In the coming decades, NASA and China intend to send the first crewed missions to Mars. Given the distance involved and the time it takes to make a single transit (six to nine months), opportunities for resupply missions will be few and far between. As a result, astronauts and taikonauts will be forced to rely on local resources to meet their basic needs – a process known as in-situ resource utilization (ISRU). For this reason, NASA and other space agencies have spent decades scouting for accessible sources of liquid water.
Finding this water is essential for future missions and scientific efforts to learn more about Mars’s past, when the planet was covered by oceans, rivers, and lakes that may have supported life. In 2018, using ground-penetrating radar, the ESA’s Mars Express orbiter detected bright radar reflections beneath the southern polar ice cap that were interpreted as a lake. However, a team of Cornell researchers recently conducted a series of simulations that suggest there may be another reason for these bright patches that do not include the presence of water.
The research team was led by Daniel Lalich, a research associate at the Cornell Center for Astrophysics and Planetary Science (CCAPS). She was joined by Alexander G. Hayes, a Jennifer and Albert Sohn Professor, the Director of CCAPS, and the Principal Investigator of the Comparative Planetology & Solar System Exploration (COMPASSE), and Valerio Poggiali, a CCAPS Research Associate. Their paper that describes their findings, “Small Variations in Ice Composition and Layer Thickness Explain Bright Reflections Below Martian Polar Cap without Liquid Water,” appeared on June 7th in the journal Science Advances.
When the first robotic probes began making flybys of Mars in the 1960s, the images they acquired revealed surface features common on Earth. These included flow channels, river valleys, lakebeds, and sedimentary rock, all of which form in the presence of flowing water. For decades, orbiters, landers, and rovers have explored Mars’ surface, atmosphere, and climate to learn more about how and when much of this surface water was lost. In recent years, this has led to compelling evidence that what remains could be found beneath the polar ice caps today.
The most compelling evidence was obtained by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument aboard the Mars Express orbiter. This instrument was designed by NASA and the Italian Space Agency (ASI) to search for water on the Martian surface and down to depths of about 5 km (3 mi). The radar returns indicated that the bright patches could be caused by layered deposits composed of water, dry ice, and dust. These South Polar Layered Deposits (SPLD) are thought to have formed over millions of years as Mars’ axial tilt changed.
Subsequent research by scientists at NASA’s Jet Propulsion Laboratory (JPL) revealed dozens of other highly reflective sites beneath the surface. The implications of these findings were tremendous, not just for crewed missions but also for astrobiology efforts. In addition to being a potential source of water for future missions, it was also theorized that microbial life that once existed on the surface might be found there today. However, the findings were subject to debate as other viable explanations were offered.
While the same bright radar reflections have detected subglacial lakes on Earth (such as Lake Vostok under the East Antarctic Ice Sheet), Mars’s temperature and pressure conditions are very different. To remain in a liquid state, the water would need to be very briny, loaded with exotic minerals, or above an active magma chamber – none of which have been detected. As Lalich said in a recent interview with the Cornell Chronicle:
“I can’t say it’s impossible that there’s liquid water down there, but we’re showing that there are much simpler ways to get the same observation without having to stretch that far, using mechanisms and materials that we already know exist there. Just through random chance you can create the same observed signal in the radar.”
In a previous study, Lalich and his colleagues used simpler models to demonstrate that these bright radar signals could result from tiny variations in the thickness of the layers. These variations would be indiscernible to ground-penetrating radar and could lead to constructive interference between radar waves, producing reflections that vary in intensity and variability – like those observed across the SPLD. For their latest study, the team simulated 10,000 layering scenarios with 1,000 variations in the ice thickness and dust content of the layered deposits.
Their simulations also excluded any of the unusual conditions or exotic materials that would be necessary for liquid water. These simulations produced bright subsurface signals consistent with observations made by the MARSIS instrument. According to Lalich, these findings strongly suggest that he and his colleagues were correct in suspecting radar interference. In essence, radar waves bouncing off of layers too close together for the instrument to resolve may have combined, amplifying their peaks and troughs and appearing much brighter.
The team is not prepared to rule out the possibility that future missions with more sophisticated instruments could find definitive evidence of water. However, Lalich suspects that the case for liquid water (and potential life) on Mars may have ended decades ago. “This is the first time we have a hypothesis that explains the entire population of observations below the ice cap, without having to introduce anything unique or odd. This result where we get bright reflections scattered all over the place is exactly what you would expect from thin-layer interference in the radar. The idea that there would be liquid water even somewhat near the surface would have been really exciting. I just don’t think it’s there.”
If so, future missions may be forced to melt polar ice deposits and permafrost to get drinking water or possibly chemical reactions involving hydrazine (a la Mark Watney). In addition, astrobiology efforts may once again be placed on the back burner as they were when the Viking Landers failed to find conclusive evidence of biosignatures in 1976. But as we’ve learned, Mars is full of surprises. While the results of the Viking biological experiments were disappointing, these same missions provided some of the most compelling evidence that water once flowed on Mars’ surface.
Moreover, scientists once suspected that the Red Planet was geologically dead, but data obtained by NASA’s InSight Lander showed that it is actually “slightly alive.” This included evidence that hot magma still flows deep in the planet’s interior and that a massive magma plume still exists beneath the Elysium Planitia region, which may have caused a small eruption just 53,000 years ago (the most recent in Martian history). Perhaps the same will hold true for briny patches of liquid water around the poles and the equatorial region.
With any luck, some of these patches may even house countless microorganisms that could be related to life on Earth. How cool would that be?
Further Reading: Cornell Chronicle, Science Advances