AUSTIN, Texas—New research by scientists at The University of Texas at Austin and NASA’s Johnson Space Center on a 4.57 billion-year-old Martian meteorite has established the age of 3.9 billion years for a carbonate vein deposited inside it.
NASA scientists three years ago announced that this carbonate vein contained odd formations, resembling terrestrial bacteria, that might prove to be evidence of ancient Martian life. Researchers have been working ever since to unravel the secrets of the meteorite.
Scientists working independently in labs in Austin and Houston used two different techniques to establish when the carbonate minerals were formed, according to Dr. James N. Connelly, assistant professor at UT Austin’s department of geological sciences.
“We believe that we have produced the first convincing age of the carbonates,” Connelly said. “We took great care in devising a method (of dating) based on sound chemical principles, and we have been able to replicate a similar age using two independent systems at two laboratories.”
At least two models have been proposed for the formation of the carbonate vein. One suggests that water passed through an existing fracture, depositing the carbonate minerals. An opposing view suggests that it formed from a high-temperature, carbon dioxide-rich fluid during massive fracturing caused by an asteroid impact on Mars. Because water is considered essential for the existence of life, the presence or absence of water on Mars when the carbonate vein formed is central to the issue of life on Mars, as well as to the formation of the carbonate vein.
Independent evidence suggests that early Mars had a warm, wet climate. But surface water has not existed on Mars for the last 3.8 billion to 3.5 billion years. A date of 3.9 billion years for the carbonate vein, and the strange shapes inside it, places them within the time range when surface water is thought to have existed on the planet.
The research was published Friday (Oct. 1) in Science, a weekly journal of the American Association for the Advancement of Science. Dr. Lars E. Borg, a research scientist at the Meteoritics Institute at University of New Mexico in Albuquerque, is lead author and Dr. Larry E. Nyquist, of the Planetary Sciences Branch of the Johnson Space Center’s Earth Science and Solar System Exploration Division, is the principle investigator. Dr. Chi-Y. Shih, Henry Wiesmann and Young Reese, all of Lockheed Engineering and Science in Houston, are co-authors with Connelly.
The 4.2 pound, grapefruit-sized Martian meteorite is believed originally to have been an igneous rock ejected from the Martian surface after a large object, such as an asteroid, crashed into it. Martian material was thrown far out into space, floating as space debris for about 16 million years. Scientists believe the meteorite fell to Earth about 13,000 to 15,000 years ago.
The object was discovered in Antarctica in 1984 during a scientific expedition to collect the meteorites that regularly emerge from the eroding ice of Antarctica’s Allen Hills. It was identified as Martian in origin in 1994.
It is one of 13 meteorites so far identified as Martian because they contain inclusions (minute bubbles) of gas with the same distinctive composition found in the Martian atmosphere. Information on Martian atmospheric gas was transmitted back to Earth in 1976 by unmanned vehicles that landed on Mars during NASA’s Viking program.
In 1996, the Johnson Space Center announced the carbonate material contained slender, squiggly formations that could be fossils of organisms resembling bacteria. Photos of the formations can be viewed on the Internet at http://sn-charon.jsc.nasa.gov/alh84001/photos.htm.
Connelly and research scientist associate Kathryn Manser dated the carbonate samples using techniques roughly similar to Carbon 14 dating of artifacts by archeologists.
Uranium decays into lead over time. Measuring the amounts of uranium compared to the amounts of lead found in the meteorite today reveals the age of the material. Scientists at the Johnson Space Center measured the ratio of rubidium compared to strontium, materials which go through a similar process of decay.
Connelly explained that the project was especially challenging because of the minute amounts of Martian material available to the scientists. The carbonate material formed in a fracture no wider than the thickness of a fingernail. The meteorite itself was cut into grape-sized pieces that were distributed to research groups.
“The carbonates in the fracture represented only one percent of that one piece,” Connelly said.
The carbonate was dissolved with various acid strengths and analyzed for uranium and lead using a mass spectrometer in the Isotope Laboratory in the department of geological sciences at UT Austin.
“The amounts of lead we analyzed are less than the amount inhaled in a single breath in an urban center, requiring all the work to be done in ultra-clean conditions,” Connelly said.
NOTE: For additional information, contact Dr. James N. Connelly at the UT Austin department of geological sciences, (512) 471-6166.
