Asteroid rain in the Earth-Moon system 800 million years ago revealed by lunar craters

A research team led by the University of Osaka investigated the formation ages of 59 lunar craters with a diameter of approximately 20 km using the terrain camera (TC) aboard the lunar orbiter spacecraft Kaguya.

Kaguya (formerly SELENE, for SELenological and ENgineering Explorer), is a lunar orbital mission of the Japanese Space Agency (JAXA).

This group showed that a 100 km diameter asteroid was interrupted 800 million years ago (800 Ma) and that at least (4-5) × 10^sixteen kg of meteoroids, approximately 30-60 times more than the Chicxulub impact, must have sunk into the Earth-Moon system. The results of his research were published in Nature’s Communications.

Since a thin layer of iridium enrichment (Ir) (a rare earth element) 65.5 Ma had been detected worldwide, an asteroid 10-15 km in diameter is believed to hit Earth and cause or contribute greatly measure to the mass extinction of the Cretaceous.

The probability of an asteroid of this size hitting Earth is believed to be once every 100 million years. Earth impact craters created before 600 Ma are known to have been erased over the years by erosion, volcanism, and other geological processes. Therefore, to discover ancient meteorite impacts on Earth, they investigated the Moon, which has almost no erosion.

They investigated the formation age distribution of 59 large craters with diameters greater than about 20 km by examining the density of 0.1-1 km diameter craters on ejection of these 59 craters. One of these examples is the Copernicus crater (93 km in diameter) and its surrounding craters. The density of 860 craters with a diameter of 0.1-1 km (shown in green) was examined to determine the age of the Copernicus crater. As a result, 8 out of 59 craters were found to be formed simultaneously (17 by a herringbone pattern), a world first.

Considering the crater scale laws and the probabilities of collision with the Earth and the Moon, at least (4-5) × 10^sixteen kg of meteoroids, approximately 30-60 times more than the Chicxulub impact, must have hit the Earth immediately before the Cryogen (720-635 Ma), which was an era of great environmental and biological changes.

Furthermore, given the age of disruption and orbit elements of existing asteroid families, disruption of the main body of the C-type asteroid Eulalia is highly likely to cause an asteroid shower. A type C asteroid is a class that is expected to contain carbon in analogy to carbonaceous chondrites (meteorites).

Because the reflectance of Eulalia’s surface is similar to that of the near-Earth type C asteroid Ryugu, Eulalia has drawn attention as a parent body to a Type C debris pile, a celestial body consisting of numerous pieces of rock close to Earth. (Sugita et al. 2019)

Ryugu was probed by asteroid explorer Hayabusa2, an asteroid sample return mission operated by JAXA.

From these considerations, they concluded that sporadic meteorite bombardment due to the disruption of 800 Ma asteroids caused the following:

• Some of the resulting fragments fell on the terrestrial planets and the Sun.
• Others stayed in an asteroid belt like the Eulalia family, and
• The remnants had orbital evolution as a member of near-Earth asteroids.

This research suggests the following possibilities:

1. An asteroid shower may have brought a large amount of phosphorus (P) to Earth, affecting the Earth’s surface environment,
2. A recent Type C asteroid shower may have contaminated the lunar surface with volatile elements,
3. The Eulalia family, the parent body of a near-Earth Type C asteroid, may have brought an asteroid shower to Earth and the Moon.

Lead author Professor Terada says: “The results of our research have provided a novel perspective on earth science and planetary science. They will produce a wide range of positive effects in various fields of research.”