Perchlorates on Mars Provide Fertile Ground for Researchers
NEW YORK — NASA’s Phoenix Mars lander made an impressive array of measurements and discoveries that will help fine-tune scientists’ understanding of Mars’ chemistry and environment, perhaps none more important than the detection of an odd type of salt that some think could have an important impact on the planet’s water cycle and ability to support life.
In a set of papers presented March 23-27 at the 40th Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas, several Phoenix team members put forth their ideas on how the class of salts, called perchlorates, might affect Mars’ water cycle; how it might boost or inhibit potential martian life; how it might form a sludge underneath Mars’ polar caps, lubricating them and allowing them to flow, as glaciers do on Earth; and how the salt got there in the first place.
Phoenix landed in the Vastitas Borealis plains of Mars on May 25, 2008, and spent its five-plus-month tenure digging up samples of martian dirt and subsurface water ice and analyzing them for signs of the planet’s past potential habitability.
The lander unearthed plenty of unanticipated findings: alkaline dirt; snow falling from the sky; and sticky dirt that clumps when scooped up.
But the detection of perchlorates, made by the lander’s Microscopy, Electrochemistry and Conductivity Analyzer (MECA) wet chemistry lab, may have some of the most interesting implications for Mars’ soil chemistry, water cycle and potential habitability.
The finding was “totally unexpected,” said Phoenix principal investigator Peter Smith of the University of Arizona, Tucson.
Once the detection was made, several Phoenix scientists began considering what perchlorates’ presence on the red planet could mean for martian surface chemistry.
Michael Hecht of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and the lead scientist for the instrument that first detected the perchlorates, and David Fisher of the Geological Survey of Canada were rooming together in Tucson during the mission and began discussing some of those implications. In particular, they formulated a potential mechanism that could allow the martian polar caps to move.
On Earth, heat is constantly escaping from the planet’s molten interior, which warms the bottom of glaciers and ice sheets, melting the ice, “and the ice sheet literally can slide,” Hecht explained.
Hecht and Fisher suspect that the perchlorates could be allowing the same thing to happen on Mars.
Some observations of Mars’ polar ice caps show characteristics that suggest that the ice could be sliding, though not everyone agrees that ice flow is the only plausible explanation. But at Mars’ poles, it is far too cold for ice at the base of the cap to melt in the same way that it does on Earth.
Perchlorates, however, can melt ice even at the frigid temperatures seen at the bottom of the martian ice caps. “It’s like super road salt,” Fisher explained.
Phoenix‘s measurements suggest that perchlorates makes up about 1 percent to 2 percent by weight of the surface dirt in Phoenix‘s landing area. Mars’ present polar caps are about 95 percent ice and 5 percent soil.
The perchlorates in that soil could either sink down through the ice cap or could be deposited on the surface as the ice sheets wax and wane every few million years.
“Wherever there’s an ice cap, there’s a tendency to build up whatever’s in it,” Fisher said.
This buildup of perchlorates would melt enough ice to eventually create a layer of brine below the ice cap on which the ice cap could slide.
Fisher has modeled the possible ice cap flow and said that it’s hard to make the ice move without such a sludge. At the conference, other scientists agreed that the sludge was a plausible mechanism for ice flow, though not all agreed that the ice actually did flow.
The best way to find evidence to support the idea would be to detect the sludge around the polar cap. Raymond Arvidson of the Washington University in St. Louis and his graduate student have already been using the Mars Reconnaissance Orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument to probe the surface around Phoenix‘s landing site.
“We did a pretty intense search,” Arvidson said. But so far CRISM has not found any perchlorates signatures.
“None. Zero. Nada. Zilch,” Arvidson said.
But Fisher points out that CRISM needs a concentration of at least 5 percent of the surface soil to see the perchlorates; the instrument also needs the compound to be hydrated. If it is desiccated or present at a lower percentage, the instrument cannot see it.
Even if there is not enough of the perchlorates to create this sludge, the compound could still interact with water on the red planet.
“It’s conceivably important in the water cycle,” Arvidson said.
Moving Martian water
Perchlorates salts may play a key role in regulating the flow of water between the atmosphere and surface on Mars because “these are compounds that suck up water,” Arvidson explained.
This is a particularly interesting characteristic in the northern plains where Phoenix landed that are now known to have a layer of water ice just a few centimeters below the surface.
In essence, you have “a significant amount of perchlorates in soil that’s sitting on top of ice,” Hecht explained. “It’s awfully hard to imagine that there aren’t mixtures” of perchlorates and water.
Where the concentrations work out, there would likely be areas where the ice and salt would form a brine.
But whether this would just result in a dampness of the soil or actual brine pools, “I wouldn’t begin to guess,” Hecht said.
Phoenix team member Nilton Renno of the University of Michigan presented a paper at the conference that proposed that a set of “little globules” attached to struts on Phoenix ‘s legs – as photographed by the lander’s robotic arm camera — might have been liquid water that was splashed up onto the spacecraft as it landed.
Hecht thinks that while this explanation is not out of the question, it is not the likeliest one, which he says is that water vapor released from exposed patches of ice stuck to the lander legs.
It is likely not the right point in Mars’ climate cycle for liquid brine to form at or near the surface, Hecht explained. During periods when Mars might have been just a few degrees warmer, perchlorates rinds could perhaps have melted water ice.
But Phoenix did observe that the martian dirt gets damp during the day, which may account for its clumpiness. The water would be released again into the atmosphere at night. “It’s a lot easier to make soil a little damp than to melt a chunk of ice,” Hecht said.
This dampness is not necessarily what would be called damp by Earth standards because of Mars’ inherent dryness.
“This is not wet enough to grow a chrysanthemum in, but it’s not like you baked it in an oven either,” Hecht said.
Impact on life
Perchlorates’ presence in the soil and interaction with water could also have implications for any potential martian microbes.
At high temperatures, perchlorates is “a very aggressive oxidant,” Hecht said, but since Mars is so cold, this is not likely to threaten life there. In fact, “perchlorates is quite benign with respect to microbes,” he said. It could actually act as an energy source for them.
“A lot of microbes eat perchlorates for lunch,” Hecht said. It’s “a wonderful PowerBar for microbes.”
But it is also a powerful desiccant that sucks up any water within its grasp, which would put it in competition for this substance that is essential to all life as we know it.
Bottom line: “There are aspects of perchlorates that are good for life; there are aspects of perchlorates that are bad for life,” Hecht added.
Where it comes from
Figuring out where the perchlorates comes from and how it is deposited on the martian surface is another puzzle the Phoenix scientists are trying to piece together.
Though perchlorates is found on Earth, it only shows up in the very arid places, such as the Atacama Desert wedged between the Pacific Ocean and the Andes, and the stratosphere. Though Mars is similarly dry, these Earth analogs only provide so much help in understanding the origin of perchlorates on Mars.
“We don’t completely understand it on Earth either,” Hecht said.
Hecht said that scientists know that most of the perchlorates originates in the atmosphere, falling from the sky as perchloric acid in precipitation. It then reacts with silicate minerals on the surface to become perchlorates.
But how the chlorine that reacts to form the perchloric acid gets injected into the atmosphere in the first place is not known.
In that Atacama Desert, sea spray that blows over the plains can provide the chlorine. Volcanoes can also contribute the element to the air when they erupt. But, of course, there are no seas on Mars right now, which leaves volcanic eruptions and reactions of chloride-containing minerals, either on the surface or aerosols – tiny particles suspended in the air.
Ozone is necessary to the reactions that form perchlorates on Earth. Whether or not this is true of Mars is not known, but if it is, it could mean that perchlorates would only be common at the colder, high latitudes of Mars, which see the only ozone concentrations high enough to fuel the reactions.
Planet-wide perchlorates hunt
The upcoming Mars Science Laboratory rover mission, set to launch in 2011, could help answer the question of whether the perchlorates is a peculiarity of the Phoenix landing site or a much more widespread constituent of the martian soil.
Intriguingly, the NASA Viking landers detected chlorine at their landing sites, which were closer to the martian equator than Phoenix‘s site. The landers could not determine the compound that chlorine came from, but it could have been perchlorates, Arvidson said.
“Now we have a specific compound to search out and find” when the large car-sized Mars Science Laboratory, which will also explore more mid-latitude sites, lands on the planet, he said.