The discovery of an unexpected chemical in Martian soil—one that is considered hazardous here on Earth—says little by itself about the possibility of life on Mars, NASA researchers announced this afternoon.

They confirmed that Phoenix Mars Lander had found evidence in soil samples of perchlorate (ClO4), a highly reactive chemical that can occur naturally on this planet in areas such as Chile's Atacama Desert, an extremely arid environment that researchers have studied as a proxy for Mars.

Researchers once believed that life could not survive the Atacama's harsh conditions, suggesting that perchlorate and life do not mix. But in fact some microbes consume it for fuel and some plants collect it the way they would nitrates, for the oxygen it stores, researchers said.

Perchlorate is toxic to people only in the sense that it can disrupt the production of thyroid hormone, an important growth hormone needed by babies in the womb for normal development.

"We have substantial evidence that our soil samples contain perchlorate," Peter Smith, lead Phoenix investigator at the University of Arizona Lunar and Planetary Laboratory (LPL) in Tucson, said during a press conference today. "In itself it is neither good nor bad for life."

The perchlorate story began last week, when reported that NASA had briefed the White House on findings related to the habitability of Mars that the agency had not disclosed in recent press briefings. reported yesterday that White House Office of Science & Technology Policy, to which was referring, denied receiving any briefing.

NASA issued a statement yesterday confirming that Phoenix's wet chemistry laboratory (WCL), part of the Microscopy, Electrochemistry and Conductivity Analyzer (MECA) instrument, had discovered perchlorate in two soil samples analyzed in June and July.

WCL dissolves soil in water samples brought from Earth and then uses a series of electrodes to "taste" for ions (charged atoms) of chlorine, sodium, potassium and other elements.

Phoenix researchers said they did not announce the possible perchlorate finding because they were still trying to reconcile it with results from the Thermal and Evolved-Gas Analyzer (TEGA), which bakes samples and "sniffs" the vapors that come off for different elements.

In one TEGA test, soil released oxygen at a high temperature, consistent with perchlorate, which exists as a salt joined with another element such as magnesium or sodium. Some of these salts would also give off chlorine; a second TEGA study, however, did not detect that element. That rules out perchlorate salts of magnesium, calcium or iron, said William Boynton, lead investigator for TEGA at LPL.

The soil probably contains a mix of salts, added Michael Hecht, lead investigator for WCL at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Phoenix researchers said they also wanted to rule out the possibility that perchlorate had somehow infiltrated the experiment prior to landing on Mars. The third stage of the Delta 2 rocket that gave Phoenix its final boost to Mars burned perchlorate-based solid fuel.

Researchers said it seemed unlikely that the perchlorate had contaminated the sample in that way, given that the water Phoenix carried with it, which was used to dissolve soil in the WCL, did not show signs of the compound.

Smith said the finding caught him by surprise: "Nobody had ever mentioned the possibility of perchlorate in our soil to me."