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Why Mars is a lightweight

Red Planet too far from the action to grow big, astronomers contend

6:53pm, October 4, 2010
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There was a whole lotta movin’ and shakin’ going on in the inner part of the early solar system, according to two new simulations. If the models are correct, this new and dramatically more violent view of terrestrial planet formation could have consequences for understanding several puzzles, including why Mars isn‘t as heavy as Earth or Venus.
Two teams of researchers who presented their models of the early solar system on October 4 at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences in Pasadena, Calif., began their work by trying to address a long-standing mystery. According to the standard model of planet formation, Mars ought to be five to 10 times as massive as it actually is.

The standard model assumes that the swirling disk of gas, dust and ice that circled the young sun and out of which the planets condensed had a continuous, relatively smooth distribution of material. David Minton and Hal Levison of the Southwest Research Institute in Boulder, Colo., challenged that assumption with a model in which the planet-forming disk had a gap in it at about the distance from the sun where Mars now resides.

Simulations by the team show that as grains of dust run into each other and coalesce in the disk, the region closest to the sun forms moon-sized planetary embryos faster than the outer regions do. The density of grains is higher in the inner regions and material there orbits the sun faster, so it is more likely to collide and stick to make bigger bodies.

The gravitational interactions among moon-sized embryos near Earth and Venus stirred up and gravitationally scattered grains, or planetesimals, in the disk. The grains in turn exerted forces back on the planetary embryos. The forces from stirred-up grains on either side of most embryos cancelled out, and those planetary embryos stayed put.

However, for the outermost embryo in the group, the forces did not cancel out. With more stirred-up grains on one side of the embryo, toward the sun, than on the side facing the outer solar system, this embryo was pushed out through the grains of the disk in just a few hundred thousand years, packing on mass as it journeyed. At about 1.5 times the Earth-sun distance, where Minton and Levison believe a gap had opened up in the planet-forming disk, the traveling embryo came to a halt and could no longer accumulate any more mass because the material simply wasn’t there.

“It’s as if this embryo escaped from Alcatraz, leaving behind all the violence and action in the Earth-Venus zone,” says Minton.

Stranded, the embryo became as massive as Mars, but then stopped growing. In contrast, the planetary embryos embedded closer to the sun, where no gap existed, were able to accumulate more mass and became Venus and Earth, Minton suggests. The model also indicates that Mars ought to have a composition and atmosphere different from Earth and Venus because it acquired most of its bulk in a region farther out than the other terrestrial planets.
By plowing through the disk, Mars would also have delivered much of the material that ended up in part of the asteroid belt, which lies between Mars and Jupiter, the team notes.

“Their suggestion that Mars populated the inner asteroid belt with planetesimals seems plausible,” says John Chambers of the Carnegie Institution for Science in Washington, D.C.

In a different model, Kevin Walsh of the Southwest Research Institute and his colleagues, including Alessandro Morbidelli of the Observatoire de la C´te D’Azur in Nice, France, explain Mars’ slimness by calling on the action of giant Jupiter. According to the researchers, Mars initially had plenty of material to pack on — until Jupiter forced its way into the inner solar system, dragged there as gas in the disk spiraled inward towards the sun. Jupiter then got thrown back into the outer solar system as its nearest large neighbor, Saturn, formed. Jupiter’s gravitational influence might also explain the origin of the gap in the disk proposed by Minton and Levison.

Walsh declined to speak to reporters because his team has submitted the work to a journal.

“Both models are consistent with one another, but they are admittedly speculative at this early stage,” says Bill Bottke, also at the Southwest Research Institute, who was not part of either study.

Walsh and his collaborators note that their model, in which Jupiter butts into the inner solar system and then migrates out again, can also explain why different parts of the asteroid belt have different populations. Material in the inner part of the belt would have originated from bodies formed at distances no greater than three times Earth’s distance from the sun, while the outer asteroid belt was populated with bodies that originated beyond Jupiter and Saturn.
“Our model links the origin of the inner solar system — explaining both the mass of Mars and the properties of the asteroid belt — to a realistic evolution of the giant planets,” the team notes in a conference abstract. This scenario differs significantly from the ‘standard model’ in which essentially all of the material in the inner solar system formed where it now resides, the team adds. The work may also shed light on how Earth acquired its water.
Both models seem to be heading “in the right direction,” says Bottke. “We need to know how to make Mars to understand the formation of Earth, the moon and Venus.”

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