ANN ARBOR---While the NASA Pathfinder rover, Sojourner, sniffs rocks on the surface of Mars, University of Michigan geologists have completed their own analysis of Mars rocks here on Earth. Results of a U-M analysis of tungsten isotopes in Martian meteorites, published in this week's issue of Nature, show that Sojourner is sitting on a planet whose internal structure has remained essentially unchanged since the earliest history of our solar system.
"The tungsten isotopic composition of the eight meteorites analyzed in our study indicates that large-scale convection, which drives plate tectonic motion and mixes the Earth's mantle, appears to have been unimportant during most of the history of Mars," said Alexander N. Halliday, a U-M professor of geological sciences. "The data also suggest that Mars formed fast and differentiated early in the solar system's history---about 20 to 40 million years faster than the Earth's own differentiation into a dense metal core, partially molten silicate rock mantle and thin surface crust."
Scientists believe the planets in our solar system began forming about 4.57 billion years ago from a huge cloud of interstellar gas, dust and debris leftover from the birth of the sun. The Earth and other rocky planets in the inner solar system built up gradually over millions of years as their gravitational pull attracted larger and larger chunks of material from the cloud.
"Mars appears to have formed over a 10-million-year period very early in the solar system's existence," said Der-Chuen Lee, a U-M post-doctoral research fellow in geological sciences and co-author of the study. "During this formation period, energy released from the incoming rock and debris and decay of short-lived radioactive nuclides would have quickly built up inside the growing planet. This interior heat may have produced a shallow magma ocean near the Martian surface."
Metallic liquids in this magma ocean would have settled to the planet's center to form the core, while lighter silicates floated to the top, according to Lee. The process was essentially complete sometime between 10 and 30 million years after the solar system formed.
"Since then, geologic activity on Mars has been sluggish, episodic and localized compared to activity on Earth. Without the constant churning, melting and mixing of mantle and surface crust produced by active plate tectonics, some features of the chemical and isotopic composition of the Martian interior have been preserved since the core formed more than 4.53 billion years ago," Halliday said.
Segregation of a metallic core on Earth was not completed for another 20 to 30 million years, according to Halliday. In previous tungsten isotopic studies, Lee and Halliday found evidence indicating that the Earth's core was not formed until at least 50 million years after the solar system began.
"It is likely that this protracted development on Earth included a collision with a massive object---triggering wholesale melting and mixing of material in the growing planet. Our current study, however, produced no evidence for such a late collision on Mars," Halliday said.
Halliday and Lee used a new technique called multiple-collector, inductively-coupled plasma mass spectrometry to measure relative amounts of tungsten isotopes in Martian meteorites and acondrites---silicate-rich debris from asteroids formed soon after the solar system developed.
"Hafnium-182 is an extinct radioactive isotope, which was relatively abundant in the early solar system. By comparing relative amounts of hafnium and tungsten with the relative enrichment of the daughter isotope tungsten-182 in these meteorites, we can calculate how quickly hafnium-to-tungsten ratios changed in early solar system objects," Halliday said.
"Hafnium tends to be incorporated into silicate minerals in rocks, while tungsten has an affinity for iron. When dense iron-rich melts separate from silicate melts, tungsten sinks into a planet's metallic core, while hafnium concentrates in the mantle. If you remove part of the tungsten early, the effects of the entire subsequent decay process are altered. We can tell when this occurred by measuring the abundance of tungsten-182. With the new mass spectrometry technique of multiple collector ICPMS, we can detect differences in isotopic ratios in as little as a few billionths of a gram of tungsten."
The research was funded by the National Science Foundation, the U.S. Department of Energy, NASA and the University of Michigan. Meteorites analyzed in the study were from the collections of NASA, the Smithsonian Institution in Washington, D.C., Museum National d'Histoire Naturelle in Paris, and the Field Museum in Chicago.