Mercury and the Moon are shrinking, but what about Earth? Understand the phenomenon

As a natural part of the geological evolution of celestial bodies, they all shrink . This occurs mainly due to the gradual cooling of their cores, and can take, depending on their characteristics, up to billions of years.

This process is universal for all celestial bodies that have a solid or semi-solid structure and that have enough internal heat to generate significant changes in their structure over time.

When they are “born,” planets and moons are extremely hot due to the heat of accretion (formation) and the decay of their radioactive elements. But over time, this internal heat is lost to space, and as the materials cool, they tend to contract.

Logically, the nature of the materials that make up the celestial body influences this process. And that is why planets with large metallic cores, such as Mercury , are more susceptible to this shrinkage, because, when it cools, the metal contracts more than the rocks.

There are other examples of volumetric reductions in the Solar System, such as our Moon, as well as satellites of gas planets, and even dead stars, such as some white dwarfs, shrink as they cool.

But what about Earth? The answer is yes: our planet is cooling and , although smaller than Mercury, it also has a solid core with 85% iron. However, some characteristics make its cooling, and consequently, its shrinkage, a little slower. Check it out.

How is Mercury shrinking?

More than 50 years ago, scientists discovered that Mercury was shrinking. The first evidence was detected by NASA's Mariner 10 mission, which flew by the planet in 1974 and 1975 and discovered "lobate scarps."

These landforms are steep, cliff-like slopes that are up to three kilometers high and hundreds of kilometers long. Their shape is curved and irregular, like sinuous lobes or undulations.

In an article for The Conversation, Professor David Rothery from the Open University explains that as Mercury's interior shrinks, its surface has less and less area to cover.

“This is like the wrinkles that form on an apple as it ages, except that an apple shrinks because it is drying out, while Mercury shrinks because of thermal contraction of its interior,” the planetary geoscientist compares.

Although the heat on its surface reaches 430°C during the day, the fact that it has a large core, which occupies 80% of the planet's volume , means that its relatively thin rock layer (mantle and crust) is unable to adjust to the shrinking of the metallic core.

NASA's Messenger spacecraft, which orbited the planet closest to the Sun from 2011 to 2015, revealed that Mercury had contracted by around seven kilometers since its formation 4.5 billion years ago.

The Moon is also shrinking

The finding that the Moon is shrinking was announced in 2010 by astronomers at the Smithsonian Institution's National Air and Space Museum (NASM), also through the observation of images of lobed scarps taken by cameras on board Apollos 15, 16 and 17.

They were compared with 14 new scarps detected by cameras on NASA's Lunar Reconnaissance Orbiter in 2019. The new formations showed that the structures were not restricted to just the lunar equator, as shown in the Apollo photos.

In a statement at the time, planetary scientist Tom Watters of NASM noted that “One of the remarkable aspects of lunar scarps is their apparent young age.”

So-called “thrust faults,” geological structures that form when one piece of rock is violently pushed over another, have caused an estimated global contraction of the lunar surface of about 100 meters in the recent past, Watters says.

He also analyzed eight moonquakes that occurred in 2019, and concluded that they were produced by still-active fault lines, which may be “producing moonquakes today as the Moon continues to gradually cool and shrink .”

What about the Earth? How are we shrinking?'

Unlike Mercury, which has an extremely thin atmosphere, and the Moon, which has virtually no atmosphere, Earth has a thicker gaseous layer. This helps trap material falling from space and stops the escape of light gases such as hydrogen and helium.

Although many large meteors burn up upon entering the atmosphere, it is estimated that about 40,000 tons of space dust and particles enter Earth annually.

However, this volume is considered insignificant when compared to the amount of gas that escapes from the atmosphere into space over time. In other words, despite the gain in external material, the Earth also continually loses mass.

According to science communicator Chris Smith, “physicists have shown that the Earth is losing about three kilograms of hydrogen gas every second. That’s about 95,000 tons of hydrogen that the planet is losing every year,” the microbiologist explains to the BBC.

In his analysis of the factors that really affect the Earth's mass, Smith considers that, on the one hand, the Earth's core loses energy as it cools, on the other hand, it accumulates heat from the greenhouse effect and climate change.

The balance of all this results in an annual loss of 50 thousand tons of mass. It may seem like a lot, but it is very little when compared to the mass of the planet, representing only 0.00000000000000001% of the total.

In terms of size, the calculation was carried out by NASA scientists using a series of techniques that included satellite laser ranging with millimeter precision.

The results estimated an average change in the Earth's radius equivalent to 0.1 millimeters per year, something like the thickness of a human hair, which is considered practically insignificant in practical terms.

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