Is there life on Mars? That's the wrong question.
Every time scientists from NASA, the European Space Agency, or any other terrestrial organization bring news back from Mars of fresh discoveries in the hunt to find evidence of life on the fourth planet, I find myself mildly annoyed that the general populace's focus is trained exclusively on the quest to find the tiniest microresidue of exobiota. That's a narrow angle, one that crops out everything in the background that makes the quest possible or worthwhile. Mars is fascinating because of its broader planet-scale dynamics. It is both earthlike (the two planets rotate at roughly the same speed, their inner layers and elemental compositions are roughly equivalent, and they both have moons), and alien (Mars is relatively small and light, has no atmosphere, and orbits the sun very slowly) in some important ways. Also, the larger-order mysteries about the state of life on Mars are mostly settled. Whatever lived there has been dead and gone for billions of years, unless one generously expands the parameters of "lived." The better question is something like: Why would there have been life on Mars?
Is this question even more enticing? We have to be honest: no, of course not. Nothing down that road is as cool as life. But there is still cool stuff. I am talking of course about Mars's glacial spiders.
The most significant difference between the blue planet and the red one is that only we have a thick atmosphere that protects us from solar radiation, warms our surface, and creates the conditions for liquid water to exist. Mars used to have a real-deal atmosphere, but the production of one is reliant on constant churning at the center of the planet. Mars is smaller and composed of different elements than Earth, so that all shut down billions of years ago. Without a magnetic field, this process left solar wind to denude Mars of its atmosphere in a process called sputtering. Mars' atmosphere is a thin, cold, shrinking layer of carbon. But it wasn't always like this. Billions of years ago, there rivers and lakes, and the research suggests there was probably life, albeit only in the simplest sense. The Martian atmosphere didn't last very long, cosmically speaking. There almost certainly weren't little guys running around; more like microbes.
But! Water still exists on the surface of Mars, locked up in the form of ice. Like Earth, both of Mars's poles are covered by ice caps. That's not where all the stuff happens, but it's where the coolest stuff happens. There is ample evidence of past glaciation throughout the Martian tropics—even under different gravitational and atmospheric conditions, glacial geomorphology (or in this case areomorphology) is extremely distinct—and today, there are a bunch of glaciers in the middle latitudes.
However, those glaciers aren't really doing anything. They're just sitting there, neither growing nor shrinking. On Earth, glaciers are the most dynamic forces of erosion, cutting down the tallest mountains that plate tectonics (another thing Mars doesn't have anymore) can throw up. But that requires them to melt. It's too cold on Mars for that to happen, and the pressure is so low that any temperature increase would see the solid ice sublime. Sublimation, in the scientific and non-Freudian sense, refers to the process by which a solid turns into a gas while entirely bypassing the liquid state. So the mid-latitude glaciers are just hunkering down, covered in rock and dust, and will probably do so for another few billion years.
Mars's polar glaciers are active, though not in the erosive sense. Its north pole is relatively less interesting, simply because it's at lower elevation and northern winter is shorter, though it is larger than its southern counterpart. Like Earth, Mars's orbit is tilted, so it has pronounced hemispheric seasons. In polar winter, a thin layer of carbon dioxide ice forms atop water-ice glaciers. In polar summer, it sublimes away. When I was studying this stuff, there were many popular theories that the glaciers were themselves largely composed of carbon dioxide ice (more popularly known as dry ice), though recent studies have established the surprising purity of Martian glaciers.
This was the furthest and most specialized I ever got in my study of the geologic arts. The fluid physics of glaciers are fascinating, chiefly because they occupy a middle rung of the timescale between tectonics (slow) and water erosion (fast), and because they tear away at the Earth's surface, rather than accommodate its preferred structures and arrangements. You can always recognize a glacial valley: they are u-shaped, the imprints of earth-scooping glaciers of yore. What I liked most was the study of glaciers in extreme environments, like the tropics, New Zealand, or Mars. Whether they are the fading traces of a colder age, or the last refuge of water on a dry world, glaciers the most significant tool the atmosphere has for shaping the planet. I wanted to see the very edge of the tool.
What makes Mars's south pole so interesting is the spiders. The longer winter and stranger areological composition mean that carbon dioxide ice forms directly on the ground, not atop frozen water glaciers. In the southern springtime, temperatures increase. Due to relative difference in albedo, the ground heats up faster than the top layers of the carbon dioxide ice, causing the bottom layers to sublimate. But that gas is trapped by the rest of the still-frozen ice above it, and it has nowhere to go. So pressure builds up until it reaches a breaking point, and, bang, geyser. The composition of the geysers is mostly gas and underlying dust, which gives them their distinctive araneiform appearance. They look like spiders. David Bowie was right!
The spiders suggest a simple truth: Something doesn't need to be alive to be active. The Martian atmosphere is gone, but there is still something dynamic happening down on the surface of the planet. There might not be life on Mars, but Mars is alive all the same.
