3. What is life?

3.4. Why is water so important for life as we know it?

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Lower Grade

Grades K-2 or Adult Naive Learner

Have you noticed that everything alive needs water? Your pets, trees, and your family, too. So why do you think that is? It’s true that our bodies and other living things are made from all sorts of different things but water makes up a lot of it. Also, when you look at a globe of Earth, there really is a lot of water! Scientists have found that all living things need water. So, if we would like to try to find living things from some other planet, then maybe we should look for places that also have water.

Grades 3-5 or Adult Emerging Learner

All living things on Earth need water. Everything from people, plants, animals, and even mushrooms and things too small to see need water to survive. Even things like cacti in a desert need some water to live. Water seems to be very important for life. Have you ever looked at a globe – there is a lot of water! Since all living things that we know of need water and many of us want to know if there could be other living things out there beyond Earth, then one thing we can do is investigate places with water beyond Earth. Mars had rivers and lakes of water a long time ago and there could still be some water deep underground there today. Also, there are some moons around other planets that have lots of water. We need to continue to investigate these places and more because if there is water, then there might be life, too.

Grades 6-8 or Adult Building Learner

One attribute that is common to all living things that we know of is the need for water to survive. Even organisms that live in very dry places need water to live. Why? Why is water so important to living things? One reason that water is so important is because it’s a liquid. This means that it can move around the materials that are needed for chemical reactions. These reactions include the ability for cells to get energy in and to move waste out.

Water is made up of two atoms of hydrogen and one atom of oxygen that make a molecule. But water is a certain type of molecule called a “polar molecule”. Polar molecules have one side of the molecule that’s a bit more positively charged and the other side a little more negatively charged. This allows water to more easily break apart or dissolve other molecules. That’s why you can dissolve salt in water! There are actually a lot of other liquids that can do this but water has other advantages as well. Water is very plentiful. About 70% of the surface of Earth is covered by water, from our oceans and our lakes to our rivers and streams. It turns out that water is also plentiful in the solar system. Comets have a lot of water within them, there is ample evidence that liquid water existed on ancient Mars in great quantities, and many moons have water under their surfaces.

Water is super important for life as we know it! There might be other forms of life out there that don’t rely on water the way we do, but looking for alien life in places with lots of water is one good way to start searching for possible extraterrestrial life.

Grades 9-12 or Adult Sophisticated Learner

All life on this planet needs water to survive. Some life can live with very little water in extremely dry places but they still need water. As we strive to find life beyond Earth, it is important to consider what life on Earth tells us about where to look. Why is water so important for life? Water supports cell functions. All organisms are made of cells, from microbes to the largest animals. All of life’s functions are completed within cells. Life needs chemical reactions to take place in order to gain energy, grow, and get rid of waste. Water is a liquid which allows the chemistry of life to take place. It is also a polar molecule which allows most other molecules to be dissolved. Because of this, we call water a “solvent”. Having such a good solvent as water is critical for the functions of life. But there are also some other reasons why water is so important:

Water is plentiful! Hydrogen is the most plentiful element in the universe and oxygen the most plentiful in Earth’s crust. On Earth, about 70% of the surface is covered by water. But there’s also lots of water in other places in our solar system. For instance, we’ve found many lines of evidence that lots of water existed on the surface of Mars during its early times, and Mars currently has a lot of frozen water under its surface. Comets contain mostly water ice. There are lots of moons in our solar system that are made of a lot of water ice, and there are even some moons with liquid water oceans under their icy crusts (like Europa and Enceladus).

Water still has other advantages as a solvent for life. For instance, water stays in the liquid phase over a large range of temperatures compared to some other solvents. That allows more places to have the potential for liquid water. It also has a high heat capacity. This means that water offers some protection to organisms from quick or drastic temperature changes.

Water also has an interesting property with regard to the density of ice. For many molecules, the solid has a higher density than the liquid. So, for most molecules, the solid would sink in the liquid. But this isn’t the case with water. For water, ice is actually less dense than liquid water. This is why ice floats! If this didn’t happen, then all of the organisms that live in the bottoms of lakes in the winter time would be completely frozen. But, even worse, during times in our planet’s history when the world has become very cold (causing what we call Snowball Earth), if frozen water sank, then all of Earth’s ocean life would have become frozen and maybe died!

If we want to understand how life works, then it’s really important to understand the chemistry of water. And astrobiologists who are wondering if we’re alone in the universe need to be aware of the potential for water to be important for other kinds of life as well. Right now, we’re investigating worlds like Enceladus and Europa, Mars, and other solar system bodies that show signs of water. Also, beyond our solar system, we’re looking for exoplanets that have the potential for liquid water at their surfaces, since they might be important places for us to look for possible extraterrestrial life.

Higher Grade

Storyline Extensions

Water is so cool!

The range of temperatures at which water stays liquid is rather large compared to most other common solvents. For instance, at sea level methane freezes at -182 Celsius © and boils at -162 C (a range of 21 C) and ammonia freezes at -78 C and boils at -34 C (a range of 44 C), meanwhile water freezes at 0 C and boils at 100 C (a range of 100 C). This means that the range of temperatures where water is liquid is more than twice that of ammonia and almost five times more than that of methane.

Water has a high surface tension. This means that that the molecules at the surface of a body of water are attracted to each other and hold each other together. You can see this yourself by filling a glass with water to the very tippy top and then seeing how many more drops of water you can get into it. You’ll be surprised to find that you can actually get a good bit more water into the glass! The high surface tension of water is also why some insects, like water striders, are able to move around on top of water without sinking into it. It also is related to something called “capillary action”, which is used by many plants to draw water up from the ground against gravity.

It’s actually really rare for a solvent to be more dense as a liquid than as a solid. We know of other elements and molecules that are more dense as liquids, but it’s far more common for the solid form of a substance to be more dense.

[note: this is fairly advanced] Oxygen is a member of the group of elements known as the “oxygen family” (also sometimes called the chalcogens). These are the elements in the periodic table that are in group 16 (the vertical column starting with oxygen and going down). They include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). All of these elements can form bonds with two hydrogen atoms. However, none of the other chalcogens come close to oxygen in its large range of temperatures where it is a liquid. This is because oxygen is much more electronegative (much greedier for electrons) and makes a far more polar molecule than the others. This greater polarity leads to stronger hydrogen bonding and the greater range of temperatures for liquid water.

WATER & LIFE

Why is water so important for life as we know it?