Astrobiology Top 10: Liquid Water in the Martian North? Maybe.
Perchlorate. Never heard of it? Join the club. But NASA’s Phoenix spacecraft has found it in the soil in the icy northern plains of Mars. And now that it’s been found, scientists are scrambling to explain how it got there, and what, if anything, its presence means about the habitability of the martian north.
Phoenix didn’t go to Mars to find perchlorate. It went looking for evidence of liquid water. From orbit, NASA’s Mars Odyssey in 2002 discovered water ice in the martian north, lying just inches beneath the surface. Very cold, very hard ice. Far too cold to support life.
But Mars’s polar regions aren’t always so cold. The angle at which Mars tilts changes over time, and every hundred thousand years or so the planet leans so far over that its north and south poles take turns facing the sun as the planet travels through its orbit. When this happens, the polar regions get increased sunlight, and some of the subsurface ice may melt, and leave behind telltale mineral signs in the martian soil. Those signs are what Phoenix is looking for.
NASA’s MER rovers have both found evidence, at sites near the martian equator, of rocks that were altered by the action of liquid water. But most scientists agree that those alterations occurred quite early in Mars’s history, perhaps as long ago as 4 billion years.
In the northern plains, where subsurface ice is prevalent, liquid water may have been around more recently. As recently as the last time Mars wobbled over onto its side.
Has Phoenix found evidence of liquid water? The jury is still out. But it has found perchlorates.
Perchlorate is a chemical compound, a negatively charged ion, that contains a single chlorine atom and four oxygen atoms. It combines with potassium, magnesium, or any of a number of other elements, to form perchlorate salts, or simply, perchlorates.
Perchlorates are incredibly soluble. That’s why, on Earth, it’s rare to find large natural deposits of them. Such deposits can exist only in very arid environments, such as Chile’s Atacama Desert. Water no doubt played a role in concentrating those deposits initially, but even a little bit of rain will cause perchlorates to dissolve and wash away. That’s why they’re more commonly found in rivers and lakes.
So where there is perchlorate, there is a water story. On Earth. On Mars, it turns out, perchlorates don’t necessarily imply water.
In nature perchlorates form photochemically in the atmosphere, and then settle randomly on a planetary surface. No water is involved in their creation. So merely finding perchlorates on Mars doesn’t say anything one way or another about liquid water.
Finding a concentration of perchlorates would argue that liquid water had been involved. "If we find a deposit of perchlorate, one can speculate that water had melted at some point and had collected it into an accumulation," says Richard Quinn, a Phoenix researcher with the SETI Institute and NASA Ames Research Center. But Phoenix hasn’t yet found a concentrated deposit of perchlorates.
Alternatively, if Phoenix found some sort of perchlorate gradient – say it saw only a small trace of perchlorate in a sample from the surface, but it saw a larger quantity in a second sample from a few inches below the first, at the boundary between the soil and the ice – one could be fairly certain that liquid water was responsible. But Phoenix hasn’t found a gradient, either.
Phoenix has two different instruments that can detect perchlorates, but they go about it in different ways. The spacecraft’s Wet Chemistry Lab (WCL) analyzes a soil sample by putting it in a small beaker of water, and then looking to see what dissolves. The beaker’s walls contain some two dozen electrochemical sensors, each of which is sensitive to the presence of a particular ion, perchlorate being one of them.
Phoenix’s TEGA (Thermal and Evolved Gas Analyzer) uses a different approach. It heats the sample, in stages, and then "sniffs" at the fumes that burn off at different temperatures. TEGA can’t detect perchlorate directly; instead it records a release of oxygen when the perchlorate breaks down. The temperature at which the oxygen is detected gives a clue to which perchlorates were present in the original sample. Some (but not all) perchlorates also release chlorine gas when heated.
WCL has detected perchlorate unambiguously in both of the samples it has tested.
The TEGA results are a bit fuzzier. In its first sample, which came from a location several feet away from the WCL samples, TEGA saw a release of oxygen consistent with the presence of perchlorate. But at that point – it was early in the mission, WCL hadn’t found perchlorates yet, and no one was expecting to see them – TEGA didn’t look for a release of chlorine.
When TEGA analyzed its second sample, which came from roughly the same location as WCL’s second sample, TEGA was programmed to look primarily for evidence of organic material. As a result, says William Boynton, the science lead for TEGA, "We didn’t see the oxygen." But, he adds, "We also heated the sample differently¡¦. The oxygen-bearing compound, presumably perchlorate, might have been there, but we might have destroyed it when we were looking for organic compounds."
TEGA is now examining a third sample, taken from the surface very near the source of the first WCL sample. This time around, Boynton says, "We’re not going to be looking for the organic compounds; we’re going to use the same heating plan that we did on the first sample." And "in addition to looking for oxygen, we’re going to look for chlorine."
Even if TEGA confirms the presence of perchlorates, however, that will only be an early step in understanding the role that water has played in the martian arctic. Additional samples will need to be analyzed, both by WCL and by TEGA, to determine whether the perchlorates have simply been blown in by the martian wind, or they have been moved around and concentrated by water.
Researchers are actively debating where those samples should come from. "We’ve really got to sit down and think about what are our last two WCL samples going to be, and can we find any gradient," Quinn says. "That’s going to be key to really saying something about water."