March 28, 2019

• The Mars paradox: Why we still don't understand water on Mars

  

  How did your water flow?NASA/JPL-Caltech/Univ. of Arizona

  By Chelsea Whyte

  SOMETHING doesn’t add up. Mars has ice caps, and there is evidence in the terrain that water flowed in rivers and lakes there billions of years ago. We have a decent understanding of how water behaves on Earth, and there’s no reason to think the laws of physics are different on Mars. And yet, we can’t figure out how water could have existed in liquid form on young Mars.

  Every time we try to replicate the conditions under which the liquid water could have existed, a new complication throws a wrench into our models. Last week, yet another paper tried to chip away at the mystery (PNAS, doi.org/bzjh). And like so many before it, instead of resolving the problem, it introduced another.

  This 40-year-old mystery is known as the Mars paradox. If and when we resolve it, we might need to throw away a lot of textbooks.

  Today, Mars’s cold, rocky terrain is dry and blanketed in dust. But observations of clay minerals and remnants of lake and river deposits are unequivocal: water flowed freely between 3.5 and 4 billion years ago

  The trouble starts when you look at the conditions on Mars at that time. Even today, Mars’s thin atmosphere and distance from the sun keep it, on average, at about -61°C, cold enough to hold existing water in permanent polar deposits. Billions of years ago, under a younger, less-heating sun, it was even colder.

  So given that the freezing point of water is the same on Earth as Mars, how was Mars ever warm enough for liquid water to flow? One plausible explanation is that greenhouse gases trapped heat in the way they do on Earth. These gases could have been produced by many sources, including volcanic eruptions. The gas with the best track record of trapping heat is carbon dioxide, because we know how much high concentrations heat Earth.

  “Every time we think we understand it, a new complication throws a wrench into the models”

  The problem is, no amount of CO₂ can warm Mars enough for liquid water. Even with an atmosphere of pure CO₂, the closest you get is -33°C.

  Not that the early Martian atmosphere was pure CO₂. Last week’s paper examined sediments laid down 3.5 billion years ago, and found that the atmosphere then contained only scant amounts of carbon dioxide.

  Perhaps the maths could work if you added some methane or hydrogen? No. With that little CO₂, it doesn’t matter how much hydrogen or methane or other gases you add into the equation. You need a thick atmosphere to begin with to shield these sensitive greenhouse gases from solar radiation.

  Last week’s paper offered another alternative: water salty enough to remain liquid even at water-freezing temperatures. Then the atmosphere wouldn’t need much CO₂.

  But this too could fall short. Ultra-saline water can flow – on Earth at least – but a Mars that cold wouldn’t allow enough precipitation to account for the standing water etched into Mars’ sandstone and mudstone over millions of years.

  So is there some planetary mechanism we still don’t understand? A mixture of greenhouse gases we haven’t yet hit on? Perhaps the real trouble is our understanding of water itself. We already know it can bedevil a few laws of physics, like when colder water flows to the top of a glass. Whatever the answer, we’re running out of obvious solutions. We’re going to be in truly alien territory when the mystery is solved.

  This article appeared in print under the headline “Why water on Mars still doesn’t make sense”

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