Lack of carbonates on the Martian surface once sparked theories that ancient Mars had acidic oceans.
But the Phoenix mission has found so much evidence of carbonate clays that Michael Hecht, a Phoenix scientist based at NASA's Jet Propulsion Laboratory (JPL), thinks carbonates account for not less than one percent of the Martian surface.
"We really have enough information to begin rewriting the book of Martian geochemistry," he said.
And the presence of carbonates suggests that water was a dominant force in Mars's early chemistry. (Related: "NASA Images Add a Billion Years to Mars's Wet Period?" [September 26, 2008].)
Among other implications, Hecht said, the finding means that water likely played a role in the regulation of surface acidity, which researchers had previously attributed to other chemicals.
William Boynton, of the University of Arizona, said the mission has indicated, but not confirmed, the presence of silicates, minerals rich in silicon and oxygen that are also typically associated with liquid water.
In addition, Phoenix found more evidence for perchlorates, oxidizing salts that had been spotted previously and were thought to be possible contaminants brought with the lander from Earth.
Perchlorates are used in rocket fuels and explosives, and they lower the freezing point of water.
If the chemicals occurred with water on Mars, perchlorates may have collected in low-lying areas and formed briny deposits, JPL's Hecht said. They may also serve as an energy source for yet-to-be-discovered Martian microbes.
Still, one mission objective continues to elude the research teams: detection of organics—compounds mostly made of carbon and hydrogen—on the Martian surface. Further analysis could reveal the presence of organics, but the researchers aren't optimistic.
Phoenix, which landed on Mars on May 25, officially ended its prime mission on August 26 and is now in extended operations.
Barry Goldstein, the Phoenix project manager at JPL, said the lander remains strong.
"Every day it's waking up on command and communicating with Odyssey," he said, referring to the NASA Mars orbiter. But with each passing Martian day, or sol, the sun is staying below the horizon longer.
"As Martian winter approaches, we lose sunlight," Goldstein said.
Eventually the temperature will approach -238 degrees Fahrenheit (-150 degrees Celsius), and Phoenix is not expected to produce enough solar power at that point to keep its instruments from freezing.
"Before the end of October, there won't be enough energy to keep using the robotic arm," Goldstein said.
"By the time we get to April, there will be no sun up there. That hurts the vehicle."
Analysis of data Phoenix has already collected will be ongoing, the researchers said, including the search for the hoped-for organics.
If Phoenix does survive the winter encased in ice, it is programmed with a "Lazarus mode" that will prompt the craft to start collecting energy and eventually ping mission control.
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