Editorial: Lakes on Mars

Gale crater

gale craterGale from orbit (Viking imagery) Image Credits: NASA JPL/MSSS/Arizona University/USGS & ESA/DLR

By Nathalie A. Cabrol, Director of the Carl Sagan Center

I thought I was going to share my thoughts with you on the new study that was just published by Science. I was busy typing. I hope you will enjoy this commentary, and better, that it will bring you some insight into that story.

Twenty years ago, the only orbital data that existed was the now old Viking imagery. Its average resolution was about 200 meters per pixel (about two football fields). We marveled at the details when Viking finally delivered some “high” resolution of selected regions, and here, I am talking about 40-70 meters per pixel. Today, we would consider these images to be context data – at best. The Viking topography was “precise” to within +/- 1 kilometer. You can then imagine how easy it was to make hydrogeological models…In some ways, it was, since the data left so much room for speculation. Limnology, the study of lakes, and sedimentology were then in the realm of this intellectual and unattainable Shambhala of planetary science.

Yet, if channels existed, water had to have ponded somewhere. This sounded like a reasonable hypothesis. But how to prove it when the only tool we had was only visual imagery with so little spatial resolution? There was one way: we all agreed that impact craters were big holes in the ground and, if some of them showed valley networks converging towards them, there was a good chance that water would have ponded in there. And then came the lava hypothesis, and then the CO2 hypothesis, and other exotic fluids like clathrates. Considering the physicochemical conditions on Mars, water was still, however, the most reasonable fluid to envision.

This is how the notion of impact crater lakes on Mars came to be. At the end of the 90s, we mapped them, and measured everything we could with what we had. It was not pretty but we were starting to explore and literally, barely scratching the surface. We were on the right track, though. We knew it, but science takes more than gut feelings. We had to convince the rest of the community. That took another decade.

The largest, and most promising, of these potential impact crater lakes were Gusev and Gale. Does this ring a bell? Spirit landed in Gusev in 2004. It took the team seven years to find evidence of the lake, but it found it and then some. Gusev ended up showcasing a number of ancient habitable environments that could have been favorable for life as we know it. Way back then, 3.7 billion years ago, the crater basin was the site of intense volcanic and hydrothermal activity. Water was ponding there and left carbonates behind, the first carbonate deposits found from the ground at Mars.

At about the same time, some serious action was also taking place 1,000 km to the East of Gusev, at Gale crater. We were convinced of that even from the low resolution Viking imagery, and some of us published what we thought was evidence of ancient runoffs and lakes. This is why we started to push for the Mars Exploration Rover missions (MER) to land in those very special places. MER was about understanding whether early Mars had provided suitable environments for life. If they were anything like terrestrial lakes, Gusev and Gale would have been some of the most favorable sites on Mars.

In the meantime, the Mars Global Surveyor and Mars Odyssey missions were finally providing the data we had been missing so badly before, including mineralogy and so much more. Studies were revisited, improved, and conclusions questioned. Gusev was selected as the landing site for the Spirit rover. Gale was its backup. It ranked very high but the engineers could not fit the landing ellipse safely in the crater basin. It was just a matter of time, and Gale finally had its day with the Mars Science Lab (MSL) mission. It was a good thing. The payload was perfectly suited for what was to come and for the type of site Gale had to offer.

Gale from the groundGale from the ground (Curiosity imagery)

From the day it landed, Curiosity kept stunning us, not only with the magnificent views but also with her findings in “layerland”. The design of her science payload was completely tuned towards understanding early habitability (environmental conditions for life, not the biological part), organics, geology, sedimentology, and limnology. But it took a relentless detective job by the science team to stay on the trail of lakes that had disappeared billions of years ago.

Sol after Sol, the science team followed stratigraphy, grain size, mineralogy, texture, composition, and more. They had to think through where to find the next evidence in the planning of their traverse. Once the pieces were there, they had to put the puzzle back together, and they did it, in an outstanding way.

The paper that was just published in Science by Grotzinger et al. is some of the most exquisite sedimentologic studies I have ever seen, period…and it is on Mars. That says a lot about how far we have come in planetary exploration. I can only commend the MSL team for bringing together orbital and ground data in such a brilliant fashion, and also doing literally the boot (wheel?) work of documenting their hypotheses as they went with the rover payload. This is what Curiosity was built for, and this is what they delivered in full with that study: Science and technology coming together, 150 million kilometers away.

Some of the processes the science team discusses can involve dry eolian and wet inter dunes. From experience, I would say, they most likely involve both. Lacustrine environments are complex and extremely sensitive to changes, and changes can be rapid in the type of watershed Gale represents. Therefore, it would not be surprising to see both processes playing key roles alternatively. 

The case for the presence of deltas is made very convincingly. The discussion about the age and the type of climate is, for me, the fascinating part. As I mentioned, lakes are fragile environments, and considered “sentinels of climate change”. There are examples of very long-lived lakes on Earth, but most of them are tectonic and glacial lakes. However, they remain the exception. Also, they are usually very deep, unlike the few meters suggested by the study at Gale. A shallow lake is harder to maintain over long periods of time and requires sustained groundwater circulation. Residual heat from the impact that formed Gale (which could have lasted over 10,000s years or more) could have helped that circulation. A frozen surface could also have delayed evaporation and sublimation processes.

Now, let's look at what the evidence says about habitability and the potential for life at Gale in this early environment. Clearly, in my fieldwork in the most arid places of our planet, I have seen puddles being colonized in 24 hours after a flood. The question is not if a lake, whether it is 100 years old or 10,000 years old (as suggested in the MSL study) is a good place for life. In this case, the question might be whether life was already around when the lakes formed. Obviously, this is a moot argument if life never started on Mars but let's be positive and consider it did.

If we take the terrestrial model for the chronology of life development, then, stratigraphy at Gale tells us that it is possible that life was already around when those lakes formed – it was on Earth. If that was the case (and that’s a big “if”), then Gale would have made a great habitat. It had water and most likely residual energy from the impact that formed the basin and that energy could have lasted over tens of thousands of years. Life would have found shelter in the water column and/or in the wet sediment.

On Earth, the type of environmental conditions the lake has to offer (light, water temperature, pH, water transparency, nutrients, others) decide what type of life will colonize…not if life will colonize. That will be for future missions on Mars to tell us if life developed on Mars. However, what this study has done in great details, and so have Spirit and Opportunity at Gusev and Meridiani, is to bring us a lot more information about the type of sedimentary rocks that were formed on early Mars, at which temperature, their mineralogy, composition, and the evolution of the environment around them, and how it impacted their own evolution. Why is this important? If life ever appeared on Mars, this is exactly the kind of information we need to understand where biosignatures might have been best preserved, and how to search for them with Mars 2020 and ExoMars. This is how each mission builds on the shoulders of the previous.