One of the myriad of unsolved mysteries about the Red Planet is why why ancient Mars had liquid water. Early in the planet’s history, Mars only received a third of the sunlight of present-day Earth, which shouldn’t be enough heat to maintain water. But in past, ancient millennia, huge rivers flowed across the planet’s surface, when its atmosphere was thicker and warmer, cutting gullies and channels on the silent, desolate landscape, unchanged for millions of years that are visible today to orbiting spacecraft. Scientists have long known that water was abundant on ancient Mars, but there has been no consensus on whether liquid water was common, or whether it was largely frozen in ice.
In 2013, planetary scientists at the European Space Agency released 3D images of the striking upper part of the Reull Vallis region of Mars, which reveal a 1500 kilometer long river running from the Promethei Terra Highlands to the vast Hellas basin. The image data from ESA’s Mars Express spacecraft shows that, at some points, the river bed is seven kilometers wide and 300 meters deep. The stereo cameras on board the satellite have also revealed “numerous tributaries” that fed the gigantic river. Today’s low atmospheric pressures on the Red Planet mean that any surface water would boil away. But recent discoveries reveal that water survives not frozen in polar ice caps and in subsurface ice deposits but also in a massive network of ancient buried lakes.
In January of 2020, Caltech astronomers probed a mysterious feature at the South Pole of Mars –a massive deposit of CO2 ice and water ice in alternating strata, like the layers of a cake, shown at the top of the page, that extend to a depth of one kilometer, buried under a thin cover of CO2 ice. This strange feature was preceded in 2018 by discovery of evidence suggesting that far beneath the deeply frozen ice cap at Mars’s south pole lies a lake of liquid water—the first found on the Red Planet. Detected from orbit using ice-penetrating radar of Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), the finding resembles the interconnected bodies of water buried under several kilometers of ice in Greenland and Antarctica, where a network of 400 lakes have been detected.
MARSIS, an instrument on the European Space Agency’s Mars Express orbiter, which launched in 2003, beams down pulses of radio waves and listens for reflections. Some of the waves bounce off the surface, but others penetrate up to 3 kilometers and can be reflected by sharp transitions in the buried layers, such as going from ice to rock.
Several years into the mission, MARSIS scientists began to see small, bright echoes under the south polar ice cap—so bright that the reflection could indicate not just rock underlying the ice, but liquid water. The researchers doubted the signal was real, however, because it appeared in some orbital passes but not others.
The spacecraft’s computer was averaging across pixels to reduce the size of large data streams—and in the process, smoothing away the bright anomalies. “We were not seeing the thing that was right under our noses,” says Roberto Orosei, a principal investigator (PI) for MARSIS at the Italian National Institute for Astrophysics in Bologna.
“It’s a very exciting result: the first indication of a briny aquifer on Mars,” said geophysicist David Stillman of Southwest Research Institute in Boulder, Colorado. The findings, if confirmed, would mark the detection of the largest body of liquid water on Mars reported Marina Koren in The Atlantic..
A lake of liquid water surrounded by smaller ponds may be buried under 1400 meters of ice near the south pole of Mars reports Nature. New measurements offer more evidence of its existence, according to Elena Pettinelli at Roma Tre University in Italy and her colleagues who used the MARSIS radar instrument then applied criteria that were used to search for buried lakes in Greenland, –where New research has increased the number of known lakes lurking beneath the ice sheet from just four to a total of 60–to examine an area called Ultima Scopuli near the Red Planet’s south pole.
The researchers spotted a liquid lake measuring about 20-by-30 kilometers, along with at least three smaller ponds, each a few kilometers across. But the resolution of the radar measurements wasn’t high enough to determine their depth.
“It was probably originally a larger, wet area, and this is the remnant of that in smaller ponds,” says Pettinelli. For the water to remain liquid at the frigid temperatures, her team suggests that it is most likely a salty brine.
“There are bacteria that can live in very awkward situations,” says Pettinelli. “In Antarctica, they found bacteria living happily in the water of the underground lakes and between the crystals of the ice, and Antarctica is our closest analogue to Mars.”
Image credit top of page: the ice-capped Martian south pole, pictured here by the Mars Express spacecraft that also carries the MARSIS radar instrument. (ESA/DLR/FU Berlin / Bill Dunford)