Ghost rivers, hidden lakes: The long search for water on Mars

The quest to send humans to Mars is on: US President Donald Trump talked about it in his inauguration speech this year. Such an epic endeavor could help to answer fundamental questions about the Red Planet, including the biggest question of all: Did Mars once host life — and does it still?

Central to those questions is the status of liquid water — the stuff of life — on the planet. It is now indisputable that there was once water flowing on Mars, but how much, and even why, is a matter of debate. More contentious still is whether liquid water may lurk somewhere on Mars even today.

Bruce Jakosky, a planetary scientist at the University of Colorado, Boulder, has studied Mars from its deep interior to its surface for nearly 50 years: He helped with the 1976 Viking mission as an undergraduate student and has been involved with a number of other NASA Mars projects since. Knowable Magazine spoke with him about the hunt for water on Mars, a topic he has written about for the Annual Review of Earth and Planetary Sciences.

This interview has been edited for length and clarity.

How has the thinking about water on Mars changed over time?

Around the turn of the last century, astronomer Percival Lowell did an analysis that suggested there were canals on Mars that must have been created by intelligent beings. He made the front page of the New York Times talking not about whether there were canals and whether there were intelligent beings, but what they might be like, and how might we communicate with them. It was a very different time.

His ideas were never accepted by the astronomy community; it was always thought to be a trick of the eye that connected things and made it look like there were straight lines on Mars. When we finally sent spacecraft there, we didn’t see any features that aligned with his map of canals.

Starting in the 1940s, astronomers were able to make measurements that began to tell us about the actual nature of the Mars environment. When the first spacecraft flew by Mars in 1965, we determined that the atmospheric pressure was only around half a dozen millibars, and the average temperature was 50 degrees Celsius below freezing. That demonstrated that liquid water could not be stable; if I put a bucket of water out there, it would take a while, but it would freeze or evaporate.

Those were the last nails in the coffin for the idea that it could still be a warm, wet planet. But that didn’t kill the prospect of Martian life.

Mars missions have not found alien-built canals. But they have found plenty of other features that look like they were carved by liquid water. Just recently there were headlines saying that images taken by the Curiosity rover in 2022 show ripples from ancient, ice-free lakes; more recently there was news about signs of coastal sediments from an ancient ocean.

The evidence for liquid water around 4.3 billion to 3.5 billion years ago is now absolutely compelling. We’ve seen abundant evidence for ancient lakes within impact craters — I think they number in the hundreds now. The most compelling evidence is in Jezero crater: Where water entered the lake, it deposited a delta, a fan of sediment that had been carried by the water.

We also see evidence for mountaintop glaciers and valley networks that look like river tributary systems. And we see evidence for water having blurped up from within the crust in catastrophic flood channels. They released a lot of water. There are also minerals on Mars that would have needed water to form.

What was it like on Mars back then, 3 billion or 4 billion years ago?

Temperatures must have been warm enough to allow liquid water. But we don’t know exactly how warm. And we don’t know why it was warmer. But other than that, we really have a handle on it (laughs).

We certainly need a greenhouse gas to warm a planet, and carbon dioxide is a great greenhouse gas, so we think there must have been more carbon dioxide back then. But the Sun was 30 percent dimmer 4 billion years ago, and in our computer models you can’t put enough carbon dioxide in the atmosphere to raise the temperature enough. So you need another greenhouse gas on top of that to consider in the simulation. We are at the stage of “Let’s pick a gas and ask how much we need, and then ask, ‘Is it possible?’”

Also, the tilt of the Mars polar axis changes over time. What did that do to the climate? We don’t know.

How much water has there been on Mars, in total?

My 2024 paper calculates that if you took all the water that was ever present on Mars and spread it out, you could coat the planet in a uniform layer somewhere between 380 meters to 1,970 meters (0.2 to 1.2 miles) thick. On Earth, for comparison, the same kind of inventory would be a layer 1,400 meters thick. This isn’t a totally fair comparison: For Mars, that’s all the water from all time added up together, it wasn’t all on the surface at the same time. On Earth, most of it is in the ocean still today.

Long ago, Mars was much wetter than it is today. But even at its wettest, the Martian land was probably very dry: In these valley networks with runoff channels, the number of channels per square kilometer is similar to some of the driest areas on Earth, like the Sahara.

And today — how much water is on Mars now?

There’s a significant amount of water present as ice. At the poles, orbiters have seen ice a few kilometers thick; we can tell it’s water from spectrometers, and we can see the structure from radar. If you spread that ice globally, it would be a layer about 20 to 30 meters thick. There’s also some ice in mid latitudes.

There are trace amounts of water vapor in the atmosphere, orders of magnitude less than on Earth. But it can form clouds that affect the temperature, that affect the climate. There’s a robust water cycle: It can snow, but it’s too cold to rain.

The biggest uncertainty is whether there’s any water present deep within the crust. We think that there should be water there because it would have percolated into the crust from an early wet environment (this happened on Earth, too). We can estimate how much we think might be present there — it might be up to half of the total Martian water inventory. But we don’t know if it’s there.

Where did all the water go?

All the water from the large outflow channels flowed into a spot in the northern lowlands, and this would have formed a large lake — what happened to that is unclear. Did it evaporate, or did it freeze in place and get covered up by dirt and dust? We don’t know.

A lot of the water that was on Mars billions of years ago has been incorporated into minerals. And we calculated that a lot of the ancient water — at least 110 meters thickness if you were to spread it all over the planet — has been lost to space.

Why was so much water lost to space?

We used to think that the main driving force was that Mars lost its magnetic field and this led to a loss of its atmosphere, which allowed water to be stripped away. We now know it’s a lot more complicated than that. We’re still in the middle of trying to sort this out.

Is there liquid water anywhere on Mars today?

That’s the million-dollar question. In mid to low latitudes, we think there are trace amounts of liquid in the dirt, stabilized by minerals. With the Phoenix lander in the ’90s, we saw droplets of water kicked up by the landing. The amount of this water, though, is lower than is required for any Earth life.

There could be subsurface brines. But given the cold temperatures, it requires a lot of salt, and a very specific composition of salt, to have liquid water stable at the surface. Since we haven’t detected it, we don’t know what that might be like.

There’s a long discussion going back several years on whether they’ve detected layers of liquid water beneath the south polar cap. It comes out of the European radar instrument, and their interpretation was that, a kilometer down, there must be a layer of liquid water. But that has been under much debate, so I’m not willing to accept it yet.

We have also seen features called recurring slope lineae: deposits that look like dark streaks that seem to grow and extend downhill. Orbiters have taken photos before and after their growth. These might be fed by fast-rising groundwater, perhaps, or a little bit of melting ice during summer that lubricates the flow. But a water origin of those features is not accepted yet. It could be dry avalanches. We’ve seen a lot of things on Mars, and on other planets, that have no good analogs on Earth.

If you wanted to look for liquid water, where would you look, and how?

If I wanted to look for larger amounts of liquid water near the surface, I would look for recent volcanism; we might find hydrothermal vents. We know that volcanism has been active up until geologically recent times, meaning the last few million years. We haven’t seen active volcanism, but that doesn’t mean it’s not there.

In the crust, if you get 2 to 3 kilometers deep, geothermal heating would raise the temperature to above the melting point of ice. It might be sitting in pore spaces microns to millimeters across, but that could still add up to a significant amount of water.

You could do electromagnetic sounding, the way that’s used to detect liquid water on Earth. So, if you put a rover down and had it do a loop and lay out an electrical cable over a kilometer or a couple of kilometers, you might be able to detect deep liquid water. I know people are thinking about it, but there hasn’t been an opportunity to propose a mission like that yet.

There’s evidence for life on Earth going back as far as 4 billion years ago. So, what are the odds that Mars had life back then, too?

That’s the question. Ancient Mars meets the environmental requirements for life: liquid water; access to “biogenic” elements like carbon, hydrogen, oxygen, nitrogen and a dozen others (which are still there today); and a source of energy from chemical reactions. Mars had all that.

What’s my bet? Either there was or there wasn’t, so it must be 50/50 (laughs). I decline to make a guess. Mars meets the requirements. It could have life. The only way we’re going to know is by going there and really looking.

What has the search for life on Mars found so far?

We tried twice with Viking landers in the 1970s. In hindsight, those experiments were ill- conceived. They were basically predicated on the basis of: Let’s take a sample of dirt and add in liquid water and organic nutrients and see if we get chemical reactions indicative of life.

They had hints of positive results but it was complicated, and in the end, the consensus, the majority view, is that it did not detect life.

But today we know that 99.99 percent of microbes can’t be cultured in the lab because we’ve taken them out of their environment, and there’s something that’s missing, we just can’t get them to grow. So, what are the odds that we can go to Mars, throw in a random sample of organic molecules and culture any organisms? I think it’s very unlikely.

The Allen Hills meteorite, collected in Antarctica, came from Mars. In the 1990s, a group at Johnson Space Center put forward the idea that there was evidence within the rock for life. That’s no longer considered compelling or convincing, but we learned a lot about how to look for life.

What would convince you?

You need multiple measurements that all point, through different means, to life being present. So, for ancient life, if we found morphological fossils and organic molecules and isotopic evidence that all suggested life, that would be pretty convincing. For present day life, if we could identify organisms that are really different from terrestrial organisms, then we would know it’s not contamination, that we didn’t accidentally bring it to Mars from Earth, ourselves.

Unlike with the Moon, humanity hasn’t yet brought any samples back from Mars. What’s the plan?

There’s a plan for the Perseverance rover to collect samples and bring them back to study. We’ve sent the rover to a place where we know there was liquid water.

It’s a complicated mission. Perseverance is collecting samples, in tubes about the length of a cigarette and a centimeter wide, and leaving them on the surface in well-marked locations. Then we’ll send a spacecraft that will collect them, put them into a container about the size of a volleyball, put it onto a Mars ascent vehicle and launch it into orbit. Then a different spacecraft will orbit the planet, collect that volleyball and bring it back to Earth, but not until after 2030.

China is also planning what’s called a grab sample, where they land, scoop up something, maybe pick up a rock, put it in a rocket, send it back.

There have been more than 20 successful missions to Mars since 1965, including flybys, orbiters and landers by many nations. You were principal investigator on NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission, which has been orbiting Mars since 2014. What has been its main achievement?

MAVEN is the only spacecraft that was devoted to understanding the upper atmosphere and the stripping of gases to space. It was very successful, but we still don’t know everything. Imagine trying to understand the history of the Earth from a handful of missions, without the benefit of hundreds of years of on-the-ground studies.

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What are the most exciting upcoming missions to Mars?

There are only two upcoming missions that I’m aware of. NASA’s Escapade — it’s a small orbiting mission — might launch later this year. It will look at the upper atmosphere as a complement to MAVEN.

The second is Rosalind Franklin. It was a joint ESA-Russia collaboration, originally going to be launched in 2020 but then delayed to late 2022. After the Russians invaded Ukraine that year, the European Space Agency withdrew from that collaboration. They took all the Russian hardware off the spacecraft, and they had to push back the launch date to 2028 at the earliest.

It is a lander and rover. The big thing that it’s going to do that hasn’t been done before is it will drill down 2 meters below the surface. At that depth, any organic molecules would be shielded from galactic cosmic rays or solar particles.

Should we send people to Mars?

I think, scientifically, it’s imperative. They can do so much more, so much more quickly than robotic spacecraft.

Science isn’t the only reason. Another reason is national prestige. A third reason is getting people excited about going into STEM (science, technology, engineering and math) careers.

It is doable. But it is really difficult, and it’s not cheap. The minimum numbers that are credible are $100 billion. Elon Musk is talking about being able to send people in four years. I think that ignores a lot of the problems, like life support, keeping people alive for the three years to do a round trip, and making enough fuel to get back. My bet is we are 10 to 15 years away from being able to send people.

Elon Musk has said, let’s abandon the Moon and go straight to Mars. I don’t know what President Trump is going to do.