Low retention of impact materials by the Moon

When a planetary body is formed, precious metals, such as gold and iridium, are removed from its rocky mantle and passed through its metal core. Successive impacts with other objects then re-enrich the mantle into these elements – a process called late accretion^1,2. Measurements of lunar rocks show that the Moon is very poor in precious metals compared to the Earth^3,4. This deficit implies that the ratio of the mass added to the Earth during late accretion and that added to the Moon is greater than 1000: 1, which is significantly different from the predicted ratio.⁵ of about 20: 1. In a paper in Nature, Zhu et al.⁶ show that the inefficient delivery of materials resulting from glance, combined with a warm early stage melted on the Moon, may explain this abnormal relationship between mass and entry.

An analysis of mass and composition⁷ of material added to a planetary body can be used to examine the formation of the body. In addition, late accumulation is related to the influx of water and other volatile elements to the Earth.⁸and these additions are probably a key factor in the livability of our planet. The low abundance of precious metals in lunar rocks^3,4,9 prompted the proposition of competing models to explain the abnormal input-mass relationship between the Earth and the Moon.

At one extreme, these models include the delivery of material by some massive impactors (of more than 2,500 kilometers in diameter) that have preferably hit the Earth.⁵. On the other hand, the focus of small objects (less than 10 m in diameter) on the Earth could have produced similar effects^ten. It has also been suggested that the difference in precious metal abundance between the Earth and the Moon was due to a decrease in the flux of impacts over the period between 4.5 and 4.1 billion d & rsquo; # 39; years.¹¹just after the formation of the solar system. These models generally assume that the Moon retains about half of the mass that has been transferred to it by the impactors.

With the help of millions of computer simulations of impact, Zhu and his colleagues examined the mass fraction of the impactor that can be restrained by planetary bodies. The authors simulated impacts at different speeds (10 to 20 km / sec) and at low (20 °) to high (80 °) angles to the body surface (Fig. 1). They found that the material of larger impactors is less well retained than that of smaller counterparts and that high-angle impacts provide the body with a larger mass fraction than low-angle impacts.

In the case of the Earth, these results imply that the mass retention of the impactor is usually high for all but the most apparent impacts with the most massive objects. For the Moon, whose mass is only about 1% of that of the Earth, the lower the angle of impact and the more massive the impactor, the more likely it is that material disappears without ever reaching the surface of the Moon or penetrating its surface. inside. Use of crater diameters¹² establish the frequency and size of the impactors striking the Moon, Zhu et al. found that the mass retention of the impactors probably changed slightly over time and that the average retention was about 20%, about three times less than previous estimates.

Ineffective retention of materials from objects striking the Moon partially compensates for the difference between the Earth / Moon input / mass ratios determined theoretically and geochemically. Zhu and his colleagues then argue that about 4.35 billion years ago, about 50% of the mass of late accumulation inputs were lost in the interior or heart of the moon, and that this loss explains any remaining discrepancies. Later, once the moon has cooled, a late accumulation accumulation mass has been distributed in the lunar mantle and crust. The authors further suggest that 300 impact craters more than 300 km in diameter may have existed on the Moon, but less than 30% of these craters are preserved today due to impact erosion. or progressive slump (viscous relaxation). ) of the first craters of the lunar warm crust.

The suggestion of inefficient mass retention due to sweeping impacts eliminates the need for proposed time varying impact streams.¹¹. However, the idea that precious metals were lost in the deepest interior or core of the Moon 4.35 billion years ago is more problematic. In the absence of evidence of the crater's impact on the Moon at this time, geochemistry is the only valid test of this idea. The loss of precious metals in a metal core can lead to the separation (splitting) of these elements¹³. However, this chemical effect has not yet been detected in the rocks of the lunar interior.^3,4. In addition, the low abundances of precious metals estimated for the lunar mantle make it difficult to envision how such fractionation signatures could have been erased by further late accumulation.

Zhu et al. also assume that the impactors penetrate the lunar crust, about 40 km thick¹⁴, would lead to the penetration of the entire impactor material retained in the mantle. But, in reality, this material would pollute both the crust and the mantle. Finally, as only a relatively small number of lunar rocks have been analyzed, models such as those of the authors, capable of reproducing the abundances of precious metals in the Moon by means of simulations, have a limited resolution.

Nevertheless, the new models will be useful for understanding the evolution of planetary bodies, especially Mars. Current estimates of late entry mass of accretion expressed as a function of total body mass are from 0.02%, from about 0.5% and up to 0.7%. % for the Moon.⁴, Earth² and Mars¹⁵, respectively. The estimate for Mars has been explained by the early formation of the planet in relation to the Earth and the Moon, as well as a constant mass flux in the first 50 to 100 million years of the solar system .¹⁵, and impacts involving massive objects^5,16.

The extrapolation of Zhu's models and his colleagues suggests that Mars, which represents only about 11% of Earth's mass, retains less material from large impactors than the Earth. Assuming both bodies were subjected to a similar number of impacts, this modeling would imply that Mars has a late accretion flux proportionally greater than that of the Earth. The combination of geochemistry and training models will undoubtedly continue to improve our understanding of the creation of the Earth and its closest neighbors.