SOMETHING doesn鈥檛 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鈥檚 no reason to think the laws of physics are different on Mars. And yet, we can鈥檛 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, ). 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.
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Today, Mars鈥檚 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鈥檚 thin atmosphere and distance from the sun keep it, on average, at about -61掳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.
鈥淓very time we think we understand it, a new complication throws a wrench into the models鈥�
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掳C.
Not that the early Martian atmosphere was pure CO2. Last week鈥檚 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鈥檛 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鈥檚 paper offered another alternative: water salty enough to remain liquid even at water-freezing temperatures. Then the atmosphere wouldn鈥檛 need much CO2.
But this too could fall short. Ultra-saline water can flow 鈥� on Earth at least 鈥� but a Mars that cold wouldn鈥檛 allow enough precipitation to account for the standing water etched into Mars鈥� sandstone and mudstone over millions of years.
So is there some planetary mechanism we still don鈥檛 understand? A mixture of greenhouse gases we haven鈥檛 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鈥檙e running out of obvious solutions. We鈥檙e going to be in truly alien territory when the mystery is solved.
This article appeared in print under the headline 鈥淲hy water on Mars still doesn鈥檛 make sense鈥�
