From Antarctica to India, Western Sahara to central France, rock hunters have found some rather unusual specimens—rocks with compositions not quite like anything else on Earth. That’s because they came from Mars.
These rocks, which were blasted from the Martian surface by impacts and arrived on Earth as meteorites, were previously used to determine the Red Planet’s composition and age. Now, in addition to providing clues about Mars, these meteorites are helping scientists understand the early orbits of Jupiter and Saturn.
“Different initial shapes of the giant planets’ orbits would have different effects on Mars’s formation speed and Mars’s bulk composition,” said Jason Woo, a planetary scientist at the University of Zürich and lead author of the new study. Using this connection, Woo and his colleagues worked backward to figure out what the early orbits of the gas giants looked like on the basis of data from the Martian meteorites.
From Mars to the Gas Giants
Using the highest-resolution simulations of planetary formation to date, they tested dozens of different orbits for Jupiter and Saturn to see whether any could reproduce a system that matched the composition and age of Mars gleaned from the meteorites. Their results, published in Astrophysical Journal Letters, suggest that after the dissipation of the solar nebula—the gaseous disk from which the solar system is thought to have formed—both of the gas giants had very circular orbits for about 10 million years. Only later on, maybe within 80 million years after the birth of the solar system, did the planets move into the more elliptical orbits we see them in today, though this phase was not simulated in the recent study.
“[The paper] definitely highlights the consensus that the field is coming together on—that the giant planets were formed and then they went from circular to eccentric [orbits] rapidly within at least 100 million years,” said Matt Clement, a planetary scientist at the Carnegie Institution for Science who was not involved in the new study. “But there’s definitely some problems with circular and eccentric [orbits] that still need to be figured out.”
To the Gas Giants and Beyond
In addition to a fuller understanding of our own solar system, the new work may also help scientists understand the formation of gas giants in exoplanet systems—long considered a holy grail in planetary science.
“We’re never going to have meteoric data from exoplanet systems,” Clement said. Instead, scientists have to make use of the chemistry and orbital dynamics from our own system to understand others.
Ultimately, although Woo and his colleagues add another piece to the puzzle of planet formation, the new findings don’t complete it. For that, scientists will likely need to retrieve samples from the solar system’s other rocky planets.
“We need meteorites or samples from Mercury and Venus, too,” Woo said. “With additional information from these two inner planets, we can develop a more accurate model for terrestrial planet formation.”
—Mara Johnson-Groh ([email protected]), Science Writer