Science

Rocks brought by China show that the Moon maintained its volcanism until more recent dates

Since 1976, when the Soviet mission Lunik 24 returned from the Moon with a few grams of earth, no one had gone to the satellite for more. It was the Chinese, with their probe Chang’e-5 , who in December of last year returned from there with about two kilos of lunar material. Now, the journal Science publishes the results of the study of two of those rocks. They are volcanic, similar in composition to Earth’s, and have turned out to be much more recent than previously believed. And this changes the history of the neighboring star and they have their connections with Earth.

The Apollo missions from the United States were brought almost 400 kilograms between earth and lunar rocks. The analysis of the first, such as the baptized stone Genesis , allowed us to date the origin of the Moon in about 4 years ago. 500 millions of years. Although with a variable composition, its constituent elements also exist on Earth. And most of them are of volcanic origin, magma emerged through fissures, crystallized and cooled a long time ago.

All this served to rewrite the history of the satellite, of which it was thought to have formed by accretion, accumulating particles, like many other celestial bodies. But, as said in this article by 2019 Rick Carlson, geochemist and director of the department of terrestrial magnetism at the Carnegie Institution for Science, “ with the first samples of the Apollo 11 , we realized that the Moon formed hot, possibly completely molten. Upon cooling from this initial state, it generated a thick crust by floating crystals in cooling magma. ” The Americans estimated that the basaltic rocks brought to Earth were created about 3 years ago. millions of years, time in which the lunar hot phase passed into the current cold era, in which a reduced metallic core and a thermally dead mantle complete the internal structure. But now Chinese and American researchers have found even younger volcanic rocks.

Scientists from the SHRIMP Center in Beijing, under the Chinese Academy of Geological Sciences, have published in Science the petrographic and geochemical analysis of two of the small rocks that the space probe Chang’e was brought from Moon. The first thing they have discovered is that they crystallized in their solid state 1. 963 millions of years ago, 50 millions up or down. This means that either the surface of the Moon was still molten, at least partially, or it had a thermally active mantle about 1. 000 millions of years later than previously believed.

Una de las dos rocas lunares analizadas en el estudio. Ambas fueron recogidas del norte del Oceanus Procellarum.
One of the two moon rocks analyzed in the study. Both were collected from the northern Oceanus Procellarum.

The director of the McDonnell Center for Space Sciences at Washington University in St. Louis (United States) Brad Jolliff has been collaborating with the scientists of the Chang’e missions. Jolliff, co-author of the present study, recalls that “all the volcanic rocks collected by the Apollo had more than 3. 000 millions of years”. At the same time he adds that “all recent impact craters dated from the analysis of these samples have less than 1. 000 millions of years”. So, he concludes, “ Chang’e-5 samples fill a critical gap.”

Indeed, to write the history of the Moon, scientists have relied on two elements: on the one hand, the age of the stones, which is estimated by variations in certain radioactive elements present in them, such as lead isotopes. The other mainstay of the lunar chronology are its craters. In a simple version of this dating technique, the more impacts a given area of ​​the satellite has, the older it must be. In fact, those responsible for the mission Chang’e-5 chose an area north of the Oceanus Procellarum for the moon landing as it is one of the areas with the lowest density of craters. , that is, it is more recent. Now, with the new stones, they will be able to improve the knowledge of the lunar past.

The study also details the composition of the two lunar rocks. The most abundant mineral is ilmenite, a compound of metallic elements. But they are followed in abundance by olivine, clinopyroxenes, plagioclase or silica. Except for the first one, the others are very frequent in terrestrial volcanic rocks, as the analyzes of the lava from the La Palma volcano have shown.

But these two pebbles are less of four millimeters could also shed light on still dark episodes in the evolution of the Earth. On the planet, the earth’s crust floats on a more or less fluid mantle. It is cracked into tectonic plates and it is where they are found, such as oceanic ridges or subduction zones, in which one plate sinks under another, where magma emerges. But there are other ways that the mantle releases heat and pressure: so-called hotspots. These are areas of high volcanic activity but far from ridges and collisions between plates. On them and due to them, a good part of the oceanic islands have been formed: Hawaii, Easter, Azores or the Canary Islands.

The professor of Petrology at the University of Barcelona Domingo Gimeno “Everyone thought it was dead from a thermal point of view 1. 000 millions of years before these rocks. So what happened to get 1. Authors do not give the answer, but it is likely that hot spots similar to those of the Earth were maintained in very localized areas of the Moon, giving rise to these volcanic rocks.

“Thanks to plate tectonics you can go back 1. 000 millions back, but There comes a time when we have no evidence of how things worked here. On the moon you have something like a hotspot that has 2. 000 millions of years. These give you an idea of ​​what the flow of heat and matter towards the earth’s crust could be like when it was still not working with plate tectonics ”, says Gimeno. For him, in his assessment of this research, “it is not so much what they have found, but what others are going to see [en estas rocas] as a key to reading an ancient Earth of which we barely conserve evidence.”

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