A mission to Mars has launched today, called ExoMars. It's aiming to look for evidence of micro-organisms, which is really exciting!
It'll be doing this by looking at methane on Mars; methane breaks down in sunlight so why is it there? It's either being produced by some kind of chemical reaction between rocks and water, or by biological processes. Most methane on Earth is biological, often created by methanogenic archaea or bacteria, which are found all over the place; the bottom of the sea, deep down in the rocks of the Earth's crust, and even in rumen of cattle. Everyone knows cow burps are bad for the environment, but it's not the cows making the methane; it's bacteria!
So why do bacteria make methane? What's in it for them? They make methane by reducing carbon dioxide with hydrogen. This enables the cell to produce ATP, the same thing all cells (from bacterial to human cells) use to provide energy for all the chemical reactions they need to do. So instead of using oxygen like us (or like anaerobic bacteria) they use CO2 and make methane!
Could they be found on Mars? Yes! The conditions there would definitely support species like those found here on Earth (but obviously evolved for Martian conditions rather than Earth's), especially the extremophilic archaea and rock-dwelling organisms. If they are found there, it will create more exciting questions like are they related to life on Earth? Or did they come from a separate origin of life? Whatever happens, it will lead to really interesting missions and science in the years and decades to come. It's a really exciting time to be alive! (And to be a microbiologist!) I'll be spending the next few years eagerly awaiting the results of this mission! Good Luck team ExoMars!