The wolf should be obvious: why I think we really found water on Mars this time

As I mentioned on Friday, when I first heard about the Italian Space Agency’s announcement of water on Mars, I was skeptical. Various space agencies have cried wolf on major discoveries before – most famously, with “NASA Confirms Evidence That Liquid Water Flows on Today’s Mars (it’s actually sand) and Discovery of “Arsenic-bug” Expands Definition of Life (it wasn’t, and it doesn’t). This is not a conspiracy — it’s just overexcitement. Scientists work hard to keep themselves free of confirmation bias, but they’re still human, and sometimes they still see what they want to see.

Given this history, how do we know that it really is a wolf this time? I’ve found that it helps to ask the obvious question.

Aside… This is what bothers me most about global warming deniers. They will go on for pages and pages about July temperatures in Paraguay, without even trying to answer the obvious question: why did global temperatures start to increase at exactly the time when we started releasing into the atmosphere a gas that is known to increase temperatures?

In the case of water on Mars, here is the obvious question. We know for sure that there is liquid water on one of the nine planets in the Solar System: here on Earth. The research team claims that there is liquid water under the polar ice caps on Mars. Could the same techniques they used have detected water under Earth’s polar ice caps, where we know there is water?

It’s right there in the second sentence of the paper that published the announcement (Orosei et al. 2018): “Radio echo sounding (RES) is a suitable technique to resolve this dispute, because low-frequency radars have been used extensively and successfully to detect liquid water at the bottom of terrestrial polar ice sheets.”

The technique they used is the IN SPAAAAAAAAAAAACE version of a commonly-used technique called ground-penetrating radar (GPR). GPR involves transmitting radio waves into the ground, then listening for the echoes of those waves reflecting off various underground layers. The strength of the return signals reflected off each layer tells you what the layer is made of, and nothing reflects quite like water. And that water-related pattern is exactly that kind of reflection that the research team saw on Mars.

The radar image that proves there is water under Mars's south polar cap; it shows up as an underground layer that strongly reflects radio waves
(A) The radar reflection profile found by Mars Express. “Surface reflection” shows the radio waves reflecting off Mars’s surface, while “Basal reflection” shows the radio waves reflecting off the water layer

(B) The same reflection measurements shown as a more traditional graph.

Source: Orosei et al. 2018. Click on the image for a larger version.

Obvious question answered, wolf found. We really did it this time!

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