Carbonate Conundrum

By Henry Bortman

Mars is good at keeping its secrets. As a case in point, consider the discovery by NASA's Phoenix lander of carbonates in the planet's northern polar plains. The goal of the Phoenix mission was to understand the historic role of water in the frozen martian north, and to assess the region's habitability. Carbonates form in the presence of water. So one might think that their discovery is cause for celebration.

"Carbonates are commonly formed by the interaction of liquid water with carbon dioxide gas," says William Boynton, principal investigator for Phoenix's TEGA instrument. And liquid water is a requirement for life as we know it. So finding carbonate at the landing site means the possibility of water there, and perhaps life as well.

Or maybe not. As TEGA co-investigator Doug Ming points out, "it's possible that [the carbonate] formed in situ at the landing site," but it's equally possible that it "formed somewhere else" and, literally, blew in with the wind. "I don't think we have enough data from the Phoenix mission to say one way or the other."

Whether or not carbonate formation is a local phenomenon, the discovery is significant because it marks the first time ever that carbonates have been detected directly on the Red Planet. Spectrometers on orbiting spacecraft and on the MER rovers Spirit and Opportunity have previously seen indications that small quantities of carbonates were present in martian soil, but spectroscopic analysis is often a bit of a guessing game. Phoenix's detection was made by performing chemistry experiments directly on martian soil, and it was made by two different instruments: TEGA, which heated soil samples and sniffed for released gases, and MECA-WCL, a wet chemistry lab. The Phoenix results are unambiguous.

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