MOSCOW, August 10, Vladislav Strekopytov. Scientists have found that the heat spot on the far side of the moon is associated with an array of hot granites — rocks that form only in terrestrial conditions. And this is not the only mystery of lunar volcanoes, which were considered extinct long ago.
Volcanic seas
Medieval astronomers thought that the large dark spots on the surface of the moon were seas filled with water. Later it turned out that these are volcanic fields — flat plains covered with hardened basaltic lava. Soviet and American missions in the 1960s and 1970s collected and brought to Earth quite a lot of samples of volcanic rocks. All of them turned out to be ancient — more than three billion years old. It is now established that the Moon was formed 4.5-4.4 billion years ago.
Based on these data, scientists have suggested that for several hundred million years after the appearance of our satellite, an ocean of molten rocks existed on it. As the melt cooled, less and less dense and refractory minerals gradually precipitated from it. This is how the inner shells of the moon were formed. In parallel, the lunar crust formed on the surface of the cooling magma ocean.
The era of active volcanism, when most of the lunar seas appeared, according to scientists, falls between 3.8 and three billion years ago. At this time, the liquid melt was preserved only in the form of a thin layer, sealed between the solid core and the solidified outer crust. The pressure of magmatic gases led to the fact that the crust cracked, and lava poured onto the surface along the resulting faults.
Slow Cooling Hypothesis
The theory existed practically unchanged for almost half a century — until in 2020, the Chang'e-5 automatic interplanetary station delivered new samples of lunar rocks to Earth. The Chinese mission took samples for the first time on the far side of the moon.
After analysis, scientists were surprised to discoverthat the age of the basalts in the marginal part of the Ocean of Storms, where the apparatus landed, is about two billion years. This did not fit in with the idea that the Moon completely cooled down 1-1.5 billion years after its formation. Petrographic observations confirmed that after 2.5 billion years, active volcanism continued on its surface. Pyroclastic glass was found in the samples, which is formed during explosive eruptions.
There was no explanation for this. Unlike the Earth, in the depths of which even now, 4.5 billion years after its formation, tectonic-magmatic processes continue, the Moon was traditionally considered a geologically dead body. The internal heat fueling lunar volcanism must have disappeared long before these eruptions occurred.
Chinese scientists conducted a comparative analysis of archival and newly delivered samples and established that younger basalts are enriched in calcium oxide and titanium dioxide compared to older ones. Experiments have shown that these compounds significantly reduce the melting point of the melt.
Based on the data obtained, the authors constructed a hypothesis of slow cooling, according to which the conditions for magma formation in the interior of the Moon persisted for one billion years longer than previously thought.
< br />«We found that the igneous rocks collected by Chang'e 5 were formed at the same depth as the Apollo samples,» according to the press release the words of the first author of the study, Dr. Bing Su from the Institute geology and geophysics of the Chinese Academy of Sciences — But over the period from three to two billion years ago, temperatures there gradually decreased by about 80 degrees.
April 27, 08:00
The mystery of the lunar granites
The study of the moon brings new surprises. So, for example, it turned out that some isolated or grouped in chains domed uplifts and cones, traditionally interpreted as extinct volcanoes, are partially located within relatively young fields, whose age, according to geological data, does not exceed one billion years.
Recently, American scientists established: Despite the fact that there are no active volcanoes on the Moon today, there is a possibility that local pockets of hot magma remain under the surface. Moreover, in terms of composition, this may not be a basalt melt, which is usual for the Moon, but granite.
The assumption that there are granites on the satellite of our planet first arose in 1998. Then the gamma spectrometer of the American automatic interplanetary station Lunar Prospector recorded an anomaly of thorium, a typical element of granites, on the far side of the Moon between the craters Compton and Belkovich.
The surface inside the anomaly is composed of rocks whose reflectivity differs from that of the surrounding basalts. In the relief, it is expressed in the form of a cone-shaped structure typical of volcanoes, but with very steep slopes — up to 25 degrees. This suggests that the lava that formed the structure was very viscous, similar to granite. The angles of inclination of most basalt volcanoes on the Moon usually do not exceed seven degrees.
The authors of the study received new confirmation that the thorium anomaly is associated with a cooling chamber of granitic magma. Summarizing the data of microwave measurements of the Chinese devices «Chang'e-1» and «Chang'e-2», as well as NASA's lunar orbital stations Lunar Prospector and Lunar Reconnaissance Orbiters, scientists have compiled a map of thermal radiation in the region. It clearly shows that the region of the anomaly is about ten degrees Celsius warmer than the surroundings, and a sharp temperature gradient appears at its borders.
Researchers believe they are dealing with an extinct volcano that last erupted approximately 3.5 billion years ago. And below it is a granite batholith — a frozen magmatic body about 50 kilometers across.
«The only possibility that explains the thermal anomaly is a large granite igneous body that is gradually cooling down,» says lead author Matthew Ziegler of the Planetary Science Institute in Tucson, Arizona. «Compared to other rocks in In the lunar crust, granite has a high concentration of radioactive elements such as uranium and thorium, which leads to the heating that we feel on the surface.
Granites are considered to be exclusively terrestrial rocks. Their geochemical «fingerprints» have never been found anywhere in the solar system before. Scientists explain this by the fact that the formation of granites is associated with the melting of lithospheric plates plunging into the mantle. That is, for granitic magma formation, plate tectonics is needed, and this mechanism of global dynamics operates only on Earth. In addition, granitic magma contains water. How granites could have appeared on the Moon remains to be seen.
August 8, 08:00