In 1964, Mariner 4 became the first spacecraft to successfully arrive at Mars. While some scientists of the time were expecting a world much like our own, one filled with rivers, oceans, and maybe even life, Mariner’s images instead unveiled a heavily-cratered surface, not unlike the Moon’s. That’s why it was such a surprise when later NASA missions, such as Viking, benefiting from improved technology, revealed a martian landscape more like our Earth’s. These newer images showed large fluvial features in ancient terrains, including the ancient river valley networks, inferred to have formed nearly 4 billion years ago. Therefore, many have argued that Mars may have once possessed a warmer climate, perhaps with a thicker atmosphere that could have supported potentially habitable conditions. Such a warmer early Mars may have been more “Earth-like” in many ways. So, it may be natural to wonder: How Earth-like does a planet need to be to host life? Did Mars ever have life? Does Mars have life now? Scientists operating on Mars could address these and similar questions that ultimately stem from one overriding question: Are we alone in the universe?
The size and scope of the ancient martian valley networks are impressive. Often hundreds of meters deep, and extending for hundreds of kilometers, these immense fluvial features likely required a climate that was warm enough, at least transiently, to have produced enough rain (or snow) to carve them. These valleys would have been difficult to form in a cold climate because much of the water would have been present as ice instead. To be fair, salty water ― known as brines ― perhaps flowing from underground sources, could remain in liquid form at below freezing temperatures. However, it is difficult to envision how a sufficient volume of briny flows to form these massive features could be produced in a cold climate with little precipitation. If the atmosphere was also significantly thinner than the Earth’s, even if surface life had arisen in such a cold climate, that life would have had to contend with less protection from harmful radiation.