Surprising finding: the moon is probably older than we previously thought

According to current knowledge, the moon was formed during the formation phase of the solar system from the debris of the collision of the primordial Earth with the Mars-sized celestial body Theia. But when exactly was that?

New studies of lunar dust brought to Earth by the astronauts of the Apollo 17 mission now indicate a minimum age of 4.46 billion years. This means that Earth’s satellite is 40 million years older than assumed based on the best previous measurements, an international research team reports in the journal ‘Geochemical Perspectives Letters’.

Theia’s collision with primordial Earth was so powerful that large amounts of rock liquefied or even vaporized and were catapulted into space. The moon that formed from rubble was initially composed entirely of liquid rock.

Only when this ‘lunar magma ocean’ had cooled sufficiently at the surface could solid rocks and therefore zircon crystals form there. Because such crystals are virtually immune to subsequent changes, they are particularly suitable for determining age: their age provides a minimum value for the age of the Earth’s satellite.

The team led by Jennika Greer from the University of Glasgow in Great Britain has now examined moon dust for the first time using a so-called tomographic atomic probe. To do this, the researchers first ‘cut’ the zirconium crystals, which are only a few thousandths of a millimeter in size, using an ion beam.

The resulting very sharp point makes it possible to remove individual atoms from the sample. “We used an ultraviolet laser to vaporize individual atoms at the tip,” Greer explains.

The vaporized atoms pass through a special analysis device – a mass spectrometer – thus revealing their identity: depending on the weight of the atoms, they move at different speeds. “In this way we discover, atom by atom, exactly what the crystals are made of,” Greer explains. Researchers can determine the age of the crystal based on the mixing of certain elements in the crystals.

Greer and her colleagues used uranium and lead. Because a certain type of uranium – the isotope uranium-238 – turns into lead through radioactive decay, with a half-life of 4.5 billion years.

Since the formation of the solar system, about half of the uranium has been converted into lead. By accurately measuring the frequency of uranium and lead atoms in the zircon crystals, researchers can determine the age of the crystals. The result: 4.46 billion years. The previous most accurate measurement on another rock sample indicated an age of 4.42 billion years.

The crystals that Greer and her colleagues examined are 40 million years older. “It’s an incredible feeling to know that we have found the oldest piece of the moon yet,” says Greer. Because the crystals could not form until the moon’s surface solidified, Earth’s satellite itself must be slightly older. The researchers emphasize that it emerged in the first 100 million years of the solar system.

Knowing the exact birth date of the moon is important, emphasizes Phillip Heck from the University of Chicago in the US: “The moon stabilizes the Earth’s axis of rotation, is responsible for the length of our days, provides the tides – without the moon, there would be a completely different life on Earth!” The exact time the moon was formed determines when gravity influenced the development of the Earth.

(dpa/t-online)