Evidence of water on Mars could be the best indication yet that liquid flows on the Red Planet.

Key takeaways

  • Scientists discovered a brackish water lake beneath Mars’ south pole, hinting at ongoing water flow.
  • The lake was detected by the Mars Express probe using ground-penetrating radar.
  • Evidence shows that Mars still has liquid water today, not just in its ancient past.
  • Different radar tools faced challenges in detecting the lake, causing a lengthy investigation process.
  • The lake’s existence raises questions about potential habitats for microbial life on Mars.

Just a mile or two beneath the surface, near Mars’ south pole, lies a reservoir of brackish water swirling and churning beneath layers of ice and rock.

This subglacial lake, found by a ground-penetrating radar on the Mars Express probe, is roughly 20 kilometers (12.4 miles) broad and maybe no deeper than a meter. Its finding is the most recent piece of evidence that implies water was not only present on Mars in the past, but is continuously flowing in some form today. If further observations corroborate the findings, it would be the most important discovery of liquid water on Mars to date.

Water was not only present on Mars in the past but is still flowing in some capacity today

Scientists had little doubt that there was once liquid water on Mars, given to small spherical deposits found by the Opportunity rover in 2004 and extensive mineralogy investigations undertaken by the Curiosity rover. Evidence implies that enormous lakes and rivers dominated Mars’ surface billions of years ago. Furthermore, fascinating signs continue to point to the presence of liquid water on Mars today. Condensation was recorded on the Phoenix lander in 2009, and black streaks on Martian dunes may be evidence of briny water (although more recent research says they are avalanches of dry sand).

A report in Science describes the finding of a subsurface water deposit, which shows that water exists beneath Mars’ red dunes. Perhaps the Red Planet possesses complete subterranean lake systems, such as those found underneath Antarctica.

A Breakthrough 15 Years in the Making

Mars Express, a European Space Agency spacecraft, has been circling the fourth planet from the sun since December 2003. A year and a half after its arrival, the vessel erected two 20-meter radar booms, resulting in a 40-meter antenna. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) equipment became online. Since then, MARSIS has used radar signals to understand more about Mars’ inner structure and composition.

NASA/JPL/Corby Waste The Mars Express spacecraft with its 40-meter MARSIS antenna deployed.

In 2007, the instrument discovered something fascinating. Strong radar signal reflections, or discrepancies between two layers known as echoes, were discovered near Mars’ southern polar ice cap. Strong echoes suggest a significant variation in dielectric permittivity.

There are microorganisms that are capable of surviving well below zero

“In the case of Mars, and also in the case of Earth, liquid water is the material that would certainly produce the strongest reflections,” says Roberto Orosei, co-investigator of the MARSIS instrument at the University of Bologna in Italy and lead author of the new study.

The powerful radar echoes observed in 2007 were caused by a layer of ice carbon dioxide on Mars’ surface rather than liquid water beneath. “CO2 ice is very transparent and allows the radar pulse to penetrate into the ice much better than pure water ice,” Orosei explains, which is why it replicated the signal expected from subsurface liquid water. However, he argues, many people thought the CO2 findings “were not the end of the story.”

Continuous observations proved tough. “The real problem was that observations over the same place in different moments would reveal different results,” Orosei reflects. “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.”

Finally, the MARSIS engineers determined what was preventing their observations. It was how their own spacecraft handled the data. “The radar by itself will produce a very high amount of data… [so] it’s necessary to sum together many pulses, more than 100 at a time,” Orosei notes. Because of this approach of averaging the information, the data occasionally retained a small, high reflectivity patch, while other times it inexplicably vanished.

“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,” Orosei says.

Even after this discovery, the team spent years doing more observations of the south pole area. Mars Express does not repeat the exact same orbit of Mars, therefore significant periods of time can separate flights over the same location. Furthermore, the crew was only able to collect relevant measurements at low altitudes and when the spacecraft was on Mars’ night side, as the light stimulates the planet’s ionosphere, which blocks radar transmissions. Overall, it took three and a half years to collect 29 observations of the target area.

Giovanni Picardi, the scientist who built MARSIS and remained sure that the signals referred to a subglacial lake, died during those years. “Professor Picardi… died in 2015, just a few months before completing the data collection above the area. So it was quite unfortunate that he did not survive to see the completion of this effort,” Orosei explains.

Even if Picardi wasn’t present, the MARSIS team fulfilled his ambition by revealing a freezing pool of liquid water beneath Mars’ southern polar ice cap. The ramifications for complex subsurface lake systems might reignite the debate over whether microbes can still survive beneath Mars’ surface today.

“It was a long march, a sort of long march through the desert, and we are finally out of it,” Orosei says. “It’s like a dark cloud has gone away.”

USGS Astrogeology Science Center/Arizona State University/ESA/INAF/Davide Coero Borga Artistic impression of the Mars Express spacecraft probing the southern hemisphere of Mars,superimposed to a color mosaic of a portion of Planum Australe. The study area is highlightedusing a THEMIS IR image mosaic. Subsurface echo power is color coded and deep bluecorresponds to the strongest reflections, which are interpreted as being caused by the presence of water.

Lakes Upon Lakes?

According to the study, Mars’ subglacial lake is quite cold and salty. Water ice at the melting point is opaque to radio waves, implying that the ice above the lake is much below freezing. As a result, the subglacial water must be at least -10°C.

To make this viable, the reservoir must be saturated with salts, most likely sodium, magnesium, and calcium salts found on the Martian surface. These salts may lower the melting point of water to minus 74 degrees Celsius, therefore the subglacial lake is most likely between this temperature and negative 10 to 20 degrees.

“From what I think we have learned about this subglacial lake, the most likely analogue for this environment is the subglacial lake that was recently discovered in Canada… in which the lake itself is in contact with a deposit of salt, and so it is very very salty,” Orosei says.

ESA/INAF/Davide Coero Borga Artistic impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed on a radar cross section of the southern polar layered deposits. The radar cross section has been tilted 90°. The leftmost white line is the surface radar echo, while the light bluespots along the basal radar echo highlight areas of very high reflectivity, interpreted as being caused by the presence of water.

Subglacial lakes on Earth have been shown to host life in some situations. “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.”

The most exciting idea is that this Martian lake is part of a larger network that extends over Mars’ south pole. “Data provide some hints that this single lake is not a unique finding,” Orosei argues. “We do see bright spots in other areas.” However, MARSIS lacks the technological power to search for channels or rivers that connect a network of lakes.

“It would be a very different story if this was just an isolated patch of water produced by some thermal anomaly in the crust, let’s say, or if indeed the conditions under the polar caps are such that you have a fully connected hydraulic system like you have in Antarctica, in which most of the subglacial lakes are connected to each other so that material, and even life, potentially, could move from place to place,” according to Orosei.

However, before we consider such a possibility, follow-up observations must prove that this single lake exists. Because, despite all of the interesting data gathered by MARSIS, another radar circling Mars, the Shallow Radar (SHARAD) on NASA’s Mars Reconnaissance Orbiter (MRO), was unable to locate the subterranean reservoir.

“The problem is SHARAD is operating at a much higher frequency, and we were almost shocked to discovered that SHARAD was seeing absolutely nothing on the areas in which we were seeing the bright reflections with MARSIS,” according to Orosei. The MARSIS team believes that abnormalities on the surface of the southern polar ice cover may deflect SHARAD radar signals, which operate at 20 megahertz compared to MARSIS’s four or five megahertz, preventing it from spotting the water reservoir.

NASA/ESA/The Hubble Heritage Team/STScI/AURA​ An image of Mars taken with the Hubble Space Telescope. The southern polar ice cap is clearly visible on the planet, stretching for hundreds of kilometers.

Daniel Nunes, the Instrument Scientist for SHARAD at NASA’s Jet Propulsion Laboratory, who did not participate in the current work, confirmed that SHARAD has had difficulty imaging Mars’ southern polar ice cover.

“There’s something intrinsic in the southern polar cap that causes the SHARAD signal to see not as deeply as it does in the northern polar cap,” Nunes told the popular magazine Popular Mechanics. “As a result, using SHARAD, we do not perceive the interface between the ice and the underlying surface. So if there were a lake there, SHARAD would not see it.”

However, Nunes emphasizes that we must thoroughly examine the MARSIS team’s results before drawing any firm judgments.

“I think there’s going to be a healthy debate on whether this interpretation is correct,” Nunes advised. “One thing we need to be careful about is that there are different types of materials that can produce large reflections… [and] I think the new processing has to withstand verification by the community,” he says, referring to the MARSIS team’s new data processing to remove errors from averaging.

Regardless, Nunes describes the MARSIS team’s results as “a very exciting development.”

A new Mars spacecraft under development may be able to unravel the enigma. The 2020 Chinese Mars Mission will contain a radar sounding equipment that will operate at frequencies between MARSIS and SHARAD. If this Chinese mission detects radar reflections indicating a subglacial lake in the same area as MARSIS, the ramifications for potential water ecosystems beneath Mars’ surface will be substantial.

“I can only say that there is a new open door we have to go through and explore what’s behind that, and from the results we get, we will learn if Mars really is connected to the Earth somehow in terms of the persistence of a habitat for life,” Orosei tells me.

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