What raindrop imprints reveal about Earth’s history – and distant planets

If you look at the development of the solar system and that of the Earth together, you quickly come across an obvious paradox: As a main sequence star, the sun does not shine with the same brightness throughout its entire life. With age, their energy production increases – that’s why our central star shines about a fifth brighter today than it did two billion years ago. Although the earth could take up at that time however clearly less solar energy, it was already approximately as warm as today – the Palaontologen know from their finds. In fact, the power of the sun should not even have been enough to warm the oceans above the freezing point of water: Not optimal conditions for the development of the first living creatures.

How to solve the dilemma? On the one hand, the energy absorption naturally depends on the part of the irradiated solar energy that is reflected again. If the Earth had a particularly low albedo at that time, for example, because there was hardly any cloud cover and coarse, dark oceans without ice cover, then a coarser energy absorption could be explained. The competition theory refers to the composition of the earth’s atmosphere: climate gases such as CO2 or methane could have helped to heat our planet. A cheer for climate warming?

It is difficult to find out which of the two theories is right and to what extent. Man records the composition of the atmosphere only recently. Drill cores in Antarctic ice, for example, do not yet reach the time periods that are important in this context. A team of researchers from the University of Washington has come up with a clever idea to shed light on the matter in the scientific journal Nature. The scientists use an everyday and at the same time ancient phenomenon: raindrops. In fact, millions of years ago precipitations have left their traces, which can be examined today.

Raindrops, it is clear, reach a certain maximum size in the earth’s atmosphere. This is due to the fact that above a certain speed, the droplet begins to grind because the surface tension and internal hydrostatic forces can no longer counteract the aerodynamics. These effects have not changed throughout the history of the Earth, raindrops are still no coarser than 6.8 millimeters in diameter. The energy with which they hit the ground depends on their coarse and terminal velocity.

The latter is a mab for the density of the atmosphere – this is the point where it became interesting for the researchers. Depending on the energy of the impact and on the characteristics of the ground, the drops have left different rough traces. These traces can be measured. And if you know the material that forms the ground, you can infer the speed of the drops and thus the density of the atmosphere.

Scientists can show that the air prere 2.7 billion years ago was not significantly different from today’s values. This does not completely solve the Young Sun’s riddle – but it excludes a few solutions. In particular the explanation with the help of simple climatic gases. It presupposes that the density of the atmosphere must have been quite a bit higher at the time. This leaves only the particularly efficient climate changers: methane, ethane or carbonyl sulfide. Sie konnten zum Beispiel durch Vulkanausbruche in der Atmosphare freigesetzt worden sein. Alternativ kommt auch noch die Albedo-Theorie in Frage.

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