Scientists find a shrinking world of Mercury

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

The planet Mercury, a small, rocky world closest to the sun, has been slowly but surely shrinking for billions of years.

However, there’s more to the story than its gradual decrease in size. It’s a story that involves not just the planet’s distant past, but also its present and possible future, which scientists are unraveling piece by piece.

A Wrinkling Surface Tells a Story

In the 1970s, when spacecraft Mariner 10 sent back images of Mercury to Earth, scientists spotted large cliff-like structures called scarps winding their way across its surface. This was our first clue that the planet was shrinking.

Years later, a spacecraft named Messenger provided even more detailed images, revealing more of these scarps scattered across the planet.

The shrinking isn’t because Mercury is losing mass. Rather, it’s all about cooling and contracting. Imagine a chocolate truffle – once it cools from its molten state, it becomes a bit smaller and wrinkles might form on its surface.

Mercury behaves in a somewhat similar way, though the processes are vastly different and occur over billions of years.

The planet’s interior has been cooling and, as a result, contracting, leading to the surface crumpling and forming scarps.

The age of the scarps is often estimated by counting the number of impact craters on their surface – the more craters, the older the surface is thought to be.

This isn’t a perfect method because the rate of asteroid impacts has changed over time, but it gives scientists a general idea. So far, it seems like most of the scarps are pretty ancient, being around 3 billion years old.

Ongoing Movement and New Discoveries

The question that intrigued scientists for years is whether these scarps have stopped moving or if they’re still active today.

A recent study has brought new information to light, thanks to a keen observation by a Ph.D. student, Ben Man.

He noticed small fractures on some of the scarps, which were interpreted as a sign that they’ve experienced movement in more recent geological times, even if that still means millions of years ago.

These smaller features are less than 1 km wide and less than about 100 meters deep, and are expected to be much younger than the scarps themselves.

Scientists think that these newer formations, called grabens, are less than about 300 million years old. This suggests that Mercury may still be geologically active, though at a very slow and gradual pace.

It’s like the planet is still subtly rearranging its outer layer, forming new features on the surface as it continues to cool and contract inside.

Looking Ahead: The BepiColombo Mission

The joint European/Japanese BepiColombo mission is set to bring us even clearer images of Mercury by 2026.

It won’t be landing on the planet, but it’s expected to show these small grabens more clearly from orbit and might even provide additional evidence of the planet’s recent geological activities.

Comparatively, Earth’s moon has offered some insights into this phenomenon, as it has also cooled and contracted, forming scarps of its own.

Unlike Mercury, we have direct seismic data from the moon, showing that some of its scarps are indeed still active today.

The on-going exploration and study of Mercury’s shriveling surface deliver not just exciting discoveries about the planet itself, but also about the broader, dynamic universe we dwell in.

Understanding the intricacies of how celestial bodies like Mercury change over time gives us a window into the past and offers a glimpse into the mechanisms that drive the evolutionary processes of planets, and perhaps, the future of our own world.

It’s a reminder that even in the seemingly steady firmament above, change is the only constant, and there’s always something new to discover, as long as we keep looking and wondering.

The research findings can be found in Nature Geoscience.

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