Mars' buried polar 'lakes' may just be frozen clay |
- Are lakes on Mars frozen?
- Does Mars have frozen water underground?
- Is there clay on Mars?
- Why is the water on Mars frozen?
For decades, scientists have suspected
that water lurks below the polar ice caps of Mars, just as it does here on
Earth.
In 2018, researchers using the MARSIS radar sounder instrument on the European Space Agency's Mars Express spacecraft detected evidence of a lake hidden beneath the Red Planet's south polar ice cap, and in 2020, they found signs of a number of super-salty lakes there. If these lakes were remnants of water that were once on the surface, these reservoirs may have once harbored life and may still, the scientists noted.
However, in order to form and maintain
liquid water at this spot on Mars, an implausible amount of heat and salt may
be needed, given what is currently known about the Red Planet, according to the
lead author of the new study, Isaac Smith, a planetary scientist at York
University in Toronto, and his colleagues.
York and his team say that clay minerals
known to exist in the south polar region of Mars can explain these radar
reflections without invoking lakes of water.
"Among the Mars community, there has
been skepticism about the lake interpretation, but no one had offered a really
plausible alternative," Smith told Space.com. "So it's exciting to be
able to demonstrate that something else can explain the radar observations and
to demonstrate that the material is present where it would need to be. I love
solving puzzles, and Mars has an infinite number of puzzles."
The scientists focused on minerals known
as spectates, a kind of clay whose chemical composition is closer to volcanic
rock than to other types of clay. Spectates form when eroded volcanic rocks
undergoes mild chemical changes after interacting with water. These clays can
hold large amounts of water, they noted.
Spectates are extremely abundant on Mars,
concentrated mostly in its southern highlands. "On Earth, they're commonly
found near volcanoes in Alaska or Central America, but can be found on every
continent," Smith said.
In the lab, the researchers cooled spectates
to minus 45 degrees Fahrenheit (minus 43 degrees Celsius), the kind of cold one
might find on Mars. They found that water-laden spectates could generate the kind
of bright radar reflections detected by MARSIS (short for "Mars Advanced
Radar for Subsurface and Ionosphere Sounding"), even when mixed with other
materials.
When Smith and his colleagues analyzed
previous visible and near-infrared data collected from Mars's south pole, they
also found evidence of spectates there. They suggested that spectates formed at
Mars' south pole during warm spells when the area was covered by water.
These water-loaded clays were later buried in underwater ice.
"Looking backward in time, to
when Mars was much wetter, this supports evidence that liquid water was
present over a larger area than we anticipated," Smith said. "Because
these clays are at and beneath the south polar cap, it must have been warm
enough there long ago to support liquids."
All in all, the researchers suggested that
spectates are a more viable explanation for the bright radar reflections seen
there instead for super-salty lakes.
"Science is a process, and scientists
are always working towards the truth," Smith said. "Showing that
another material besides liquid water can make the radar observations doesn't
mean that it was wrong to publish the first results in 2018. That gave a lot of
people ideas for new experiments, modeling, and observations. Those ideas will
translate to other investigations of Mars and already are for my team."
In the future, "I would like to repeat the measurements at even colder temperatures and with a more diverse set of clays," Smith said. "There are other types of clays found on Mars that I suspect can also make these reflections, and it would be good to follow up with them."
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