If you were planning an ice-fishing trip to the Martian south pole and its sub-surface lakes observed by radar in 2018, don’t pack your parka or ice auger just yet. In a research letter published earlier this month in Geophysical Research Letters by I.B. Smith et al., it seems that the Martian lakes may be nothing more smectite, that is, a kind of clay. Should the findings of the paper, titled A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice (a solid title if you ask me), turn out to be correct, it would be a significant setback for those hoping to find life on the red planet. So why were these supposed lakes so critical for the search for life on Mars? How were they discovered in the first place? Why have our dreams of Martian ice-fishing turned to dust (or, more correctly, clay)?
In 2018, the European Space Agency (ESA) announced that its Mars Express orbiter had discovered evidence of liquid water lakes below the surface of the Martian south pole. Understandably, the discovery bolstered hopes for finding extremophile organisms surviving in the icy water similar to bacteria surviving under 4 kilometers of ice in Antarctica’s Lake Vostok.
Lake Vostok, roughly the size of Lake Ontario, is buried under several kilometers of ice in Antarctica yet has been found to support life. Credit: Nicolle Rager-Fuller / NSF
Like Mars, Antarctica had a warm and wet past. As geological and tectonic processes migrated the great continent to the south pole, it underwent extreme glaciation. Microbes adapted to the radical climate change and eventually gave rise to the ecosystem that thrives there today. While the glaciation of Antarctica was driven by the tectonic action of continental drift, the climate change on Mars was global and likely due to the loss of the atmosphere from erosion by the solar wind. It is not unreasonable to imagine microbes adapting to this extreme climate change and clinging stubbornly to life in subsurface lakes at the Martian poles.
Computer-generated image depicting ESA’s Mars Express in orbit above the surface of Mars. The MARSIS instrument on Mars Express famously showed evidence of subsurface lakes in the southern polar region of Mars in 2018. Credit: NASA/JPL/Corby Waste
Mars Express utilized Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument or MARSIS. The radar was pulsed and carefully measured, revealing reflectivity data for the surface and below to a depth of 1.5 kilometers. An exceptionally bright area, roughly 20 kilometers wide, was consistent with what would be expected if a large body of liquid were present.
The authors of the recent paper disputing the validity of the claims of Martian lakes raise some questions that cannot be answered by radar reflectivity alone. They claim that the required amounts of salt and heat needed to sustain the supposed lake are not plausible. Mars is too cold, and while there is salt present on the planet, there is no known mechanism that would concentrate it to the salinity levels necessary for liquid water to persist. They also estimate that the local geothermal flux (would Mars-thermal flux be a more appropriate term?) is one-sixth that is required to maintain liquid as well.
The south pole of Mars as seen by Mars Express. Credit ESA/DLR/FU Berlin
In science, especially when the stakes are as high as identifying possible biomes for extraterrestrial life, it is crucial to seek out the simplest explanation for an observation. The authors posit that rather than a portion of the south pole exhibiting highly anomalous salinity and heating levels, a particularly reflective clay called smectite that is abundant on Mars is the more likely culprit for the radar results seen by MARSIS.
Somewhat ironically, the formation of the smectite is thought to have been formed by various weathering processes that involve groundwater, hydrothermal systems, or surface water bodies. The clay also likely contains water, but critically, frozen solid and distributed throughout the layer of extremely cold smectite. The mineral is common on Mars and has even been detected by the Curiosity rover!
This is not the first time in Martian history (or rather in the history of Earthly observation of Mars) that a tantalizing feature has turned out to be non-existent. Famously, late 19th and early 20th century observers like Giovanni Schiaparelli and Percival Lowell observed so-called canals on the planet’s surface. These were imagined to be works of engineering constructed by a civilization of highly advanced Martian life forms. The widespread belief in Martian life from this time inspired countless works of science fiction, including the HG Wells work The War of the Worlds.
Illustrations showing supposed canals on Mars made by Percival Lowell. These features were later found to be optical illusions and are not present on the planet. Credit: Percival Lowell
While it is easy to get swept away in the excitement of imagining alien civilizations or even just subsurface lakes on Mars, it is critical to do good science in our explorations of the solar system. Discovering Mars as it truly is, in reality, is an extraordinary achievement and our findings inform us of the nature of the solar system. Despite crushing our hopes of bodies of liquid water, there’s no understating how extraordinary Mars Express is, and the incredible sophistication required to analyze the data displayed by the team in this recent paper is out-of-this-world.
Lead image: Radar imagery provided by Mars Express showing a supposed body of liquid water in the subsurface. This is now thought to be a mineral layer of smectite clay. Credit Context map: NASA/Viking; THEMIS background: NASA/JPL-Caltech/Arizona State University; MARSIS data: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei .et al. 2018
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A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice