A one-of-a-kind meteorite from Mars has an unexpected chemistry that could refine scientists’ models of how terrestrial planets form, according to a new study of the old space rock. 

Chemical clues from this far-flung sample hint that Mars and Earth–often viewed as would-be twins because they are rocky worlds and solar system neighbors–were birthed in very different ways: Earth formed slowly, and Mars much faster.

Current hypotheses about the creation of a rocky planet, like Mars or Earth, suggest that some elements in the planet’s interior should have the same chemical characteristics as those in the planet’s atmosphere. That’s because, in the early days of our solar system about 4.5 billion years ago, the rocky planets were covered in a magma ocean. As the planets cooled and their molten mantles solidified, the process probably released the gasses that became atmospheres. 

Those gasses weren’t just any chemicals. They were volatiles, chemical elements and compounds that vaporize very easily. Volatiles include hydrogen, carbon, oxygen, and nitrogen, as well as noble gasses, which are inert elements that don’t react with their environment. On Earth, those chemicals eventually allowed our world to develop and support life.

To look for signs of that process on Mars, Sandrine Péron, a postdoctoral fellow in the Institute of Geochemistry and Petrology at ETH Zürich compared two Martian sources of the noble gas krypton. One source was a meteorite that originated in the Martian interior. The other was  krypton isotopes sampled from Mars’ atmosphere by NASA’s Curiosity Rover. Unexpectedly, the krypton signatures did not match. And that could change the sequence of events for how Mars got its volatiles and atmosphere in the first place.

“This is kind of the opposite to the standard model of volatile accretion,” Péron says. Her results are described in a paper published Thursday in the journal Science. “Our study shows that it’s a bit more complicated.”

The planets in our solar system formed from the debris of our sun’s birth. Clumps of material coalesced in the swirling disk of gas and dust, called a solar nebula, around the new star. Some clumps, which accumulated through gravity and collisions, grew large enough to become planets and develop complex geological processes. Others remained small and inactive as primitive asteroids and comets. 

[Related: Mysterious bright spots fuel debate over whether Mars holds liquid water]

Scientists think that volatiles were first incorporated into the new worlds directly from the solar nebula in the earlier stages of planetary development. Later, as the solar nebula dissipated, more volatiles were delivered from bombardments of chondritic meteorites, small chunks of stony asteroids that remain unchanged from the earliest days of the solar system. Those meteorites then melted into the magma oceans.

If the atmosphere was delivered by space rock, planetary scientists would expect the volatiles in a planet’s atmosphere to match those from chondritic meteorites, not the solar nebula. Instead, Péron found that the krypton from the Martian interior is nearly purely chondritic, while the atmosphere is solar. 

As such, perhaps Mars was bombarded by chondritic meteorites early on and then solidified while there was still enough solar nebula to form an atmosphere around the hardened Red Planet, Péron suggests. She explains that the nebula would have dissipated around 10 million years after the sun formed, so the accretion of Mars would have had to be completed well before then, perhaps in the first 4 million years. 

Mars photo
A sample of the Chassigny meteorite that revealed the Martian interior contains chondritic volatiles. Courtesy of Sandrine Péron

“It looks like Mars acquired its atmosphere from the primordial gas that permeated the solar system as it was forming,” says Matt Clement, a postdoctoral fellow studying terrestrial planet formation at the Carnegie Institution for Science who was not involved in the study. “This generally fits in with our picture. We think Mars formed much, much faster than the Earth did.”

Scientists often look to Mars to study the early solar system precisely because of how fast it is thought to have formed. Mars, which is a tenth of the mass of Earth, is also far less geologically active, which means the Red Planet probably preserves a lot of the conditions of our planetary neighborhood’s earliest days. 

However, to study the chemistry of Mars, scientists either have to send mechanical envoys like the Curiosity Rover to the planet or examine pieces of Mars that have broken off, hurtled through space, and landed on the surface of Earth. There are only a few hundred such meteorites.

The meteorite that Péron studied is unique. In 1815, it plummeted through Earth’s atmosphere, fracturing into pieces over Chassigny, France. Since then, scientists studying the fragments of the Chassigny meteorite determined that it likely came from the Martian interior—unlike all other Mars meteorites. 

This study highlights how much there is still to learn about planetary formation, Clement says. “We still don’t really understand fully where the volatiles on our own planets and the closest couple planets to us came from,” he says. “The further we dig into the formation of the planets we can measure the best, the more complicated that process seems to be.”

Each new distinction between Earth and Mars hints at even more diversity among planets elsewhere, Clement adds. “If it’s that easy to form planets that are that different so close to each other,” he says, what weird worlds might scientists find orbiting other stars?