PRESENTATION OUTLINE
Atmosphere
Earth
One of the reasons we can live on Earth is because the atmosphere has oxygen in it that all living things need. There is also water in the atmosphere even though it may be in a gas form. The atmosphere protects the Earth. For instance, if an asteroid was heading for the planet, it would most likely burn up before it hit the surface unless it was very large.
Atmosphere
Mars
Mar's atmosphere is made mostly of carbon dioxide (95.3% compared to less than 1% on Earth). Mars has much less nitrogen (2.7% compared to 78% on Earth). It has very little oxygen (0.13% compared to 21% on Earth). The red planet's atmosphere is only 0.03% water vapor, compared to Earth, where it makes up around 1%. On average, it exerts only 6.1 millibars of surface pressure (Earth's average sea-level surface pressure is 1,013.25 millibars)
But in the end none of this really matters... humans are smart, and we have space suits.
Climate
Earth
Obviously the climate is an important factor in the ability of humans being able to live on earth. The limit for humans in heat is 128 F; the coldest is -97 F. The average temperature for the planet earth is 47 F, but that is because of the freezing of Antarctica on a regular basis. So the Earth's average temperature is well within the limit of human ability to survive.
Climate
Mars
The average temperature on Mars is -80 F, so even if you could breathe without oxygen (which is impossible) you would freeze to death without a space suit. You would want to be in the warmest area (equator) which is 70 F at noon...a pretty good spot. Otherwise, you would need to have a way to stay within the human heat boundries (128 F to -97 F.)
Water
Earth
Water is believed to be the most important chemical nessesary for human existance. Obviously we can't go very long without it or we would die of thirst, but also it contains oxygen. It doesn't burn skin like chemicals with acids. We use water for many things; for example, some foods need to be boiled in water for us to eat them without being sick.
Water
Mars
Mars is too cold to form water. If you were in the cold areas, your blood would freeze but in the warm areas where it can be 70 F at noon, water and blood wouldn't freeze and would be suitable for living.
Light
Earth
Light is another huge factor in surviving on Earth. Plants that make oxygen for living organisms to breathe need light as food (photosynthesis) to provide oxygen. If all the plants died we would all die. Plants are also used as food, so light is extremely important for the survival of the plants. Another thing about light is that it lets us see. Without this, we would all be blind and have difficulty surviving.
Light
Mars
Light on Mars really only affects Mars in very small ways compared to Earth. It does provide heat to the planet. However, there are no plants on Mars that need light to live. Similar to Earth, light on Mars gives humans the ability to see.
Movement
Mars
The movement of Mars around the sun makes life possible because if it was thrown off it's rotation it would float away from the sun causing two major events on the planet. One, it would lose all the light making it impossible to live on. In addition, it would also make the temperature drop so much that the planet would freeze and eventually it would be so cold the ice would break and the planet would cease to exist.