Carbonate Conundrum

An artist’s rendering of the Phoenix lander. Phoenix is searching for signs of habitability in the martian northern plains.
Credit: NASA

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.

In the test performed by MECA, acid was added to a soil sample that had been dissolved in solution. When the acidity of the solution didn’t change, scientists knew that some type of carbonate must be present, absorbing the acid. You can conduct a similar chemistry experiment the next time you have heartburn. The active ingredient in antacid is calcium carbonate, which neutralizes stomach acid.

Massive carbonate formations, like the white cliffs of Dover on the English coast, are widespread on Earth, but only trace amounts of carbonate have been found on Mars.

TEGA took a completely different approach to sample analysis. It slowly heated a soil sample, and then measured the gases released at different temperatures. When one of its samples released carbon dioxide after being heated to a high temperature, scientists knew for certain that the sample contained calcium carbonate.

On Earth, carbonates appear in massive deposits, such as the white cliffs of Dover and the Dolomite mountains in northern Italy. These formations were produced underwater over long periods of time; the carbonates of which they are comprised are what remains of the shells of small marine animals that lived in ancient, warm, shallow seas. But carbonate deposits can also form abiotically.

Scientists, intrigued by martian landforms that appeared to have been shaped by massive quantities of water – deep branching channels, delta-like structures – spent many years searching for large carbonate deposits on Mars. To date, however, no such deposits have been found.

Instead, says Phil Christensen, “we find it in the dust.” Christensen is the principal investigator for the TES spectrometer, which first detected martian carbonate from orbit, and for the Mini-TES spectrometers on Spirit and Opportunity, which found carbonate at the two rovers’ landing sites. “I don’t think carbonates form on Mars by standing water and oceans and lakes,” he added.

“It probably all comes down to temperature. Today carbonates are forming in very shallow, very warm seas on the Earth, in places like the Bahamas,” says Christensen, who is a professor of geology at the University of Arizona at Tempe. “Carbonates don’t form in the North Sea” because it is too cold. And Mars, he suspects, even at its warmest, has always been cold, perhaps never warming up to more than a few degrees above freezing.

The Phoenix lander recovered soil from trenches as deep as 12 inches below the martian surface, to determine whether the chemical makeup of the soil varies with depth.
Credit: NASA/JPL-Caltech//University of Arizona/Texas A&M University

So how do carbonates form on Mars? Christensen suggests that it happens through “a weathering process” involving “the interaction of water vapor in the atmosphere with CO2 and basalt.” Much of the fine dust on Mars is composed of tiny basalt particles; when they weather, “you get these minute films or thin coatings of carbonates forming on these dust particles… We see [carbonates] everywhere we see the fine-grained dust.” Christensen cited as support for this notion a laboratory experiment conducted in the 1970s, in which researchers placed basalt sand in a bell jar under martian atmospheric conditions and exposed it to sunlight. “At the end of a month, they had formed small amounts of carbonate as coatings on all these basalt sand grains.”

So the discovery of carbonates in the soil at the Phoenix landing site, while intriguing, he says, “does not require anything unusual going on at the Phoenix site with regard to liquid water.” That doesn’t mean that there isn’t a habitability story still to be found in the martian north. It just means that the presence of carbonates in the soil there doesn’t tip the habitability scale one way or the other.

Although Phoenix has now gone silent and its mission on the surface of Mars has ended, scientists are still hopeful that data from the lander will reveal a clearer picture of the role of liquid water near Mars’s north pole.

One question the Phoenix science team would particularly like to answer is whether there are concentrations of water-related minerals in the soil. Is there, for example, a greater concentration of carbonate or perchlorate (a mineral that was found at the site earlier in the mission) a few inches below the surface than right at the surface? So far, little variation has been found between one sample and another.

To shed light on this question, scientists focused toward the end of the Phoenix mission on obtaining a set of samples, both for TEGA and MECA, from different depths. Experiments were run on those samples and the resulting data was transmitted to Earth before Phoenix lost power, but analyzing that data is a complex process that will take several more months.

“The smoking gun,” says Hecht, “would be if we were to find a significant deposit of salts, particularly the perchlorates,” because “they would tend to precipitate out in the last place liquid water had been.” If you found an accumulation of white stuff that was not ice, then you’d say, ‘Aha, we had liquid water here.’ As simple as that. As straightforward as that. That would be the big story.”


Related Web Sites

Astrobiology Roadmap Goal 2: Life in Our Solar System
Liquid Water in the Martian North? Maybe.
Snow Falls on Mars
Perchlorates, Perchance?
Opal’s Sparkle Indicates Water