Impact structures are dominant landforms on many of the Solar System’s solid celestial bodies. Like any other planet, Earth has a long history of meteorite impacts that have greatly affected the atmosphere, the climate and the evolution of life on our Blue Marble.
All significant asteroid impacts leave behind traces – impact structures (e.g., craters) and specific impact material, with former being the most valuable for research. However, tectonic movements and erosion eventually erase the craters, so there are no impact depressions that are older than about 2 billion years. That makes the ancient impact-induced evolution of Earth hard to track down, as their ties to climatic and geological shifts remaining largely elusive.
The Yarrabubba impact structure is located in Western Australia. It is an eroded remnant of a former impact crater that sits on one of the most ancient sections of Earth’s crust – the Yilgarn craton. Yarrabubba was first described in 2003, after the discovery of minerals that were characteristic of a meteorite impact at the site. But before the latest research published in Nature Communications this January, Yarabubba’s age was unknown and debated. It was clear that it was one of the older impact structures on Earth – but not exactly how old. The estimates ranged from 1.1 to 2.6 billion years.
How are impact craters dated?
By using a radiographic dating technique that measures isotopes of uranium and lead on small impact-shocked crystals at the site, the Curtin University team lead by Timmons M. Erickson concluded that the Yarrabubba Crater formed 2.229 billion years ago (+/- 5 million years).
In their statement, the team gave a more detailed explanation of their dating technique:
“Geologists date events using “isotopic clocks” in minerals like zircon and monazite. These minerals contain small amounts of uranium, which gradually decays into lead at a known rate. Asteroid strikes raise the temperature in rocks they hit, causing minerals to lose their accumulated lead, which resets the clock. After impact, the isotopic clocks start ticking again as new lead accumulates. So by measuring the isotopes of uranium and lead in these minerals, we can calculate how much time has passed since the impact.”
Evidence of older asteroid impacts are out there, but they are scattered. Scientists have previously discovered impact material older than 2.4 billion years elsewhere in Western Australia and in South Africa; however, no craters that corresponded to this age were ever found. That makes Yarrabubba the most ancient asteroid impact structure on Earth.
Besides its ancient quality, the newly-confirmed age of Yarrabubba has another interesting component to it. The age of the crater fairly coincides with the age of the youngest Palaeoproterozoic glacial deposits – the Rietfontein diamictite in South Africa. That means that this particular impact might have greatly shifted the Earth’s climate at the time and became the beginning of the end of the Paleoproterozoic glaciations, which include the infamous Huronian glaciation – the oldest and the longest Ice Age on Earth. That period in Earth’s geological history is often dubbed the “Snowball Earth.”
Numerical simulations suggest that an impact that could leave a 70 km (40 miles)-diameter crater in a continental glacier also could have instantaneously released between 8.7 × 1013 and 5.0 × 1015 kilograms of water vapor into the atmosphere.
“Our models show that if the Yarrabubba asteroid hit an ice sheet 5 kilometers thick (not an unreasonable estimate), more than 200 billion tons of water vapour would be ejected into the atmosphere. That’s about 2% of the total amount of water vapour in today’s atmosphere, but would have been a much bigger fraction back then. Water vapour is a serious greenhouse gas. It’s responsible for about half of the heat absorption from solar radiation today.”– the research team explained in the joint statement for The Conversation.
Since climate models for the Proterozoic Earth do not exist yet, it is not certain if it was the Yarrabubba impact that had pushed Snowball Earth into a thawing phase. Still, the Yarrabubba dating study makes a significant contribution to the research of the puzzling Paleoproterozoic glaciations. It provides new estimates of the impact-produced water vapor abundances for the models, and highlights the possible role of impacting on the shaping of the Earth’s climate.
Erickson, T.M., Kirkland, C.L., Timms, N.E. et al. (2020) Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure. Nat Commun 11, 300 . https://doi.org/10.1038/s41467-019-13985-7
“A 2.2-billion-year-old crater is Earth’s oldest recorded meteorite impact.” Science News. 21 January 2020. https://www.sciencenews.org/article/australia-crater-is-earth-oldest-recorded-meteorite-impact
“We found the world’s oldest asteroid strike in Western Australia. It might have triggered a global thaw”. The Conversation. 21 January 2020. https://theconversation.com/we-found-the-worlds-oldest-asteroid-strike-in-western-australia-it-might-have-triggered-a-global-thaw-130192