A Mars orbiter has detected a wide lake of liquid water. That lake lies hidden below the planet’s southern ice sheets. There had been tiny, brief signals of water on the Red Planet before. But if confirmed, this lake marks the first discovery of a long-lasting store of liquid water, not just ice.
“This is potentially a really big deal,” says Briony Horgan. She’s a planetary scientist at Purdue University in West Lafayette, Ind. “It’s another type of habitat in which life could be living on Mars today,” she explains.
The lake is about 20 kilometers (12.4 miles) across. That’s what planetary scientist Roberto Orosei of the National Institute of Astrophysics in Bologna, Italy and his colleagues reported online July 25 in Science. But the lake is buried under 1.5 kilometers (almost a mile) of solid ice.
Orosei and his colleagues spotted the lake by combining data collected over more than three years. The observations had come from the European Space Agency’s orbiting Mars Express spacecraft. An instrument called MARSIS — which stands for Mars Advanced Radar for Subsurface and Ionosphere Sounding — aimed radar waves at the planet. These were able to peer beneath the ice.
As the radar waves passed through the ice, they bounced off different materials embedded in the glaciers. The brightness of the returning echo told scientists about the material doing the reflecting. Notably, liquid water makes a far brighter echo than either ice or rock.
Orosei’s team combined 29 radar observations. They were made between May 2012 and December 2015. A bright spot emerged in the ice layers near Mars’ south pole. It was surrounded by much less reflective areas. The researchers considered other explanations for the bright spot. Perhaps the radar had bounced off of some carbon dioxide ice at the top or bottom of the sheet, for instance. In the end, the team decided such alternative explanations options wouldn’t produce the same radar signal or were too much of a stretch to be likely.
That left one option: A lake of liquid water.
Lakes have been discovered in the same way beneath the ice in Antarctica and Greenland.
“On Earth, nobody would have been surprised to conclude that this was water,” Orosei says. “But to demonstrate the same on Mars was much more complicated.”
A big, cold, salty pool
The lake is probably not pure water. One reason: Temperatures at the bottom of the ice sheet are around –68° Celsius (-90.4° Fahrenheit). At that temperature, pure water would be frozen, even under the pressure of so much ice. But if a lot of salt were dissolved in the water, the freezing point could prove much lower. Salts of sodium, magnesium and calcium have been found elsewhere on Mars. If they were here, too, they might help to keep this lake liquid.
The pool also could be more mud than water. Still, Horgan says, that might be an environment able to support life.
Previously, scientists have discovered extensive sheets of solid water ice under the Martian soils. There also were hints that liquid water once flowed down cliff walls (although those might have been tiny dry avalanches). The Phoenix lander saw what looked like frozen water droplets near Mars’ north pole in 2008. Scientists suspect, however, that the water was melted by the lander itself.
“If this [lake] is confirmed, it’s a substantial change in our understanding of the present-day habitability of Mars,” says Lisa Pratt. She’s NASA’s planetary protection officer. (Such people look to keep spacecraft from contaminating planets with life from elsewhere.)
How deep the newly discovered lake is remains unclear. Still, its volume dwarfs any previous signs of liquid water on Mars, notes Orosei. The lake has to be at least 10 centimeters (4 inches) deep for MARSIS to have noticed it. That means it could contain at least 10 billion liters (2.6 billion gallons) of liquid water. That’s roughly the volume of water contained by 4,000 Olympic size swimming pools.
“That’s big,” Horgan says. “When we’ve talked about water in other places, it’s in dribs and drabs.”
A decades-long hunt
Under-ice lakes on Mars were first suggested in 1987. The MARSIS team has been searching since Mars Express began orbiting the Red Planet in 2003. Sill, it took the team more than a decade to get enough data to convince themselves the lake was real.
For the first several years of observations, limits in the spacecraft’s computer forced the team to average hundreds of radar pulses together before sending those data back to Earth. That tactic sometimes cancelled out the lake’s reflections, Orosei says. The result: On some orbits, the bright spot was visible. On others, it wasn’t.
In the early 2010s, the team switched to a new technique. This one let them store the data, then send it to Earth more slowly. Three years ago, months before the end of the observing campaign, the experiment’s principal investigator died unexpectedly.
“It was incredibly sad,” Orosei says. “We had all the data, but we had no leadership. The team was in disarray.”
To have finally turned up the lake is “a testament to perseverance and longevity,” says Isaac Smith. He’s a planetary scientist in Lakewood, Colo., who works for the Planetary Science Institute. “Long after everyone else gave up looking,” he notes, “this team kept looking.”
Still, there is room for doubt, says Smith. He works on a different radar experiment for NASA’s Mars Reconnaissance Orbiter, or MRO. It has seen no sign of the lake, even in 3-D views of the poles taken with CT-like scans. It could be that MRO’s radar is scattering off the ice in a different way. It’s also possible that the wavelengths it uses don’t penetrate as deeply into the ice. The MRO team will look again. Having a specific spot to aim for is helpful, he says.
“I expect there will be debate,” Smith says. “They’ve done their homework. This paper is well earned.” Still, he adds, “We should do some more follow-up.”
3-D Short for three-dimensional. This term is an adjective for something that has features that can be described in three dimensions — height, width and length.
Antarctica A continent mostly covered in ice, which sits in the southernmost part of the world.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
calcium A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter burns (including fossil fuels like oil or gas). Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
colleague Someone who works with another; a co-worker or team member.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
echo To bounce back. For example, sound bouncing off walls of a tunnel, and returning to their source. Radio waves emitted above the surface can also bounce off the bedrock underneath an ice sheet — then return to the surface.
environment The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).
glacier A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. In June 2009, Greenland became an independent nation. Ice covers roughly 80 percent of it. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet).
habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
ice sheet A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
ionosphere A layer of Earth’s atmosphere lying around 75 and 1,000 kilometers (47 and 620 miles) above Earth’s surface. It absorbs the sun’s harmful extreme-ultraviolet rays. That energy strips electrons from atoms and molecules, creating a zone full of free-floating ions. The share of ions present, here, affects radio and other signals passing through it.
lander A special, small vehicle designed to ferry humans or scientific equipment between a spacecraft and the celestial body they will explore.
magnesium A metallic element that is number 12 on the periodic table. It burns with a white light and is the eighth most abundant element in Earth’s crust.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
NASA (Short for the National Aeronautics and Space Administration) Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
orbiter A spacecraft designed to go into orbit, especially one not intended to land.
perseverance The quality that allows someone to continue trying to do something even though it is difficult
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
planetary science The science of planets other than Earth.
poles (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates.
pressure Force applied uniformly over a surface, measured as force per unit of area.
radar A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.
Red Planet A nickname for Mars.
reflective Adjective that refers to the ability of something to reflect light strongly. Such objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake.
salt A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine.
sodium A soft, silvery metallic element that will interact explosively when added to water. It is also a basic building block of table salt (a molecule of which consists of one atom of sodium and one atom of chlorine: NaCl). It is also found in sea salt.
wave A disturbance or variation that travels through space and matter in a regular, oscillating fashion.
wavelength The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.