A tiny chunk of rock that once broke away from Mars and found its way to Earth may hold clues that reveal startling details about the Red Planet’s formation.
A new analysis of the Chassigny meteorite, which fell to Earth in 1815, suggests that how Mars obtained its volatile gases – such as carbon, oxygen, hydrogen, nitrogen and noble gases – contradicts our current models of planet formation.
Planets are born, according to current models, from the remnants of stars. Stars form from a nebular cloud of dust and gas when a dense clump of matter collapses under the effect of gravity. Spinning, it rolls up more matter from the cloud around it to expand.
This material forms a disk, swirling around the new star. In this disk, dust and gas begin to clump together in a process that grows a baby planet. We’ve seen other baby planetary systems form this way, and evidence in our own solar system suggests it formed the same way around 4.6 billion years ago.
But how and when certain elements were incorporated into the planets has been difficult to piece together.
According to current models, the volatile gases are absorbed by a molten planet that forms from the solar nebula. Because the planet is so hot and mushy at this point, these volatiles get sucked into the global magma ocean that is the planet-forming, before later being partially outgassed into the atmosphere as the mantle cools.
Later, more volatiles are delivered by meteor bombardment – volatiles bound in carbonaceous meteorites (called chondrites) are released when these meteorites break up upon their introduction to the planet.
Thus, a planet’s interior should reflect the composition of the solar nebula, while its atmosphere should primarily reflect the volatile contribution of meteorites.
We can tell the difference between these two sources by looking at the ratios of noble gas isotopes, in particular krypton.
And, because Mars formed and solidified relatively quickly in around 4 million years, compared to up to 100 million years for Earth, it’s a good record for these very early stages of the planetary formation process. .
“We can piece together the history of volatile delivery over the first million years of the solar system,” said geochemist Sandrine Péron, formerly of the University of California, Davis, today at ETH Zurich.
That is, of course, only if we can access the information we need – and that’s where the Chassigny meteorite is a gift from space.
Its noble gas composition differs from that of the Martian atmosphere, suggesting that the chunk of rock broke away from the mantle (and flew into space, hastening its arrival on Earth), and is representative of the interior planetary and therefore of the solar nebula.
Krypton is quite difficult to measure, however, so precise isotope ratios have eluded measurement. However, Péron and his colleague, fellow UC Davis geochemist Sujoy Mukhopadhyay, used a new technique using the UC Davis Noble Gas Laboratory to make a precise new measurement of krypton in the Chassigny meteorite.
And this is where it gets really weird. Isotope ratios of krypton in the meteorite are closer to those associated with chondrites. Like, remarkably closer.
“The Martian interior composition of krypton is almost purely chondritic, but the atmosphere is solar,” Péron said. “It’s very distinct.”
This suggests that meteorites were delivering volatiles to Mars much earlier than scientists thought, before the solar nebula was dissipated by solar radiation.
The order of events would therefore be that Mars acquired an atmosphere from the solar nebula after its global magma ocean cooled; otherwise, the chondrite gases and the nebular gases would be much more mixed than what the team observed.
However, this presents another mystery. When solar radiation eventually burned up the remains of the nebula, it should have burned up the nebular atmosphere of Mars as well. This means that the atmospheric krypton present later must have been kept somewhere; perhaps, the team suggested, in the polar ice caps.
“However, this would require Mars to have been cold immediately after accretion,” Mukhopadhyay said.
“While our study clearly points to chondritic gases inside Mars, it also raises interesting questions about the origin and composition of Mars’ early atmosphere.”
The team’s research has been published in Science.