The search for evidence of past life on Mars has produced several studies investigating whether liquid water still exists on the planet today as it did in its ancient past. In recent years, several studies have presented evidence that liquid water could exist under layers of ice on the Red Planet. Now, however, a new study published July 15 in the journal Geophysical Research Letters, suggests frozen clay—not liquid water—may be buried beneath these so-called, ice-covered “lakes.”
Beginning in 2018, researchers published a ground-breaking study in Science detailing a mysterious feature thought to be a reservoir of liquid water underneath ice on Mars' south polar ice cap, also known as the south polar layered deposits (SPLD). In 2020, a follow-up study published in Nature Astronomy found a network of three underground lakes within the same region.
The studies detected the elusive underground lakes using data collected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument (MARSIS) aboard the European Space Agency's Mars Express Orbiter. The radar detected stronger and brighter signals from the planet, an effect water is known to have, NASA researchers explained in a press release. However, Mars is extremely cold, with temperatures averaging minus 81 degrees Fahrenheit and some scientists are puzzled over how water could remain as a liquid form in such extreme temperatures.
A new analysis from a different study published June 16 found that some of the shiny patches detected by MARSIS were in locations too cold for liquid water to exist, reports Science Alert's Michelle Starr. For liquid water to exist on the Red Planet, the water needs to be infused with large amounts of salt or warmed by a heat source like geothermal activity, Science Alert reports. In 2019, a study found that no amount of salt present on Mars is enough to melt the Martian south pole, and while the planet does have evidence of volcanic activity, it does not occur on either of the poles.
Based on this, Isaac Smith, a planetary scientist at York University, and his team suspect that instead of liquid water riddling Mars' south pole, the radar may be detecting deposits of frozen clay called smectites, reports Isaac Schultz for Gizmodo. Their newly published Geophysical Research Letters study provides another hypothesis on what MARSIS is picking up on the Red Planet's south pole.
"Among the Mars community, there has been skepticism about the lake interpretation, but no one had offered a really plausible alternative," Smith explains to Space.com's Charles Q. Choi. "So it's exciting to be able to demonstrate that something else can explain the radar observations and to demonstrate that the material is present where it would need to be. I love solving puzzles, and Mars has an infinite number of puzzles."
The team analyzed smectites, a type of clay that is more similar to volcanic rocks than other kinds of clay. This type of clay forms when volcanic rocks interact with water and, in general, the clay retains a lot of water, per Space.com. Smectites are found on the planet's southern highlands.
"Because these clays are at and beneath the south polar cap, it must have been warm enough there long ago to support liquids," Smith tells Space.com.
To test out their hypothesis, the team chilled smectites to a frosty minus 45 degrees Fahrenheit, an average temperature on Mars. Using a wave propagation model, the team found that the smectite could generate the radar reflections picked up by MARSIS, even if it's mixed with other materials, Space.com reports. Smith and his team also found evidence of smectites along Mars's south pole after analyzing visible and infrared light data collected from the region. The scientists suggest the smectites may have formed during warm spells on the planet when the south pole had liquid water. Over time, the clay was buried under ice.
"Science is a process, and scientists are always working towards the truth," Smith said to Space.com. "Showing that another material besides liquid water can make the radar observations doesn't mean that it was wrong to publish the first results in 2018. That gave a lot of people ideas for new experiments, modeling, and observations. Those ideas will translate to other investigations of Mars and already are for my team."
To know for sure what lies underneath Mars's south pole, researchers will need sophisticated instruments that can dip directly into the Martian pole, Gizmodo reports. For future studies, Smith and his colleagues plan to cool smectites at colder temperatures and repeat the experiment with other types of clays. Per Space.com, Mars has a diverse set of clays scattered on the planet, and the team suspects they may also reflect light.