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."
advertisement
