Unfortunately, There are Other Viable Explanations for the Subsurface Lakes on Mars

Mars’ south polar ice cap. Credit: ESA / DLR / FU Berlin /

Ever since 1971, when the Mariner 9 probe surveyed the surface of Mars, scientists have theorized that there might be subsurface ice beneath the southern polar ice cap on Mars. In 2004, the ESA’s Mars Express orbiter further confirmed this theory when its Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument detected what looked like water ice at a depth of 3.7 km (2.3 mi) beneath the surface.

These findings were very encouraging since they indicated that there could still be sources of liquid water on Mars where life could survive. Unfortunately, after reviewing the MARSIS data, a team of researchers led from Arizona State University (ASU) has proposed an alternative explanation. As they indicated in a recent study, the radar reflections could be the result of clays, metal-bearing minerals, or saline ice beneath the surface.

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Maybe Mars Didn’t Lose its Water After All. It’s Still Trapped on the Planet

Credit: Daein Ballard/ESA

Roughly 4 billion years ago, Mars looked a lot different than it does today. For starters, its atmosphere was thicker and warmer, and liquid water flowed across its surface. This included rivers, standing lakes, and even a deep ocean that covered much of the northern hemisphere. Evidence of this warm, watery past has been preserved all over the planet in the form of lakebeds, river valleys, and river deltas.

For some time, scientists have been trying to answer a simple question: where did all that water go? Did it escape into space after Mars lost its atmosphere, or retreat somewhere? According to new research from Caltech and the NASA Jet Propulsion Laboratory (JPL), between 30% and 90% of Mars’ water went underground. These findings contradict the widely-accepted theory that Mars lost its water to space over the course of eons.

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You’re Going to Need a Bigger Drill. The Best Place for Life on Mars is Deep, Deep Underground

A vertically exaggerated, false-color view of a large, water-carved channel on Mars called Dao Vallis. Image: ESA/DLR/FU Berlin, CC BY-SA 3.0 IGO. 3D rendered and colored by Lujendra Ojha

For decades, robotic missions have been exploring Mars to learn more about the planet’s geological and environmental history. Next year, the Perseverance rover will join in the hunt and be the first mission to send samples back to Earth and by the 2030s, the first crewed mission is expected to take place. All of these efforts are part of an ongoing effort to find evidence of past (and maybe even present) life on Mars.

According to a new study from Rutgers University-New Brunswick., the most likely place to find this evidence is located several kilometers beneath the surface. It is here (they argue) that water still exists in liquid form, which is likely the result of geothermal heating melting thick subsurface sheets of ice. This research could help resolve lingering questions like the faint young Sun paradox.

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Mars Express Finds Even More Ponds of Water Under the Ground on Mars

Radar data collected by ESA’s Mars Express point to a pond of liquid water buried under layers of ice and dust in the south polar region of Mars. Credit: ESA

Evidence of Mars’ watery past is written all over the surface of the planet. Between dried-up river valleys, outflow channels, and sedimentary deposits, it is clear that Mars was once a much different place. But until recently, the mystery of where this water went has remained unsolved. This changed in 2018 when data obtained by the ESA’s Mars Express probe indicated the existence of water beneath the south pole of the planet.

According to the Mars Express probe’s Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), this body of water is in a 20 km (~12.5 mi) wide area about 1.5 km (~1 mi) beneath the surface. And now, further analysis of the data by a team led by the Roma Tre University has revealed the existence of three new ponds, the largest of which measures about 20 x 30 km (~12.5 x 18.5 mi) and is surrounded by many smaller ponds.

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There Might be Enough Oxygen Below the Surface of Mars to Support Life

Artist's impression of water under the Martian surface. Credit: ESA/Medialab

The possibility that life could exist on Mars has captured the imagination of researchers, scientists and writers for over a century. Ever since Giovanni Schiaparelli (and later, Percival Lowell) spotted what they believed were “Martian Canals” in the 19th century, humans have dreamed of one day sending emissaries to the Red Planet in the hopes of finding a civilization and meeting the native Martians.

While the Mariner and Viking programs of the 1960s and 70s shattered the notion of a Martian civilization, multiple lines of evidence have since emerged that indicate how life could have once existed on Mars. Thanks to a new study, which indicates that Mars may have enough oxygen gas locked away beneath its surface to support aerobic organisms, the theory that life could still exist there has been given another boost.

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New Study Could Help Locate Subsurface Deposits of Water Ice on Mars

Mars Express' view of Meridiani Planum. Credits: ESA/DLR/FU Berlin (G. Neukum)

It is a well-known fact that today, Mars is a very cold and dry place. Whereas the planet once had a thicker atmosphere that allowed for warmer temperatures and liquid water on its surface, the vast majority of water there today consists of ice that is located in the polar regions. But for some time, scientists have speculated that there may be plenty of water in subsurface ice deposits.

If true, this water could be accessed by future crewed missions and even colonization efforts, serving as a source of rocket fuel and drinking water. Unfortunately, a new study led by scientists from the Smithsonian Institution indicates that the subsurface region beneath Meridiani Planum could be ice-free. Though this may seem like bad news, the study could help point the way towards accessible areas of water ice on Mars.

This study, titled “Radar Sounder Evidence of Thick, Porous Sediments in Meridiani Planum and Implications for Ice-Filled Deposits on Mars“, recently appeared in the Geophysical Research Letters. Led by Dr. Thomas R. Watters, the Senior Scientist with the Center for Earth and Planetary Studies at the Smithsonian Institution, the team examined data collected by the ESA’s Mars Express mission in the Meridiani Planum region.

Artist’s impression of a global view of Mars, centered on the Meridiani Planum region. Credit: Air and Space Museum/Smithsonian Institution

Despite being one of the most intensely explored regions on Mars, particularly by missions like the Opportunity rover, the subsurface structure of Meridiani Planum has remained largely unknown. To remedy this, the science team led by Dr. Watters examined data that had been collected by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the ESA’s Mars Express orbiter.

Developed by researchers at the University of Rome in partnership with NASA’s Jet Propulsion Laboratory (and with the help of private contractors), this device used low-frequency radio pulses to study Mars’ ionosphere, atmosphere, surface, and interior structure. The way these pulses penetrated into certain materials and were reflected back to the orbiter was then used to determine the bulk density and compositions of those materials.

After examining the Meridiani Planum region, the Mars Express probe obtained readings that indicated that the subsurface area had a relatively low dielectric constant. In the past, these kinds of readings have been interpreted as being due to the presence of pure water ice. And in this case, the readings seemed to indicate that the subsurface was made up of porous rock that was filled with water ice.

However, with the help of newly-derived compaction models for Mars, the team concluded that these signals could be the result of ice-free, porous, windblown sand (aka. eolian sands). They further theorized that the Meridiani Planum region, which is characterized by some rather unique physiographic and hydrologic features, could have provided an ideal sediment trap for these kinds of sands.

Artist’s impression of the Mars Express rover, showing radar returns from its MARSIS instrument. Credit: ESA/NASA/JPL/KU/Smithsonian

“The relatively low gravity and the cold, dry climate that has dominated Mars for billions of years may have allowed thick eolian sand deposits to remain porous and only weakly indurated,” they concluded. “Minimally compacted sedimentary deposits may offer a possible explanation for other nonpolar region units with low apparent bulk dielectric constants.”

As Watters also indicated in a Smithsonian press statement:

“It’s very revealing that the low dielectric constant of the Meridiani Planum deposits can be explained without invoking pore-filling ice. Our results suggest that caution should be exercised in attributing non-polar deposits on Mars with low dielectric constants to the presence of water ice.”

On its face, this would seem like bad news to those who were hoping that the equatorial regions on Mars might contain vast deposits of accessible water ice. It has been argued that when crewed missions to Mars begin, this ice could be accessed in order to supply water for surface habitats. In addition, ice that didn’t need to come from there could also be used to manufacture hydrazine fuel for return missions.

This would reduce travel times and the cost of mounting missions to Mars considerably since the spacecraft would not need to carry enough fuel for the entire journey, and would therefore be smaller and faster. In the event that human beings establish a colony on Mars someday, these same subsurface deposits could also used for drinking, sanitation, and irrigation water.

A subsurface view of Miyamoto crater in Meridiani Planum from the MARSIS radar sounder. . Credit: ESA/NASA/JPL/KU/Smithsonian

As such, this study – which indicates that low dielectric constants could be due to something other than the presence of water ice – places a bit of a damper on these plans. However, understood in context, it provides scientists with a means of locating subsurface ice. Rather than ruling out the presence of subsurface ice away from the polar regions entirely, it could actually help point the way to much-needed deposits.

One can only hope that these regions are not confined to the polar regions of the planet, which would be far more difficult to access. If future missions and (fingers crossed!) permanent outposts are forced to pump in their water, it would be far more economical to do from underground sources, rather than bringing it in all the way from the polar ice caps.

Further Reading: Smithsonian NASM, Geophysical Research Letters