Layering, Not Liquid: Astronomers Explain Mars’ Watery Reflections

There is water in many places on Mars, including most of the polar ice caps — all in the frozen form. But recently, the European Space Agency detected bright reflections beneath Mars’ South Pole Layered Deposit (SPLD), a 1.4-kilometer-thick (0.87 miles) formation of relatively pure water ice’s Mars Express orbiter.

Some scientists interpreted the observations collected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument as evidence of liquid water. However, Dan Lalich, a research associate with the Cornell Center for Astrophysics and Planetary Science in the College of Arts and Sciences (A&S), said that while such a possibility is exciting, he and other Cornell researchers argue the strong reflections are not necessarily evidenced that the SPLD contains liquid water.

Mars’ south pole ‘water’ may be reflections between geological layers.

They’ve devised an alternative explanation, which they detail in Explaining Bright Radar Reflections Below the South Pole of Mars Without Liquid Water, published in Nature Astronomy. They demonstrate that similar strong reflections can be generated by interference between geological layers without liquid water or other rare materials. The research team, which includes Alexander Hayes, associate professor of astronomy, director of CCAPS, director of the Spacecraft Planetary Image Facility, and the Louis Salvatore ’92 Faculty Leadership Fellow; and Valerio Poggiali, CCAPS research associate, wrote:

Lalich said:

But the fact remains, Lalich said, that the bright reflection exists and requires an explanation. Lalich uses radar data to study planetary surfaces, and he’s particularly interested in the recent climate evolution of Mars. He’s focused recently 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 one-dimensional modeling procedure commonly used to interpret MARSIS observations. He created simulations with layers composed 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 CO2 layers, separated by a layer of dusty ice — produced reflections as bright as the actual observations. Lalich said:

From the models, the researchers determined that the thickness of the layers and how far apart they are have a more significant impact on reflection power than the composition of the layers. While no simplified stratigraphy in the paper can explain every observation, the researchers wrote: “We have shown that it is possible to create bright reflections without liquid water.”

In 2021, Lalich contributed to research that found that under the right conditions, a class of minerals called smectites, common on Mars, could produce a reflection similar to the one observed from MARSIS. It’s essential to figure out what’s not liquid water on Mars, Lalich said, because the stakes are so high: He said:

Liquid water could also have important implications for the age of the polar cap, the internal heating of Mars, and how the planet’s climate has evolved in the geologically recent past — and Lalich does not rule it out entirely. Lalich concluded:

Provided by Kate Blackwood, Cornell University [Note: Materials may be edited for content and length.]

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