Mars, now cooled into crusty sphere, once sizzled with oceans of magma for millions of years. New research suggests it was red-hot tens of millions of years longer than previously thought.
Rare chunks of Martian rock flung to Earth as meteorites hint at an extended molten status, for which scientists think a thick, early atmosphere was responsible.
"The most recent physical models for magma oceans suggest they solidify on timescales of a few million years or less, so this result is surprising," said Alan Brandon, a geochemist at NASA's Johnson Space Center in Houston.
Brandon, who co-authored the study detailing the new findings in the Nov. 22 issue of the journal Nature, said that a crusty surface alone can not explain what the Martian rocks reveal.
"Some type of insulating blanket, either as a rocky crust or a thick atmosphere, is needed as an insulator to have kept the Martian interior hot," he said.
Brandon and his colleagues said that understanding how slowly Earth cooled is difficult, as our rocky home continuously melts down its geologic history. Because Mars is smaller and cooled faster, however, it harbors valuable information about the distant past of planet formation.
"These rocks were lavas that were made by melting deep in Mars and then erupted on the surface," Brandon said of nine Martian meteorites his team examined. "They were delivered to Earth ... following impacts on Mars that exhumed them and launched them into space."
Called shergottites, the rare meteorites were named after the first one that landed in Shergotty, India in 1865. Study co-author Vinciane Debaille, a planetary scientist at the Lunar and Planetary Institute in Houston, dated the rocks' radioactive metals to determine when the fiery entrails of early Mars formed them.
"We expected to find that their sources all formed at the same time," Debaille said. "But what we found instead was that the shergottite sources formed at two different times."
She explained that the oldest formed 35 million years after the solar system began to condense from ice and dust into large planets, about 4.567 billion years ago. The youngest formed about 110 million years after planets began to accrete.
Debaille and her colleagues think a global magma ocean existed in the final stages of Mars' formation, then slowly solidified over this time period. To slow that cooling, she thinks an atmospheric blanket once insulated the planet.
"The primitive atmosphere was composed mostly of hydrogen left over from accretion into a rocky planet," Debaille said. "But [it] was removed, probably by impacts, about 100 million years after the planet formed."