A unique meteorite from Mars has unexpected chemistry that could refine scientists’ models of terrestrial planet formation, according to a new study of ancient space rocks.
Chemical evidence from this widely scattered sample suggests that Mars and Earth — often thought of as potential twins because they are rocky worlds and neighbors in the solar system — were born in very different ways: Earth formed slowly and Mars much faster.
Current hypotheses about the formation of a rocky planet like Mars or Earth suggest that some elements in the planet’s interior should have the same chemical properties as those in the planet’s atmosphere. Because in the beginning of our solar system about 4.5 billion years ago, the rocky planets were covered by a magma ocean. As the planets cooled and their molten envelopes solidified, the process likely released the gases that became atmospheres.
These gases were not just any chemicals. They were volatile substances, chemical elements and compounds that vaporize very easily. Volatile materials include hydrogen, carbon, oxygen, and nitrogen, as well as noble gases, which are inert elements that do not react with their environment. Eventually, on Earth, these chemicals enabled our world to evolve and support life.
To look for signs of this process on Mars, Sandrine Perón, postdoc at the Institute for Geochemistry and Petrology at ETH Zurich, compared two Mars sources of the noble gas krypton. One source was a meteorite originating from the interior of Mars. The other was isotopes of krypton taken from the Martian atmosphere by NASA’s Curiosity rover. Unexpectedly, the krypton signatures did not match. And that could change the sequence of events of how Mars got its volatiles and atmosphere in the first place.
“It’s sort of the opposite of the standard model of volatile accretion,” says Péron. Their results are described in a piece of paper published Thursday in the journal Science. “Our study shows that it’s a bit more complicated.”
The planets in our solar system were formed from the debris of the birth of our sun. Clumps of material merged around the new star in the swirling disk of gas and dust dubbed the solar nebula. Some clumps, accumulated by gravity and collisions, grew large enough to become planets and develop complex geological processes. Others remained small and inactive as primitive asteroids and comets.
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Scientists believe that volatiles were first absorbed 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 released by bombardments from chondritic meteorites, small chunks of stony asteroids that have remained unchanged since the earliest days of the solar system. These meteorites then merged in the magma oceans.
If the atmosphere were supplied by space rock, planetary scientists would expect the volatiles in a planet’s atmosphere to match those of chondritic meteorites, not those of the solar nebula. Instead, Péron found that the krypton from the interior of Mars is almost purely chondritic, while the atmosphere is solar.
So Mars may have been 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 cleared about 10 million years after the Sun formed, so the accretion of Mars should have been complete long before that, perhaps in the first 4 million years.
“It looks like Mars got its atmosphere from the primordial gas that permeated the solar system during its formation,” he says Matt Clemens, a postdoctoral researcher studying terrestrial planet formation at the Carnegie Institution for Science and not involved with the study. “That basically fits our picture. We think Mars formed much, much faster than Earth.”
Scientists often look to Mars to study the early solar system precisely because it is believed to have formed so rapidly. Mars, which is one-tenth the size of Earth, is also far less geologically active, meaning the Red Planet likely preserves many of the conditions of our planetary neighborhood’s earliest days.
However, to study the chemistry of Mars, scientists must either send mechanical envoys like the Curiosity rover to the planet, or study pieces of Mars that broke off, flung through 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 crashed through Earth’s atmosphere and broke into pieces over Chassigny, France. Since then, scientists examining the fragments of the Chassigny meteorite have determined that it likely came from inside Mars — unlike all other Martian meteorites.
This study shows how much there is still to learn about planet formation, says Clement. “We still don’t fully understand where the volatiles come from on our own planet and the planets closest to us,” he says. “The further we delve into the formation of the planets we can best measure, the more complicated this process seems to be.”
Each new distinction between Earth and Mars points to even greater diversity among planets elsewhere, adds Clement. “If it is so easy to form planets the different, so close together,” he says, what strange worlds might scientists find orbiting other stars?