visible in the distance.
Courtesy of NASA
Apollo 17 astronaut Jack Schmitt collecting lunar samples. In the image, he is shaking soil out of a rake after making a pass through the surface soil. A variety of tools were used to collect samples on the Moon’s surface, including drills, hammers, rakes, tongs, and core tubes.
Courtesy of NASA
In the podcast, Anand recounts how Jack Schmitt noticed and collected material that looked unusual. This turned out be volcanic glass that contained the first water discovered on the Moon. The rock was found in the Shorty Crater within the orange soil at bottom-right in the photograph at right. The photomicrograph of the soil at left shows orange and black glass spheres having sizes of 25 to 45 microns.
In his journal, Schmitt wrote: “It was volcanic material, but it was volcanic glass that had been spewed out of some fire-fountain-like eruptions 3.5 billion years ago that somehow had been protected from mixing with anything else, even though it was now at the surface. It had almost certainly been covered almost immediately by a lava flow, so that it was protected from meteor disruption and stirring. And then, when Shorty formed, somehow the pyroclastic ended up in the rim and a few other places in nearly pure form.”
Courtesy of NASA
Cratering Curve
As mentioned in the podcast, in 2022, the Chang'e 5 mission returned a sample of lunar basalt to Earth, where a radiometric age of 2 billion years was obtained. This is much younger than previously dated samples, indicating that volcanism persisted much longer on the Moon than was previously thought. The result also provided a valuable anchoring point (red dot) for calibrating the middle portion of the cratering curve, which plots the number of craters per unit area on the Moon against the age of the surface. This improves the accuracy of the crater-counting method of dating surfaces of the Moon as well as other inner Solar System bodies such as Mars.
Chen, Y. et al. (2023), Innovation Geoscience 1(1): 100014
Water on the Moon
The presence of water on the Moon was confirmed in 2009 by NASA’s LCROSS spacecraft (illustrated at left). A section of the rocket that was used to launch the spacecraft accompanied the spacecraft in its journey to the moon and was crashed into the lunar surface into a crater near the South Pole. The debris cloud released by the impact was analyzed as the spacecraft passed through the debris. The LCROSS spacecraft was then itself crashed into the Moon, releasing another plume of surface material, which in turn was analyzed by another spacecraft, NASA’s Lunar Reconnaissance Orbiter. The results showed that crater floors in permanent shadow contained frozen water, perhaps as much as hundreds of millions of tons. The likely distribution of such water around the lunar South Pole is indicated by the blue shading.
Courtesy of NASA
Artist’s concept of astronauts prospecting for water ice near the lunar south pole.
Courtesy of NASA
Image captured from lunar orbit of the sunlit rim of the Shackleton crater. Twenty kilometers across, this crater is deep enough to have an interior that is in permanent shadow where a substantial amount of water ice is thought to have accumulated.
Courtesy of NASA, GSFC, and Arizona State University
Living on the Moon
A virtual reality model of a lunar colony prepared for an exhibition at the 2019 Royal Society Summer Science Exhibition.