An instrument aboard NASA's Phoenix Mars lander has detected what may be perchlorate in the soil of the red planet's northern latitudes, an unexpected, still tentative discovery that has touched off a flurry of speculation about whether Mars is now, or has ever been, habitable. While highly oxidizing perchlorates can be toxic, some terrestrial plants thrive on the compound and its presence on Mars would not confirm or challenge the planet's overall habitability, either now or in the distant past, scientists said today.

"We have substantial evidence that our soil samples contain perchlorate, that's chlorine with four oxygen (atoms)," said Peter Smith, the manager of the Phoenix lander project at the University of Arizona. "On the Earth, perchlorates are found in the Atacama Desert in Chile in association with nitrates that are mined for fertilizer. The desert is a hyper-arid environment that rarely sees rain and has no vascular plants. It is often used by scientists, as a matter of fact, as a martian analogue site.

"These compounds are quite stable in soil and water and do not destroy organic materials under normal circumstances. In fact, there are species of perchlorate-reducing microbes that live on the energy provided by this oxidant. Therefore, this is an important piece in the puzzle as we attempt to determine if habitable conditions exist for microbes on Mars. In itself, it is neither good nor bad for life."

Either way, Smith said, the results have not yet been confirmed by a second instrument aboard the Phoenix lander. And while perchlorate is defined as a toxic compound "with powerful oxidizing properties," it is too early to draw any conclusions about the habitability of the landing zone or of Mars in general based on data from Phoenix.

"How this perchlorate in the soil affects habitability is a complex question that we certainly don't have the final answer on," Smith said. "It is a very stable material, it's not likely to tear apart organic materials unless you heat it to very high temperatures. It really doesn't limit us in our search for habitability in this icy soil. And if we were lucky enough to see some organic signatures (in future soil samples), it would not be a huge surprise to think they were co-existing with perchlorate."

The Phoenix lander is not equipped with instruments capable of direct detection of life. But by analyzing the chemical makeup of soil near now-confirmed ice deposits, scientists hope to draw conclusions about the past and present habitability of the red planet. That data, in turn, will help NASA planners select landing sites and experiments for future missions.

During a news conference last week, Phoenix scientists reported confirmation that ice has been found in the soil near the lander. That was not a surprise given earlier photographs from the lander that showed what appeared to be ice sublimating and vanishing in sunlight.

But in a subsequent story, Aviation Week & Space Technology magazine, quoting sources, reported the White House Presidential Science Advisor's office had been briefed on a new discovery concerning the habitability of Mars. The story did not say whether those results supported or challenged habitability.

A NASA spokesman said today the agency had not briefed the White House at that time. But NASA apparently received enough enquiries from the news media in the wake of the Aviation Week story to warrant a follow-up teleconference today to discuss the issue.

"We are here today to announce a non-announcement," said Michael Meyer, chief scientist of the Mars Exploration Program at NASA headquarters. "More experiments and time are needed to resolve the interesting findings from the soil chemistry."

Smith described the teleconference as "an unusual step and break with scientific tradition."

"Today we're opening a window into the project to allow the public to see our scientific process in action," he said. "Tradition has been bypassed because of the extreme interest that has been exhibited toward Phoenix ... in our search for a habitable environment on the northern plains on Mars.

"Rather than the speculation that has become rampant on the web, I promise you insight into the investigations that are underway inside the operations center. However, I must caution you that we have not completed our process and ... we're about halfway through the data collection phase."

Asked repeatedly about the potential implications of perchlorates if, in fact, the presence of the compound in one or more forms is confirmed, Phoenix officials said they simply did not know.

"I can say they could potentially keep a lot of graduate students busy for a long period fo time," said Michael Hecht, science lead for the Microscopy, Electrochemistry and Conductivity Analyzer, or MECA, instrument.

Over the past few weeks, two soil samples have been analyzed by MECA's wet chemistry laboratory. Results indicate the possible presence of perchlorate in the soil. Another instrument, called the Thermal and Evolved-Gas Analyzer, or TEGA, uses small ovens to bake out and "sniff" soil constituents. Scientists are still analyzing data from TEGA to find out if it, too, detected perchlorate.

Along with analyzing data from the MECA and TEGA instruments, the Phoenix team also is working to make sure the spacecraft did not somehow introduce perchlorate, a component of solid rocket propellant, into the martian environment.

"So why all the excitement because our chlorine atoms are surrounded by a few more oxygen atoms than expected?" asked Hecht. "Well, in part, because as scientists we're fascinated by things like that. That's why we had trouble getting dates in high school. But mostly it's because different types of perchlorate salts have interesting properties that may bear on the way things work on Mars if - and that's a really big if - the results from our little teaspoon full of soil are representative of the whole planet or even significant portions of the whole planet."

He said perchlorates would affect "many things, from how land forms to whether in the distant pass there ever was precipitation."

"There are widespread implications and they may all turn out to be nothing, or they may turn out to be very important," Hecht said. "It's just that this has opened up a whole new research chapter for us. We could talk about any one of those potential applications at length, I think we are not ready to do so yet."