Giant impact may have erased evidence of Mars' formation

March 2, 2021

Mars is there ... but how did it form? (AP Image/JPL)

A Yale University physicist has proposed that Mars’ billion-year-old formation may be the consequence of a giant terrestrial impact because it appears to have a uniform composition, and only such a massive collision could induce energy at a high-enough level to erase any physical evidence of the merges that all planets experience.

There are three steps to planet formation: minuscule space particles produce planetesimals, such as asteroids; accumulation of planetesimals develops stranded planetary embryos, such as the moon; and finally, some embryos coalesce, and huge planets such as Earth emerge. Observations seemed to suggest that Mars stopped at step two, but a paper published Feb. 9 in Advancing Earth and Space Science argues that it could truly have moved to step three.

During their formation, stranded planetary embryos are thought to generate so much energy that they completely melt into what is called a magma ocean. Magma oceans remove traces of the various bodies that add to a planetary mass, giving it a uniform composition. Because Mars is assumed to be a stranded planetary embryo, its homogeneous makeup was unsurprising.

But rather than follow the assumption, Zhongtian Zhang, a Ph.D. student at Yale University and lead author of the study, wanted to find true proof for the accretion of stranded planetary embryos. Naturally, he kept a presumed one in mind: Mars.

To do this, he and his team developed a model that detects the progression of stranded planetary embryo phase transitions from solid to liquid. Referred to as two-phase models, they ended up indicating something unexpected: Stranded planetary embryos don’t really become magma oceans. 

It appeared that the embryos did not melt enough to generate a homogeneous composition. The models showed that there was insufficient energy, contrary to what was previously assumed, because some of the energy was released outward. This cooled the planetary body.

“The mantle of a growing planetary embryo may stay ‘mostly solid’ during the body’s accretion,” Zhang said. “Given this result, we may need to rethink about some questions regarding the internal evolution of early planetary bodies.”

The findings beg the question: If stranded planetary embryos don’t have homogeneous compositions, then why would Mars have one?

The physicists figured that the only way to create a sufficient amount of energy to melt a planetary body, and erase evidence of a collision, would be with a giant impact — not unlike the one found in the final step of planet formation. 

“The previous view is that the formation of Mars was exclusively a second-stage process," Zhang explained. “Here, we suggest that it may have extended to the third stage.”

The opposite of a homogeneous composition in a planetary body is called nucleosynthetic heterogeneity. It is the visible existence of different isotopes, or chemical compositions, of planetesimals, particles and other smaller bodies, but within one big planetary body.

“Nucleosynthetic isotope compositions can be considered as ‘fingerprints’ of planetary bodies,” Zhang said. “Different planetary bodies exhibit different nucleosynthetic isotope compositions.”

Basically, Mars should have shown some semblance of nucleosynthetic heterogeneity if it were a stranded planetary embryo that halted at step two of planet formation. That’s because it should have maintained its nucleosynthetic isotope due to it not fully melting into a magma ocean. 

“The homogeneity in nucleosynthetic isotope compositions among Martian meteorites itself seems to be a surprising observation,” Zhang remarked.

Another important observation of the study was that there wasn’t any silicate differentiation — which is associated with the formation of planet layers such as the mantle and crust — on Mars for the first 15 million years after the solar system became intact. 

This observation is inconsistent with predictions for stranded planetary embryos, which are thought to have extremely early silicate differentiation. The lack thereof suggests that the giant impact — step three — erased hints of potential early differentiation, too.

Zhang says that his work may shed light on early evolution of the solar system, as well, including planetesimal formation theory and protoplanetary disk conditions that are currently unfolding through 3D simulations.

“I am sure that there will be debate about the ideas proposed in our study, but I believe that this process itself will help us better understand the observations that we have, and how they can help us to constrain the theory of early solar system evolution,” he said.

At the very least, Zhang hopes it will entice researchers to continue working on understanding how the planets in the solar system came to be. 

The paper, “A two‐phase model for the evolution of planetary embryos with implications for the formation of Mars,” was published Feb. 9 in Advancing Earth and Space Science. It was authored by Zhongtian Zhang and David Bercovici, Yale University; and Jacob S. Jordan, Rice University. 

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