The mysteries of the cosmos are not only found in the farthest bodies from our planet. The Earth-Moon system still holds many enigmas for science.
The most widely accepted hypothesis about the formation of the Moon is that a body the size of Mars (Theia) collided with a forming Earth. The debris from this collision eventually formed the Moon, which, being smaller, likely cooled rapidly and froze, geologically speaking. In principle, the hypothesis is accepted that the Moon did not have enough mass to host all the compatible processes for life that did thrive on Earth.
But the apparent early dynamism of the Moon now challenges ideas of how it was formed, and makes it possible for us to have to discard the Theia hypothesis , albeit still without alternatives to supplement it.
Our moon is a relatively cold rocky body with a limited amount of water and little tectonic processing. Decades of observations have shown that lunar history was much more dynamic than expected, with volcanic and magnetic activity occurring just a billion years ago , much later than expected.
Differences between the far side and the visible side of the Moon
One of the biggest mysteries for different generations of astronomers, who have been studying the Moon for decades, is why the hidden side and the visible side are morphologically different. On the visible side, dark and light spots can be seen with the naked eye. Early astronomers called these dark regions Maria , a Latin term for ‘seas’, thinking they were bodies of water by analogy with Earth. Using telescopes, scientists were able to discover more than a century ago that these were not seas, but rather craters or volcanic features. Back then, most scientists assumed that the far side of the Moon, which they would never have been able to see, was more or less like the visible side.
In the late 1950s and early 1960s, unmanned space probes launched by the USSR obtained the first images of the other side of the moon, and scientists were surprised to find that the two sides were very different. The other side hardly had these maria: only 1% of the far side was covered with these craters compared to 31% of the visible side. Scientists were puzzled, but suspected that this asymmetry offered clues about how the Moon formed.
Between 1969 and the early 1970s, NASA’s Apollo missions launched six spacecraft to the Moon, and astronauts brought 382 kg of lunar rocks to try to understand the origin of the moon through chemical analysis. Holding samples in hand, the scientists quickly discovered that the relative darkness of these patches was due to their geological makeup and was, in fact, attributable to volcanism.
They also identified a new type of rock signature that they called KREEP: short for potassium-enriched rock (chemical symbol K), rare earth elements (REE, which include cerium, dysprosium, erbium, europium, and other elements that are rare in the Earth) and phosphorus (chemical symbol P), which was associated with Maria craters. But why volcanism and this KREEP signature should be so unevenly distributed between the visible and hidden faces of the Moon presented an enigma.
Now, using a combination of observation, laboratory experiments, scientists from the Institute of Terrestrial Life Sciences, Tokyo Institute of Technology, the University of Florida, the Carnegie Institution for Science, Towson University, the Johnson Space Center of NASA and the University of New Mexico have uncovered new clues about how the moon gained its asymmetry of the visible and hidden faces. These tracks are linked to an important property of the KREEP rocks.
Due to the relative lack of erosion processes, the moon’s surface records geological events from the early history of the solar system. In particular, the regions on the visible side of the moon have concentrations of radioactive elements like U and Th unlike anywhere else on the moon. Potassium (K), thorium (Th), and uranium (U) are radioactively unstable elements. This means that they occur in a variety of atomic configurations that have varying numbers of neutrons. These atoms of varying composition are known as isotopes, some of which are unstable and break apart to produce other elements, producing heat.
The heat from the radioactive decay of these elements can melt the rocks in which they are contained, which may partly explain their co-location.
The idea is that, in addition to warming, the KREEP rocks also lowered their melting temperature, increasing volcanic activity asymmetrically. Because most of these lava flows were located early in lunar history, this study also implies that the time of evolution of the Moon and the order in which various processes occurred on the Moon must be rewritten.
Earth’s natural satellite, along with the Sun, offers many observable features that provide evidence for how the planet and the solar system formed. Most of the planets in the solar system have satellites. For example, Mars has two moons, Jupiter has 79, and Neptune has 14. Some moons are frozen, some are rocky, some are still geologically active, and some are relatively inactive. How the planets got their moons and why they have the properties they do are questions that could shed light on many aspects of the evolution of the early solar system. Evidence for such non-symmetrical processes could be found on other moons in our solar system, and it can be ubiquitous in rocky bodies throughout the universe.
Stephen M. Elardo et al, Early crust building enhanced on the Moon’s nearside by mantle melting-point depression, Nature Geoscience (2020). DOI: 10.1038/s41561-020-0559-4
