The Two-Sided Moon: Two endings of a massive impact
The study simulated the impact process in the Antarctic-Aitken Basin, and the results showed that the impact would create a column of hot mantles in the mantle, which carried deep radioactive elements to the lunar crust and lunar surface. It is worth mentioning that the simulation traversed the various direct and grazing impacts that produced the South Pole-Aitken Basin, and found that regardless of the impact, the area and direction in which the mantle was produced had the same result: it only occurred in the direction of the front of the Moon facing the Earth.
Since the time of Apollo, scientists have known that the moon has two sides: one side facing the Earth, the other side always facing the Earth, the side facing the Earth is relatively flat, and the side facing the Earth is uneven, covered with thousands of impact craters.
Why the two sides of the moon are so different is one of the many mysteries of the moon.
Recently, scientists have proposed a new interpretation of the two "faces" of the moon: billions of years ago, the huge impact of the South Pole-Aitken Basin (SPA) on the far side of the moon generated a huge amount of heat that spread throughout the moon, contributing to the melting of the lunar mantle material, in which rare earth (REE) and radioactive thermogenic elements thorium (Th), potassium (K), phosphorus (P) were carried to the lunar surface symmetrical with the impact area, forming a KREEP rock, distributed in and around the storm ocean on the front of the moon. The concentration of radioactive heat-generating elements creates lava flows on the lunar surface that eventually form volcanic plains on the lunar front. The findings were published in the journal Science Advances.
Ping Jinsong, a researcher at the National Astronomical Observatories of the Chinese Academy of Sciences, told science and technology daily reporters that the new study gave a more convincing analysis of the causes of revealing both sides of the moon.
The kreeep rock on the lunar surface is associated with a basin on the back of the moon
KREEP rock is one of the focal points of scientists and is thought to be responsible for the moon's volcanic activity for hundreds of millions of years. KREEP rocks are named for their abundance of incompatible elements such as potassium (K), rare earth (REE), and phosphorus (P), as well as radioactive thermogenic elements such as uranium (U) and thorium (Th), which provide a heat source for the moon's ongoing volcanic activity.
Xiao Long, a professor at the Institute of Planetary Sciences of China University of Geosciences (Wuhan), told reporters that studies have shown that KREEEP rocks and ilmenite are terminal products of the magma ocean consolidation process, and some KREEP rocks may be mixed with ilmenite, distributed at the boundary of the shell mantle, and formed after the consolidation of the lunar crust.
According to Ping Jinsong, KREEP rocks were first discovered in Apollo-12 samples, and later found fragments in almost all lunar return samples, which exist in the magmatic rock region of the lunar front. At the same time, in 1994, the American Clementine probe discovered that there was an abnormal region of composition on the front of the moon known as the Storm Ocean Crippe.
The mainstream hypothesis is that the Moon's youngest basalt (the product of volcanic activity) is primarily filled with stormy Ocean Klip bodies on the moon's front. But the hypothesis that the radioactive heat generation of KREEP rocks causes the moon's youngest volcanic activity has not been tested.
Why are KREEP rocks concentrated in storm oceans on the lunar front? The new study offers an explanation for its association with SPA on the far side of the moon.
SPA is the largest, deepest and oldest impact basin on the Moon, with a diameter of about 2500 km and a depth of 6-8 km, and is considered the area most likely to expose the deep material of the Moon.
Ping Jinsong said that the new study simulated the impact process of the SPA, and the results showed that the impact would create a hot mantle column in the lunar mantle, which brought deep radioactive elements to the lunar crust and lunar surface. It is worth mentioning that the simulation traversed the various direct and grazing impacts that produced the SPA, and found that regardless of the impact, the area and direction in which the mantle was generated had the same result: it only occurred in the direction of the front of the Moon facing the Earth.
Talking about the internal heat dynamic mechanism of this process, Xiao Long said that after the impact of the southern SPA of the moon, a large amount of heat was rapidly transferred to the north, and the temperature of the nuclear mantle boundary in the lower part of the SPA exceeded 1800 ° C. Driven by the temperature difference, the mantle material flows from the high temperature area to the low temperature area. Low-viscosity ilmenite layers are pushed into the Storm Ocean and surrounding areas under this action. The ilmenite layer has a higher density, and under gravity, most of the ilmenite layer slowly sinks to the nuclear mantle boundary. Due to the large base, the Storm Ocean and the surrounding area will still retain considerable ilmenite and KREEP material.
Ping Jinsong added that the huge impact that formed the SPA produced enough thermal and kinetic energy to spread throughout the moon (from the lunar nuclear mantle to the lunar crust), contributing to the melting of the lunar mantle material. Through the process of lunar mantle flipping, the lunar front is promoted to form a mantle source region rich in ilmenite and KREEP rocks, and the materials in the lunar interior such as rare earths and radioactive thermogenic elements are centrally carried to the near-Earth side of the lunar symmetry of the impact area by means of the ignification process, resulting in the lava flow on the lunar surface that we see, so that the material on the front and back of the moon shows an asymmetrical distribution.
There are three explanatory models of KREEP rocks enriched in storm oceans
Why are KREEP rocks concentrated in and around the Storm Ocean? Xiao Long introduced that there are currently three possible models, namely SPA impact model, storm ocean impact model and endogenous model. However, no model has yet been widely accepted.
As one of the models of the formation of the Crieppite in the Lunar Storm Ocean, the SPA impact model provides an idea for understanding the formation of the Crippeite in the Storm Ocean. Xiao Long also pointed out that the establishment of the SPA impact model, in addition to explaining the distribution of KREEP rocks, also needs to explain other characteristics of the Storm Ocean Krippite, such as the thinner lunar crust (less than 30 kilometers) and the giant linear structure.
It is understood that there is no relevant research to support the hypothesis that the SPA impact event can thin the thickness of the lunar crust of the Storm Ocean Crippe body or produce a giant linear structure.
Due to the correlation between the SPA impact event and the Storm Ocean Crippe body, Xiao Long noted that the sampling return of the SPA may be able to verify the model. In addition, if the impact time of the SPA can be determined, combined with the internal structure of the moon at that time and the geothermal gradient, the SPA impact model can be greatly improved and more reliable results can be obtained.
It is worth mentioning that the storm ocean impact model can explain both the thinning of the lunar crust and the redistribution of KREEP rocks, so it has been supported by a large number of researchers. However, Xiao Long said that if there is a storm ocean impact event, it is difficult to confirm because its products are covered by late impacts.
In terms of explaining the giant linear structure, foreign researchers observed the giant linear stretch structure surrounding the Storm Ocean Clipper through the gravity data of the "Holy Grail" lunar probe, and then proposed an endogenous model of the formation of the Storm Ocean Kriepp body. The model suggests that no additional impact events are needed to produce the Storm Ocean Crippe. Xiao Long said that to validate the model, it may also be necessary to perform higher-resolution gravity simulations of the Storm Ocean Crippe.
There are dichotomies on both sides of the Moon in many ways
For thousands of years, the moon has only one side facing the Earth, and the other side has always been facing the Earth. This is one of the important discoveries of human lunar exploration. What is even more amazing is that the two sides of the moon facing and facing the earth with their backs to the earth are very different, which is the so-called lunar dichotomy.
Studies have found that "in addition to the dichotomy of the morphology, geology and chemical elements of the moon, there are also dichotomies in gravity, magnetism, heat and other characteristics, and the mechanism of production is different." Ping Jinsong said.
The Moon is not an even density object, and scientists have found that there are many "mass tumors" in the Lunar body, and its distribution law is dichotomous. Ping Jinsong explained that there is a high density of material under the lunar sea area on the front of the moon, forming many huge "mass tumors" with higher gravity than the surrounding areas, and the "mass tumors" are fewer and much smaller on the far side of the moon. Ping Jinsong pointed out that the "mass tumor" on the front of the moon is more heavy, resulting in the moon's mass center and shape center do not coincide, the former is closer to the earth direction of about 2 kilometers than the latter, the torque tidal action caused by the lock moon rotation and orbit around the earth chirality is consistent, the same period, so that the moon side faces the earth for a long time, the other side long-term back to the earth.
According to reports, the current strength of the moon's magnetic field is less than one-thousandth of the Earth's magnetic field. Ping Jinsong pointed out that whether there is a strong magnetic field in the history of the formation and evolution of the moon is still controversial, but the magnetic field on the lunar surface also has the dichotomy characteristics of weak and small front, slightly stronger and wide distribution range on the back side.
In recent years, the topic of small bodies and micrometer impacts threatening the earth is not uncommon, in fact, the probability of them hitting the front and back of the moon is not the same. "In contrast, the far side of the moon is more likely to be threatened." Ping Jinsong said that part of the reason is that The Earth's gravitational pull on small bodies and micrometers from space is greater, helping the moon to avoid some of the impacts.
Mainland scientists have found that the front of the moon is also several percentage points higher than the back side in receiving sunlight, making the front slightly more weathered than the back, and the shallow surface is slightly hotter.
"The mainland lunar exploration project will use methods such as return exploration and future lunar surface research station plans, which will surely unveil the mystery of lunar duality more clearly." Ping Jinsong said.
(Science and Technology Daily)