A long-ago inward migration by Jupiter during the formation of our solar system could explain why Mars is small in relation to Earth and Venus, according to a paper published in Nature.

Researchers have long sought to explain the small mass of Mars, which has remained an outstanding problem in terrestrial planet formation, said David P. O’Brien, a research scientist at the Planetary Science Institute and co-author of “A Low Mass for Mars from Jupiter’s Early Gas-Driven Migration” that appears in Nature.

“This work not only solves a difficult problem in solar system formation,” O’Brien said, “it shows that the solution lies in the giant planets of our solar system undergoing significant early migration, which was generally thought to only have occurred in extrasolar planetary systems.”

Simulations of the formation process of the four inner planets in the solar system — Mercury, Venus, Earth and Mars — generally produced a version of Mars far more massive than the real planet.

“We tried a large variety of simulation parameters to solve this problem, but nothing seemed to work,” O’Brien said.

A 2009 paper by Brad Hansen from UCLA offered a new clue: Hansen showed that if the initial distribution of solid material in the solar system was assumed to have an outer boundary at 1 astronomical unit (1 AU being the current average distance from the Sun to Earth), a smaller Mars could form.

The presence of a sharp outer boundary at 1 AU required in Hansen’s work was hard to explain, given the existence of the asteroid belt between 2 and 4 AU, the giant planets between 5 and 30 AU and the Kuiper Belt beyond that.

However, it has been shown in numerical simulations over the past decade that Jupiter and Saturn could migrate in the early solar system when gas was still present, and in some cases could move inwards and then back outwards to roughly their current locations.

“Rapidly the pieces of the story came together,” said Kevin J. Walsh, lead author of the paper who began work on the project at the Observatoire de la Cote d’Azur in Nice, France, and is now at the Southwest Research Institute in Bounder, CO. “If Jupiter had moved inwards from its birth place down to 1.5 AU from the Sun and then had turned around because of the formation of Saturn, eventually migrating outwards towards its current location, it would have truncated the distribution of solids in the inner solar system at about 1 AU, as required to explain the small mass of Mars.”

Jupiter now orbits the Sun at 5.2 AU.

“The problem was to understand whether the inward and outward migration of Jupiter through the 2-4 AU region could be compatible with the existence of the asteroid belt today,” Walsh said. “So we started to do a huge number of simulations.”

“The asteroid belt, which was a priori our main problem, turned out to be the main strength of our model,” said O’Brien.

“The result was fantastic,” Walsh said. “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.”

The passage of Jupiter depleted then re-populated the asteroid belt region, with inner-belt bodies originating between 1 and 3 AU and outer belt bodies originating in a very distinct region between and beyond the giant planets, naturally producing the significant compositional differences existing today across the belt.

The model was called the “Grand Tack Scenario” with Jupiter’s motion similar to a sailboat tacking around a buoy.

Other authors are Alessandro Morbidelli (Observatoire de la Cote d’Azur, France), Sean N. Raymond (Observatoire de Bordeaux, France) and Avi M. Mandell (NASA Goddard).

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O’Brien’s work was funded by a grant to PSI from NASA’s Planetary Geology and Geophysics research program.

The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to solar system exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972. PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork in North America, Australia and Africa. They also are actively involved in science education and public outreach through school programs, children’s books, popular science books and art. PSI scientists are based in 15 states, the United Kingdom, Switzerland, Russia, Australia and Canada.