I have been interested in discoveries since childhood. Our local newspaper had a weekly page that summarized scientific advances, and I still remember a special about scientists looking for life on Mars. Much to my disappointment, they weren’t looking for little green men; instead, they were looking for water. Almost 30 years later, they’re still looking.
Why is water so important? Mainly, it is because life, as we know it, depends on water. Not all lifeforms on Earth need oxygen or light, but all of them require water.
However, is the quest for outer space water just a shot in the only direction that we know? Doesn’t it ignore the fact we depend on water only because we evolved on a` planet covered with it? If life would have evolved without water, it should not require water, right?
Well, I hear this claim now and then, and it seems logical only on the surface. However, there is more to water as the cradle of life than just pure coincidence. We can assume that the laws of physics and chemistry are consistent across the universe. They are universal.
If this is so, water will be as important out there as it is down here. Here are four reasons why water is essential for life:
- Water is an excellent solvent. Many molecules dissolve in water. This means substances disintegrate down to the level of individual molecules when in water. That makes these molecules available for chemical reactions with other dissolved molecules. In fact, life depends on chemical reactions. Millions of them occur in the human body, and chemical reactions need a solvent. Water is among the best ones out there. Of course, they are not the only ones. Solvents, such as liquid ammonia or even hydrogen sulfide (H2S instead of H2O), are proposed, yet they may lack other properties that are essential to bear life.
- Water molecules stay together with the right amount of force. Not too strong and not too weak, water molecules are slightly polar, which means they have negative and positive ends. They are similar to magnets, but not anywhere near as strong. This means that water molecules form brief bonds with other water molecules, just to let them go and create brief bonds with other molecules around them. That makes water, how to say it, watery. It flows as a whole, but it also splashes. If they flow, they don’t break apart and travel as a whole, yet they don’t cling to each other so much as to slow the flow. This is essential for the transportation of materials, such as oxygen and nutrients and the building blocks of bodies, inside living things.
- Water needs a lot of heat to warm up. Maintaining a stable temperature is an essential task for life, and living inside the water (and consisting mainly of water) makes this task easier. In physics terms, water has a high heat capacity. This means that, compared to alternative other substances, water needs to gain a lot more energy to heat up and must shed a lot more energy to cool down. Place a metal object in the sun, and it is likely to become too hot to stand on rather soon. Place a glass of water in the sun, and it will unlikely become too hot to drink. All in all, this means that water’s temperature is more stable than that of other liquids, such as methane or ammonia, which are also proposed as potential life harbors. For organisms that consist of water, it means less effort is needed to maintain body temperature. For the Earth, this means a more stable climate.
- Water becomes lighter when it freezes. Most substances, when they become solids, become denser. That derives from the natural properties of all materials—they shrink when it gets colder. But not water, no. Water becomes less dense, and thus lighter, when it freezes. The consequences are straightforward: ice floats. That is why lakes and oceans freeze from top to bottom, not the other way round. Under the North Pole, there is a large amount of water full of life. This would not be possible if ice were sinking into the water. Without ice floating, life would not survive for long. Even more so, it probably would never arise in the first place.
In addition to the above, water has other properties that are essential for life. However, most of the others may be crucial for life as we know it. What I have listed above are the most fundamental properties that make water the most suitable substance for life, regardless of where in the universe it evolves.
While other liquids, such as liquid ammonia or the methanol lakes on Saturn’s moon Titan are also on extraterrestrial life-seekers’ radar, water remains their best bet.