Astronomers don't know how it happened, but Mars appears to have sat out the game of planetary billiards that marked the solar system's early days, surviving as a quickly formed, diminutive and relatively unaltered embryo planet, a new study shows.
The finding offers an explanation to the long-standing mystery of why Mars, the fourth planet from the sun, is so small, lead researcher Nicolas Dauphas, with the University of Chicago, tells Discovery News.
"There was this problem of explaining the small size of Mars," Dauphas said. "There were ideas floating around in the scientific community, but the question was what do you measure to tell whether these ideas are a fairy tale or whether it corresponds to what happened."
Dauphas and colleagues decided to attack the problem by figuring out how fast it took Mars to form, a study that entailed painstaking analysis of radioactive materials in 20 Mars meteorites and comparative studies of 30 chondrite meteorites, which are believed to be leftover remains from the solar system's earliest days.
With the ratios of radioactive tracer elements in hand, the scientists then turned to computer simulations to figure out how fast Mars formed. They found the planet reached half its present size in 1.8 million years -- or less.
In contrast, Earth, which is close to twice the size of Mars, took 50 to 100 million years to form.
The key difference appears to be access to new material. Earth bulked up on planetoids and other bodies that crashed into its surface, while Mars starved.
Computer models suggest Earth and Venus may have deflected objects away from Mars, or objects may have been scattered outward before they could be incorporated into Mars, said University of Miami researcher Ali Pourmand.
"It appears that Mars was a lucky survivor and escaped collision with other similar bodies," Pourmand wrote in an email to Discovery News.
Mars' rapid growth means heat from the decay of radioactive aluminum 26, another element present in the early universe, would have had time to temporarily turn Mars into an ocean of magma, Dauphas added.
"It's an important difference if a planet forms as a solid ball or if it's molten. What you see on the surface can be influenced by the large-scale geology of the planet, including whether the planet has a magma ocean. That's a very important outcome of our work," he said.
Whether the planet's rapid growth impacted its chance to host life is not known.
"A shorter accretion timescale could potentially give life the chance to evolve earlier on Mars. But I must emphasize this is pure speculation," wrote Pourmand. "Every planet has a different evolutionary history."
The research is published in this week's Nature.