Salty water everywhere, nor any drop to drink!

In recent years, we have observed tantalizing evidence that Mars may presently have liquid water, which raises important questions about its habitability. Under present-day conditions, though, only salty liquids could exist on Mars. Could these be suitable environments for terrestrial organisms?

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Although our paper worked towards answering this question, our study actually started with a different goal. We started by investigating the role of brines in producing some of the flow features that had been recently observed on Mars. Of these, recurring slope lineae (RSL) were the first strong, persistent evidence that liquid water might be occurring on Mars.

RSL were observed to form seasonally during the spring and summer. They were seen mostly on equator-facing crater walls, which are warmer because their orientation allows them to receive more sunlight. RSL would emanate from bedrock as dark streaks, grow over time, and later fade away during autumn and winter. These properties strongly supported the idea that liquid water, most probably brines, could be part of their formation.

Warm-Season Flows on Slope in Newton Crater
Series of images showing active RSL in Newton Crater, Mars. Image credit: NASA/JPL-Caltech/Univ. of Arizona

In fact, the near certainty of liquid water’s role in the formation of some flow features on Mars became an unexpected challenge for the Mars Science Laboratory (MSL) rover. In 2016, dark streaks were observed along its planned path. Suddenly, the MSL operations team and NASA faced a difficult decision. On one hand, getting an up-close look at these active features would resolve a Martian mystery, but on the other hand, approaching them with the rover might contaminate the potentially wet streaks with terrestrial organisms, ruining our chance at answering a fundamental question, “Is there life beyond Earth?”. 

To consider these issues, NASA has a Planetary Protection Office, currently led by Dr. Lisa Pratt. The office coordinates for NASA the application of planetary protection policies, which are a set of internationally agreed upon practices that help protect planetary environments from contamination by human activity. These policies are developed by the Panel on Planetary Protection of the Committee on Space Research (COSPAR) and are informed by what we know about life on Earth, and what we know about the diverse environments in our Solar System. 

In 2010 and then in 2014, COSPAR and MEPAG (Mars Exploration Program Analysis Group for NASA), worked to define the conditions under which environments on Mars should undergo extra precaution during their exploration. These so-called Special Regions were defined as areas that could host liquid water that had a temperature above 250 K (-23°C) and water activity above 0.5. (When thinking about brines, water activity can be thought of as a measure of “saltiness” where a water activity of 1 is pure distilled water, 0 is pure salt, and typical sea water is 0.98). These limits are based on findings of what life can tolerate on Earth - that is to say the conditions known organisms can continue to metabolize and replicate.

Since their discovery in 2011, though, views on RSL and other dark slope streaks have changed. They are now thought to be dry flows, and so would not be considered “Special Regions”. However, there is still a real possibility of brine formation on present-day Mars. We even think we observed briny droplets on the struts of NASA’s Phoenix lander shortly after touchdown. This possibility has motivated many researchers to recreate Mars-like environments in the lab to constrain how brines could form and remain stable on Mars, and understand whether these liquids are suitable environments to life as we know it.

Phoenix and the Holy Cow
Picture of the struts of NASA's Phoenix lander taken using its Robotic Arm Camera showing the exposed icy patches (center), as well as potential briny droplets on the strut (top left). Image credit: Marco Di Lorenzo/Kenneth Kremer/NASA/JPL-Caltech/UA/Max Planck Inst./Spaceflight

So, our work changed direction in light of the new information we had. Not only did we want to better understand when, where, and for how long brines could exist on Mars, but we now also wanted to know whether the locations of these liquids could be considered Special Regions. Our team grew beyond our expertise on brines, both lab-based (Vincent Chevrier) and modeling (myself), to incorporate researchers on Mars climate (Alejandro Soto) and scientists on lander environmental instruments (Germán Martínez), so that we could better tackle this important question. 

In our work, we combined the simulated atmospheric conditions from a Mars weather model with what we have learned from experiments on brines and what we know from in-situ measurements on Mars to understand the stability and chemistry of potential brines. We even accounted for metastability, that is the fact that we have found brines to exist under conditions that they are not expected to be liquid. 

Our work shows that while brines may exist across the Martian surface, they never reach temperatures above 225 K (-48° C), far below the Special Regions temperature limit of 250 K. Thus, our work would suggest a reduced risk of contaminating Mars with terrestrial life; however, it does not completely take away the risk because we could still, in the future, discover life on Earth flourishing under extremely cold conditions.

The journey we took trying to answer one question but ending with answering another is not unexpected. Often in science you refine your study as you work based on new discoveries. In planetary science, new discoveries happen often as we continue to explore our Solar System. The hope is that inevitably your research leads to a better understanding of how the Universe works. 

My hope is that our work helps inform planetary protection policies and our exploration of Mars, as well as motivates other follow-on work. Maybe we’ll find life surviving even more extreme conditions. Maybe we’ll find new (meta)stable brines. I think there’s really only one rule in science, and that is, prepare to be surprised!

Edgard G. Rivera-Valentín

Planetary Scientist, Lunar and Planetary Institute (USRA)

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