Swiss team unfolds new angle on formation of Moon around 4.5 billion years ago
By Kiran N. Kumar
How did the Moon originate? A long-held belief among scientists is that a massive collision of barely-formed Earth with a Mars-sized planet named Theia, around 4.5 billion years ago, resulted in a vast spray of debris that eventually took the form of the Moon.
New evidence put forward by experts at ETH Zurich sheds light on the proof that the moon was formed after the violent collision. In their study just published in the journal, Science Advances, the research team showed that the Moon has noble gases of helium and neon found in the Earth’s mantle and rule out solar wind as the source of these gases.
This discovery is based on lunar meteorites, or lunaites, which are found on Earth, including Antarctica. They studied six fragments recovered from Antarctica, which are part of the same original meteoroid, and consist of a very specific kind of rock: unbrecciated – that is not a ‘fruitcake’ of multiple rock types, as many meteorites are.
The team found the isotope ratios of the neon were very similar to isotope ratios of neon inside the Earth that are likely undisturbed since the planet formed, 4.5 billion years ago. This similarity suggests that the gases came from Earth and ruled out the solar wind as the source, the researchers concluded.
The scientists found that isotopes of the noble gasses helium and neon trapped in lunar meteorites recovered from Antarctica were similar with a signature argon isotope concentration, they concluded that those gasses were inherited from Earth, when the two bodies were one.
In the absence of an atmosphere to protect, asteroids keep pelting the Moon’s surface. A high-energy impact might have ejected the meteorites from the middle layers of the lava to the vast plains known as the Lunar Mare and eventually the rock fragments made their way to Earth in the form of meteorites.
Many of these meteorite samples were taken from NASA’s collection picked up in the “cold desert” of Antarctica where they are easier to spot in the landscape.
The meteorites consist of basalt rock that formed when magma welled up from the interior of the Moon and cooled quickly. Additional basalt layers covered and protected them from cosmic rays, especially the solar wind. The cooling process resulted in the formation of lunar glass particles among the other minerals found in magma.
The team found the glass particles retain the chemical fingerprints (isotopic signatures) of the solar gases: helium and neon from the Moon’s interior and propose that the Moon inherited noble gases indigenous to the Earth.
When the team examined them in the Noble Gas Laboratory at ETH Zurich where a noble gas mass spectrometer (Tom Dooley) is available and in fact, the only instrument in the world capable of detecting such minimal concentrations of helium and neon, they were able to detect these noble gases in the grains.
“Finding solar gases, for the first time, in basaltic materials from the Moon that are unrelated to any exposure on the lunar surface was such an exciting result,” said cosmochemist Patrizia Will, formerly of ETH Zurich in Switzerland, now at Washington University in St. Louis.