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.