Mars and Mercury Formed From Planetary Scraps

Ker Than
for National Geographic News
January 22, 2009
Mars and Mercury were formed from the scraps of Earth and Venus, according to a radical new theory of rocky planet formation.

The model could explain some characteristics of Mars and Mercury that have long puzzled scientists, said Brad Hansen, an astronomer at the University of California, Los Angeles.

"In this picture, Mars and Mercury are essentially byproducts" of Earth and Venus, said Hansen, who presented his research at a recent meeting of the American Astronomical Society in Long Beach, California.

Merging Mountains

Scientists generally agree that Earth and the other rocky planets in the solar system formed from a wispy disk of gas and dust that ringed the infant sun some 4.5 billion years ago.

(Related: "Planet-Forming Disk Spotted Around Dead Star" [April 5, 2006].)

Over time, the microscopic dust particles coalesced into pebble-size clumps. The pebbles became boulders that became mountain-size "planetesimals," which merged into full-fledged planets.

In computer simulations of this process, scientists typically assume that the initial dust particles were distributed evenly in a disk around the sun.

"While this is a logical first guess, there are some problems," said Andrew Youdin, a planetary modeler at the Canadian Institute for Theoretical Astrophysics (CITA) who was not involved in the research.

If the rocky planets formed from a homogenous debris disk, they should all be roughly the same size and orbit the sun in similar circular orbits, Youdin explained.

In reality, however, Venus and Earth are much more massive than Mercury and Mars, and the orbits of the latter two planets are more elliptical, or eccentric, than expected.

"The traditional explanation is that this is luck—or more scientifically, 'chaos'—with Jupiter's proximity to Mars perhaps playing a role," Youdin said, referring to the large gravitational pull of a planet as big as Jupiter.

Planetary Afterthoughts

Hansen has a different interpretation. He proposes that the dust disk fragmented into bands of debris at various distances from the sun—much like the rings of Saturn.

According to this scenario, Earth and Venus formed within one particularly thick band, or annulus, in the inner solar system.

As the young Earth and Venus circled the sun, they waded through a sea of pebble- and mountain-size debris. The two planets captured and assimilated some of this debris, but hurled other chunks out of the annulus.

Most of these ejected particles eventually circled back, returning to the annulus. But other bits collided with one another during their exile.

"If this happens, the particles are put on a new orbit," Hansen said. "They become decoupled from the main annulus and don't come back."

Computer simulations by Hansen suggest Mercury and Mars could have formed from such separated debris.

Hansen estimates that about 90 percent of the debris in the annulus went into the formations of Earth and Venus, while the leftovers formed Mercury and Mars.

"Crazy or Inspired"

The new model is "simple and elegant," said Youdin of CITA.

In Hansen's model, "Earth and Venus are massive and [have circular orbits] because they form from the majority of the nearby ring material," Youdin said. "Mercury and Mars are smaller and eccentric because less material is expelled further from the ring."

Phil Armitage, an astrophysicist at the University of Colorado in Boulder who also did not participate in the research, said Hansen's idea is "very provocative."

"Brad's model is an extremely original attempt to solve these problems starting with a hypothesis that's either crazy or inspired," Armitage said.

Hansen said his model makes specific predictions that could be scientifically tested.

"It predicts that all the terrestrial planets more or less came from the same parent reservoir," he said.

If Hansen's model is correct, the composition of the solar system's four rocky planets should be strikingly similar.

Armitage agreed. "In the standard model the composition varies with distance from the sun," he said.

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