The Mars rover Spirit is now dead in its tracks (JPL) but the planet under it continues to rumble, in theoretical overhauls and anomalies.  Mars has been much on the mind of news reporters this week after a new paper speculated that the red planet grew up fast and then stopped as a runt.
In Nature,1 Dauphas and Pourmand studied ratios of isotopes of hafnium (Hf) and tungsten (W) to envision a history of Mars much different than previously assumed.  Their model makes Mars form in about one-fifth or less the time previously assumed to be required.  In the same issue ofNature,1 Alan Brandon summed up the new idea: “It seems that Mars had grown to near its present size by 2 million to 4 million years after the Solar System began to form,” he said.  “Such rapid growth explains why the planet is much smaller than Earth and Venus.”
Any explanatory gains, however, appear to be offset by puzzles, according to Bloch’s Law, “Every solution breeds new problems.”  Brandon said, “The authors finding that rocky bodies the size of Mars accreted within 2 million to 4 million years has ramifications for models of early planetary history.”  Some of these ramifications confirm earlier theories, while others contradict them:

With such an early time for Mars accretion, which probably led to the formation of a global magma ocean, how do we explain the times for magma-oceansolidification of around 100 million years after the Solar System began to form that are obtained from measurements of Lu (lutetium)-Hf and Sm-Nd chronometers in Martian meteorites?  Magma oceans are not supposed to take that long to solidfy [sic].  This suggests that, although Dauphas and Pourmand have provided us with a key constraint on the early formation and evolution of our planets, we still have much to learn….

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