Mars a Stranded Planetary Embryo That Ceased to Grow Unlike Earth

Scientists still wrestle with large uncertainties in the composition of Earth’s mantle because of processes such as melting.

“We have the same problem for Mars,” Dauphas said.

Analyses of martian meteorites provide clues about the mantle composition of Mars, but their compositions also have changed.

Solving some lingering unknowns about the composition of chondrites, a common type of meteorites, provided the data needed.

As essentially unaltered debris left over from the birth of the solar system, chondrites serve as a Rosetta stone for deducing planetary chemical composition.

Cosmochemists have intensively studied chondrites, but still poorly understand the abundances of two categories of elements they contain, including uranium, thorium, lutetium and hafnium.

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.

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.

The relationships between hafnium, thorium and tungsten dictated that the hafnium-thorium ratio in the mantle of Mars must be similar to the same ratio in chondrites.

To derive the martian mantle’s hafnium-tungsten ratio, they divided the thorium-tungsten ratio of the martian meteorites by the thorium-hafnium ratio of the chondrites.

“Why do you do that? Because thorium and tungsten have very similar chemical behavior,” Dauphas said.

Once Dauphas and Pourmand had determined this ratio, they were able to calculate how long it took Mars to develop into a planet.

A computer simulation based on these data showed that Mars must have reached half its present size only two million years after the formation of 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.”

A quickly-forming Mars would help 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.

The short formation history of Mars further raises the possibility that aluminum 26, which is known from meteorites, turned the red planet into a magma ocean early in its history.

Aluminum 26 has a half-life of 700,000 years, so it would have disappeared too quickly to contribute to the internal heat of Earth.

If Mars formed in two million years, however, significant quantities of aluminum 26 would remain. “When aluminum 26 decays it releases heat and can completely melt the planet,” Pourmand said.

The research was also funded by the National Aeronautics and Space Administration and the Packard Foundation.