Water is important to life
Glass of water by Allie Ford is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
There are two big assumptions that we often make when looking for life: it will need liquid water, and it will be carbon based. Various science fiction stories have investigated life based on chemistries other than those we are familiar with (Wikipedia includes an extensive list). We’ll look at the role of carbon in a future entry. In this post, we’re going to focus on water.
What is water?
Water is a compound made up of two hydrogen atoms covalently bonded (by sharing of electrons) to an oxygen atom, hence its formula: H2O. These elements are both very common (hydrogen is the most common element in the Universe).
Why is water so important for life on Earth?
All life on Earth that we know of, uses water in some way. Even xerophiles (creatures that can survive without water for long periods) such as tardigrades still need access to water in order to function; in low-water situations they tend to enter a form of hibernation until water is once again available.
There are several chemical properties of water that make it useful for sustaining life on Earth, and possibly elsewhere:
- Water is polar
- Water has a high specific heat
- Ice is less dense than water
- Water is liquid at fairly high temperatures (compared to other possible solvents)
Let’s look at each of these properties in turn.
Water is polar
Oxygen (blue) is better at attracting negatively charged electrons than hydrogen (grey), so one end of the molecule is more negative than the other. This is called ‘polarity’.
Polar Water Molecule Image by Allie Ford is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.]
The oxygen atom in each water molecule is more attractive to negative electrons than the hydrogen atoms, meaning the oxygen end of the molecule appears more negative to any passing particles. Various other compounds are also polar, including a number of vitamins and minerals. Other compounds, such as many fats, are non-polar, with an even distribution of charge over their own molecules.
In chemistry we often say that ‘like dissolves like’, meaning that polar solvents tend to dissolve polar substances, but are less good at dissolving non-polar compounds, and vice versa.
Lipids, fat-related molecules which are important in many biological organisms, include both polar and non-polar regions in their molecules. This leads to interesting properties; the polar parts of the molecule tend to be attracted to water (hydrophillic, meaning water loving) while the non-polar parts are hydrophobic (water fearing), and will preferentially position themselves away from water.
When the hydrophobic ends of many lipid molecules clump together to avoid water, a primitive type of cell forms, with a hydrophillic layer on the ‘outside’ near the water, and the hydrophobic ends in the centre. Eventually more complex structures can form out of larger collections of lipids. So the polarity of water was important in the formation of cells and membranes in living systems.
Water has a high specific heat capacity
The specific heat capacity of a substance is the amount of energy required to change the temperature of 1kg that substance by 1oC. For water, the specific heat capacity is 4181 J/kg/oC. Compare this to oxygen (918 J/kg/oC), or lead (128 J/kg/oC). What this means is that it takes a lot more energy to heat 1kg of water than it does to heat oxygen or lead. Other common solvents such as ethanol and methanol also have lower specific heat capacities than water (around 255 J/kg/oC each). This property is useful. It helps with our own internal temperature maintenance systems, meaning we don’t lose heat from our internal organs quickly, even when it is cold. It is also useful in other natural systems, such as lakes and oceans, allowing them to change temperature gradually with environmental conditions, rather than the sea temperature suddenly jumping several degrees on a particularly sunny summer day. For early life, this thermal stability was probably very important.
Ice is less dense than water
While this doesn’t seem particularly useful (except maybe for allowing you to easily show off amusing ice cubes in your summer drinks), it could have been very important earlier in our evolution. There have been numerous ice ages on Earth, for various reasons. During some, it is possible that the oceans froze almost completely over, even with the high specific heat capacity of water. If ice formed on the surface – which would be cooler than the sea floor, where Earth’s internal heat would help to maintain some warmth in the water – and sank, the process could repeat until all the water in the oceans froze. Because ice is less dense than water, it floats. This forms an insulating layer at the top of the ocean, making it more likely that the water below stayed liquid; good news for any organisms swimming around!
Water is liquid at higher temperatures than other solvents
| Solvent | Freezing point | Boiling point | Liquid range |
|---|---|---|---|
| Water | 0 | 100 | 100 |
| Ammonia (NH3) | -78 | -33 | 45 |
| Methane (CH4) | -182 | -164 | 18 |
| Ethane (C2H6) | -183 | -87 | 95 |
All temperatures in degrees Celcius.
We can see that methane, ethane and ammonia are all liquids at colder temperatures than water (we’ve found good evidence that there are methane and ethane lakes on Saturn’s moon, Titan), but there’s a slight problem; chemical reactions happen more slowly at lower temperatures. While the liquids present on Titan could facilitate chemical reactions, and even lead to creation of primitive membranes, it would likely take a lot longer for any degree of complexity to arise. Any creatures living in the lakes of Titan would likely have fairly slow metabolisms as they would have evolved to function at the very low temperatures – this makes it likely that any life we do find in the lakes of Titan would be very primitive.
Water as a defence from UV
Finally, water has probably had another key role in our evolution: it protected us from high-energy ultraviolet (UV) radiation from the Sun. Water in both our atmosphere and the oceans was probably important to early life forms on Earth. These days a lot of our protection comes from the ozone layer 30-something kms above us – but this didn’t form until life had been around for quite some time, at least long enough to produce significant amounts of oxygen which could react to form a thin band of ozone to deflect UV away from the surface. Everything from simple molecules to complex organisms would have been at risk of damage (in the case of complex creatures) or destruction (in the case of molecules) from high levels of UV radiation. Without some form of protection, it seems unlikely that life could have arisen at all. Most evidence seems to point for life having an origin deep in the oceans around thermal vents. (More on that in the future).
Conclusion
Hopefully you now have a better appreciation of the wonder of water, and its importance in making us what we are today. While it is by no means impossible for extraterrestrial life to exist in an environment without liquid water, the combination of benefits that water alone can provide seems to make it more likely that if we do find alien life, it will be in what we consider a ‘habitable’ environment: one where liquid water is common.
Background and header image credit: NASA, ESA, T. Megeath (University of Toledo) and M. Robberto (STScI)
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