Mars’ growth stunted by an early giant planet instability


  • 800 direct numerical simulations of a giant planet instability occurring during the process of terrestrial planet formation.
  • Naturally explains Mars size and formation timescale.
  • Simulated asteroid belts are largely depleted, seldom form a planet and broadly match the orbital structure of the actual belt.
  • Many systems simultaneously match success criteria for both the inner and outer solar system.
  • Most accurate terrestrial systems are formed when the giant planets attain their correct orbits.


Many dynamical aspects of the solar system can be explained by the outer planets experiencing a period of orbital instability sometimes called the Nice Model. Though often correlated with a perceived delayed spike in the lunar cratering record known as the Late Heavy Bombardment (LHB), recent work suggests that this event may have occurred much earlier; perhaps during the epoch of terrestrial planet formation. While current simulations of terrestrial accretion can reproduce many observed qualities of the solar system, replicating the small mass of Mars requires modification to standard planet formation models. Here we use 800 dynamical simulations to show that an early instability in the outer solar system strongly influences terrestrial planet formation and regularly yields properly sized Mars analogs. Our most successful outcomes occur when the terrestrial planets evolve an additional 1–10 million years (Myr) following the dispersal of the gas disk, before the onset of the giant planet instability. In these simulations, accretion has begun in the Mars region before the instability, but the dynamical perturbation induced by the giant planets’ scattering removes large embryos from Mars’ vicinity. Large embryos are either ejected or scattered inward toward Earth and Venus (in some cases to deliver water), and Mars is left behind as a stranded embryo. An early giant planet instability can thus replicate both the inner and outer solar system in a single model.
A University of Oklahoma astrophysics team explains why the growth of Mars was stunted by an orbital instability among the outer solar system's giant planets in a new study on the evolution of the young solar system. The OU study builds on the widely-accepted Nice Model, which invokes a planetary instability to explain many peculiar observed aspects of the outer solar system. An OU model used computer simulations to show how planet accretion (growth) is halted by the outer solar system instability. Without it, Mars possibly could have become a larger, habitable planet like Earth.

"This study offers a simple and more elegant solution for why Mars is small, barren and uninhabitable," said Matthew S. Clement, OU graduate student in the Homer L. Dodge Department of Physics and Astronomy, OU College of Arts and Sciences. "The particular dynamics of the instability between the giant planets kept Mars from growing to an Earth-mass planet."

Clement and Nathan A. Kaib, OU astrophysics professor, worked with Sean N. Raymond, the University of Bordeaux, France, and Kevin J. Walsh, Southwest Research Institute, to investigate the effect of the Nice Model instability on the process of terrestrial planetary formation. The research team used computing resources provided by the OU Supercomputing Center for Education and Research and the Blue Waters sustained peta-scale computing project to perform 800 computer simulations of this scenario.

The goal of this study was to investigate simulated systems that produced Earth-like planets with Mars analogs as well. Recent geological data from Mars and Earth indicates that Mars' formation period was about 10 times shorter than Earth's, which has led to the idea that Mars was left behind as a 'stranded planetary embryo' during the formation of the Sun's inner planets. The early planet instability modeled in this study provides a natural explanation for how Mars emerged from the process of planet formation as a 'stranded embryo.'

The success of the terrestrial planetary formation simulations for this study were found to be tied to the detailed evolution of the solar system's two giant planets -- Jupiter and Saturn. Systems in the study where Jupiter and Saturn's post-instability orbits were most similar to their actual current orbits also produced systems of terrestrial planets that resembled the current solar system.