If Mars Had Water, Where Did It Go?

It’ll be a fine day for Poland Spring, when Mars is finally colonised: bottled’s the only option, when you’re living on a planet whose last substantial traces of flowing liquid water disappeared a few billion years ago. That ancient water has occasioned much study and debate, and provided the name for at least one French-Candian psych rock band. The fact that it existed, at one point, is a large part of why dreams of annexing Mars have flourished.

But where did it go, exactly? By what majestic geological processes do massive bodies of space-liquid just disappear? For this week’s Giz Asks, we talked to a number of Mars experts to find out.

Professor, Geoscience, Virginia Tech, who studies the formation and evolution of planets

There are a number of lines of evidence that at some time in the past there was more water on the surface of Mars compared with the cold desert conditions we observe today. Where this water went is one of the great puzzles in our solar system. As a geophysicist who spends a lot of time thinking about subduction zones on Earth, I’m even more surprised than most people by the disappearance of water from the surface of Mars. Here’s why. On Earth, water reacts with rocks on and below the ocean floor. Those water-altered rocks are carried into subduction zones by the motion of tectonic plates. This moves 150-300 metric tons of water a year from the surface to the interior of the Earth—a pretty efficient way to remove water from the surface. That mechanism doesn’t work on Mars because there is no plate tectonics or subduction. The orbiters and robots that we have sent to Mars have identified rocks and minerals that formed in the presence of water, including some of the same minerals and rocks found on Earth’s ocean floor. We know that some of these rocks and minerals only form at pressures and temperatures deep below the surface of Mars; water must have been present deep below the surface. As a participating scientist on the InSight mission, I am calculating densities and seismic properties for likely compositions of the Martian surface rocks in order to identify signatures of water-altered rock that can be detected by seismic waves. Data from the InSight mission could place limits on how much water might be hidden in plain sight — within the altered rocks we have observed.

These dark, narrow, 100 meter-long streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Hale crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water. The blue colour seen upslope of the dark streaks are thought not to be related to their formation, but instead are from the presence of the mineral pyroxene. -NASA (Image: NASA/JPL/University of Arizona)

There is and was quite a bit of water on Mars. Even today, the Martian polar caps are made of enough water ice that if you melted it all and spread it evenly around the planet, the global ocean would be at least 22 meters deep! However, the atmospheric pressure on Mars is so low that liquid water is unstable, so the water today is only in the form of ice and a small amount of gas. In the past, Mars had significantly more liquid water, and it formed rivers, lakes, and possibly even oceans on the surface. The Curiosity rover has recently investigated more than 300 meters of rock that formed at the bottom of a lake that appears to have been stable on Mars’ surface for more than 1 million years, about 3.5 billion years ago. This shows there must have been a thicker atmosphere and more water early in Mars history, but we still don’t completely understand how much there was or how long it was stable. So where did the water go? Some of it was lost to space (Mars doesn’t have a magnetic field to protect it from solar wind), some of the water reacted with volcanic rocks and then got trapped in minerals, and some of the water is still there today, frozen into the ice caps and in permafrost layers below the ground.

“Some of it was lost to space… some of the water reacted with volcanic rocks and then got trapped in minerals, and some of the water is still there today, frozen into the ice caps and in permafrost layers below the ground.”

Professor of Physics and Deputy Director (Solar System) at Mullard Space Science Laboratory, University College London

Mars has changed significantly in the 4.6 billion years since its formation. About 3.8 billion years ago, Mars was much more Earth-like, with volcanism, a magnetic field, water on the surface and a thick atmosphere – at a time when life was starting on Earth. The evidence for ancient water on the surface has been building up — starting with orbiter imaging from Viking, in-situ direct evidence that water was on the surface with mineral analysis from Opportunity and Curiosity, evidence for neutral acidity water from Curiosity, and water-rich minerals and clays on the older surface regions, mapped by Mars Express. Mars now is cold and dry, and has a thin carbon dioxide-atmosphere, with a harsh surface environment and a thinning atmosphere unprotected by a global magnetic field. Mars Odyssey and Phoenix found evidence for subsurface water ice, Mars Reconnaissance Orbiter found Recurring Slope Lineae which may be signs of water seeping from the sub-surface (or alternatively dust falls) and last year Mars Express found evidence for a liquid water ‘lake’ underneath the South pole using radar measurements. All this shows that water has been and still is present on Mars—but some of the water went underground and some escaped to space as seen by Mars Express and Maven. But the potential for life on Mars was best 3.8 billion years ago. That’s why with the ESA-Russia Rosalind Franklin (ExoMars) rover we’ll be drilling up to 2m underneath the harsh Martian surface to search for signs of past, or less likely present, life in-situ. Also, Mars 2020 will gather samples for an eventual Mars sample return.

“Mars now is cold and dry, and has a thin carbon dioxide-atmosphere, with a harsh surface environment and a thinning atmosphere unprotected by a global magnetic field.”

Professor, Astronomy, Vanderbilt University, and the author of ‘Life on Mars,’ from which the below is drawn

If we take all the water on a planet, put it on the surface of the planet, and spread it out evenly over 100% of the surface area, we would have what planetary scientists call a ‘global ocean.’ This concept helps us easily visualise the total volume of water on that planet. Fairly robust estimates indicate that the total amount of water planetary scientists have now found on Mars, primarily in the polar ice caps, would create a global ocean with a depth of 70 to 100 feet. That’s how much water we know Mars has today. We can state that with a great deal of confidence. We also know that Mars has lost a great deal of water. By using the abundances of certain important trace gasses in Mars’ atmosphere today, scientists estimate that Mars once had a global ocean with a depth of about 137.16m. On the basis of this evidence from atmospheric gasses, we know that Mars has lost 75% to 85% of the water it started with. All of that water is gone forever, lost to space. Again, I think we can state this with a great deal of confidence. However, if, in addition to the evidence from atmospheric gasses, we use the visual evidence for flowing water on the surface of Mars, which is clear in the form of dried up river valleys and outflow channels that scar the ancient surface of the red planet, we can estimate that Mars once had enough water to generate a global ocean with a depth of 1,500 to 3,000 feet. If we use this evidence from the ancient river valleys and outflow channels, we would necessarily conclude that 40% to 80% of the water Mars started with is not lost to space, all of that water is hiding from us, inside Mars and not locked into the polar ices. That is a whole lot of water. In total, the evidence (in the current atmosphere) appears to suggest that Mars lost 10% to 30% of the water it had 4 billion years ago. Of the remaining 70% to 90% of its water inventory, no more than 5%-10% of that water has been found in the polar caps. The remaining water, perhaps as much as 90% of the water Mars started with, is in underground reservoirs.

A southward-looking panorama combining images from both cameras of the Mast Camera (Mastcam) instrument on NASA’s Curiosity Mars Rover shows diverse geological textures on Mount Sharp. (Image: NASA/JPL-Caltech/MSSS)

Professor, Planetary Physics, University of Louisville

Mars is the only other planet in our solar system that has the potential to be habitable to humans, and so it is no wonder that every detail that is similar or different with Earth is being closely studied. Even though Mars is smaller than Earth, it has the same surface area in terms of dry land (because Earth’s surface is two-thirds oceans), which helps explain the size of the task of exploring the geology of Mars. After more than a half-century of interplanetary exploration, we have many independent lines of evidence that water once flowed on the surface of Mars in abundance. From orbit (remote sensing), we have high-resolution images showing fluvial features in now-dry river channels. From on-the-ground rovers, we have aqueous chemistry detected in several different kinds of minerals, which do not form without liquid water, and even smooth pebbles. We even have movies of briny water flowing today on the surface of Mars, where it is warmest near the equator in the middle of the day. This was confirmed by spectroscopy, which found the signal of hydrated salts — very dilute milk of magnesia! — right where these damp flows appear, and not where they do not. But otherwise, where is all the surface water on Mars? A big part of the answer, perhaps most of it, is the fact that Mars is not quite large enough to have a planetary magnetic field. Earth’s molten iron-nickel core generates a dynamo that gives the home planet a strong magnetic field, which deflects the endless stream of harmful charged particles streaming by from the sun, the solar wind. In stark contrast, Mars has been blasted by the solar wind relentlessly, most likely for billions of years. NASA’s MAVEN spacecraft is currently in orbit around Mars making detailed measurements of this process, and has confirmed that the solar wind steadily strips away volatiles from Mars. The picture that is emerging is that every detail one can list for Earth is in large or small measure beneficial to life, and missing even a few of these makes having life appear and thrive next to impossible. Beneficial features that Earth has that are lacking on Mars include a strong magnetic field, a large moon (to provide tides that agitate the ocean’s chemistry, and to stabilise the obliquity or tilt of the planet, and hence its seasons), and plate tectonics (to recycle oxygen and other resources back into the ocean crust). But, the more we learn about Mars, the more intriguing the planet becomes. The latest big mystery is there is a strong and uneven amount of methane in the atmosphere of Mars, much more than was expected. On Earth, this is caused in part by geothermal vents, but predominantly by the biosphere. Planetary scientists are currently devising ways to decipher what is causing the excess methane on Mars, so stay tuned (and join in)!

“Mars is the only other planet in our solar system that has the potential to be habitable to humans.”

Professor, Geological Sciences, University of Colorado, and Principal Investigator on the Mars Atmosphere and Volatile Evolution (MAVEN) mission whose research focuses on understanding the nature of planetary surfaces and atmospheres and the possibility for the existence of life in the universe

Evidence for liquid water on ancient Mars is seen in the morphology of the surface—features that look like runoff channels for surface water, lakes that filled ancient closed basins created by impact craters, a general degradation of the surface that is most consistent with the presence of an active hydrological cycle, and flow features that suggest the occurrence of large-scale flooding. In addition, minerals have been identified at the surface by the rovers that can form only in the presence of liquid water. Some of these are in the form of “concretions”, round nodules of minerals that form when water flows through the ground and can dissolve minerals and redeposit them elsewhere. On Mars today, we have identified a type of chemical called “perchlorates” mixed in with the soil. These minerals can take water vapour out of the atmosphere and dissolve in it to produce small amounts of liquid water that is stable at the surface today at some times of the Martian day. More controversial are features like “gullies” and flow-like features termed “recurring slope lineae” that may be due to recent water or may be caused by dry flow. And radar has detected what appears to be a wet layer about a kilometer beneath the surface near the south pole that may involve a buried groundwater layer. There is water still on Mars today, in the form of atmospheric water vapour, ice in the polar caps, ice buried beneath the surface in non-polar regions, and water bound up as part of minerals globally. There also could be additional water beneath the surface, perhaps present as widespread or globally distributed groundwater. Although possible, we have no direct evidence for its existence. Each of these has been detected using remote-sensing observations or directly by imaging. Much of the water has been broken up into its component hydrogen and oxygen atoms and lost to space. We know that this has happened, because it leaves a distinctive signature behind: Deuterium is a heavier form of hydrogen, having a neutron in addition to a proton; as a result of being heavier, it escapes to space less readily and leaves the deuterium relatively more abundant in the water remaining on Mars. This enrichment in “D/H” tells us that between 85-95% of the water near the surface of Mars has been lost to space.

“There is water still on Mars today, in the form of atmospheric water vapour, ice in the polar caps, ice buried beneath the surface in non-polar regions, and water bound up as part of minerals globally.”

Assistant Professor, Chemistry and Biochemistry, Georgia Tech, whose research focuses on developing instruments for in situ organic analysis in the search for extraterrestrial life, among other things

The water on Earth is as yet unexplained. The general problem is that the Solar System appears to be a giant distillation column, with volatile compounds being largely evaporated off planetary bodies that receive more heat and then accrete on planetary bodies that are further out and colder. The “ice line” for water appears to be further out than Earth, so explaining why we have so much of could be a bigger challenge than explaining why Mars has so little of it. The small size of Mars cannot be easily explained without the migration of Jupiter and Saturn inwards and then out to their current positions, so the original position of Mars cannot be known with 100% accuracy until our models and understanding of the entire solar system is improved. It is challenging to therefore know how big a problem the Earth-Mars water ratios actually are, as Mars may have been at any number of locations relative to the Sun prior to Jupiter and Saturn migrating to their present positions. Another issue is that Mars lost its magnetic field relatively early due to its relatively small size. This results in solar wind hitting the atmosphere, ionizing it, and then blasting off free protons or molecular hydrogen gas, and even water vapour as a molecular cloud. The MAVEN mission is studying this interaction currently.

“Water really is the ink in the story of Mars.”

Assistant Professor, Earth, Atmospheric and Planetary Sciences, Purdue University, whose research program uses data from NASA satellites and rovers, along with lab and field work back on Earth, to understand the surface processes that have shaped Mars and the Moon