During its stint in the Martian Arctic, NASA's Phoenix Mars lander made an impressive array of measurements and discoveries that will help fine-tune scientists' understanding of the chemistry and environment of the red planet.
Perhaps no discovery was more surprising than the detection of an odd type of salt that Phoenix scientists think could have an important impact on the Martian water cycle and the planet's ability to support life.
In a set of papers presented last week 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 cap, lubricating them and allowing them to flow, as glaciers do on Earth; and how the salt even got there in the first place.
An unexpected detection
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 (where previously it was thought that Martian regolith was acidic); snow falling from the sky; and sticky dirt that clumps when scooped up.
But the detection of perchlorate, made by the lander's Microscopy, Electrochemistry and Conductivity Analyzer's (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 during a review of the mission after the lander lost power.
Once the detection was made, several Phoenix scientists began considering what perchlorate's 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 perchlorate, 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 perchlorate 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 necessary to explain the features. But at Mars' poles, it's far too cold for ice at the base of the cap to melt in the same way that it does on Earth.
Perchlorate can get around this problem because it can melt ice even at the frigid temperatures seen at the bottom of the ice caps.
"It's like super road salt," Fisher explained.
Phoenix's measurements suggest that perchlorate makes up about 1 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, which means that in 1,000 meters (3,280 feet) of ice, there could be roughly 0.5 meter of perchlorate.
The perchlorate 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 perchlorate 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 LPSC, 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. Arvidson 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 hasn't found any perchlorate 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 perchlorate; the instrument also needs the compound to be hydrated. If it is desiccated or present at a lower percentage, the instrument can't see it.
But even if there isn't enough of the perchlorate 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
Perchlorate salts may play a key role in regulating the flow of water between 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 inches below the surface.
In essence, you have "a significant amount of perchlorate in soil that's sitting on top of ice," Hecht explained. "It's awfully hard to imagine that there aren't mixtures" of perchlorate 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 presented a paper at the LPSC 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 isn't the likeliest one, which is that water vapor released from exposed patches of ice stuck to the lander legs.
It's likely not the right point in Mars' climate cycle (which changes as the planet's rotation axis wobbles) for liquid brine to form at or near the surface, Hecht explained. During periods when Mars might have just a few degrees warmer, perchlorate 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.
Though this dampness isn't 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
Perchlorate's presence in the soil and interaction with water could also have implications for any potential Martian microbes.
At high temperatures, perchlorate is "a very aggressive oxidant," Hecht said, but since Mars is so cold, this isn't likely to threaten life there. In fact, "perchlorate is quite benign with respect to microbe," he said. It could actually act as an energy source for them (perchlorates provide the boom to most fireworks and rocket propellants).
"A lot of microbes eat perchlorate for lunch," Hecht said. It's "a wonderful PowerBar for microbes."
But it's 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 was we know it.
Bottom line: "There are aspects of perchlorate that are good for life; there are aspects of perchlorate that are bad for life," Hecht added.
Where it comes from
Figuring out where the perchlorate comes from and how it is deposited on the Martian surface is another puzzle the Phoenix scientists are trying to piece together.
Though perchlorate 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 perchlorate on Mars.
"We don't completely understand it on Earth either," Hecht said.
Hecht said that scientists know that most of the perchlorate originates in the atmosphere, falling from the sky as perchloric acid in precipitation. It then reacts with silicate minerals on the surface to become perchlorate.
But how the chlorine that reacts to form the perchloric acid gets injected into the atmosphere in the first place isn't 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 perchlorate on Earth. Whether or not this is true of Mars isn't 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 perchlorate hunt
The upcoming Mars Science Laboratory (MSL) mission, set to launch in 2011, could help answer the question of whether the perchlorate is a peculiarity of the Phoenix site or a much more widespread constituent of the Martian regolith.
Intriguingly, the Viking landers detected chlorine at their landing sites, which were closer to the Martian equator than Phoenix's site. The landers couldn't determine what compound that chlorine came from, but it could have been perchlorate, Arvidson said.
"Now we have a specific compound to search out and find" when MSL, which will also explore more mid-latitude sites, lands on the planet, he said.
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