Looking Up – Bob Eklund

Wet & Mild: Caltech Researchers Take the Temperature of Mars’s Past —

Researchers at Caltech have directly determined the surface temperature of early Mars for the first time, providing evidence of a warmer and wetter Martian past.

By analyzing carbonate minerals in a four-billion-year-old meteorite that originated near the surface of Mars, the scientists determined that the minerals formed at about 18 degrees Celsius (64 degrees Fahrenheit). “The thing that’s really cool is that 18 degrees is not particularly cold nor particularly hot,” says Woody Fischer, assistant professor of geobiology and coauthor of a paper published online in the Proceedings of the National Academy of Sciences (PNAS) on October 3. “It’s kind of a remarkable result.”

The finding in the Martian meteorite is just one data point—but it’s the first and only one to date. “It’s proof that early in the history of Mars, at least one place on the planet was capable of keeping an Earthlike climate for at least a few hours to a few days,” says John Eiler, the Robert P. Sharp Professor of Geology and professor of geochemistry, and a coauthor of the paper.

To make their measurement, the researchers analyzed one of the oldest known rocks in the world: ALH84001, a Martian meteorite discovered in 1984 in the Allan Hills of Antarctica. The meteorite likely started out tens of meters below the Martian surface and was blown off when another meteorite struck the area, blasting the piece of Mars toward Earth. The potato-shaped rock made headlines in 1996 when scientists discovered tiny globules in it that looked like fossilized bacteria. But the claim that it was extraterrestrial life didn’t hold up upon closer scrutiny. The origin of the globules, which contain carbonate minerals, remained a mystery.

“It’s been devilishly difficult to work out the process that generated the carbonate minerals in the first place,” Eiler says. There have been countless hypotheses, he adds, and they all depend on the temperature in which the carbonates formed.

Finding the temperature through independent means would therefore help narrow down just how the carbonate might have been formed. The researchers turned to clumped-isotope thermometry, a technique developed by Eiler and his colleagues.

In this case, the team measured concentrations of the rare isotopes oxygen-18 and carbon-13 contained in the carbonate samples. Carbonate is made out of carbon and oxygen, and as it forms, the two rare isotopes may bond to each other—clumping together, as Eiler calls it. The lower the temperature, the more the isotopes tend to clump. As a result, determining the amount of clumping allows for a direct measurement of temperature.

The temperature the researchers measured, 18 ± 4 degrees Celsius, rules out many carbonate-formation hypotheses. “A lot of ideas that were out there are gone,” Eiler says. For one, the mild temperature means that the carbonate must have formed in liquid water. “You can’t grow carbonate minerals at 18 degrees other than from an aqueous solution,” he explains.

Could this wet and warm environment have been a habitat for life? Most likely not, the researchers say. These conditions wouldn’t have existed long enough for life to grow or evolve—it would have taken only hours or days for the water to dry up. Still, these results are proof that an Earth-like environment once existed in one particular spot on Mars, at least for a short time.

You can contact Bob Eklund at [email protected], or visit his websites at www.bobeklund.com and http://firststarbook.com.

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