A team of scientists announced today that they found a large body of liquid water beneath the southern ice caps of Mars. The discovery is the result of a decade-long effort to find liquid water on Mars and one that may rewrite the odds of finding life on the Red Planet.
The body of water is about 12.5 miles across and sequestered beneath nearly a mile of ice at the south pole. Its waters are likely so salty that they remain liquid despite the frigid -90˚ F conditions at the bottom. The discovery was reported in the journal Science.
If confirmed, this would be the most significant body of liquid water found on Mars to date. The only other evidence of liquid water on Mars has been of ancient bodies of water and transient seasonal flows. Finding a current stable body of water, similar to the subglacial lakes found under Antartica, sparks hope that we may be closer than ever to discovering life on our planetary cousin.
“This lake on Mars, even if salty, is now the best site for finding extant life on Mars,” said Chris McKay, a senior scientist at NASA.
John Priscu, a microbiologist at Montana State University, says that liquid water may be the missing ingredient for life on Mars. “We can show that there’s enough energy to drive chemotrophic life—life that doesn’t need sun, but lives on chemistry,” he said. “There’s enough energy, but the problem we ran into is we didn’t have a hydraulic system to keep it going.”
Not everyone is convinced, however. A NASA spacecraft with the same mission and similar technology hasn’t detected the body of water, suggesting that it may be transient and not the permanent source that life would need to survive.
“It’s exciting and has super interesting implications if it’s validated,” says Jack Holt, a professor at the University of Arizona’s Lunar and Planetary Laboratory and an expert in the radar technology used to detect the liquid water. But, he cautions, “I’d say it’s not quite the smoking gun.”
Searching for Life
In recent years, planetary scientists and astrobiologists have been scouring the Red Planet for water, one of the main requirements for life as we know it. In their search, they’ve turned to the icy polar regions of the planet, which contain ice caps and CO2 that may insulate lakes of liquid water underneath. Researchers are keenly interested in such reservoirs since they are reminiscent of Antarctica, where subglacial lakes teeming with microbial life have recently been discovered.
Antarctica’s ice sheets were once considered a biological desert, particularly beneath its enormous volumes of ice, said Brent Christner, a microbiologist at the University of Florida. “Now, 30 years later, we know that Antarctica is essentially the largest wetland on the planet, and we view the subglacial environment as an oasis for life,” he said.
Even if this new discovery is validated, experts are lukewarm about whether the body of water on Mars would be similarly suitable for life.
“If there is microbial life operating there, it’s operating under conditions that would be at the very limits of what we know life operates under here on Earth,” said Christner.
The location and depth of the finding suggests significantly harsher conditions than in Antarctica. For one, the site is closer to the frigid surface of Mars—just under a mile down versus two and a half miles below the surface for Lake Vosok, Antarctica’s largest known subglacial lake. For water to remain liquid at those temperatures, it would have to be incredibly briny.
“The estimated temperature, which is to be debated to some degree, at the depth at which this water is occurring is said to be 205 K [-90˚ F],” said Vlada Stamenković, a research scientist at NASA’s Jet Propulsion Laboratory. “If that were to be liquid water, it would be only feasible if there will be large concentrations of salts within it.”
And high levels of salt and low temperatures do not usually bode well for microbial life. Antarctica has salty lakes, including Don Juan Pond, which is 18-times saltier than seawater, Priscu said. But in the two decades he’s spent studying it, he hasn’t been able to coax life from it. “I see cells in it, but it’s so briny that the cells—I can’t get them to metabolize,” he said, referring to the chemical processes that all organisms undertake to survive.
Scouring the Poles
To detect liquid water in the polar regions of Mars, researchers needed to be able to remotely penetrate sometimes miles-thick layers of ice.
To do so, the team gathered data using a sophisticated radar sensor known as MARSIS aboard the Mars Express spacecraft. It took them three and a half years from May 2012 to December 2015—to acquire 29 radar profiles. That’s because the radar works best at night and when it’s within 500 miles of the planet’s surface. But the spacecraft’s eccentric orbit oscillates between distances of 220 miles and 6,200 miles from the Martian topography, giving the team relatively brief periods of time to probe beneath the ice.
But in those 29 profiles, they found what they say are clear evidence of a stable body of water beneath Mars’ southern polar ice cap. And finding liquid water beneath the surface of Mars, specifically under ice sheets, is exactly what scientists and engineers had hoped MARSIS would do.
The MARSIS instrument consists of a radar sounder coupled with a 40-foot antenna that sends and receive radar pulses that both bounce off the planet’s surface and penetrate up to three miles underground. It’s similar to other ground-penetrating radars used here on Earth to explore beneath the ice sheets of Greenland and Antarctica.
Ground-penetrating radar works by measuring what’s known as permittivity, which describes the electrical properties of a material through which radar travels. Certain materials, like dry rocks or CO2, have low permittivity while others, like liquid water, produce high values. That’s because polarized materials like water—which has a positive charge on the hydrogen side and a negative charge on the oxygen side—change two parameters of each radar pulse, the amount that is reflected and the speed at which radar waves move through the material. For example, a radar wave that travels through rock will reflect the penetrating pulses back to the antenna more quickly than those that travel through water. The antenna on the spacecraft receives the radar waves returned by the rock before the pulse returned by the water. That difference in time gives scientists a clue about what type of matter the radar encountered.
Mars Express scientists Roberto Orosei and his team found a 12.5 mile patch of Mars with high permittivity—so high that the numbers like that are almost always associated with liquid water, including lakes lurking under glaciers in Greenland and Antarctica. The data also ruled out liquid CO2, which is not polarized like water and thus has low permittivity.
The Mars Express hasn’t been alone in its quest. The NASA Mars Reconnaissance Orbiter also has been pinging the Martian surface with radar waves looking for subsurface water. Its radar instrument, known as SHARAD, has not detected the bright batch of permittivity found by MARSIS.
However, the SHARAD technology varies slightly from the MARSIS onboard the Mars Express. “The main difference between the two radars is the wavelength,” Holt said. MARSIS uses a wavelength that is over 300 feet and can penetrate deeper, whereas SHARAD’s is 50 feet and scours closer to the surface in more detail.
Orosei doesn’t think that SHARAD can go deep enough to pick up the water, but Holt disagreed. “If there’s really liquid water at the base, it should be showing up in SHARAD,” Holt said.
A third eye in the Martian sky could clear up this disagreement. “The Chinese mission to Mars that will be launched in 2020 carries a radar sounder that will transmit at frequencies that are intermediate betwee those of MARSIS and SHARAD,” Orosei said. “Perhaps this experiment will be able to complement MARSIS observations and continue them after the end of the Mars Express mission.”
Ultimately, confirming the findings, and detecting other sources for liquid water on Mars, may mean moving beyond radar technology, which is limited to detecting large quantities of water close to the surface.
“Even with those limitations, we’ve now found some good evidence that there might be liquid water in the Martian subsurface” Stamenkovic said. “However, that also indicates that there might be much more liquid water in the Martian subsurface in other regions which we cannot detect easily with MARSIS and SHARAD.”
Physically sampling the water would answer many outstanding questions. Drilling down to the reservoir would be a straightforward solution, but the technical challenges of drilling miles deep on a foreign planet are daunting.
For example, to bore into the ice in Antarctica to reach the life below, Priscu said he had to truck in a million pounds of equipment and fuel. Sending such a payload to Mars would be prohibitively expensive.
Still, those challenges don’t appear to discourage some scientists. “This is very big news for astrobiology on Mars,” said NASA’s McKay. “Drill, baby, drill!”
Updated July 27, 2018: Added comment from Roberto Orosei.