What the moon's craters reveal about the Earth's history: U of T expert and lead author of new study

noreen.rasbach — Fri, 18 Jan 2019 17:36:40 +0000

noreen.rasbach Fri, 01/18/2019 - 12:36

Cutline

A look at the southern rim of the Copernicus crater on the moon (photo by NASA/GSFC/Arizona State University)

Sara Mazrouei

Topic

Global Lens

Earth Sciences

Faculty of Arts & Science

Global

Moon

Research & Innovation

Most scientists believe the rate at which the moon and Earth have been bombarded by meteorites has remained constant for the past two to three billion years. Understanding the age of craters on the moon can help us better understand the age of our own planet because the Earth would have received similar numbers of impacts.

It’s been assumed that the rarity of young craters on Earth (those created 300 to 600 million years ago) is attributed to preservation bias – craters have been erased over the years by erosion and the movement of the Earth’s plates. Since then, however, using a new method to date craters on the moon, my colleagues and I have determined that the rarity of craters 300 to 600 million years old is due to a lower bombardment rate. In fact, the bombardment rate has increased by a factor of two to three in the past 300 million years.

To test this idea, we compared the Earth’s crater record to the moon’s in an article published in the journal Science . We suggest that the scarcity of terrestrial craters that are 300 to 650 million years old is simply due to a lower bombardment rate during that period – and not due to preservation bias.

Using rock abundance data from the Lunar Reconnaisance Orbiter to determine ages for lunar craters (image by Rebecca Ghent, ��Ƶ, and Thomas Gernon, University of Southampton)

Dating craters

The moon’s surface serves as a time capsule, helping us to detangle Earth’s history. There are tens of thousands of craters on the moon and the only way to see if the bombardment rate has changed is to have an age for every single crater.

Stable terrain and crater erosion

Analysis shows young craters with numerous metre-sized fragments are easy to pick out from older craters with eroded fragments. As time goes by, these large rocks get broken down by future small impactors. Eventually, over the course of about a billion years, all of the rocks form into lunar regolith (a fine layer of dust covering the moon’s surface), providing an inverse relationship between rock abundance (the rockiness of a crater’s ejecta) and crater age. As craters get older, they become less rocky.

Using measured rock abundance values, we computed ages for 111 lunar rocky craters larger than 10 kilometres in diameter that formed between 80°N and 80°S over the last billion years. Using the ages of these young craters, we determined that the production rate of large lunar craters (more than 10 kilometres in diameter) increased by a factor of two to three in the past ~300 million years. Thus, the near-Earth objects population has increased over the last billion years.

The size and age distributions of lunar and terrestrial craters larger than 20 kilometres over the last 650 million years have similar shapes. This implies that large crater erasure must be limited on stable terrestrial terrains. It also implies that the observed deficit of large terrestrial craters between 290 and 650 million years is not preservation bias, but a reflection of a distinctly lower impact rate. If we had observed more dominant erosion, the age distribution of terrestrial craters would be strongly skewed toward younger ages.

Using data from the recent study on moon craters, SYSTEM Sounds created this video and accompanying soundtrack

Support for limited erosion on cratered terrains also comes from records of kimberlite pipes on Earth. Kimberlite pipes are carrot-shaped pipes that extend a couple of kilometres below the surface and are often located on the same stable regions where we would find preserved impact craters. These underground pipes have been widely mined for diamonds, providing scientists with plentiful information about their location and erosion state.

Records show that kimberlite pipes have not experienced much erosion since forming around 650 million years ago. Therefore, the large young impact craters found on the same stable terrains must also be intact, providing us with a complete record.

Asteroid breakup?

The cause of this increase in the bombardment rate is still unknown. However, a hypothesis is that an asteroid family breakup caused a larger amount of debris to leave the asteroid belt and head towards our region of the Solar System. The loss of most craters older than 650 million years could be due to erosion from Snowball Earth, when most of Earth’s surface was frozen around 650 million years ago.

We predict that the rare extinction-level event type craters like Chicxulub, which may have lead to the extinction of the dinosaurs, were a byproduct of the current high bombardment rate. These new findings could have implications for the evolution of Phanerozoic life — our current geologic era — and the history of life including extinction events and evolution of new species.

Sara Mazrouei is a sessional lecturer and planetary scientist at the ��Ƶ.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Sara Mazrouei

What the moon's craters reveal about the Earth's history: U of T expert and lead author of new study

Dating craters

Read more about the research at U of T News

Stable terrain and crater erosion

Asteroid breakup?