"We know that microbes can exist quite happily in oxidizing conditions," said Phoenix scientist Richard Quinn of the NASA Ames Research Center. "The story possibly could turn out to be the same for Mars. We don't know yet."
The surprising find comes after scientists previously reported that the soil near Mars' north pole was Earth-like where plants such as asparagus, green beans and turnips could thrive. The presence of perchlorate, if confirmed, would appear to make the soil more exotic than previously believed.
But scientists insisted that has no bearing on the red planet's habitability.
"In itself, it is neither good nor bad for life," chief scientist Peter Smith of the University of Arizona in Tucson said of the chemical.
Some researchers who have no role in the $420 million mission were less enthusiastic.
"Perchlorate is not a particularly nice thing to find in the soil," said astrobiologist Kenneth Nealson of the University of Southern California. "No one hunting for life would be happy to see it in any sort of abundance."
Although some microbes on Earth thrive on perchlorate, "it is not a molecule of choice for most life," Nealson said.
Phoenix landed in the Martian arctic plains on May 25 to study whether the region could support life, past or present. It confirmed the presence of ice at the landing site, but it has yet to uncover organic, or carbon-based, building blocks of life.
The spacecraft detected perchlorate in two soil samples taken from different shallow trenches in June and July using its onboard chemistry lab. Scientists are working to confirm the signal because another Phoenix instrument, also capable of sniffing out the chemical, failed to turn up any evidence in a test last week.
It's unknown whether the detected perchlorate occurs naturally on Mars. NASA is investigating whether the contaminant could have hitchhiked aboard Phoenix during launch preparations. It's unlikely the chemical leaked from the spacecraft's thrusters since they carried hydrazine fuel. Engineers said there's a remote possibility that Phoenix may have been contaminated by the rocket that launched it.
Perchlorate is used in solid rocket fuel, fireworks, pyrotechnics and other explosives. In the United States, perchlorate contamination has been found in the waterways of at least 25 states, according to the federal Environmental Protection Agency. The toxin can interfere with thyroid function and poses developmental health risks, particularly to fetuses.
While oxidizers can be harmful to living things, perchlorate is relatively stable in soil and would not react with organics unless triggered, said geochemist David Parker of the University of California, Riverside.
The last time NASA searched for organics was during the twin Viking missions in the 1970s. The spacecraft turned up empty. Some scientists theorized that an oxidant, perhaps peroxide, in the Martian soil may be responsible for breaking down carbon compounds if they had existed.
