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Jupiter robbed Mars of building material
Posted: 08 June 2011

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A young Jupiter may have migrated to within 1.5 AU of the Sun, starving Mars of its building blocks and resulting in its stunted growth.

The new computer simulations are published in Nature this week, and compliment another Nature report published last week that found that Mars had reached its final size, just 11 percent the mass of Earth, in just 2-4 million years, much faster than the 50-100 million years that Earth took, which provided an explanation for its small size (read our report here).

Jupiter's migration could explain Mars' small size and compositional differences in the asteroid belt. Image: Jupiter: NASA/JPL/Space Science Institute; Asteroid Belt graphic: NASA.

Kevin Walsh of the Southwest Research Institute lead the international team that found a solution as to why Mars is so small.

“If Jupiter had moved inwards from its birthplace down to 1.5 AU [1 AU is the Earth-Sun separation] from the Sun, and then turned around when Saturn formed as other models suggest, eventually migrating outwards towards its current location, it would have truncated the distribution of solids in the inner Solar System at about 1 AU and explained the small mass of Mars,” he says. “The problem was whether the inward and outward migration of Jupiter through the 2 to 4 AU region could be compatible with the existence of the asteroid belt today, in this same region.”

Today Jupiter resides at 5.2 AU, but models of early Solar System formation suggest that the outer gas planets shuffled around on their orbits before settling in their final locations, redistributing material throughout the Solar System as they did so. The asteroid belt contains both very dry and ice-rich bodies, such as the so-called Main Belt Comets, but the simulations showed that Jupiter's movement depleted and then re-populated the asteroid belt region with inner-belt bodies originating between 1 and 3 AU as well as outer-belt bodies originating between and beyond the giant planets.

“Rapidly the pieces of the story came together,” says Walsh. “The simulations showed that the migration of Jupiter was consistent with the existence of the asteroid belt, but it also explained properties of the belt never understood before.”

Planets in other solar systems are observed at widely varying distances from their host stars, supporting the idea that migrations are common throughout the Universe.