When news “broke” about the discovery of liquid water gushing over the slopes of Mars this week, the world went Red Planet crazy… again. Water (in a liquid state) has been confirmed to exist on the parched planet’s surface, turning a once desert world into a veritable oasis. That’s what the headlines led us to believe, at least.

PHOTOS: When Liquid Water Gushes on Mars

Of course, this isn’t the first time water has been discovered on Mars. We know that water is laced within Mars regolith; we know there’s water locked in Mars’ ice caps and below the surface; we know that Mars once possessed rivers, lakes and even seas. The last few years of discoveries by our Mars orbiter, lander and rover armada have revolutionized our understanding of Earth’s smaller neighbor. But this time NASA’s Mars Reconnaissance Orbiter has made comprehensive measurements of what appear to be seasonal flows of liquid water on a global scale on modern Mars.

And this could be the most exciting Mars water discovery yet. But it could also be the most frightening, making us wish liquid water didn’t exist at all.

Follow the Water

Returning to Earth for a moment, our planet is the only place in the universe where we know life exists. Earth occupies a very privileged orbit around the sun, known as the “habitable zone,” where the distance from our nearest star allows water to persist in a liquid state. And this is key for Earth Brand™ life; all life as we know it uses liquid water in its metabolism in some way. Where there’s water, there’s life.

But does this logic hold for other worlds in the universe? Does it hold for other planets and moons in our solar system? Does it hold for Mars?

Well, until recently, the only evidence for water on Mars has been in a frozen or gaseous state. The planet’s atmosphere is too cold and too thin to sustain liquid water (on the surface at least). This wasn’t always the case — Mars was a blue planet before Earth in the early evolution of our solar system; it once had a thick atmosphere, but in its ancient past something happened to Mars that likely “switched off” its internal dynamo that provided a global magnetic field. Without a strong magnetosphere, Mars shed its atmosphere under the constant erosion of the solar wind, sending the planet into a deep, dry freeze.

The Martian: Science vs. Fiction

Water on the surface froze, sublimed (passing from ice to vapor without turning into a liquid) and the vapor was then lost to space. But a huge supply of water was likely locked underground as ice in permafrost-like zones and held in the icecaps. Until recently, it was assumed that’s all there was in the way of water — frozen artifacts of a once wet world. But in 2011, MRO scientists announced the discovery of strange seasonal dark tracks appearing on warming Mars slopes in the southern hemisphere. Although circumstantial, planetary scientists likened these “recurring slope lineae” (or RSL) with recent outflows of liquid water on the Martian surface.

This week -- coincidentally in time for the release of the Matt Damon movie "The Martian" on Friday -- their hunch was confirmed. Using a spectrometer on the MRO, the signature of hydrated salts was detected in these dark RSL lanes, meaning RSLs are indeed formed by liquid water — but the only reason this water is held in a liquid state is because they are brines — or water saturated in perchlorate salts.

These are exciting times; there really is liquid water flowing on a current-day Mars — but the very fact that it’s in a liquid state is troubling. In fact, it could be deadly.

Mars Antifreeze

It’s hard to see beyond the hype that liquid water on Mars could be a bad thing, but for hypothetical future astronauts, you really wouldn’t want to drink the wet stuff straight from the source.

In the 1970s when NASA sent its two Viking landers to explore Mars, they did so boldly. These missions were sent to specifically search for life. Experiments were carried out by these early robots on the Martian soil and one experiment, which was designed to seek the metabolic signature of Mars bacteria, actually tested positive. After adding a small quantity of nutrients to a sample of Mars regolith, a sudden burst of gas was detected, a reaction that hinted at the presence of microbial life fizzing away like an antacid in water. However, the experiment was declared a dud when no corresponding organic chemistry was detected in the sample.

ANALYSIS: Did We Meet Martians 36 Years Ago?

A lesson to come from the Viking Program was that NASA shouldn’t look directly for life on Mars, as the proposition is a lot harder (and more difficult to get political backing or funding when previous attempts have failed) than thought. Perhaps future missions should look for evidence for past and present “habitable environments,” a mentality that persists today.

But in 2008, NASA’s Phoenix Mars arctic lander made the breakthrough discovery of perchlorates on the surface — a highly toxic type of chemical that is used in a range of industrial processes, including being a key oxidizer in rocket fuel. Interestingly, like sprinkling salt on your path during cold weather to lower the freezing point of water (thereby avoiding icy surfaces), perchlorates are a powerful form of salt, lowering the freezing point of water in Mars’ frigid environment — it’s a naturally-occurring antifreeze. Phoenix complemented its perchlorate discovery with fascinating images of possible droplets of suspected perchlorate-rich brines forming on its landing struts.

Most recently, NASA’s Mars rover Curiosity confirmed Phoenix’s findings, experimenting on soil, in an equatorial region (inside Gale Crater) also abundant in perchlorates.

The Dark (and Light) Side of Perchlorates

Being a powerful oxidizer, when heated, it is known that perchlorates destroy organic compounds. As the Red Planet is covered in perchlorates and organic compounds have since been confirmed by Curiosity, were the Vikings’ early life-detection experiments duds at all? Not detecting organics was the key reasoning why Viking didn’t find life, but in the 1970s we had little idea that perchlorates were there, possibly sterilizing the samples. It’s an interesting thought.

Perchlorates are highly toxic to humans — this family of chemicals is known to trigger thyroid problems when exposed to very small doses. It's also a possible carcinogen. But Mars is perchlorate rich, possibly overshadowing all other dangers our future Mars astronauts face, even the radiation threat. It would be the mother of all decontamination efforts. Mars is, for the most part, a dry world, dominated by dust. And we now know that the dust is filled with perchlorates that is known to cause life-threatening conditions. With the nearest hospital millions of miles away, we wouldn’t want future Mars explorers breathing that stuff in on a daily basis.

ANALYSIS: The Hills Have Ice… on Mars, That Is

Of course, this doesn’t preclude our future on Mars (in fact, it is lucky that our robot explorers have detected the toxic compound, so we can better develop the technologies to deal with it), it just makes the proposition that little bit harder. Although these chemicals are scary, there’s a flip-side to the perchlorate coin of risk: some hardy microbes on Earth use perchlorates as an energy source and that little fact might be enough for us to send a human Mars expedition to the Red Planet, despite the dangers.

As highlighted by Mars scientists on Monday’s much-hyped press conference, it would be highly unlikely that we would send a robot to investigate. These slopes are extreme and landing any unmanned mission in these regions would be nigh-on impossible (at least with current technology). Also, astrobiologists are keenly aware that these outflow regions have just became prime locations for existing life on Mars, so contamination could be a problem — hardy microbes could hitch a ride on robots and could trigger false positives or even contaminate these slopes. One has to wonder about the likelihood of any hitchhiking bacteria setting up home in the toxic brines of Mars, but it would be a shame if the first life we detect on Mars actually originated from Earth.

NEWS: This Scoop of Mars Soil is Two Percent Water

Knowing how hard it is to find life on Mars, investigating the sources of RSLs and the possibility of life will likely be the prime incentive for NASA’s planned manned expedition to Mars in the late 2020s or early 2030s.

Although the source of RSLs is not currently known, their life-giving potential is clear, let’s find out once and for all whether life exists on Mars and develop the technologies to live on Mars. It may be toxic, but it’s about time we follow the water to the slopes of Mars.