NASA's Phoenix spacecraft has detected the presence of a chemically reactive salt in the Martian soil, a finding that if confirmed could make it less friendly to potential life than once believed.
Scientists previously reported that the soil near Mars' north pole was similar to backyard gardens on Earth where plants such as asparagus, green beans and turnips could grow.
But preliminary results from a second lab test found perchlorate, a highly oxidising salt, that would create a harsh environment.
The first test "suggested Earth-like soil. Further analysis has revealed un-Earthlike aspects of the soil chemistry," chief scientist Peter Smith of the University of Arizona in Tucson said in a statement on Monday.
On Earth, perchlorate is a natural and man made contaminant sometimes found in soil and groundwater. It is the main ingredient in solid rocket fuel and can be found in fireworks, pyrotechnics and other explosives.
It's unclear how perchlorate forms on Mars or how much there is of it. NASA is investigating whether the substance could have gotten there by contamination before launch. Phoenix used another fuel, hydrazine, to power its thrusters and land on the red planet on May 25.
Phoenix detected the salt through a chemistry experiment. The lander mixed soil with water brought from Earth into a teacup-size beaker and stirred it. Two dozen sensors inside the beaker detect the soil's pH and probe for traces of mineral nutrients.
The first test determined the soil was slightly alkaline and contained nutrients such as magnesium, sodium, potassium and chloride necessary for living things. The second test found the highly reactive perchlorate.
Scientists want to confirm their results because another Phoenix instrument that bakes and sniffs soil samples found no evidence of perchlorate during a run on Sunday.
Brown University geologist John Mustard, who has no role in the mission, said judgment about the soil's potential to support life should be reserved until all the data are in.
But at first glance, "it is a reactive compound. It's not usually considered an ingredient for life," Mustard said.
The latest soil finding comes less than a week after NASA extended Phoenix's three-month mission by another five weeks through the end of September.
Since arriving at Mars, the three-legged lander has impressed scientists by confirming that ice exists in the Martian arctic plains.
Its main task is to study whether the landing site could be a habitable zone for primitive life forms to emerge.