A one-of-a-kind meteorite from Mars has unexpected chemistry that could refine scientists’ models of how terrestrial planets form, according to a new study of old space rock.
Chemical clues from this distant sample suggest that Mars and Earth, often seen as would-be twins because of the rocky and neighboring worlds of the solar system, were born in very different ways: Earth formed slowly and Mars much faster.
Current hypotheses about the creation of a rocky planet, such as Mars or Earth, suggest that some elements within the planet should have the same chemical characteristics as those in the planet’s atmosphere. This is because, at the dawn of our solar system, some 4.5 billion years ago, the rocky planets were covered with an ocean of magma. As the planets cooled and their molten mantles solidified, the process likely released the gases that became atmospheres.
Those gases weren’t just chemicals. They were volatile substances, chemicals and compounds that vaporize very easily. Volatiles include hydrogen, carbon, oxygen and nitrogen, as well as noble gases, which are inert elements that do not react with their environment. On Earth, those chemicals eventually allowed our world to develop and sustain life.
To look for signs of that process on Mars, Sandrine Péron, a postdoctoral fellow at ETH Zurich’s Institute of Geochemistry and Petrology, compared two Martian sources of the noble gas krypton. One source was a meteorite that originated in the interior of Mars. The other was made up of isotopes of krypton sampled from the atmosphere of Mars by NASA’s Curiosity Rover. Unexpectedly, krypton’s signatures did not match. And that could change the sequence of events for how Mars got its volatile elements and atmosphere in the first place.
“This is somewhat the opposite of the standard volatile growth model,” says Péron. His findings are described in an article published Thursday in the journal Science. “Our study shows it’s a bit more complicated.”
The planets of our solar system were formed from the debris of the birth of our sun. Clusters of material have coalesced in the swirling disk of gas and dust, called the solar nebula, around the new star. Some clusters, which have accumulated due to gravity and collisions, have become large enough to become planets and develop complex geological processes. Others remained as small and inactive as primitive asteroids and comets.
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Scientists think that the volatiles were first incorporated into the new worlds directly from the solar nebula in the early stages of planetary development. Later, as the solar nebula dissipated, more volatiles were released from the bombardments of chondritic meteorites, small fragments of stony asteroids that remain unchanged from the early days of the solar system. Those meteorites then merged into the magma oceans.
If the atmosphere were carried by space rock, planetary scientists would expect the volatiles in a planet’s atmosphere to match those of chondritic meteorites, not the solar nebula. Instead, Péron found that the krypton of the Martian interior is almost purely chondritic, while the atmosphere is solar.
As such, perhaps Mars was bombarded by chondritic meteorites at first and then solidified while there was still enough solar nebula to form an atmosphere around the hardened Red Planet, Péron suggests. He explains that the nebula would have dissipated about 10 million years after the formation of the sun, so the accretion of Mars should have completed well before then, perhaps in the first 4 million years.
“It appears that Mars acquired its atmosphere from the primordial gas that permeated the solar system as it was forming,” says Matt Clement, a postdoctoral fellow studying the formation of terrestrial planets at the Carnegie Institution for Science, who was not involved in the study. “This generally fits our image. We think Mars formed much, much faster than Earth. “
Scientists often look to Mars to study the early solar system precisely because of the speed at which it is thought to have formed. Mars, which is one-tenth the size of Earth, is also far less geologically active, meaning the Red Planet likely retains much of the condition of the early days of our planetary neighborhood.
However, to study the chemistry of Mars, scientists must send mechanical envoys such as the Curiosity Rover to the planet or examine pieces of Mars that broke off, crashed into space, and landed on Earth’s surface. There are only a few hundred such meteorites.
The meteorite studied by Péron is unique. In 1815 it plunged through the Earth’s atmosphere, fracturing into pieces on Chaassigny, France. Since then, scientists who have studied the fragments of the Chassigny meteorite have determined that it likely came from within Mars, unlike all other meteorites on Mars.
This study highlights how much there is still to be learned about planetary formation, says Clement. “We still don’t quite understand where the birds on our planets and the pair of planets closest to us come from,” he says. “The more we dig into the formation of planets that we can best measure, the more complicated the process seems to be.”
Each new distinction between Earth and Mars suggests even greater diversity between planets elsewhere, adds Clement. “If it is so easy to form planets that they are That different so close to each other, “he says, what strange worlds might scientists find orbiting other stars?