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 the old space rock.
Chemical evidence from this distant sample suggests that Mars and Earth – often seen as potential twins because they are rocky worlds and neighbors of the solar system – were born in many different ways: Earth formed slowly and Mars formed much faster.
Current hypotheses about the creation of a rocky planet, such as Mars or Earth, suggest that some elements inside the planet must have the same chemical characteristics as those in the planet’s atmosphere. This is because in the early days of our solar system about 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 probably released the gases that became atmospheres.
These gases were not just chemicals. They were volatile, chemical elements and compounds that evaporated very easily. Volatile substances include hydrogen, carbon, oxygen and nitrogen, as well as noble gases, which are inert elements that do not react with the environment. On Earth, these chemicals eventually allowed our world to thrive and sustain life.
To look for signs of this process on Mars, Sandrine Perron, a doctoral student at the Institute of Geochemistry and Petrology at ETH Zurich, compared two Martian sources of the noble gas krypton. One source is a meteorite that originates from the interior of Mars. The other was krypton isotopes taken from the atmosphere of Mars by NASA’s Curiosity Rover. Unexpectedly, the krypton signatures did not match. And that could change the sequence of events about how Mars got its volatiles and atmosphere in the first place.
“This is the opposite of the standard variable accumulation model,” says Perron. Its results are described in an article published Thursday in the journal Science. “Our research shows it’s a little more complicated.”
The planets in our solar system were formed from the remnants of the birth of our sun. Lumps of material merge into a rotating disk of gas and dust called a solar nebula around the new star. Some lumps, accumulated by gravity and collisions, have become large enough to become planets and develop complex geological processes. Others remained small and inactive, such as primitive asteroids and comets.
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Scientists believe that volatile substances were first introduced to the new worlds directly from the solar nebula in the earlier stages of planetary development. Later, as the solar nebula dissipated, more volatile matter was delivered by the bombardment of chondritic meteorites, small pieces of rocky asteroids that have remained unchanged since the earliest days of the solar system. These meteorites then melted into the oceans of magma.
If the atmosphere was delivered from a cosmic rock, planetary scientists would expect volatile matter in the planet’s atmosphere to coincide with that of chondritic meteorites, not the solar nebula. Instead, Perón found that the krypton from inside Mars was almost pure chondrite, while the atmosphere was sunny.
As such, Mars may have been bombarded by chondritic meteorites in the beginning and then solidified while there was still enough solar nebula to form an atmosphere around the solidified Red Planet, Peron suggested. She explains that the nebula would have dissipated about 10 million years after the formation of the sun, so the accumulation of Mars had to be completed long before that, perhaps in the first 4 million years.
“Mars appears to have acquired its atmosphere from the primary gas that penetrates the solar system as it forms,” said Matt Clement, a doctoral student studying the formation of terrestrial planets at the Carnegie Institution of Science who was not involved in the study. “It simply came to our notice then. We believe that Mars formed much, much faster than Earth.
Scientists often turn to Mars to study the early solar system precisely because of how quickly it is thought to have formed. Mars, which is one tenth the size of Earth, is also far less geologically active, which means that the Red Planet probably retains many of the conditions from the earliest days of our planetary quarter.
However, to study the chemistry of Mars, scientists must either send mechanical messengers like Curiosity Rover to the planet, or study pieces of Mars that broke off, flew into space and landed on Earth’s surface. There are only a few hundred such meteorites.
The meteorite Peron studied is unique. In 1815, it fell sharply through the Earth’s atmosphere, shattering over Chassini, France. Since then, scientists studying fragments of the Chassigny meteorite have found that it probably came from inside Mars – unlike all other meteorites on Mars.
This study highlights how much remains to be learned about the formation of the planet, says Clement. “We still don’t fully understand where the volatiles on our own planets and the planets closest to us come from,” he said. “The deeper we delve into the formation of the planets we can best measure, the more complex this process seems.
Any new distinction between Earth and Mars hints at even greater diversity between planets elsewhere, Clement added. “If it’s so easy to form planets that are that different so close to each other, ”he says,“ what strange worlds would scientists orbiting other stars discover?