Why Mars Is a Planetary Runt

by Richard A. Kerr

Big sister, brother. Mars (right) failed to grow up to the size of neighbors Venus (left) and Earth (middle).

Credit: NASA

Something stunted the growth of Mars in the earliest years of the solar system, planetary scientists report today in Nature. And other researchers are offering two new ways that a nascent Mars could have been starved of the building blocks it needed to grow into a full-size rocky planet. Both ways involve shuffling young planets to and fro as they grew.

With a mass 11% that of Earth, Mars is definitely undersized. But planetary dynamicists have struggled to explain why, as in their computer models of the swirling gas, dust, and rubble of the early solar nebula, Mars tends to grow to the size of Earth or Venus. In the models, kilometer-size planetesimals agglomerate into moon- to Mars-sized “embryos” out where Mars is today. These embryos, in turn, keep on agglomerating until an Earth-size Mars has formed. Obviously, the models have been missing something.

Unlike Mars in the models, the real Mars simply stopped growing once it reached the embryo stage, according to planetary geochemists Nicolas Dauphas of the University of Chicago in Illinois and Ali Pourmand, who is now at the University of Miami in Florida. The two came to that conclusion after they made a new estimate of how long it took the planet to form.

Many researchers had gauged how long Mars took to form using the steady decay of radioactive hafnium-182 to tungsten-182, but the answers were all over the place. The problem had been that the hafnium-tungsten dating technique depends not only on measuring the relevant isotopes in meteorites long ago blasted off Mars but also on knowing the relative proportion of hafnium and tungsten in the deep martian mantle. But martian meteorites are bits of martian crust, a rock derived from the mantle by melting, which alters the ratio of hafnium to tungsten. Researchers’ estimates of the extent of alteration and, thus, the time it took to make Mars varied wildly.

To shrink the uncertainty, Dauphas and Pourmand went looking in a variety of non-martian meteorites for a stand-in for the hafnium-tungsten ratio that would not be altered. They found it in the ratio of hafnium-176 to hafnium-177, which are identical chemically. When they redid the dating calculation using published isotopic ratios and their more precise elemental ratio, they found that it took just 2 million to 4 million years to form Mars, not the tens of millions of years Earth must have taken to agglomerate. Mars therefore must be an embryo that for some reason stopped merging with other embryos and failed to become a full-size rocky planet.

So why did Mars grow no further? Two studies presented at last March’s Lunar and Planetary Science Conference in Houston, Texas, suggest that the best way to arrest Mars’s development was to starve it of building material. In simulations run by planetary dynamicists David Minton and Harold Levison of Southwest Research Institute (SwRI) in Boulder, Colorado, Mars grows rapidly by colliding with kilometer-sized planetesimals close to the sun. But at the same time, Mars is nudged outward by trillions of close encounters with planetesimals that give infinitesimal gravitational kicks to the growing Mars without hitting it. When the embryo reaches about where Mars now orbits, its growth by planetesimal accretion slows, but the other embryos have been left behind, closer to the sun. Starved of its only building materials, Mars remains embryo-size while Earth and Venus loiter closer to the sun and continue to grow.

Or Jupiter could have starved Mars, according to planetary dynamicist Kevin Walsh, who is also at SwRI in Boulder, and his colleagues. They modeled an early solar system in which the gas that briefly surrounded the sun dragged Jupiter toward the sun, as appears to have happened to gas giants in most known exoplanetary systems. An inward-migrating Jupiter would have flung planetesimals out of the way to clear a gap. The first embryo to wander into the gap would become Mars after Saturn gravitationally latched on to Jupiter and both moved outward again.

“It seems crazy,” Minton says, but “there are all these ways of moving planets around early in the solar system’s history when there was a lot going on.” Exactly how the planetary shuffling played out must await much more modeling.