WHAT IS LIFE If we are going to find life beyond Earth we will need to know how to recognize it Life is found everywhere on Earth from the coldest to the hottest places the highest peaks and the deepest parts of the ocean Life spans a massive range of forms including microbes fishes birds trees and mammals like us A Common Definition There is no definition of life in general but all life has things in common It has complex chemistry Life forms respond to changes in the environment around them like recoiling from something too hot or cold They make copies of themselves through reproduction They evolve or change over long periods of time as a result of natural selection They use up resources to eat and drink so that they can grow and they give out waste materials They are made up of tiny bits called cells Carbon can take more than one form Coal is almost pure carbon and the graphite we use in pencils is made from carbon Carbon can turn into diamond under extreme pressure and temperatures 6
All About Carbon One other thing all life has in common is the element carbon One of the 118 known elements carbon like the others has its own unique properties Different elements combine together in chemical reactions to make everything we know For example the gas oxygen likes to react with iron to make iron oxide or rust Carbon is able to make combinations of materials by reacting with other elements more than any other element All the chemistry needed for life involves carbon and most scientists agree that any alien life would also need this element Carbon is common on Earth and throughout the universe and so carbon based life is likely to be the only type there is The element most similar to carbon is silicon Some astronomers have argued that silicon based life which may be unrecognizable to us could exist However experiments in laboratories have shown that silicon cannot make the variety of chemicals needed for life If you could see the water content of your body you would see that we are made mostly of water Along with carbon water is an essential ingredient for life on Earth 7
ALIVE OR DEAD Life is complex Even the simplest life forms on Earth have more complex chemistry going on inside them than all nonliving chemical reactions Some life is hard to spot Many sea creatures that live inside shells can look like rocks and many of them anchor onto the same spot for life like barnacles If you poke a barnacle with a stick you will see it quickly close up its shell as it senses danger A tree is alive even though it never seems to move Its branches grow and it drops seeds Contained within each cell in every living thing are coded instructions that determine what kind of plant or animal it is part of We call this DNA DNA is unique to each species and each individual plant or animal 8
Be a Space Scientist Are crystals and fires alive Think about the properties of life on the previous two pages and the information below and consider if the crystals or fire have these properties Which two properties do they not have Crystals If you dissolve salt into water and then let the water slowly evaporate into the air the salt is left behind It starts to crystallize Tiny bits of solid salt start to appear but instead of many small bits forming the first small bits start to grow They form a crystal with flat sides making a cube With our salty water experiment you can change how the crystal grows by adding more water or more salt Just like a barnacle or a tree the crystal has changed to suit its surroundings Fires Like any living thing fire can grow eat and give out waste in the form of ashes and carbon dioxide Like animals it needs oxygen to survive Fire also moves as the flames change shape depending on their surroundings We can see a fire respond to what is around it by pouring water or more fuel on it and it recoils from wind You can use a flame to light additional fires so like life fire can reproduce crystals fire 9
