formation, asymmetry, ice, earthquakes… A look back at four lunar mysteries

We have already set foot on its soil, we see it almost daily in the sky, but the Moon still hides secrets. With the first part of the Artemis mission launched toward it, Monday, August 29, NASA begins a new chapter in the conquest of space. The mission, which aims to settle permanently on Earth’s natural satellite, is an important step in the preparation of future manned flights to distant destinations such as Mars. It will also help to understand this star better. Here are four open questions that could be resolved through the Artemis mission.

1How was the moon formed?

The genesis of the Moon still has gray areas. If several scenarios are on the table, the path of the giant impact has been widely accepted for decades: a huge body would have violently struck the still-forming Earth, tearing a chunk off it. The latter and many pieces of debris would have remained in orbit around our planet. It would all have ended up coming together and giving shape to the Moon. Radio-Canada returns to this hypothesis, formulated by Canadian Reginald Daly, in this video.

Without being perfect, the giant impact hypothesis has several advantages, such as explaining the different proportions of iron observed in the composition of the Moon and Earth, since the impact would have only torn rocks poor in iron.

Another scenario tries to stand out. A study published in April 2017 in the scientific journal Nature Geosciences, put forward the hypothesis of multiple influences. It favors the idea of ​​a series of weaker collisions. Each of them would have given rise to mini-moons, which would have then merged to form the moon we know. “In the early solar system, collisions were very common, so it’s more natural that several fairly common impactors originated from the Moon instead of just one.”assessed Raluca Rufu, one of the authors of the study.

The analysis of future samples taken from the Moon will make it possible to better understand the composition of the lunar soil and further refine the main hypotheses. Currently, emphasizes Jessica Flahaut, only 4% of the Moon’s surface has been sampled. In fact, during the various Apollo missions (carried out between 1969 and 1972 on the surface of the Moon), American astronauts had landed on the visible side, mainly on plains which are not “representative” from the lunar surface.

Infographic from the US space agency that points to the places where US spacecraft from the Apollo mission have landed on the Moon.  (NASA)

With the Artemis mission, the American space agency plans to land in particular at the South Pole, an area that is still unexplored and more like the majority of the lunar soil. Also, sending humans to the surface of the Moon gives more than robots “a real added value”According to Jessica Flahaut: “A man has an easier reflex to take a sample if he finds a protrusion or an interesting rock.”

2Why is the visible side so different from the hidden side?

The moon always presents us with the same side: its visible side. Observations have shown that it is much less robust than the hidden side, the one we never see from Earth, but which probes have been able to analyze. The United States Geological Survey (USGS) presented a detailed map of the Moon in April 2020. (English) conducted with NASA and the Lunar and Planetary Institute (LPI). The document, visible below, titled “unified geological map of the Moon” (“United Geological Map of the Moon”), shows that the visible side (left) has far fewer craters than the hidden side (right).

The visible face has large “seas” (in pink and red in the picture). These are vast plains of volcanic rock. From Earth, to the naked eye, these areas appear dark. In contrast, the other areas which appear clear to the naked eye are commonly referred to as “the lands”. According to calculations, these cover about 83% of the Moon’s surface and are more on the far side. With the various craters there, the average thickness of the lunar crust on the other hand is about 60 km. This is twice as much as on the visible side.

How to explain this finding? “It’s part of the mysteries of lunar geology”, summarized Pierre Thomas, professor emeritus of planetology at ENS Lyon during a conference held in 2019 (around 52 minutes). Patrick Pinet, research director at CNRS and deputy director of the Institute for Research in Astrophysics and Planetology, discusses several avenues with franceinfo, in particular a “external cause of an asymmetric bombardment between the visible face and the hidden face since the beginning of the history of the Earth-Moon system, with the Earth being able to shield”. He also mentions “an inner cause”linked to the circulation of matter inside the Moon when it was still young.

3What is this ice that is on its surface?

“Until 2008, we thought the Moon was an anhydrous medium”, that is, without water, reminds franceinfo Patrick Pinet. The idea collapsed when small water features were discovered by probes. Ten years later, NASA confirmed the presence of ice on the Moon’s surface, which had been suspected for a long time. This ice is found at the North and South poles of Earth’s natural satellite. It lodges in craters where the sun’s rays do not penetrate. In these areas the temperature never exceeds -150°C. A visualization of the South Pole and its deep craters is shown in the NASA video below (from 1:39). This area at the South Pole called the Aitken is the largest impact basin in the solar system, about 2,500 km in diameter.

“Where does this ice come from? What is its origin? How much of it is on the surface of the Moon?” wonders Jessica Flahaut, wondering if it comes from meteorites, comets or “outgassing of lunar volcanoes”. Another unknown for this lunar ice: its composition. It remains totally mysterious as neither humans nor robots have been able to collect it until now.

“We assume it’s water ice, but is it pure water ice?”

Jessica Flahaut, lunar geologist

at france info

In the absence of analyses, the questions remain open. “Is it water ice mixed with rhyolite [de la roche volcanique] ? Or water implanted on the surface of rock grains, as can be seen at other latitudes of the Moon? lists Jessica Flahaut.

4What is its seismic and volcanic activity?

The moon is often presented as a dead star. Which deserves a little nuance. “If you compare it to Earth or Mars, it’s much less active, that’s for sure because it’s a smaller body, so it cooled faster, explains Jessica Flahaut. But volcanoes have been discovered on the surface that have very few craters. This means that at first glance they look quite young.” “The Chinese brought relatively young samples back to the Moon’s history, dated to 2 billion yearscontinues the researcher. We thought there had been no activity on the Moon for 3.5 or even 4 billion years. There may therefore have been locally late volcanism which spread over billions of years.

This lunar volcanism leads to the question of earthquakes on the Moon. Few have been measured because only the Apollo missions brought seismometers to the site. “We still find it difficult to understand the origin of these more or less deep earthquakes today.notes Jessica Flahaut. Some moonquakes may be associated with tidal effects: the Moon has tidal effects on the Earth, but the Earth also has tidal effects on the Moon.” These tremors of mysterious origin have a power that can hardly be compared to those that occur on Earth, she believes. For his part, Pierre Thomas argued that during these tremors, “The Moon released a hundred billion times less seismic energy than the Earth”during a conference held in 2019 (approx. 48 minutes).

According to Jessica Flahaut, these earthquakes also have a completely different behavior of those observed on Earth “due to the elastic properties of the lunar crust” : the attenuation is much longer and gradual. “These are new things to discover, especially since today we have seismometers that are much more sensitive than 50 years ago”, summarizes with franceinfo Jean Blouvac, head of exploration and manned flight programs at the National Center for Space Studies (Cnes). Earthlings are not done being shaken by discoveries on their natural satellite.

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