Mars might be a *really* alien planet.

Science News shows how Mars might not have formed with the rest of solid worlds of the inner solar system:

Simulating the assembly of the solar system around 4.56 billion years ago, researchers propose that the Red Planet didn’t form in the inner solar system alongside the other terrestrial planets as previously thought. Mars instead may have formed around where the asteroid belt is now and migrated inward to its present-day orbit, the scientists report in the June 15 Earth and Planetary Science Letters. The proposal better explains why Mars has such a different chemical composition than Earth, says Stephen Mojzsis, a study coauthor and geologist at the University of Colorado Boulder.

Mars, like Mercury, is a runt of the inner solar system, weighing in at only about a ninth of Earth’s mass. One of the reigning theories of planetary formation, the Grand Tack model, blames Jupiter for the Red Planet’s paltry size. In that scenario, the newly formed Jupiter migrated toward the sun until it reached Mars’ present-day orbit. A gravitational tug from Saturn then reversed Jupiter’s course, sending the gas giant back to the outer solar system.

But studies of Martian meteorites suggest that the Red Planet contains a different mix of various elements and isotopes, such as oxygen-17 and oxygen-18, compared with Earth.

Planetary scientist Ramon Brasser of the Tokyo Institute of Technology, Mojzsis and colleagues reran the Grand Tack simulations, keeping an eye on the materials that went into Mars’ creation to see if they could explain the different mix.

As with previous studies, the researchers found that the most probable way of creating a solar system with the same planet sizes and positions as seen today is to have Mars form within Earth’s orbit and migrate outward. However, this explanation failed to explain Mars’ strikingly different composition.

Another possible scenario, though seen in only about 2 percent of the team’s new simulations, is that Mars formed more than twice as far from the sun as its present-day orbit in the region currently inhabited by the asteroid belt. Then as Jupiter moved sunward, its gravitational pull yanked Mars into the inner solar system. Jupiter’s gravity also diverted planet-making material away from Mars, resulting in the planet’s relatively small mass. With Mars forming so far from the planetary feeding frenzy responsible for the other rocky planets, its composition would be distinct. While this scenario isn’t as likely as Mars forming in the inner solar system, it at least matches the reality of Mars’ makeup, Mojzsis says.

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