The secrets of Earth, the Moon and Mars are being uncovered by detailed studies of zircon crystals in ancient rocks.
John Curtin Distinguished Professor Simon Wilde and Associate Professor Alexander Nemchin, with colleagues from Curtin’s Department of Applied Geology, undertake in situ isotopic analyses of zircons and other chemically complex materials.
To do this they use Curtin’s two Sensitive High Resolution Ion Micro Probes (SHRIMPs) in the John De Laeter Centre for Isotope Research.
“The oldest zircons on Earth, the Moon and Mars – which are all close to 4.4 billion years old – have been dated using the Curtin SHRIMPs,” says SHRIMP Manager Dr Allen Kennedy.
While Wilde primarily focuses on terrestrial zircons, Nemchin – who divides his time between Curtin and the Swedish Museum of Natural History in Stockholm – has analysed zircons from the Moon and Mars.
“Previous research in the seventies discovered abundant zircon in many lunar samples delivered by the Apollo missions,” Nemchin says. “So we used zircon samples from the Moon to gain a better understanding of how to interpret our terrestrial zircon data.”
The results were illuminating: “We found the currently oldest known zircon on the Moon with an age of 4.417 billion years
– which provides the youngest limit to the formation of the lunar magma ocean.” This vast ‘ocean’ of partially melted rock
is thought to have swamped the Moon shortly after it formed.
In addition, Nemchin and his international collaborators, including NASA, identified a series of features in zircon grains that allow major lunar impact events to be dated.
They have also developed novel methods of analysing phosphates from the Moon with a precision close to a few million years. “Together, this resulted in our questioning of the terminal lunar cataclysm hypothesis.”
Also known as the Late Heavy Bombardment, the lunar cataclysm concept was put forward in the 1970s. It suggests that asteroids barraged the Moon for a short time approximately 3.9 billion
years ago, causing much of the cratering seen today on the lunar surface and having geological consequences for Earth.
Nemchin’s results instead suggest multiple cataclysmic spikes of impacts occurred throughout the history of the Solar System, separated by relatively quiet periods.
The team also dated zircon found in an ancient Martian meteorite known as Black Beauty, which was discovered in the Sahara Desert in 2011 by Bedouin tribesmen.
After they determined that the meteorite’s zircon crystals were 4.43 billion years old, the team took precise measurements that provided additional ideas about how the Martian atmosphere has changed through time.
They found that water on Mars was more abundant when the crystals formed, but something dramatically changed prior to 1.7 billion years ago, leaving the barren Martian desert that persists to this day.
– Ben Skuse