Fundamental Science: The landscape clock falls from the sky

Waiting for the answer to fall from the sky is not a recommendation that matches science. However, when it comes to calculating the age of landscapes, what falls from the sky, more than recommended, is indispensable. I’m referring to cosmic rays.

As you read this text, now, cosmic rays are passing through your body, your screen, your desk, in other words, everything around you. These same rays also penetrate mineral crystals, and this has led to a revolution in studies of the evolution of the Earth’s surface in the last 30 years.

Have you ever wondered how long a certain natural beauty has been around? For example, how long has Chapada Diamantina been around? And the Finger of God? Or Sugarloaf Mountain? Until the 1990s, answering these questions would have seemed an almost impossible task. Here, in this text that I published in 2021, I described a simplified way of estimating this calculation, but we have a precise technique, the key element of which is contained in the crystalline structure of some minerals, such as quartz grains, which are exposed to the sky.

The cosmic rays that reach the atmosphere generate a cascade of secondary rays (those that pass through us) that, when they reach the Earth’s surface, penetrate it and react with the atoms that form the minerals. The reaction produces other elements, called cosmogenic isotopes, which accumulate year after year, recording the time they are exposed to the sky. As a kind of cosmic counter, quartz is the main mineral that keeps the landscape clock. This technique was introduced by Indian geophysicist Devendra Lal in 1991, but has roots in 1912, with a balloon flight.

110 years ago, physicist Victor Hess, born in Austria, undertook several flights aboard his balloon, reaching a height of 5 km. On these flights, Hess found that as he ascended, the more radiation our atmosphere received, even during a solar eclipse. The conclusion of his research was certain: the radiation could only come from space and not from the interior of the Earth or the Sun, contrary to what had been imagined until then. The radiation Hess discovered was later attributed to cosmic rays, which are subatomic particles that bombard our atmosphere at all times at speeds close to the speed of light, a discovery that won him a Nobel Prize in 1936.

Cosmic rays are special: they are the result of the explosion of supernova stars, that is, the “death” of stars and black holes throughout the universe. When they explode, stars eject all atoms disintegrated into subatomic particles (protons, neutrons, electrons, etc.). These particles travel through the universe at speeds close to the speed of light and eventually end up inside a mineral here on Earth.

It is fascinating to imagine that inside every grain of sand we see on the ground, there is an infinitely small piece of some star in the universe. What would Hess say when he learned that the evidence of what he sought in heaven was under his feet?

Back to the landscape clock. This technique allowed us to establish ages for some of the oldest surfaces on Earth and revealed that extreme temperatures are great for preserving landscapes. For example, cosmogenic isotope levels measured in gravel in the Atacama revealed that the surface of this desert is at least 9 million years old. In Antarctica, preserved surfaces on top of mountains are 18 million years old. If these surfaces are so old, that means they’ve been unmodified all this time, meaning not even an inch of depth has been removed from them. They are true fossil landscapes, preserved in time.

Do we have, in Brazil, fossil surfaces as old as those of Atacama or Antarctica?

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Pedro Val is a geologist and professor at Queens College, City University of New York.

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