According to the prevailing theory of how the Moon formed, it all began roughly 4.5 billion years ago when a Mars-sized object (Theia) collided with a primordial Earth. This caused both bodies to become a molten mass that eventually coalesced to form the Earth-Moon System (aka. The Giant Impact Hypothesis. This theory also states that the Moon gradually cooled from the top down, with the crust solidifying and arresting lava flows early in its history. However, recent findings from samples obtained by China’s Chiang’e-5 probe indicate that lava existed at shallower depths longer than previously thought.
These samples obtained by the Chiang’e-5 lander were from the young mare basalt unit in the Oceanus Procellarum region, a vast lunar mare on the western edge of the near side of the Moon. The samples included 1.7 kilograms (3.7 pounds) of scooped and drilled material composed of basalt and igneous rock that formed roughly 2 billion years ago, making them the youngest samples obtained to date. These findings contradict the previous theory that the temperature of the outer layers of the Moon was too low for melting to occur in the shallow interior, and could revise theories about the Moon’s early evolution.
The research was led by Stephen M. Elardo, an Assistant Professor from The Florida Planets Lab at the University of Florida. He was joined by researchers from the Colorado School of Mines, the University of Rochester, the Planetary Science Institute (PSI), the Hawaiʻi Institute of Geophysics and Planetology, the University of Hawaiʻi Manoa, and the University of Oxford. The paper describing their findings appeared on July 18th in the journal Science Advances.
The Chiang’e-5 samples are examples of rock formed from rapidly cooled lava, which is characteristic of the mare region from which they were obtained. To obtain an estimate of how deep this lava came from, the team conducted high-pressure and high-temperature experiments on a lava simulant with an identical composition. Based on remote sensing from orbit, previous work from Chinese scientists showed it erupted in an area with very high abundances of radioactive, heat-producing elements, including potassium, thorium, and uranium.
In large amounts, the researchers believe these elements could generate enough heat to keep the Moon hot near the surface, slowing the cooling process over time. Before this study, it was presumed that the upper mantle cooled first as the surface gradually lost heat to space, which was based largely on seismic data obtained by the Apollo astronauts. Per this theory, younger lavas like the samples obtained by the Chang’e-5 lander should have come from the deep mantle, where the Moon would still be hot. However, these findings suggest there must have been pockets in the shallow mantle that were hot enough to partially melt rock 2 billion years ago.
As Prof. Elardo explained in a UF News release:
Using our experimental results and thermal evolution calculations, we put together a simple model showing that an enrichment in radioactive elements would have kept the Moon’s upper mantle hundreds of degrees hotter than it would have been otherwise, even at 2 billion years ago.
Lunar magmatism, which is the record of volcanic activity on the Moon, gives us a direct window into the composition of the Moon’s mantle, which is where magmas ultimately come from. We don’t have any direct samples of the Moon’s mantle like we do for Earth, so our window into the composition of the mantle comes indirectly from its lavas.
Artist’s impression of the interior structure of the Moon. Credit: Hernán Cañellas/Benjamin Weiss/MIT
This research is helping to establish a detailed timeline of the Moon’s evolution, which is critical to understanding how planets and smaller bodies form and evolve. The prevailing theory is that this process begins with accretion from a protoplanetary disk, where dust and gas coalesce due to angular momentum to form planetary bodies. Initially, these bodies are extremely hot and have molten surfaces, which gradually cool to form solid bodies composed of rock and metal, with some forming envelopes of gas or volatiles like water (depending on where they form around their host stars).
The process of cooling and geological layer formation are key steps in the evolution of these bodies. Since the Moon is Earth’s closest celestial neighbor, studying lunar samples is the easiest way to learn more about these processes. Said Elardo:
My hope is that this study will lead to more work in lunar geodynamics, which is a field that uses complex computer simulations to model how planetary interiors move, flow, and cool through time. This is an area, at least for the Moon, where there’s a lot of uncertainty, and my hope is that this study helps to give that community another important data point for future models.
Further Reading: UF News, Science Advances