Liquid Water on Mars

Radar sounding measurements from the Mars Express Orbiter and the science behind liquid water on Mars.

Mars Sunset


In a paper first released in Science on July 25th, 2018, a team of scientists from the European Space Agency (ESA) working on the Mars Express Orbiter announced the potential discovery of a large, underground liquid water aquifer under a Martian polar cap.

Mars Express Orbiter

Mars Express Orbiter Source

The Mars Express Orbiter is one of six currently active spacecraft orbiting Mars, launched from Earth in mid-2003 and inserted into Martian orbit later that year. Mars Express is comprised of seven scientific instruments that are being used to study the Martian surface and atmosphere.

Mars Express Instruments Source

One of these instruments, the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS, detected an anomalous radar signature under the Martian surface near the Martian polar caps. During a measurement campaign between 2012 and 2015, MARSIS took a sequence of twenty-nine radar sounding measurements that detected a radar signature that is plausibly consistent with a large body of liquid water.

List of Articles

By now you’ve likely seen this news in one of many different science articles, so I won’t go into the whole story here. But I encourage you to read through a couple of the articles covering this story linked below.

1. “Scientists detect giant underground aquifer on Mars, raising hope of life on the planet”, The Verge, Loren Grush (@lorengrush)

2. “Liquid water spied deep below polar ice cap on Mars”, Science, Daniel Clery (@danclery)

3. “Scientists discover evidence of the first large body of liquid water on Mars”, Wired, Robbie Gonzalez (@rtg0nzalez)

4. “Mars (probably) has a lake of liquid water”, Science News, Lisa Grossman (@astrolisa)

5. “Liquid Water on Mars! Really for Real This Time (Probably)”, Planetary Society, Emily Lakdawalla ([@elakdawalla])(

Also, since it’s always a good idea to read the source documentation, check out the actual paper published in Science below!

6. “Radar evidence of subglacial liquid water on Mars”, Science, Roberto Orosei et al

And last, check out the paper below if you want more technical detail on the MARSIS instrument itself.

7.“The Mars express MARSIS sounder instrument”, Planetary and Space Science, R. Jordan et al

Questions and Answers

Ok, let’s ask some questions about this work and pull out some answers from these papers and articles.

So, What Exactly Did They Find?

In short, European scientists discovered what appears to be an anomalous radar signature in the Southern polar cap region of Mars. Based on the evidence available, this radar anomaly appears to be a large, underground salty liquid water aquifer. About 1.5 kilometers under the Martian surface and about 20 kilometers wide (twice as large as Crater Lake in Oregon!), it’s the best and largest indication of liquid water that has been found on Mars to date.

Crater Lake

Mars Anomaly Region Source

How Did They Take These Radar Measurements?

Did They Have Any Issues With The Measurements?

Oh yeah, plenty! To name a few, the MARSIS instrument has relatively low spatial resolution making it difficult to find small features; the instrument wasn’t calibrated on Earth because of its large size, so they can only make relative, not absolute, measurements; the radar signals get attenuated by the Martian ionosphere; and it took the scientists a couple of years to realize the spacecraft was “hiding” data due to an averaging process that was intended to help minimize bandwidth of data transfers back to Earth.

Why Do They Think It’s Liquid Water?

Well, for one, the strength of the relative radar echoes they measured are similar to ones that they have been measured by other scientific studies on Earth, specifically of subsurface ice-water boundary regions in Antartica and Greenland. They also ran some computer models of what the radar signals would look like for cold, briney water (their best estimate of the conditions on Mars) and it seemed to match their measurement data.

And because of how cold it is on Mars, liquid water could only exist if its freezing point had been lowered, either due to pressure from the ice caps above it or by salts that could be present in the liquid water, both of which are theorized to be likely conditions on Mars.

Hmm, That Sounds Pretty Convincing…But Are We Sure It’s Water?

Of course not! A radar sounding is an indirect measurement when it comes to identifying a molecular compound like water. Until we drill down into the aquifer to do spectroscopic measurements, we cant be sure that the radar anomaly is liquid water.

On top of that, another radar instrument orbiting Mars, the Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter (MRO), hasn’t seen a radar anomaly in that area.

So How Do We Learn More About This Radar Anomaly?

Well for one, this discovery by the MARSIS team gives the SHARAD team a specific area of interest to target future radar measurements. SHARAD has a higher resolution than MARSIS, about 0.3 to 3 kilometers, so in theory it should be able to see the anomaly in better resolution, but it also operates at a different radar frequency that might not be sensitive to liquid water. So it remains to be seen if the SHARAD team can find the same anomaly in their measurements. But if they do, it might help corroborate the MARSIS hypothesis.

Any Other Missions to Mars That Might Help Do More Science On This?

Well, it’s probably fair to say that the location of this radar anomaly has gone to the top of the list for future science mission destinations on the Red Planet.

One mission already on its way, the InSight lander, will be focusing on temperature measurements below the Martian surface, although not nearly as deep as the radar anomaly. But, those temperature measurements will help out in our understanding of the geothermal properties of the planet, which will inform our understanding of the feasibility of liquid water under the surface.

This discovery also opens the door for new mission ideas, ones that might even include drilling down the 1.5 kilometers to try and get a sample analysis. The technology to launch and deploy a drill to Mars that could go down that deep doesn’t exist quite yet, but theoretically it’s possible.

While drilling for a sample analysis would be the ideal way to verify this discovery, there’s still a lot we have to learn about this radar anomaly. New orbiter spacecraft with advanced ground-penetrating radars could be used to analyze the anomaly in much higher-resolution and with calibrated antennae to better characterize the radar signatures to known material compositions.

I can also envision a suite of orbiters that work on a tip-and-cue model, where fast-flying orbiters do low resolution scans across the whole planet looking for anomalies and then flag regions of interest for slower-flying orbiters with higher resolution scanners.

The Tapestry of Planetary Knowledge

These types of discoveries are so fascinating to read about, first in news reports and then reading through the actual papers and work the reports are based on. They tell us the story of humankind’s efforts to understand our cosmos, and particularly our neighboring planets. These kinds of discoveries build on themselves and the work of teams all around the world.

We send a spacecraft, we learn something new and we learn more questions to ask. We send more spacecraft to answer some of those questions, then we learn something new and find even more questions. Our library of knowledge about our neighboring planets gets bigger and bigger, and our technology gets better and better. It’s a lovely story to see unfold in our lifetimes, and it’s beautiful to watch the tapestry of planetary knowledge being woven.