Earth is currently the only known place where life exists in the universe. This year, the Nobel Prize in Physics was awarded to three astronomers who proved nearly 20 years ago that planets are common around stars outside the solar system. Life exists in various forms, from humans to the ubiquitous microorganisms that inhabit almost every square centimeter of planet Earth, affecting almost everything that happens on it. It will likely be some time before it becomes possible to measure or detect life outside the solar system, but the solar system offers many places that can give us an idea of how difficult it is to find life.
Mars is at the top of this list for two reasons. First, it is relatively close to Earth compared to the moons of Saturn and Jupiter (which are also considered good candidates for detecting life outside of Earth in the solar system and are slated for research in the next decade). Secondly, Mars is extremely interesting to observe because it lacks a dense atmosphere like Venus, and there is still pretty strong evidence that temperature and pressure on the surface of Mars fluctuates around the point of liquid water - believed to be necessary for conservation life. In addition, the observed river deltas and later measurements taken on the surface of Mars provide strong evidence that liquid water did flow on Mars billions of years ago.
Scientists are increasingly convinced that Mars was habitable billions of years ago. Whether it was inhabited by someone or is it still inhabited remains a hotly debated topic. To answer these questions, scientists are trying to understand the chemistry of water that created the minerals seen today on Mars that were created billions of years ago.
Salinity (how much salt was present), pH (a measure of the acidity of the water), and redox state (roughly a measure of the content of gases such as hydrogen [H2, which are called reducing media] or oxygen [O2, which are called oxidizing media; these two types, as as a rule, mutually incompatible], are fundamental properties of natural waters, for example, the modern Earth's atmosphere is highly oxygenated (contains a large amount of O2).
Recent remote sensing measurements on Mars suggest that its ancient environment may provide insight into the early habitability of Mars. In particular, the properties of pore water in sediments apparently deposited in former lakes in the Gale Crater on Mars suggest that these rocks were formed in the presence of liquid water, the pH of which was close to that of Earth's modern salt oceans. Earth's oceans are, of course, hosts of countless life forms, so it seems compelling that the early surface environment of Mars was the place where modern earthly life might have lived, but it remains a mystery why evidence of life on Mars is so hard to find.