Earth and Mars formed in the same part of the solar system, but their chemistry suggests the two planets were built in different ways, according to new research.
The finding adds to growing evidence that suggests planets do not all form in one way. It also has implications for a long-running open question about how exactly the disk of gas and dust around the young sun formed the planets of today's solar system around 4.5 billion years ago.
"The most surprising result was that Earth and Mars appear to have formed in different ways," Anders Johansen, a professor of planetary sciences at the University of Copenhagen in Denmark who co-led the study, said in a statement. "You might have expected that two planets formed side by side in the same solar system would share a more similar formation history."
Around 4.5 billion years ago, the solar system was a crowded place. The young sun was ringed by a rotating disk of gas and dust with the first building blocks of planets taking shape within it.
In one long-standing picture, dust grains collided and stuck together to form pebbles, which grew into kilometer-wide rocky bodies known as planetesimals. Those planetesimals repeatedly crashed into one another and merged, building planets over a few tens of millions of years.
Whether Earth and Mars were built by one process or a mix of both has not been settled. The Copenhagen team set out to test the competing ideas by studying the chemistry of the planets' crust and mantle.
The researchers focused on volatile elements such as sodium, zinc and potassium, which evaporate easily at high temperatures. So whether those elements are present or missing acts as a chemical fingerprint of the heat and violence a planet went through while forming, according to the researchers.
"It is a major detective job to figure out what happened back then when most of the evidence disappeared long ago," Johansen said. "But even after 4.5 billion years, the compositions of Earth's and Mars' mantles remain the same."
What formation route suits best?
The team fed the data about the reservoirs of volatile elements of both Earth and Mars into computer models to test which formation scenarios fit best, and the results favor a hybrid model.
At least 75 percent of Earth's mass appears to come from two young planets, known as protoplanets, that grew large by gathering pebbles, while planetesimals supplied up to 25 percent, the study found.
For Mars, the team thinks that this balance was reversed, with roughly three-quarters of its mass appearing to come from planetesimals, and the remaining quarter from pebble accretion.
The exact chemical makeup of the raw material that became Earth and Mars is unknown, so the calculations depend on assumptions, the study notes. Nevertheless, adjusting those assumptions did not change the main conclusion, according to study co-lead Haiyang Wang of the University of Copenhagen.
The team argues that its method is more direct than the isotope-based approach widely used in the field, which "can often be interpreted in multiple ways," according to Wang.
The technique used by the researchers could eventually be applied to worlds beyond the solar system, extrasolar planets, or exoplanets.
As future missions find Earth-like planets around other stars, judging whether those worlds could be habitable will require some sense of what they're made of.
"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 concluded.
This research is described in a paper published Sept. 25 in the journal Nature Astronomy.
