Superposition, not liquid: Astronomers explain Mars’ watery reflections

There’s water in many places on Mars, including most of the two polar ice caps – all in frozen form.

But recently, bright highlights were detected beneath the surface of Mars’ South Pole Layered Deposit (SPLD), a 1.4 kilometer thick relatively pure water ice formation, by the Mars Express orbiter. European Space Agency. Some scientists have interpreted the observations, collected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument, as evidence of liquid water.

Dan Lalich, a research associate at the College of Arts and Sciences (A&S) Cornell Center for Astrophysics and Planetary Science, said that while such a possibility is exciting, he and other Cornell researchers argue that strong reflections don’t are not necessarily proof that the SPLD contains liquid water.

They found an alternative explanation, which they detail in “Explaining Bright Radar Reflections Below the South Pole of Mars Without Liquid Water” published Sept. 26 in Nature Astronomy.

Using computer simulations, they demonstrate that similar strong reflections can be generated by interference between geological layers, without liquid water or other rare materials.

“This result, combined with other recent work, calls into question the likelihood of finding liquid water beneath the SPLD,” wrote the research team, which includes Alexander Hayes, associate professor of astronomy, director of the CCAPS, director of the Spacecraft Planetary Image Facility. and Faculty Leadership Fellow Louis Salvatore ’92; and Valerio Poggiali, Associate Researcher at CCAPS.

“On Earth, such bright reflections often indicate liquid water, even buried lakes like Lake Vostok [under the surface of the East Antarctic Ice Sheet]”, Lalich said. “But on Mars, the prevailing opinion was that it should be too cold for similar lakes to form.”

But the fact remains, Lalich said, that the light reflection exists and requires explanation.

Lalich uses radar data to study planetary surfaces, and he is particularly interested in the recent evolution of the climate of Mars. He has recently focused on modeling radar reflections off the Martian polar caps, positioning him well to investigate a claim of liquid water on the planet.

Lalich used a commonly used one-dimensional modeling procedure to interpret MARSIS observations. He created simulations with layers made up of four materials – atmosphere, water ice, carbon dioxide (CO2) ice and basalt – and assigned each layer a corresponding permittivity, an intrinsic property of the material describing its interaction with electromagnetic radiation passing through it.

Simulations using three layers – two layers of CO2, separated by a layer of dusty ice – produced reflections as bright as real-life observations.

“I used layers of CO2 embedded in water ice because we know it already exists in large quantities near the surface of the ice sheet,” Lalich said. “In principle, however, I could have used layers of rock or even particularly dusty water ice and achieved similar results. The point of this article is really that the composition of the basal layers is less important as layer thicknesses and separations.

From the models, the researchers determined that the thickness of the layers and their distance have a greater impact on the reflectance than the composition of the layers. Although no simplified stratigraphy in the paper can explain all of the observations, the researchers wrote, “We have shown that it is possible to create brilliant reflections without liquid water.”

In 2021, Lalich contributed research that found that under the right conditions, a class of minerals called smectites, common on Mars, could produce a reflection similar to that observed from MARSIS.

Understanding what is not liquid water on Mars is important, Lalich said, because the stakes are so high. “If there is liquid water,” he said, “maybe there is life, or maybe we could use it for future human missions to Mars. .”

Liquid water could also have important implications for the age of the polar ice cap, the internal heating of Mars, and the evolution of the planet’s climate in the geologically recent past – and Lalich doesn’t rule it out entirely.

“None of the work we’ve done disproves the possible existence of liquid water there,” Lalich said. “We just think the interference hypothesis is more consistent with other observations. I’m not sure anything less than an exercise can prove either side of this debate definitely right or wrong.

The work was funded in part by NASA grants.

Kate Blackwood is a writer for the College of Arts and Sciences.

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