Jupiter and its four planet-size moons were photographed and assembled into this collage. They are not to scale but are in their relative positions. Reddish Io (upper left) is nearest Jupiter; then Europa (centre); Ganymede and Callisto (lower right).

Credit: NASA/JPL

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SYDNEY: The movements of Jupiter may have been responsible for restricting the amount of materials available to Mars as it developed in the early Solar System, according to a new study.

Planetary scientists have long wondered why Mars came to be the 'runt' of the planetary litter, being only about half the size and one-tenth the mass of its neighbour, Earth. As the planets nearby probably formed about the same time, why isn't Mars more like Earth and Venus in size and mass?

A paper published in the current issue of Nature provides the first cohesive explanation, while suggesting that Jupiter was also responsible for the composition of the asteroid belt - a region of the Solar System made up of an array of irregularly shaped asteroids located roughly between the orbits of Mars and Jupiter.

"The result was fantastic," said lead author Kevin Walsh from the Southwest Research Institute in Colarado, U.S. "Our simulations not only showed that the migration of Jupiter was consistent with the existence of the asteroid belt, but also explained properties of the belt never understood before."

Stripping Mars of material

The international team of researchers performed simulations of the early Solar System, demonstrating how an infant Jupiter may have migrated to within 1.5 astronomical units (AU, the distance from the Sun to the Earth) of the Sun, stripping a lot of material from the region and essentially starving Mars of formation materials.

"One of the beautiful things to me is that we are now seeing a need for substantial migration during the early times of our Solar System, something that is mandatory in many of the extra-solar planetary systems discovered to date," said Simon O'Toole from the Australian Astronomical Observatory, who was not involved in the study.

"This was always expected to some degree (for example to explain the position of Uranus and Neptune), but this paper investigates a much larger change in orbit for Jupiter and Saturn than previously. With this they can explain why Mars is the 'runt of the litter', so to speak."

Explaining the asteroid belt

According to Walsh, if Jupiter had moved inwards from its birthplace down to 1.5 AU from the Sun, and then turned around when Saturn formed as other models suggest, it would have eventually migrated outwards towards its current location.

In doing this it would have truncated the distribution of solids in the inner solar system at about 1 AU and hence explain the relatively small mass of Mars.

"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. So, we started to do a huge number of simulations."

Migrations of the planets

The asteroid belt is populated with two very different types of rubble, very dry bodies as well as water-rich orbs similar to comets. Walsh and collaborators showed that the passage of Jupiter 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, producing the significant compositional differences existing today across the belt.

The collaborators call their simulation the Grand Tack Scenario, from the abrupt change in the motion of Jupiter at 1.5 AU, like that of a sailboat tacking around a buoy. The migration of the gas giants is also supported by observations of many extra-solar planets found in widely varying ranges from their parent stars, implying migrations of planets elsewhere in universe.

"This paper represents an important step forward in our understanding of the Solar System's formation," said O'Toole.

"Another neat result is that not only can they explain the mass distribution of the terrestrial planets, but it comes naturally from determining the final position of the two main types of asteroids: rocky types in asteroid belt and icy types in the trans-Neptunian regions. Two long-standing questions answered in one fell swoop!"