Mars developed in as little as two to four million years after the birth of the solar system, far more quickly than Earth, according to a new study published in the May 26 issue of the journal Nature. The red planet’s rapid formation helps explain why it is so small, say the study’s co-authors, Nicolas Dauphas at the University of Chicago and Ali Pourmand at the University of Miami (Fla).
Mars probably is not a terrestrial planet like Earth, which grew to its full size over 50 to 100 million years via collisions with other small bodies in the solar system, said Dauphas, an associate professor in geophysical sciences. Mars instead is a planetary embryo that never developed into a full-fledged planet.
“Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other embryos to make an Earthlike planet,” Dauphas said. The new work provides supporting evidence for this idea, which was first proposed 20 years ago on the basis of planetary growth simulations.
The new evidence likely will change the way planetary scientists view Mars, observed Pourmand, assistant professor in marine geology and geophysics at the UM Rosenstiel School of Marine & Atmospheric Science. “We thought that there were no embryos in the solar system to study, but when we study Mars, we are studying embryos that eventually made planets like Earth.”
There had been large uncertainties in the formation history of Mars because of the unknown composition of its mantle, the rock layer that underlies the crust. “Now we can shrink those uncertainties to the point where we can do interesting science,” Dauphas said.
Dauphas and Pourmand were able to refine the age of Mars by using the radioactive decay of hafnium to tungsten in meteorites from Mars as a way of estimating their age. Hafnium 182 decays into tungsten 182 in a half-life of nine million years. This relatively rapid decay process means that almost all hafnium 182 will disappear in 50 million years, providing a way to assemble a fine-scale chronology of early events in the solar system.
“To apply that system you need two gradients,” Pourmand explained. “You need the hafnium-tungsten ratio of the mantle of Mars, and you need the tungsten isotopic composition of the mantle of Mars.” The latter was well known from analyses of martian meteorites, but not the former.
Previous estimates of the formation of Mars ranged as high as 15 million years because the chemical composition of the martian mantle was largely unknown. Scientists still wrestle with large uncertainties in the composition of Earth’s mantle because of composition-altering processes such as melting.
—Prof. Jacqueline Stewart
