If today we are already short of hours in the day, imagine what it was like 2.5 billion years ago: at that time, the day on Earth lasted only 17 hours. This is one of the findings of an international team of geologists, who were also able to intelligently determine how far the Moon was from our planet at that time.
These things are all connected: the Moon was born from a giant impact against Earth 4.5 billion years ago, shortly after our planet formed, and since then the gravitational interactions between the two bodies have been doing their stretching work. and-pull. The most notable effect of this process is the gravitational lock that keeps the same side of the Moon always facing Earth. But the game goes both ways: the Moon also wants to force the Earth to keep the same face towards itself.
As the natural satellite has a much smaller mass than the planet, the effect on Earth is much more gradual. Still, the rotation of our world is gradually being slowed down by the gravitational pull of the moon. So it was faster in the past. And, by the same process, when transferring part of the force to change the Earth’s rotation, the Moon is slowly moving away in its orbit. Today, at an average distance of 384,300 km, our companion is approximately 3.8 cm further away each year. It’s a tiny bit, but it turns into something significant after billions of years.
Enter the work by Margriet L. Lantink of the University of Utrecht in the Netherlands and colleagues, published in PNAS, the journal of the US National Academy of Sciences. The group studied a group of ancient sedimentary rocks in western Australia, which contain patterns of iron and clay deposition that make it possible to distinguish the so-called Milankovitch cycles – a phenomenon linked to periodic orbital and axial oscillations of the Earth that give rise to glaciations.
The periodicity of these cycles, in turn, is also related to the organization of the Earth-Moon system. Hence the possibility of extrapolating, from their records in sedimentary rocks, things like the Earth’s rotation and the lunar distance. This is what the researchers did, noting that the Moon, 2.46 billion years ago, orbited at approximately 321,800 km (with a margin of error of 6,500 km). The Earth, in turn, completed a rotation in 16.9 hours (with a margin of error of 12 minutes) at the same time.
The result helps to better understand the evolution of the Earth-Moon system and beats the record for the oldest record of its parameters by a wide margin, advancing more than 1 billion years in the past with respect to previous studies. It’s surprising? No, on the contrary. It is something more than expected, based on everything that could be imagined from the past. But it makes a difference not merely to find, but to know. The rocks, witnesses of the most remote events on the planet, tell their story, and this is what allows us to give an account of what was happening here long before the first human set foot on Earth.
This column is published on Mondays, in Folha Corrida.
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