Pick up a handful of soil from your garden and you’re holding material that has been recycled, eroded, subducted, and remade so many times that almost nothing of the original early Earth survives. Our planet is a relentless geological engine, constantly destroying its own past. If you want to know what was happening here 3.5 billion years ago, when life was just beginning to take its first tentative hold on the world, you have to look somewhere else. That somewhere else surprisingly, is the Moon.
The Moon shares our neighbourhood and our impact history, but it doesn’t share our geological upheaval. There’s no erosion, no plate tectonics, no weather to wipe the slate clean. What lands on the Moon tends to stay recorded in the rock. And that’s exactly what makes a small meteorite found in northwest Africa so scientifically valuable.
The landing site of Apollo 17 (Credit : NASA)
The meteorite, catalogued as NWA 12593, originated on the Moon. At some point in the more recent past, a collision knocked it off the lunar surface and sent it on a slow journey toward Earth, where it eventually fell and was found. Inside it, researchers at the University of Colorado Boulder have now identified evidence of not one but three separate impact events, each leaving its own signature in the rock like entries in a diary written in minerals and heat.
The oldest and most significant happened around 3.5 billion years ago. It was enormous and the energy released was sufficient to melt the entire surface of the surrounding area into a flowing sheet of liquid rock. The temperatures reached were so extreme that they created cubic zirconia, the same mineral used in jewellery but in this case formed naturally under conditions of almost unimaginable violence. Cubic zirconia is fragile at low temperatures and rarely survives in nature so finding traces of it in this meteorite is a fingerprint of something catastrophic.
A second, smaller impact later shattered that solidified melt sheet, mixing the broken fragments together and fusing them into the type of rock geologists call a breccia, essentially a natural concrete of crushed and rewelded material. The meteorite itself is that very breccia, the third impact, relatively recent in geological terms, that simply launched it off the Moon toward us.
Lunar breccia meteorite like that used in this recent study (Credit : James St Johns)
But it’s the timing of the first event that has really captured the attention of the scientists. That same 3.5 billion year window shows up in the impact record on Earth, and on 4 Vesta, one of the largest objects in the asteroid belt. Finding the same bombardment signature on three separate bodies in the inner Solar System in one study is rare. It suggests a shared event, perhaps the breakup of a large asteroid sending a wave of debris cascading across the inner Solar System at exactly the moment life on Earth was finding its footing.
The question of whether those impacts helped or hindered the emergence of life remains open. But knowing when they happened, and that they happened everywhere at once, is a crucial piece of the puzzle.
Source : Scientists discover a 3.5-billion-year-old asteroid impact on the Moon
