Bright reflections observed at Mars’ south pole may be clay, not water

Seeing may be deceiving.


Two years ago, planetary scientists reported the discovery of a large saltwater lake under the ice at Mars’ south pole. In 202, scientists confirmed the presence of subsurface lakes. The discovery of Mars lakes deep below the ice cap at Mars’ south pole was made using a radar instrument aboard the ESA (European Space Agency) Mars Express orbiter. 

Now, Scientists measured the area’s electrical properties with orbiting, ground-penetrating radar. Based on the data, they claim that reflections of the red planet’s south pole may be smectite, a form of hydrated clay, buried about a mile below the surface.

The research was led by Isaac B. Smith of York University, Toronto, with significant contributions by second author Dan Lalich, research associate in the Cornell Center for Astrophysics and Planetary Science in the College of Arts and Sciences.

Scientists said, “the presence of liquid water requires implausible amounts of heat and salt.”

Lalich said, “Those bright reflections have been big news over the last few years because they were initially interpreted as liquid water below the ice. That interpretation is inconsistent with other observations that imply the ice isn’t warm enough to melt, given what we know about conditions on Mars.”

“The reflection is about a mile below the Mar’s south polar subsurface, where you don’t expect as bright of a reflection. We were getting radar reflections that were much brighter than the surface. And that’s weird. It’s not something that we had seen before, and it’s not something we expected.”

The group had pored over data from the MARSIS instruments. They then used a diagnostic physical property in ground-penetrating radar called dielectric permittivity, which measures the ability to store electric energy. Then, by using the reflection strength, scientists were able to estimate the permittivity difference between the ice and the base of the polar cap.

They then compared those estimates with lab measurements of smectite.

York University’s Smith said, “Smectites are very abundant on Mars, covering about half the planet, especially in the Southern Hemisphere. That knowledge, along with the radar properties of smectites at cryogenic temperatures, points to them being the most likely explanation to the riddle.”

Lalich said“the data to confirm the hydrated clay was easily reproduced from the observed data, meaning that liquid water is not necessary to generate bright reflections. The scientists were hoping to find lakes and other geologic forms.”

“Unfortunately, that’s a bit of a downer because lakes below the ice cap would have been very exciting. We believe the smectite hypothesis is more likely, and it’s more consistent with other observations.”

Other co-authors of the study include Craig Rezza, graduate student at York University; Briony Horgan, associate professor at Purdue University; Jennifer L. Whitten, assistant professor at Tulane University; and Stefano Nerozzi, postdoctoral research associate and Jack Holt, professor, University of Arizona.

Journal Reference:
  1. I.B. Smith et al. A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice. Geophysical Research Letters, 2021; DOI: 10.1029/2021GL093618
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