The find, which took years to verify and still remains somewhat in question, is only the latest piece of evidence that liquid water both existed on Mars in the past and continues to do so today, though under extreme conditions.
The find was detailed in a paper published in Science Magazine Wednesday.
The lake is about 12.4 miles wide and is estimated to be only about 4 feet deep. It sits underneath a 1-meter-deep layer of carbon dioxide ice on Planum Australe, the vast plain that spreads across the Red Planet's southern pole.
"The presence of liquid water at the base of the martian polar caps has long been suspected but not observed," the study's abstract states, noting that "the presence of liquid water at the base of the martian polar caps was first hypothesized more than 30 years ago."
You wouldn't want to take a swim in this lake, though: the subglacial water must be at least negative 10 degrees Celsius, Popular Mechanics notes. It's kept liquid through salts of sodium, magnesium and calcium, which lower the melting point of water ice in much the same way that salting your sidewalk turns snow into slush.
Agua en Marte:— Fitter Happier (@Javieroto) July 25, 2018
Que: Un lago de agua líquida bajo la superficie, cerca del polo.
Quien: Agencia espacial Italiana en conjunto con la ESA y la U. di Roma
Como: Radar MARSIS montado la sonda Mars Express que órbita el planeta. pic.twitter.com/Kw5tlvnAA8
Those salts have been observed on the surface of Mars, too, and a 2015 study published in Nature concluded that "brine flows (or seeps) have been proposed to explain the formation of recurring slope lineae," which are dark, salty streaks spotted on Martian sand dunes.
Scientists found the lake using the MARSIS [Mars Advanced Radar for Subsurface and Ionosphere Sounding] instrument, a low-frequency, ground-penetrating radar on the Mars Express spacecraft. Operated by the European Space Agency, the spacecraft was launched on June 2, 2003, from the Baikonur Cosmodrome in Kazakhstan and began its surveying work in Martian orbit on May 4, 2005, New Scientist reported. MARSIS is complemented by the Shallow Radar Sounder (SHARAD) on NASA's Mars Reconnaissance Orbiter, which uses different wavelengths to monitor similar phenomena on the Martian surface, according to the ESA's Mars Express website.
"Radar profiles collected between May 2012 and December 2015 contain evidence of liquid water trapped below the ice of the South Polar Layered Deposits [SPLD]," the study states. "Anomalously bright subsurface reflections are evident within a well-defined, 20-kilometer-wide zone… We interpret this feature as a stable body of liquid water on Mars."
Collecting the data took a long time, since Mars Express doesn't follow a regular orbit, meaning seeing the same area of the planet again can be difficult. It took three-and-a-half years to obtain 29 observations of the target location. Plus, the results were unsteady and often confusing, making drawing firm conclusions challenging, too.
The #Mars Express radar investigation finds a subsurface feature spanning about 20km across under a 1.5km thick layer of ice & dust, interpreted as a buried pond of liquid water, which could be laden with salty, water-saturated sediments… pic.twitter.com/cjTSG9Cv9Q— ESA Science (@esascience) July 25, 2018
"The real problem was that observations over the same place in different moments would reveal different results," said Roberto Orosei, a co-investigator of the MARSIS instrument at the University of Bologna and lead author of the study. "So we would see a bright reflection on a certain area one day, and then we would fly over the same area maybe a week later, a month later… and we would not see the same strong reflection we would see ordinarily."
"It was something that would really be frustrating in hindsight because we were of course spending years and years debating the possible causes, when in fact the solution really was to change the processing scheme of the spacecraft."
Sadly, the scientist who designed MARSIS and who was one of the most avid proponents of the Martian polar subglacial lake idea, Giovanni Picardi, died in 2015, only months before data compilation was completed. "It was really tragic that he could not live to see the end of this work," Orosei told Popular Mechanics.
La sonda europea "Mars Express" detecta un lago de agua líquida bajo capas de hielo y polvo en la región del polo sur de Martehttps://t.co/VhSTQiUl85— Troposfera.org (@Troposfera) July 25, 2018
The scientists remain optimistic that their find is not unique. "The lack of previous radar detections of subglacial liquid water has been used to support the hypothesis that the polar caps are too thin for basal melting and has led some authors to state that liquid water may be located deeper than previously thought," the study says.
However, the authors note that "the limited raw-data coverage of the SPLD (a few percent of the area of Planum Australe) and the large size required for a meltwater patch to be detectable by MARSIS (several kilometers in diameter and several tens of centimeters in thickness) limit the possibility of identifying small bodies of liquid water or the existence of any hydraulic connection between them… there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location."
That's big news for the search for life on Mars, since life has been observed in similar environments here on Earth. "There are microorganisms that are capable of surviving well below zero even without being in contact with water, and there are microorganisms that can use the salt, presumably the salt in the water on Mars… for their metabolism," Orosei noted.
So if ET is out there, he may be wet, cold and salty.