Rephrase the title:An interplanetary collision might have shrunk Mercury to its current size, scientists think

But alas, while Mercury’s minuscule appearance might make it an easy target, it wasn’t always this tiny, it turns out.

Mercury, the smallest and closest planet to the Sun, has long captivated scientists with its extreme conditions and mysterious features. With temperatures swinging from scorching heat to freezing cold and a surface scarred craters, understanding Mercury’s composition and formation has proved challenging.

Further, to actually get to Mercury is a mammoth task itself. Since the planet is so close to the Sun, it takes a tremendous amount of effort to oppose the star’s gravity when near the planet. This also means a mountain of fuel is needed to decelerate spacecrafts enough so they can insert themselves into Mer-dog’s gravity without suffering a meteor’s fate.

But that doesn’t mean we’ve stopped trying. NASA has managed to fly two missions close enough to map the planet, which just made things even curiouser. For starters, they found that, for whatever reason, Mercury has an absurdly gigantic core compared to its mantle, while its crust is paper-thin — relatively speaking. The other nearrock-based planets, such as Venus, Earth and Mars, all sport bacores surrounded a thick molten mantle.

Further, analysis of Mercury revealed that it has an unusually high concentration of thorium, an element that should have evaporated in the extreme heat of the planet. Last, but not least, scientists found that, similar to Earth, Mercury too is surrounded a magnetic field. This suggests that its inner workings might be very similar to ours, and even be made of the same stuff as our core.

To keep himself from pulling his hair out, planetary researcher Nicola Mari decided to take a closer look at the planet. Forgoing fancy space-faring rockets and opting for cars and planes instead, his journey took him to Cyprus, a Mediterranean island that used to be at the bottom of the ocean 90 million years ago.

The area’s rugged terrain unearthed a geological treasure trove: boninite rocks, born from our Earth’s oozing mantle. Through meticulous analysis, Mari and his team confirmed that these rocks suspiciously mirrored Mercury’s chemical makeup almost exactly. The composition of elements such as magnesium, aluminum and iron was the exact same as that seen on the crust of the big-cored planet.

The reason this is so significant is because boninites are formed from lava that erupts from a shallow point in our Earth’s crust. If Mercury’s rocks were also formed from its mantle in similar ways to our own, it means that something could have happened that blew away the majority of its crust, leaving behind the thin exterior we see now. Something catastrophic like an interplanetary collision, perhaps.

This catastrophic clashing theory has been in the works for a while now, and planetary scientists think that Mercury might have started off being as big as the Earth. Most of its crust was wiped away post-collision, leaving what we see today.

The journey to decipher Mercury’s secrets is far from over. With BepiColombo‘s — a joint space endeavor the European Space Agency and Japan — scientific operations on the horizon, the stage is set for a new era of discovery. As Mari ventures forth to explore other terrestrial analogues, the cosmos awaits with its myriad secrets, ready to be unveiled one rock at a time.

These findings are based on a BBC article that can be accessed here.