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.
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