Modern Earth shows almost no signs of its earliest phase. The Hadean period, spanning from the planet’s formation 4.5 billion years ago until about 4 billion years ago, is poorly understood in geological terms. Only scattered ancient rocks and zircon crystals remain from that era, since most of the original crust has disappeared.
Researchers have often linked this absence to plate tectonics and its recycling of surface material. A group led by a Curtin University geologist now suggests instead that repeated asteroid strikes during the Hadean kept the crust from ever becoming stable.
The collisions delivered huge amounts of energy that heated the surface and left much of the outer layer partially molten, preventing rocks from hardening into lasting formations.
Supporting clues come from the Moon, which preserves a clear record of the same bombardment. Similar evidence appears on Mercury, Mars, and meteorites, indicating Earth was not spared. Reconstructions of impact timing and size show that the added heat reached the mantle, triggering widespread melting and magma production.
Models indicate this external energy input matched or exceeded the planet’s internal heat for long stretches of the Hadean. As a result, rock layers several kilometers down stayed fluid, delaying the growth of rigid tectonic plates and the first continents.
