New analysis of a martian meteorite that fell to Earth 13,000 years ago has revealed clues about prior habitability on the red planet. The rock, which was recovered from Antarctica 30 years ago, originated on Mars 4 billion years ago, when water is believed to have covered the planet's surface. Understanding the chemical composition of the rock will help scientists determine if Mars could possibly have sustained life. Robina Shaheen of the University of California, San Diego is lead author of the paper, which was published in the Proceedings of the National Academy of Sciences.
"Minerals within the meteorite hold a snapshot of the planet's ancient chemistry, of interactions between water and atmosphere," Shaheen said in a press release.
The potato-shaped meteorite is officially known as ALH84001, and is the oldest-known rock from Mars that we have found on Earth. It was formed from the cooled magma of a volcano, and trapped samples of minerals and carbonates, providing a snapshot of ancient martian conditions.
The relative abundances of isotopes of carbon and oxygen from the martian atmosphere allow the researchers to pinpoint the specific composition signature that existed on Mars at the time. While carbon dioxide dominates the Martian atmosphere, ozone is also present. However, the ozone is fairly odd due to the heavy isotopes of oxygen within it. These bizarre isotopes undergo a rare chemical reaction that imprints its chemical signature onto carbon dioxide.
"When ozone reacts with carbon dioxide in the atmosphere, it transfers its isotopic weirdness to the new molecule," Shaheen explained.
This "weirdness" is also transferred when carbon dioxide and water create carbonates, leaving a very specific signature that can be detected. These levels of weird isotopes can be used to determine how much water may once have existed on the planet, which could give clues about potential habitability. Small quantities of water would lead to larger amounts of the odd isotope signature, while large oceans would have a smaller signature. Ultimately, the chemicals found within the meteorite do not suggest that Mars held the vast oceans that many believe once covered the red planet. Smaller bodies of water are much more likely.
"What's also new is our simultaneous measurements of carbon isotopes on the same samples. The mix of carbon isotopes suggest that the different minerals within the meteorite had separate origins," Shaheen continued. "They tell us the story of the chemical and isotopic compositions of the atmospheric carbon dioxide."
The results suggest that Mars' atmosphere now has higher levels of carbon-13 than it did then. At the time, Mars had low levels of carbon-13 and larger levels of oxygen-18. The changes in composition are likely due to the bombardment experienced on the planet billions of years ago in addition to the loss of atmosphere.
Unfortunately, the carbonate discovered by the team does not appear to have biological origins, and these data did not yield any evidence of ancient Martian microbial life. While NASA's Curiosity rover did find traces of methane earlier this month, but this study was unable to corroborate those findings.
"We now have a much deeper and specific insight into the earliest oxygen-water system in the solar system," concluded co-author Mark Thiemens, also of UCSD. "The question that remains is when did planets, Earth and Mars, get water, and in the case of Mars, where did it go? We've made great progress, but still deep mysteries remain."