Earth and Mars are similar in many ways: Both have similar atmospheric chemistry, both have sustained polar caps, and both have strong seasonal variability due to similar rotational tilts. So why is Earth so much larger than Mars? Experts believe they have found the answer.
Why does Mars have only 10 percent of the mass of the Earth? The solution, according to Hal Levison from the Southwest Research Institute and his colleagues, lies in a new type of planetary formation modeling process in which planets can grow from smaller bodies called “pebbles.”
Writing in the Proceedings of the National Academy of Sciences (PNAS) Early Edition, Levison and his colleagues used this process, which was previously used to explain the rapid formation of gas giants Jupiter and Saturn, to reproduce the structure of the solar system’s inner planets.
“Understanding why Mars is smaller than expected has been a major problem that has frustrated our modeling efforts for several decades,” Levison, first author of the study and a scientist at the SwRI’s Planetary Science Directorate, explained in a statement earlier this week. “Here, we have a solution that arises directly from the planet formation process itself.”
Location is key when it comes to accumulating pebbles
As the researchers explain, the standard model of planetary formation states that objects of about the same size accumulate and assimilate through a process called accretion. Yet, while accretion models successfully produce planets roughly the same as Earth and Venus, they also indicate that Mars should be at least the same size as the Earth, if not larger.
However, new calculations performed by scientists at the SwRI’s Planetary Science Directorate demonstrate that the structure of the inner solar system is actually the result of a different kind of planetary growth. This new model is known as Viscously Stirred Pebble Accretion (VSPA).
In VSPA, cosmic dust grows to pebble-like objects a few inches in diameter, some of which end up gravitationally collapsing and forming primordial asteroid-sized objects. When conditions are right, these asteroids pull the remaining pebbles unto orbit, where they spiral down and fuse with the rest of the growing world and allow full-sized planets to form relatively quickly.
The new model also found that not all primordial asteroids are equal in terms of their ability to accumulate pebbles and grow. An object about the same size of Ceres (approximately 600 miles across) would have been able to grow quickly near the Earth’s current location, but not near the current location of Mars, they explained, because the aerodynamic drag would be too weak.
“This means that very few pebbles collide with objects near the current location of Mars. That provides a natural explanation for why it is so small,” said co-author Katherine Kretke. “Similarly, even fewer hit objects in the asteroid belt, keeping its net mass small as well. The only place that growth was efficient was near the current location of Earth and Venus.”
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Feature Image: NASA/JPL/MSSS
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