Rapid Formation May Have Stunted Mars’ Growth

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Somewhere between two and four million years after our solar system formed, a rocky little runt went through a rapid growth spurt. In its embryonic stage, it was much like Earth. But it didn’t end up being terrestrial. Earth ended up being twice its size through collecting other rocky bodies as they passed by. But not Mars… Oh, no. Not Mars.

“Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other embryos to form an Earthlike planet.” said Nicolas Dauphas at the University of Chicago. “Mars probably is not a terrestrial planet like Earth, which grew to its full size over 50 to 100 million years via collisions with other small bodies in the solar system.”

The latest study of Mars just released in Nature puts forth the theory that the red planet’s rapid formation helps explain why it is so small. The idea isn’t new, but based on a proposal done 20 years ago and heightened by planetary growth simulations. The only thing missing was evidence… evidence that’s hard to come by since we can’t examine firsthand the formation history of Mars because of the unknown composition of its mantle – the rock layer beneath the planetary crust.

So what has changed that gives us a new view of how Mars came to be the runt of the solar system litter? Try meteorites. By analyzing Martian meteorites, the team was able to pick out clues about the mantle composition of Mars, but their compositions also have changed during their journey through space. This debris left over from the genesis time is nothing more than a common chondrite – a Rosetta stone for deducing planetary chemical composition. Dauphas and Pourmand analyzed the abundances of these elements in more than 30 chondrites, and compared those to the compositions of another 20 martian meteorites.

“Once you solve the composition of chondrites you can address many other questions,” Dauphas said.

And there are many, many questions left to be answered. Cosmochemists have intensively studied chondrites, but still poorly understand the abundances of two categories of elements they contain, including uranium, thorium, lutetium and hafnium. Hafnium and thorium both are refractory or non-volatile elements, meaning that their compositions remain relatively constant in meteorites. They also are lithophile elements, those that would have stayed in the mantle when the core of Mars formed. If scientists could measure the hafnium-thorium ratio in the martian mantle, they would have the ratio for the whole planet, which they need to reconstruct its formation history. When the team of Dauphas and Pourmand had determined this ratio, they were able to calculate how long it took Mars to develop into a planet. Then, by applying a simulation program, they were able to deduce that Mars… Oh, yes. Mars. Reached its full growth only two million years after the solar system.

“New application of radiogenic isotopes to both chondrite and martial meteorites provides data on the age and mode of formation of Mars,” said Enriqueta Barrera, program director in NSF’s Division of Earth Sciences. “That is consistent with models that explain Mars’ small mass in comparison to that of Earth.”

And still there are questions… But fast formation seems to be the answer. It might explain the puzzling similarities in the xenon content of its atmosphere and that of Earth’s. “Maybe it’s just a coincidence, but maybe the solution is that part of the atmosphere of Earth was inherited from an earlier generation of embryos that had their own atmospheres, maybe a Marslike atmosphere,” Dauphas said.

Mars? Oh, no. Not Mars.

Source: University of Chicago, AAS