If you were planning an ice fish excursion to the South Pole of Mars and its underground lakes watched by radar in 2018, don’t pack your parka or ice snail just yet. In a Geophysical Research Letters by IB Smith et al. published research letter it seems that the Mars lakes could be nothing more than smectite, i.e. a kind of clay. Should the findings of the paper, entitled A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice (a solid title if you ask me) prove to be correct, it would be a major setback for those hoping to live find the red planet. Why were these supposed lakes so important in the search for life on Mars? How were they discovered in the first place? Why did our dreams of ice fishing on Mars turn to dust (or more correctly: clay)?
In 2018, the European Space Agency (ESA) announced that its orbiter Mars Express had detected evidence of liquid water lakes beneath the surface of Mars’ South Pole. Understandably, the discovery boosted hopes of finding extremophilic organisms that survive in icy water, similar to bacteria that survive under 4 kilometers of ice in Lake Vostok in Antarctica.
Lake Vostok, roughly the size of Lake Ontario, is buried under several kilometers of ice in Antarctica, but has been found to be life-sustaining. Photo credit: Nicolle Rager-Fuller / NSF
Like Mars, Antarctica had a warm and humid past. When geological and tectonic processes migrated the great continent to the South Pole, it experienced extreme glaciation. Microbes adapted to radical climate change and eventually gave birth to the ecosystem that thrives there today. While the glaciation of Antarctica was driven by the tectonic effects of continental drift, 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 stubbornly clinging to life in subterranean lakes in the Martian Poles.
Computer generated image showing ESA’s Mars Express in orbit over the surface of Mars. As is well known, the MARSIS instrument on Mars Express showed indications of underground lakes in the southern polar region of Mars in 2018. Source: NASA / JPL / Corby Waste
Mars Express used Mars Advanced Radar for Subsurface and Ionosphere Sounding Instrument or MARSIS. The radar was pulsed and carefully surveyed, revealing reflectivity data for the surface and below to a depth of 1.5 kilometers. An exceptionally bright area about eight miles wide was what one would expect if there was a large amount of liquid.
The authors of the recent article, which denies the validity of Marsseen claims, raise some questions that cannot be answered by radar reflectivity alone. They claim that the amounts of salt and heat required to maintain the supposed lake are implausible. Mars is too cold, and while there is salt on the planet, no mechanism is known that would focus it on the salinity required for liquid water to persist. They also estimate that the local geothermal flow (would Mars thermal flow be a more apt term?) Is one sixth, which is also needed to maintain the fluid.
The South Pole of Mars as seen by Mars Express. Credit ESA / DLR / FU Berlin
In science, especially when it comes to identifying possible biomes for extraterrestrial life, finding the simplest explanation for an observation is critical. The authors believe that a particularly reflective clay called smectite, which is abundant on Mars, is the more likely culprit for the radar results displayed by MARSIS than that part of the South Pole has highly anomalous salinity and warming values.
Ironically, it is believed that the formation of smectite was caused by various weathering processes that involve groundwater, hydrothermal systems or surface water. The clay probably also contains water, but critically frozen and dispersed in the layer of extremely cold smectite. The mineral is common on Mars and was even discovered by the Curiosity rover!
This is not the first time in the history of Mars (or rather, in the history of terrestrial observation of Mars) that an enticing trait is absent. It is known that observers of the late 19th and early 20th centuries such as Giovanni Schiaparelli and Percival Lowell observed so-called channels on the surface of the planet. These were presented as engineering works built by a civilization of highly developed Martian life forms. The widespread belief in life on Mars from this period inspired countless science fiction works, including HG Wells’ work The War of the Worlds.
Illustrations by Percival Lowell showing alleged canals on Mars. It was later found that these features are optical illusions and do not exist on the planet. Photo credit: Percival Lowell
While it is easy to get carried away with the excitement of imagining extraterrestrial civilizations or even just subterranean lakes on Mars, doing good science in our explorations of the solar system is important. The discovery of Mars for what it really is is, in reality, an extraordinary achievement and our results tell us about the nature of the solar system. While we have dashed our hopes for liquid water masses, it is not to be underestimated how extraordinary Mars Express is, and the incredible sophistication required to analyze the data presented by the team in this recently published article is not of this world.
Leading image: Radar images provided by Mars Express that show a supposed liquid body of water underground. It is now believed to be a mineral layer of smectite clay. Credit Context Card: 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 beneath the south polar ice of Mars