All Life Needs Water. Why?
Could aliens drink something else?
Nothing on our planet can survive without water. It covers 70% of the surface and about 60% of our body is composed of nothing but water.
Water is everywhere. It’s a good thing too. Because without it, life as we know it would not be possible.
But what is it about water that makes it so essential to living things?
There are organisms on Earth that do not need oxygen to live. They metabolize hydrogen or methane or a number of other compounds. Plants breathe CO2 and get much of their energy from the Sun through photosynthesis. There is life at the bottom of the oceans which have never seen the sun and get all of their energy from hydrothermal vents on the seafloor.
But all life on Earth shares one thing in common: we all need water to live. Without exception.
Good thing it’s so abundant.
Water is found everywhere on our planet.
It’s even been found on Mars. There’s probably a little bit on our Moon and Jupiter’s moon Europa is basically a big ball of frozen water.
And when we aim our spectroscopes out at the universe, water is one of the most common molecules we find.
That alone might be enough to explain why we depend on it so much. You certainly wouldn’t want to hinge the existence of life on a comparatively rare and scarce molecule.
But its abundance is only one reason why life depends on it so much.
Water has some pretty amazing properties that make it an ideal substrate for life.
For one, water is a liquid across a wide range of temperatures on Earth. A liquid is a much better transport medium for nutrients and other life-necessary compounds than a solid. It just happens that most of the water on Earth can exist in its liquid form.
Water is also a great solvent. It has been called the universal solvent because so many different kinds of substances can be dissolved in water. All life needs to take in substances to generate energy and flush out waste products and toxins. Because of water’s knack for dissolving all kinds of substances, it makes an ideal medium for both tasks.
Water is so good at being a solvent because of its polarity.
This means the water molecule has two poles of opposite electric charge. Since each water molecule is made up of two hydrogen atoms and one oxygen, the hydrogen atoms contribute a positive charge while the oxygen contributes a negative charge, giving the molecule both positive and negative poles. This makes the water molecule not only strongly attractive to other water molecules, it will attract other polar molecules as well. This why so many substances can dissolve in water. The molecules are attracted to one another.
Water’s polarity also gives it much of the properties that make it so essential for life.
Like surface tension.
Surface tension is the tendency of water molecules to adhere to one another in such way as to make its surface act as a membrane. You see surface tension at work whenever you see an insect walking on water or whenever water beads up on a surface.
This is important to life because it means water is sticky. It tends to clump together. This allows it to be sucked up as continuous stream through the roots of plants or circulated through our blood vessels against the force of gravity.
Water is the only natural substance that can be a liquid, solid, and a gas within the tight temperatures ranges of Earth. That means we get the benefits of all three phases of water.
Because water can evaporate here on Earth, it can be distributed all over the planet. Sunlight will cause water to turn into its gaseous form. Because it is less dense, the water vapor will float up into the atmosphere. But because of it’s inherent stickiness, the vapor will tend to clump up around dust particles in the air and become clouds. Water basically hitches a ride on these particles as they are blown across the planet — to be deposited far and wide in the form of rain.
Ice is peculiar for a solid.
It is less dense than its liquid form and this means that it floats in liquid water.
If it didn’t, life wouldn’t be able to exist.
Since life got its start in water, if frozen water was denser than its liquid state that would mean that ice would sink. Ponds, lakes, and the ocean would freeze from the bottom up — killing any life within it. As it is, water freezes from the top down, allowing life to survive in the relatively warmer liquid water underneath the ice.
Anybody who has ever taken a drink of ice water on a hot day can attest to water’s cooling abilities. But it does more than quench our thirst.
Water has a high specific heat.
This means it can absorb a large amount of heat without changing its temperature much. This allows the Earth’s oceans to absorb excess heat and moderate the planet’s temperature just as the water in our bloodstream helps us maintain a constant body temperature.
Water vapor in the atmosphere also acts as a greenhouse gas, keeping the planet warm enough to sustain life.
And the amazing properties of water may have protected the building blocks of life long enough for it travel the vast distances of interstellar space, making it possible for life to get here in the first place.
And it’s because water is blue.
Small amounts of water appear clear but if you get it together in large enough quantities, water has a slight blue tinge. This is because water doesn’t absorb as much blue light.
Blue light is composed of shorter more energetic wavelengths of electromagnetic radiation. This kind of radiation can sometimes be harmful to organic molecules. Space is full of this kind of radiation. Any organic molecules exposed to this radiation would quickly be destroyed. But if those same organic molecules are encased inside the frozen interior of a giant hunk of space ice (like a comet), they could feasibly be preserved from the deleterious effects of the high energy ionizing radiation of space, allowing the building blocks of life to propagate throughout the solar system — even the galaxy.
The idea that life could be spread from planet to planet via comets or other giant chunks of ice is called the Panspermia Theory. And it depends on water’s radiation shielding properties to work.
Even if the building blocks of life weren’t delivered via comet, the deep oceans of a Hadean era Earth may have afforded just enough shielding to keep the primordial soup from being cooked by the ultraviolet rays of a young and dangerously luminous Sun.
We don’t know of any kind of lifeform that can survive without water.
That’s why NASA is so focused on finding water or the right conditions for liquid water exist on other planets. It’s the only indicator we have that life could be possible.
But that doesn’t mean all life has to have water. Just life as we know it.
There could be alien life out there that uses alternatives to water. Ammonia and methane have similar characteristics that make them suitable substitutes.
Various solutions of ammonia have a wide range of uses on Earth — from fertilizer to window cleaner. But the ammonia molecule displays the same kind of polar flexibility that water is famous for. So it could be used as nutrient intake and waste disposal medium in the same fashion. Like water, ammonia is composed of hydrogen — the most common element in the universe. But instead of bonding with oxygen, ammonia substitutes nitrogen — also very common.
But astronomers have never identified large bodies of it.
Methane isn’t a polar molecule but a lot of substances can be dissolved in it and it’s pretty abundant as well.
But both ammonia and methane are only liquid at extremely low temperatures — colder than anything experienced on Earth. These conditions do exist in our solar system though.
On Saturn’s moon Titan, the average temperature is a chilly -179 degrees celsius. That’s cold enough to freeze a bottle of rubbing alcohol solid. Yet Titan is the only object in the solar system known to sustain liquid on its surface. This isn’t water though. It’s methane. Water does exist on Titan but it’s frozen and as hard as stone.
If life existed on Titan, it would use methane in place of water. It would metabolize hydrogen and this should have an effect on the composition of the atmosphere — an effect large enough to be measured.
And there is some evidence of this.
There are smaller amounts of hydrogen in the lower levels of Titan’s atmosphere than expected — just the kind of reading we would expect to get if something was metabolizing hydrogen on the surface.
But these results are only preliminary and they’re just as likely to be evidence of how much we don’t understand of the chemical processes going on in Titan’s atmosphere as they are evidence of a lifeform using a biochemistry we have never encountered before.
So for now, life as we know it will have to make do with good old fashioned water.
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