We have already set foot on its soil, we see it almost daily in the sky, but the Moon still conceals secrets. With the first part of the Artemis mission launched towards 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 preparing for future manned flights to distant destinations like Mars. It will also help to better understand this star. Here are four open questions that could be clarified through the Artemis mission.
1How did the Moon form?
The genesis of the Moon still has gray areas. If several scenarios are on the table, the track of the giant impact has been widely accepted for decades: a huge body would have violently struck the Earth still in formation, tearing a piece from it. The latter and many pieces of debris would have remained in orbit around our planet. The whole would have ended up aggregating, giving shape to the Moon. Radio-Canada returns to this hypothesis, formulated by the 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 the Earth, since the impact would only have torn rocks poor in iron .
Another scenario tries to stand out. A study published in April 2017 in the scientific journal Nature Geoscience, put forward the hypothesis of multiple impacts. It favors the idea of a succession of weaker collisions. Each would have given rise to mini-moons, which would then have merged to form the Moon that we know. “In the early solar system, collisions were very common, so it’s more natural that several fairly common impactors originated the Moon rather than just one.”estimated 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 to further refine the main hypotheses. For the moment, underlines Jessica Flahaut, only 4% of the surface of the Moon have been sampled. Indeed, 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 in plains, which are not “representative” from the lunar ground.
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With the Artemis mission, the American Space Agency plans to land in particular at the South Pole, an area still unexplored and more like the majority of the lunar soil. Moreover, sending humans to the surface of the Moon rather than robots presents “a real added value”according to Jessica Flahaut: “A man has more easily the reflex to take a sample if he finds an outcrop 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 rugged 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. (in English) carried out with NASA and the Lunar and Planetary Institute (LPI). The document, visible below, entitled “unified geological map of the Moon” (“Unified Geologic Map of the Moon”), shows that the visible side (on the left) has far fewer craters than the hidden side (on the right).
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The visible face has large “seas” (in pink and red on the image). These are large plains of volcanic rocks. 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 surface of the Moon, and are more on the far side. With the various craters there, the average thickness of the lunar crust on the far side is about 60 km. This is twice as much as that of 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, director of research at the CNRS and deputy director of the Institute for Research in Astrophysics and Planetology, discusses several avenues with franceinfo, in particular a “external cause with an asymmetrical bombardment between the visible face and the hidden face since the beginning of the history of the Earth-Moon system, with the Earth which was able to screen”. He also mentions “an internal 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 to say without water, reminds franceinfo Patrick Pinet. The idea collapsed when tiny water elements were detected by probes. Ten years later, NASA confirmed the presence of ice on the surface of the Moon, 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 of the South Pole called Aitken is the largest impact basin in the solar system, with some 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 the “outgassing of lunar volcanoes”. Another unknown for this lunar ice: its composition. It remains totally mysterious since neither humans nor robots have been able to collect it until now.
“We’re assuming it’s water ice, but is it pure water ice?”
Jessica Flahaut, lunar geologistat franceinfo
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 it has been detected volcanoes on the surface that have very few craters. This means that they look quite young on the face of it.” “The Chinese brought back relatively young samples for the history of the Moon, 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 on site. “We still have difficulty understanding the origin of these more or less deep earthquakes today.observes Jessica Flahaut. Some moonquakes could be linked to tidal effects: the Moon has tidal effects on the Earth, but the Earth also has tidal effects on the Moon.” These tremors with enigmatic origins have a power hardly comparable to those which occur on Earth, she believes. For his part, Pierre Thomas argued that during these tremors, “the Moon released a hundred billionth times less seismic energy than the Earth”during a conference given in 2019 (about 48 minutes).
These earthquakes, according to Jessica Flahaut, also have completely different behaviors 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 fifty years ago”, summarizes with franceinfo Jean Blouvac, head of exploration and manned flight programs at the National Center for Space Studies (Cnes). Earthlings have not finished being shaken by discoveries on their natural satellite.