Study Sheds New Light on Early Formation of Earth and Mars
November 28, 2007
Source: NASA and LPI
A team of scientists from the Lunar and Planetary Institute (LPI) and NASA's Johnson Space Center (JSC), both in Houston, and from the University of California Davis (UCD) has found that terrestrial planets such as Earth and Mars may have remained molten in their early histories for tens of millions of years. The findings indicate that the two planets cooled slower than scientists thought and that a mechanism to keep the planet interiors warm was required.
These new data reveal that the early histories of the inner planets in the solar system are complex and involve processes no longer observed. Because evidence of these processes has been preserved on Mars, but not on Earth, Mars probably provides the best opportunity for understanding the formation of Earth.
Vinciane Debaille (LPI), Alan Brandon (JSC), Qing-zhu Yin (UCD), and Ben Jacobsen (UCD) present these new findings in a paper published in the November 22 issue of Nature.
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,” says 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, constitute a class 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 that 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." Martian meteorites provide a treasure chest of information about the planet Mars 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 4567 million years ago. The youngest formed about 100 million years after this time."
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."
Debaille recently served at LPI as a postdoctoral fellow. Her research primarily consisted of the study of terrestrial planets such as the Earth, Moon, and Mars in a global geodynamic context as revealed by isotope geochemistry. She has returned to her native Belgium to continue to study areas of interest including the formation of chemically distinct reservoirs and mixing between these reservoirs, mantle convection, mantle plumes, the role of the source mineralogy, and early planetary differentiation.
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Last updated January 29, 2008