These new data reveal that the early histories of the inner planets in the solar system are complex and involve processes no longer observed. Evidence of these processes has been preserved in Mars, while it has been erased in Earth. So Mars is probably the best opportunity to understand how Earth formed.
The formation of the solar system can be dated quite accurately to 4,567,000,000 years ago, said Qing-Zhu Yin, assistant professor of geology at UC Davis and an author on a new paper. Mars' metallic core formed a few million years after that. Previous estimates for how long the surface remained molten ranged from thousands of years to several hundred million years.
The persistence of a magma ocean on Mars for 100 million years is "surprisingly long," Yin said. It implies that at the time, Mars must have had a thick enough atmosphere to insulate the planet and slow down cooling, he said.
Scientists think that early crust formation alone cannot account for the slow cooling magma ocean seen in large planets. This new evidence instead implies that Mars, at one time, had a primitive atmosphere that acted as the insulator. “The primitive atmosphere was composed mostly of hydrogen left over from accretion into a rocky planet, but was removed, probably by impacts, about 100 million years after the planet formed,” said Debaille.
Debaille and her colleagues performed precise measurements of neodymium isotope compositions of nine rare Martian meteorites called shergottites using mass spectrometers at JSC and UCD. Shergottites, named after the first-identified meteorite specimen that fell at Shergotty, India, in 1865, are a group of related meteorites from Mars composed primarily of pyroxene and feldspar.
The scientists examined shergottites because their large range in chemical compositions is thought to be a fingerprint of the formation of their deep sources very early in the history of Mars.
“These rocks were lavas that were made by melting deep in Mars and then erupted on the surface," said Brandon. “They were delivered to Earth as meteorites following impacts on Mars that exhumed them and launched them into space." Mars meteorites are a treasure chest of information about that planet and have been the focus of extensive research by scientists.
The metallic element samarium has two radioactive isotopes that decay at a known rate to two daughter neodymium isotopes. By precisely measuring the quantities of neodymium isotopes, Debaille was able to use these two radiometric clocks to derive the times of formation of the different shergottite sources in the Martian interior.
“We expected to find that their sources all formed at the same time,” said Debaille. “But what we found instead was that the shergottite sources formed at two different times. The oldest formed at 35 million years after the solar system began to condense from ice and dust into large planets about 4,567 million years ago. The youngest formed about 110 million years after the solar system began to condense.”
Debaille and her colleagues found that the scenario that best fits the data is one where a global-scale magma ocean formed from melting in Mars during the final stages of accretion and then slowly solidified over this time period.
“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 Brandon. “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.”