Earth and Mars formed differently, study finds

Earth and Mars formed in the same part of the solar system but their chemistry tells a different story, according to a study reported by Space.com. The findings suggest the two planets were assembled in distinct ways, adding to evidence that planet formation is not a single process.

Researchers led by Anders Johansen of the University of Copenhagen focused on volatile elements such as sodium, zinc and potassium. These elements evaporate easily at high temperatures, so their presence or absence acts as a chemical fingerprint of the heat and violence a planet experienced while forming, the team explained.

"The most surprising result was that Earth and Mars appear to have formed in different ways," Johansen said in a statement.

The researchers fed data on the volatile-element reservoirs of both planets into computer models to test competing formation scenarios. The results favor a hybrid model.

Pebbles versus planetesimals

At least 75 percent of Earth's mass appears to have come from two young planets, known as protoplanets, that grew large by gathering pebbles. Planetesimals — kilometer-wide rocky bodies — supplied up to 25 percent of its mass, the study found.

For Mars, the balance was reversed. Roughly three-quarters of its mass appears to have come from planetesimals, with the remaining quarter from pebble accretion.

Limitations and next steps

The exact chemical makeup of the raw material that became Earth and Mars is unknown, so the calculations depend on assumptions, the study notes. However, adjusting those assumptions did not change the main conclusion, according to study co-lead Haiyang Wang.

The team argues that its method is more direct than the isotope-based approach widely used in the field, which Wang said "can often be interpreted in multiple ways."

The technique could eventually be applied to exoplanets, helping judge whether Earth-like worlds around other stars could be habitable.

"If we understand how planets lose volatile elements during their formation, we can also become better at predicting how much water and other life-supporting substances they ultimately retain," Johansen said.

The research was published Sept. 25 in the journal Nature Astronomy.

• Anders Johansen
• Earth
• Haiyang Wang
• Mars
• Nature Astronomy
• planet formation
• University of Copenhagen