 about the requirements for life and I'll add that I'm going to talk about them as for life as we know that life on earth right, but some of these requirements we think are universal and would be present elsewhere in the universe so life is based on chemical reactions and those reactions depend on water water is the universal solvent as an oxygen of course and two hydrogens and it's got a little bit of a charge difference in it and It's a very special molecule and the way it catalyzes chemical reactions life The life works Life works with enzymes and those enzymes are made up of some specific elements and those include carbon Which really makes nice complicated chains of molecules hydrogen Nitrogen which is a key component of amino acids which make proteins and enzymes There's also oxygen phosphorus and sulfur, so life on earth Requires all of these particular elements and then the third thing is A life has metabolism it needs energy To be able to Control the chemical reactions to add biomass To reproduce and so this energy is Extracted from the environment So we can ask the question about why these three things for all for all life in the universe Why scientists often think that these these are the key things that are needed and it really comes down to the uniqueness of the combination of Liquid water behavior and that polar characteristics The elements the key one is The behavior of carbon and we think that life needs to be based on carbon Because the of the way carbon molecules bond with each other With hydrogen and with these other elements, so you can make various rings of carbon I drew one with Six but it can also have five it can have Hydrogen's attached to it nitrogen's attached to it oxygen and these rings and chains of carbon can form very complicated structure and So that those complicated structures that are chemically reactive are really good at Performing reactions, so there aren't any other elements That are commonly available as created in stars that basically have this Property the form of energy can vary so on earth photosynthesis and the use of light Changing it to chemical energy Supports our biosphere, but you can also have water reacting with rocks and Particularly things like iron and sulfur in rocks can give Chemical energy as well So you don't need light and photosynthesis to support a biosphere. You can basically Have reactions between water and often Carbon dioxide carbon monoxide Reacting with rocks to create that chemical energy and Basically this sources of energy are available sort of throughout the universe. So when we are actually looking for Life elsewhere for example on the surface of Mars or maybe under the ice on Europa We're usually looking for liquid water and We're looking for carbon carbon is a super abundant element It's one of the ones is is created in the interiors of stars and so Carbon is present almost everywhere So when we look at Mars, we know we have rocks And we have light so we know there's a source of energy We know carbon is present. There's carbon dioxide in the atmosphere And so a lot of what we're looking for is Where and how liquid water was distributed on the surface Mars the recent missions have demonstrated that there is frozen water on Mars and the sedimentology of Gale crater for example as well as other areas that we can see from orbit suggests that we had the good water And then it was long lived so we say that an environment is habitable if it could provide These three things right the water the schnapps elements and the energy So the surface of Mars was habitable billions of years ago The surface of Mars right now is too cold to have liquid water It just has ice and so the surface is not habitable On an ongoing basis right now So if we come back to thinking about life on earth We have liquid water almost everywhere on near the surface and kilometers deep into the continental and oceanic crust the elements of Carbon hydrogen nitrogen oxygen phosphorus and sulfur are really common on the surface of earth and We have chemical energy with the reaction of the water and rock and In the interior and surface of the earth and we also have light energy From the Sun so we would say that earth is in some sense very habitable Thanks for watching