Mars Express water discovery reopens intriguing questions for future Martian Exploration

by Chris Gebhardt

For the first time in human exploration of Mars, the presence of a stable, subterranean body of salty (briny) water has been strongly suggested by data returned by the European Space Agency’s Mars Express orbiter.  Using measurements taken by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument, a team of Italian researches announced this week the first (probable) discovery of a stable body of liquid water on Mars.  But with this discovery comes a host of questions regarding future exploration of the Red Planet.

A stable, large body of liquid water on Mars:

It’s the word “stable” that is the historic and big-ticket item here.

Prior to this week, liquid water on Mars’ surface had only been observed for very brief periods of time.  Briny water droplets were briefly observed on NASA’s Phoenix Polar Lander and orbital terrain images have revealed new surface features carved by flowing water that quickly disappeared.

But never before had liquid water in large, stable quantities been found on – or beneath – Mars.

Nonetheless, the presence of liquid water underneath the Martian polar caps has long been hypothesized.  In fact, it was first suggested as “possible” over 30 years ago, but conclusive data to either prove or disprove the hypothesis remained elusive.

Mars Express in orbit of Mars. (Credit: ESA)

Enter the European Space Agency’s (ESA’s) Mars Express orbiter, which has been at Mars since 25 December 2003.

Since then, it has been dutifully studying the Martian terrain, including its subsurface mysteries, with the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS, instrument – specifically, a region underneath Mars’ Planum Australe (south polar region) known as the South Polar Layered Deposits.

The evidence returned by MARSIS of a persistent liquid water deposit in this region came via a three and a half Earth-year observational period from May 2012 to December 2015, during which MARSIS conducted 29 different radar scans of a 200 km-wide (124 mile-wide), flat area of the Planum Australe.

The radar waves sent from MARSIS penetrated the top layers of the Martian terrain, extending a few kilometers underneath the surface.  Those waves were then reflected back toward MARSIS when they encountered various rocks, sediments, and – in this case – liquid water.

The MARSIS findings are reported by Dr. R. Orosie et al. in the article “Radar evidence of subglacial liquid water on Mars” and was published in the journal Science can be read here.

(A) Shaded relief map of Planum Australe, Mars, south of 75°S latitude. The black square outlines the study area. (B) Mosaic produced using infrared observations by the THEMIS (Thermal Emission Imaging System) camera, corresponding to the black square in (A). The red line marks the ground track of orbit 10,737 of Mars Express. The area consists mostly of featureless plains, except for a few large asymmetric polar scarps near the bottom right of (B). (Credit: R. Orosei et al. Science 2018)

The specific discovery was made by measuring what’s known as the dielectric permittivity (or the strength of the reflected radio waves) and via routine (i.e., stable) observations of a highly reflective 20 km-wide (12 mile-wide), 1.5 km (0.9 miles) deep deposit of water over the 3.5 year observational period.

For this discovery, the dielectric permittivity provided the information necessary to understand that it was liquid water causing the strong reflections seen in the MARSIS data and not something else.

In fact, other potential explanations for the high permittivity returns were explored but ultimately rejected based on sound evidence.

Moreover, the extremely strong evidence in the MARSIS data that this is, in fact, stable liquid water also points to this deposit being extremely salty because temperatures in the region are understood to be -68°C (-90°F) – far below the freezing point of freshwater.

Therefore, the water deposit would have to be salty, as the saltier the water, the lower the freezing point is.

On Mars, NASA’s Phoenix Polar Lander has found calcium, magnesium, and sodium perchlorate (salt) in abundance under the northern Martian terrain, and the U.S. space agency’s Curiosity rover operating in Mars’ equatorial region has found perchlorate salt there, as well.

Therefore, it follows that these elements are also present in abundant quantities in the Planum Australe.  And in fact, the presence of these salts would reduce the freezing point of any water present to -74°C (-110°F) – more than 6°C (20°F) colder than the temperature at the location of the discovery.

However, whether the liquid water evidenced by MARSIS is in the form of sludge (salt water mixed with soils – e.g., mud) or if it is more pure, a salt pool resting atop surface soils, will require follow-up investigations.

Moreover, while MARSIS is limited in what it can detect in terms of the size of subterranean liquid water deposits (it can’t detect very small bodies of water) – there is no evidence in the data returned that would lead scientists to conclude that this is an isolated phenomenon.

In other words, while we still have a great deal to learn about Mars’ water cycle (hydrologic cycle), this discovery points to the possibility that other stable (persistent) salt water deposits exist just underneath Mars’ surface.

Color-coded map of radar wave reflections from MARSSI. The large blue area outlined in black corresponds to the main bright area. The value at each point is the median of all radar footprints crossing that point. (Credit: R. Orosei et al. Science 2018)

And this could have significant implications for future human missions to Mars as well as the ongoing search to discover if Mars currently hosts life.

Water is a huge part of any Mars colonization and human exploration mission, and the prospect of potentially having liquid water already on the Red Planet and available for use (after desalinization) would be a massive boon to such human initiatives – especially given that all currently-on-the-table plans for human Mars exploration place a heavy reliance on near complete recycling of water no matter its form or origin.

This type of water recycling is already in use aboard the International Space Station, where urine is converted into clean drinking water for the crew via the Urine Processing Assembly.

Even more so, this discovery is tantalizing in that a stable, salt water body is precisely where life first developed and was nurtured on Earth.

While Mars is quite different – not the least in its radiation environment – from Earth, the building blocks of life have been found by Curiosity at Mount Sharp in areas that used to flow with liquid water.

Curiosity finds organics on Mars. This chart shows comparisons between the amount of organic chemical chlorobenzene detected in the “Cumberland” rock sample by NASA’s Curiosity Mars rover over time. (Credit: NASA/JPL-Caltech)

And here on Earth, scientists have found simple organisms that live in incredibly salty water (much saltier than Earth’s oceans) that exists in a liquid form at temperatures down to -50°C (-58°F) – like the Don Juan Pond, a hypersaline lake in Antarctica.

Therefore, if life did develop on Mars billions of years ago in the presence of liquid water, it is possible that despite the radiation environment of present day Mars that life could still exist on the Red Planet in a salt water pool/deposit like the one evidenced in the MARSIS data.

If (and that’s a big “if”) life were to be discovered in a subterranean Martian salt water lake (or lakes), it would raise interesting ethical and biological questions for human exploration and colonization of our second-closest planetary neighbor.

One of the tenets of interplanetary exploration is to prevent the contamination of another world with microbes and bacteria from Earth – and vise versa.

International treaties currently set the allowable levels of microbial contamination on robotic explorers sent to Mars – and those cleanliness standards are different depending on where the robot is set to land on Mars.

Warmer, wetter areas that have a greater chance of supporting present-day life on Mars have stricter standards.  And to date, only NASA’s Viking landers have met those higher standards.

But we can decontaminate a robot.  We can’t decontaminate a human because we need the host of microbes and bacteria that live on and in us to survive – microbes and bacteria that could potentially overrun and destroy indigenous Martian life.

So if we were to “follow the water” and find conclusive proof that life – indigenous Martian life – exists, what, then, are the implications for future exploration and human colonization initiatives already in work?

On Earth, where there is water, there is life.  If that is true on Mars, then any human presence on the Red Planet could devastate the life that exists there already just based on the simple fact that humans would have to bring microbes and bacteria with us to Mars.

And if we were to definitively prove that there is life on Mars before we send humans to the planet, would we then have a moral and ethical obligation to protect that life – to let if thrive on its own without potentially wiping it into extinction by Earth microbes and bacteria that would greatly infect a Mars ecosystem – however primitive – as soon as humans stepped foot on the Red Planet?

NASA’s Mars 2020 rover will specifically and directly search for signs of past life on Mars. (Credit: NASA/JPL-Caltech)

In other words, if we find life on Mars, would we have an ethical and moral obligation to never set foot on the planet and let that life follow its natural course completely undisturbed by human and/or continued robotic presence?

Human history is one of exploration and discovery – of wanting to know what’s beyond the horizon.  As European explorers pushed across the oceans, they brought with them microbes and bacteria that indigenous life in the Americas had no biological immunity to.  And that native life was greatly – and negatively – affected by that interaction.

At first, it was an accident.  But one we are now acutely aware of.

So with this knowledge, and if we found life on Mars, what should our discussions be toward colonization of the Red Planet?

Would it be a game changing find in that we’d stop and second-guess our desire to live on another world?  Would we place an equal value on native Martian life – however simple – as we do our own?

Artist’s conception of a potential future plan to terraform Mars. (Credit: SpaceX)

Or would we decide it was OK to contaminate (and potentially make extinct) any indigenous Martian life because of the security of survival a dual planet existence would provide humanity?

Human exploration is certainly filled with examples where the latter decision (in those cases, multi-continent inhabitation) was the one taken.

More so, if we do find life on Mars in or around salt water deposits like the MARSIS data indicates is present, we would need to make sure it’s not dangerous or deadly to humans before sending dozens or hundreds of people to the planet.

But also just as important: what if we don’t find conclusive evidence for present-day life on Mars – or the sterility of Mars (something looking less and less likely but is still, nonetheless, a possibility) – before sending humans?  

Artist’s render of a potential future Mars colony established by SpaceX. (Credit: SpaceX)

In other words, what if the possibility of present-day life exists but isn’t proven by the time humans get there?  With the growing evidence of past and current Mars habitability potential, should we not take every precaution possible to protect any life there even if we haven’t found it by the time we get there?

After the painstaking care to prevent Earth microbial contamination of Mars with our robotic explorers, this type of contamination would be inevitable as soon as humans reach the Red Planet.

Presently, there are no answers to these questions.  And these answers will certainly not be easy to arrive at.

But they are necessary questions for a conversation that needs to happen in earnest now that convincing evidence for a stable salt water deposit has been found – especially given SpaceX’s plans to send the first wave of human colonists to Mars within a decade (a mission that, regardless of our collective answer to the above questions, will be the most significant achievement in human history).

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