Planetary scientist puts Mars lake theory on ice with new study that
offers alternate explanation
Interdisciplinary investigation of the planet's south pole points to
clays being the likely culprit

Date:
July 29, 2021
Source:
York University
Summary:
For years scientists have been debating what might lay under the
Martian planet's south polar cap after bright radar reflections
were discovered and initially attributed to water. But now, a new
study puts that theory to rest and demonstrates for the first time
that another material is most likely the answer.



FULL STORY ==========================================================================
For years scientists have been debating what might lay under the Martian planet's south polar cap after bright radar reflections were discovered
and initially attributed to water. But now, a new study published in Geophysical Research Letters, led by planetary scientists from Lassonde
School of Engineering at York University, puts that theory to rest and demonstrates for the first time that another material is most likely
the answer.


========================================================================== Research led by Isaac Smith, Canada Research Chair and assistant professor
of Earth and Space Science at Lassonde School of Engineering and research scientist at the Planetary Science Institute, uses multiple lines of
evidence to show that smectites, a common type of clay, can explain all
of the observations, putting the Mars lake theory on ice.

"Since being first reported as bodies of water, the scientific
community has shown skepticism about the lake hypothesis and recent publications questioned if it was even possible to have liquid water,"
said Smith. Papers in 2018 and 2021 demonstrated that the amount of
salt and heat required to thaw ice at the bottom of the polar cap was
much more than Mars provides, and recent evidence showing these radar detections are much more widespread -- to places even harder to thaw
ice -- put the idea further into question.

The research team, which includes researchers from the University of
Arizona, Cornell, Purdue and Tulane universities, used experimental and modelling work to demonstrate that smectites can better explain the
radar observations made by the MARSIS instrument aboard the European
Space Agency's Mars Express orbiter.

Further, they found spectral evidence that smectites are present at the
edges of the south polar cap.

"Smectites are very abundant on Mars, covering about half the planet, especially in the Southern Hemisphere," said Smith. "That knowledge, along
with the radar properties of smectites at cryogenic temperatures, points
to them being the most likely explanation to the riddle." Experiments
done at York University measured the radar characteristics of hydrated smectites at room temperature and cryogenic temperatures. The radar characteristics in question are two numbers that represent the real and imaginary parts of the dielectric constant. Both numbers are important
for fully characterizing a material, but the 2018 study used modelling
that included only the real part of the dielectric value, leaving out
certain classes of materials from being considered -- namely clays.



==========================================================================
Once the experimental measurements were completed, data was evaluated
using code. It was in these simulations researchers found that frozen
clays have numbers big enough to make the reflections.

Smectites are a class of clay that is formed when basalt (the volcanic
rock that comprises most of Mars' surface) breaks down chemically in
the presence of liquid water.

"Detecting possible clay minerals in and below the south polar ice cap is important because it tells us that the ice includes sediments that have interacted with water sometime in the past, either in the ice cap or
before the ice was there," said Briony Horgan, co-author and associate professor in Earth, Atmospheric, and Planetary Sciences at Purdue
University. "So, while our work shows that there may not be liquid water
and an associated habitable environment for life under the cap today,
it does tell us about water that existed in this area in the past."
To support this new hypothesis, Smith conducted experiments in his lab
with equipment designed for measuring dielectric values. To simulate
the conditions beneath Mars's south polar cap as best as possible, his
team froze the clays to -50 C and measured them again, something that
had never been done before. Smith adds that the infrared absorptions attributable to these minerals are present in south polar orbital
visible-near infrared reflectance spectra. Because these minerals are
both present at the south pole and can cause the reflections, the team
believes this to be a more viable scenario than the presence of liquid
water. No salt or heat is required.

"We used our lab measurements of clay minerals as the input for a radar reflection model and found that the results of the model matched very well
with the real, observed data," said Dan Lalich, post-doctoral researcher
at the Cornell Center for Astrophysics and Planetary Science at Cornell University and second author on the study. "While it's disappointing
that liquid water might not actually be present below the ice today,
this is still a cool observation that might help us learn more about
conditions on ancient Mars." "We analyzed the MARSIS radar data and
identified observations with high-power values at the base of the south
polar layered deposits, both in the proposed lake region and elsewhere,"
said Jenny Whitten, co-author and planetary scientist in the Department
of Earth and Environmental Sciences at Tulane University.

"The first reason the bright reflectors cannot be water is because
some of them continue from underground onto the surface. If that is the
case, then we should see springs, which we don't," said Stefano Nerozzi, post-doctoral fellow in the Lunar and Planetary Laboratory and Department
of Geosciences at the University of Arizona and co-author. "Not only
that, but multiple reflectors are stacked on top of each other, and some
are even found right in the middle of the polar cap. If this were water,
this would be physically impossible." Putting the results in perspective
Smith says the answer is clear.

"Now, we have the trifecta. One, we measured dielectric properties of
materials that are known to exist on over 50 per cent of Mars' surface and found them to have very high values. Two, we modelled how those numbers
would respond in Mars' south-polar conditions and found them to match the
radar observations well. Three, we demonstrated that these minerals are
at the south pole. Because the liquid water theory required incredible
amounts of heat which is six-to- eight times more than Mars provides,
and more salt than Mars has, it was already implausible. Now, the clays
can explain the observations with absolutely no qualifiers or asterisks." ========================================================================== Story Source: Materials provided by York_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. I. B. Smith, D. E. Lalich, C. Rezza, B. H. N. Horgan, J. L. Whitten,
S.

Nerozzi, J. W. Holt. A Solid Interpretation of Bright Radar
Reflectors Under the Mars South Polar Ice. Geophysical Research
Letters, 2021; 48 (15) DOI: 10.1029/2021GL093618 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729143424.htm

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