Mercury may look like a small, silent world sitting close to the Sun, but its surface is telling a surprisingly dramatic story.

Scientists have known for years that Mercury has been slowly shrinking as its interior cools. Now, new research suggests the planet may have contracted 10% to 30% more than previous estimates indicated.

That could mean Mercury has lost as much as 23 kilometers — about 14.5 miles — from its total diameter since the planet formed billions of years ago. 

And one of the most interesting parts of the discovery is why scientists may have underestimated it for so long.

Mercury Is Literally Wrinkling

Mercury formed around 4.5 billion years ago, during an extremely violent period in the early solar system.

Its interior was once much hotter than it is today.

As the planet gradually lost heat, its enormous metallic core and surrounding layers cooled and contracted.

Imagine what happens to the skin of a grape when the inside loses moisture and gets smaller.

It wrinkles.

Mercury has experienced something somewhat similar on a planetary scale.

As the interior contracted, the outer crust was forced to adjust. This created enormous cliffs, ridges and faults across the planet’s surface.

Some of these features stretch for hundreds of kilometers.

Scientists use these geological structures to estimate how much Mercury has shrunk over its lifetime. 

But there was a problem.

Some of Mercury’s “Wrinkles” May Have Been Hidden

Mercury has also been hit by asteroids and other objects countless times during its history.

Those impacts created craters and scattered huge amounts of broken rock and debris across the surface.

According to the new study, this rough material may have buried or obscured some of the geological features caused by Mercury’s contraction.

That means scientists looking at the visible surface may not have been seeing the whole picture.

Researchers compared areas with different levels of surface roughness and found that rougher, debris-covered regions appeared to contain fewer visible contraction structures.

The likely explanation?

Some of the evidence was simply hidden underneath. 

The Difference Could Be Surprisingly Large

Previous estimates suggested Mercury had lost roughly 4 to 16 kilometers of its total diameter.

After accounting for geological features that may be hidden by surface debris, the new research suggests the total contraction could reach approximately 23 kilometers.

That represents an increase of roughly 10% to 30% compared with earlier estimates, depending on how the calculations are made. 

On Earth, 23 kilometers might sound like a relatively small distance when compared with the size of an entire planet.

But Mercury itself is only about 4,900 kilometers wide.

For planetary scientists trying to understand what is happening deep inside another world, that missing distance matters.

Why Scientists Care About a Shrinking Planet

Mercury’s surface provides clues about what has been happening beneath it for billions of years.

Scientists cannot simply drill into Mercury’s core.

Instead, they study the surface and use the planet’s faults, cliffs and ridges like clues in a giant geological puzzle.

If scientists know how much Mercury has contracted, they can better estimate how quickly its interior cooled and how the planet evolved after it formed.

The revised contraction estimate also appears to bring observations closer to what theoretical models of Mercury’s cooling predicted. 

In other words, finding evidence that Mercury shrank more than previously estimated may help scientists better understand what is happening deep inside the smallest planet in our solar system.

And Scientists May Still Be Missing Some of It

There is another twist.

The researchers say the new estimate may still be too low.

Much of what scientists currently know about Mercury’s detailed surface comes from NASA’s MESSENGER spacecraft, which orbited the planet before its mission ended in 2015.

The available data can reliably reveal only geological structures above certain sizes.

Smaller cliffs and faults may still be waiting to be discovered. 

Fortunately, scientists will soon get a much closer look.

BepiColombo Could Reveal Even More

The European-Japanese BepiColombo mission is preparing to begin its main scientific observations of Mercury.

According to the researchers, higher-resolution measurements expected from the mission could reveal smaller ridges, faults and impact structures that previous observations could not clearly detect. 

That could answer an intriguing question:

Has Mercury shrunk even more than the latest study suggests?

Scientists may soon find out.

A Planet That Is Still Full of Surprises

From Earth, Mercury can easily seem like a simple world — small, rocky and scorched by its proximity to the Sun.

But beneath that battered surface is a record stretching back billions of years.

Its enormous core has cooled.

Its crust has been compressed.

Its surface has wrinkled.

Asteroid impacts have buried some of those scars.

And now scientists are discovering that the planet’s transformation may have been greater than they realized.

Sometimes the most fascinating discoveries in space are not about finding something completely new.

Sometimes they come from realizing that the evidence was sitting in front of us all along — we just couldn’t see all of it.