It was thought that there were lakes of liquid water under the southern ice cap of Mars.
It has been found that the bright shining radar signal could have been produced by geological layers. It's not a foregone conclusion, but it does suggest that stronger evidence is needed before we can determine what's underneath the ground.
A team of researchers led by astronomer Dan Lalich of Cornell University demonstrate that similar reflections can be generated as a result of thin layer interference.
It calls into question the likelihood of finding liquid water below the south polar layer deposit.
The European Space Agency's Mars Express satellite probe picked up a signal a few years ago. Scientists found a patch of reflective material under the southern polar ice cap.
It was suggested that whatever the first patch was, it was not unique.
There were shining patches buried under the ice of the South Pole. Lake Vostok is part of a network of lakes under the ice.
Scientists concluded that there could be similar liquid storage on Mars. Mars is thought to be too cold for liquid water, even under layers of ice, and with a freezing point lowered by salts.
The shiny patches are what they are. Lalich and colleagues conducted simulations using materials known to occur on Mars.
Radio waves are bounced at an object or surface. The properties of the materials below the surface of a planet are not always revealed by how long it takes for those radio waves to be reflected back.
The simulation layers of water ice, carbon dioxide ice, basalt and atmosphere were created to reflect radar waves in different ways.
They worked out what the signals would be. There is a layer of water ice sandwiched between two layers of carbon dioxide ice.
"I used CO 2 layers embedded within the water ice because we know it already exists in large quantities near the surface of the ice cap."
I would have gotten the same results using rock layers or dusty water ice. The point of the paper is that the composition of the layers is not as important as the thickness and separation.
There are other suggestions that the signal could have been generated by something other than water. Last year, a team found that frozen clays produced the same level of radar shine, and earlier this year, a team proposed volcanic rock as a source.
The water explanation seems to be less likely with the addition of layers of rock and ice.
It would be a shame, because liquid water on Mars would have implications for the past, present, and future of the red planet.
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Lalich says that no work has disproved the existence of liquid water down there. The interference hypothesis is more in line with other observations. I don't think a drill can prove either side of the debate.
It's not likely to happen any time soon because of the deep patches, the distance to Mars, and the difficulty of transporting things there.
The research has been published.
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