Mars has always been the toddler of the rocky planet family. With a radius half that of Earth’s and a mass just over one tenth of that of our planet, it is bigger than baby Mercury but not quite as grown up as Earth and Venus. Now it seems that some unruly behaviour on behalf of Jupiter in the early solar system may be to blame for the red planet’s stunted growth.
Planets are born in protoplanetary disks. These rotating disks are full of gas and cosmic dust, and their centre is marked by a protostar that began to take shape when part of the disk underwent gravitational collapse. Most of the material in the disk goes into making the young star; the rest eventually becomes planets, moons and asteroids. We can see that this fits with the solar system today: the Sun contains 99.86% of the total mass in the solar system. Of the tiny fraction of mass remaining, the gas giants — Jupiter, Saturn, Uranus and Neptune — are the biggest contributor, making up 99% of the 0.14%.
Gas giants formed earlier than rocky planets, taking a few million years to emerge from the protoplanetary disk. The rocky planets took much longer to stop growing — around 30-50 million years. Most models that planetary scientists have so far come up with produce a Mars that is much bigger than the one we see today. Previous work has shown that a smaller Mars does make sense, but only if the planetesimal disk — containing rocks, ice and the beginnings of planets — in the early solar system existed in the space between the young Sun and where the Earth sits now. If there were any solid material further out, the resulting Mars mass would be all wrong again. This does not seem to fit with the asteroid belt that exists beyond the Earth, never mind the gas giants that comprise most of the total planetary mass in the solar system and are even further away.
Now it seems that understanding the movement of the gas giants while they and the other planets were still forming may be the key to solving the mystery of an undersized Mars. In a paper published in Nature, scientists describe how they ran computer simulations of the early solar system that show how Jupiter’s movement could have cut short the planetesimal disk from which rocky planets like Earth and Mars later formed. The simulations are able to produce a planetesimal disk that extends no further than the Earth-Sun distance, called one astronomical unit (AU), and a Mars mass that fits with what we see.
In the simulations, performed by Kevin J. Walsh and Alessandro Morbidelli at the Observatoire de la Cote d'Azur in Nice, France, Jupiter moves in from where it formed at 3.5 AU to just 1.5 AU away from the Sun. 3.5 AU is though to be a favourable distance from the Sun for the formation of gas giants, because it is past the “snow line” — the distance at which ice, ammonia and methane can condense into grains of ice. As Jupiter moves inwards, Saturn stays put. The gas giants are still growing at this point, and once Saturn reaches a mass sixty times that of present-day Earth, it too begins to migrate inwards. Once Saturn catches up with Jupiter, they begin to migrate outwards again, and drag young Uranus and Neptune with them. The gas giants’ migration slows as the disk of gas dissipates, and the final arrangement they reach in the simulation matches what we see today.
Jupiter’s movement in the simulation empties the asteroid belt, but then repopulates it. It also helps explain the difference in composition of different asteroid belt regions. The asteroid belt occupies the space between Mars and Jupiter. The inner belt is usually said to end at around 2-3 AU, and the outer belt stretches from there to beyond the gas giants. The inner belt contains more silicate-rich asteroids whereas those in the outer belt are more likely to be carbon-rich. In the simulations, the inner and outer asteroid belts come from different places in the solar system; the inner lot started out much closer to the Sun, and the outer group formed amongst the outer planets. Jupiter’s and Saturn’s migration shook the two populations up a little and created an overlap, but left them separate enough to still be noticeably different today despite them now sitting side by side.
The findings also shed light on the lives of planets outside the solar system. Scientists have known for a while that extrasolar planetary systems seem to exhibit migration. “Hot Jupiters” are gas giants similar in size and composition to Jupiter that are found close to their host star. They cannot have formed this close to the star, so are believed to have migrated inwards. Until now scientists thought that this behaviour was out of the ordinary, because it was not like the formation of our own planetary system. Now, it appears that our solar system may not be as special as we once thought.
Walsh KJ, Morbidelli A, Raymond SN, O'Brien DP, & Mandell AM (2011). A low mass for Mars from Jupiter's early gas-driven migration. Nature, 475 (7355), 206-9 PMID: 21642961