Mercury: Enormous collision between 2 round rocky bodies in space, 1 a little smaller than the other. A huge ring of bright, hot debris encircles the smaller body.
View larger. | Artist’s concept of a collision between two rocky bodies in the early solar system. A new study suggests a similar event created Mercury. Image via NASA/ JPL-Caltech/ Wikimedia Commons (Public Domain).
How did the planet Mercury form? Scientists have been pondering this question for a long time.
According to new research, Mercury originated from the massive grazing collision of two similarly-sized bodies.
Collisions like this one were common in the early solar system billions of years ago. In fact, they likely accounted for about 1/3 of all impacts.
Mercury is the smallest and innermost planet in our solar system. It looks a lot like our moon at first glance, but it’s its own world, with unique geology and history. Scientists have been trying to figure out how it formed for a long time. And now, a new study from researchers in Brazil, Germany and France has shed some new light on the question. In a new preprint paper published on March 4, 2025, they said that a grazing giant collision between two similar-sized rocky bodies likely created Mercury a few billion years ago.
Mark Thompson wrote about the latest findings in Universe Today on March 25, 2025.
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How did Mercury form?
Despite its superficial resemblance to our moon, Mercury is a unique and strange world. Researchers have found evidence for a possible 10-mile thick layer of diamonds between the core and mantle of this planet, along with salty glaciers that could even be habitable.
And until now, scientists haven’t fully understood how Mercury formed. Surrounding its iron core is a relatively thin silicate mantle. In fact, the solid inner core and the molten outer core together take up nearly 85% of the planet’s radius. That’s much more than any of the other rocky planets. This posed a mystery. As the paper states:
The origin of Mercury still remains poorly understood compared to the other rocky planets of the solar system. One of the most relevant constraints that any formation model has to fulfill refers to its internal structure, with a predominant iron core covered by a thin silicate layer.
Cutaway view of gray rocky planet with a bright yellowish middle layer.
View larger. | Diagram depicting the interior structure of Mercury as currently understood. The overall core is nearly 85% of the planet’s radius, much more than other rocky planets in our solar system. This supports the theory that Mercury formed from the grazing impact of 2 similarly-sized bodies. Image via NASA/ Goddard Space Flight Center.
Collisions in the early solar system
The early solar system was a chaotic place, with frequent collisions between rocky bodies. And Mercury’s strangely large core has led scientists to hypothesise that a collision with a much larger body might have stripped away its outer layers.
But simulations of the early solar system have found collisions between very differently sized objects to be relatively rare. On the other hand, recent simulations suggest that collisions between very similarly sized bodies are much more common. In fact, they likely accounted for about 1/3 of all impacts in the early solar system. And this, the new study says, is how Mercury likely formed.
Black and white image of a spacecraft arm with a cratered world behind it.
One of the newest images of Mercury, from the 3rd flyby of the BepiColombo spacecraft on June 19, 2023. Image via ESA/ BepiColombo/ MTM.
Did a massive collision create Mercury?
Patrick Franco at the National Observatory in Brazil led the new study into whether two similar-sized rocky bodies could form a planet similar to Mercury.
Their study used a main body – a proto-Mercury – with a mass just over 10% of Earth’s, and a 30% iron makeup. In the simulations, the researchers experimented with variously sized secondary bodies, with varying amounts of iron.
They also varied the impact velocities between the two bodies, from 2.8 to 3.8 times the mutual escape velocity. The escape velocity is the minimum speed needed for an object to escape the orbit of or contact with a primary body.
Within these parameters, the researchers experimented with collision scenarios that could have occurred billions of years ago in the early solar system.
And they eventually found a scenario that produced a planet that matched Mercury’s mass with a 5% margin. And its core was 65-75% iron, compared to Mercury’s current value of 70%. It’s strong evidence, they said, that Mercury formed from a grazing collision between two similarly sized rocky bodies.
Forming Mercury by a grazing giant collision involving similar mass bodies. Patrick Franco et. al. https://arxiv.org/abs/2503.02826
— AstroArxiv (@astroarxiv.bsky.social) 2025-03-05T05:07:08.122613+00:00
Smiling man wearing a parka and scarf.
Patrick Franco at the National Observatory in Brazil is the lead author of the new Mercury study. Image via LinkedIn.
Bottom line: A new study says Mercury was formed from a huge collision between 2 similarly-sized rocky bodies.
Source: Forming Mercury by a grazing giant collision involving similar mass bodies
Via Universe Today
Read more: Mercury may have a 10-mile-thick layer of diamonds
Read more: Mercury images from final flyby of BepiColombo!
Paul Scott Anderson
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About the Author:
Paul Scott Anderson has had a passion for space exploration that began when he was a child when he watched Carl Sagan’s Cosmos. He studied English, writing, art and computer/publication design in high school and college. He later started his blog The Meridiani Journal in 2005, which was later renamed Planetaria. He also later started the blog Fermi Paradoxica, about the search for life elsewhere in the universe. While interested in all aspects of space exploration, his primary passion is planetary science and SETI. In 2011, he started writing about space on a freelance basis with Universe Today. He has also written for SpaceFlight Insider and AmericaSpace and has also been published in The Mars Quarterly. He also did some supplementary writing for the iOS app Exoplanet. He has been writing for EarthSky since 2018, and also assists with proofing and social media.