Our atmosphere is made up of a mixture of gases. Nitrogen, oxygen, argon, and a little bit of carbon dioxide. But what if the atmosphere was made up solely of oxygen. In this situation oxygen would be more toxic than safe. When people are deep sea diving they are basically living off of oxygen. When this amount of oxygen is inhaled in this period of time it can cause cramps, lung damage, and many other conditions. The reason people can survive with tanks so long without dying from to much oxygen is because the oxygen in the tanks is diluted with helium so it is not deadly. If the atmosphere was made solely of oxygen the earth would be a giant fireball, because oxygen is an inflammable gas. In that case we would not be able to survive.
When the spacecraft leaves the atmosphere it is traveling at a rate of 6 miles per second. It has to travel this fas because to keep the space craft in orbit it has to be traveling at a high rate of speed. it is like when you come on the highway from an on ramp. before you pull out into traffic you have to be traveling at the same rate of speed so people don't hit you. This is how it works in space everything is traveling at 7 miles per second. When returning into the atmosphere you have to slow down otherwise you would hit the earth and make a huge crater. The atmosphere slows down and acts as brakes. kind of like when you are driving and come to an immediate stop you usually smell rubber which means it starts to burn and heat up. just like when a spacecraft enters the atmosphere.
Mercury is inhabitable planet. The atmosphere is not sustainable it has small traces of hydrogen, helium, oxygen, and even smaller amounts of sodium, calcium, potassium, and magnesium. It may also contain very small amounts of gas particles. At night temperatures may reach near -274 degrees fahrenheit, and during the day it may reach temperatures of 806 degrees fahrenheit. Mercury has one of the largest temperatures changes in the solar system.
Looking at Earth's atmosphere, and comparing it to other planets atmospheres. There is an atmosphere on Mars, Venus, and Pluto*. too small or too hot to retain an atmosphere. Mercury´s doesn't have an atmosphere because it is too small and too hot to retain an atmosphere. Itś exosphere is made up of Hydrogen, Helium, Oxygen, Sodium, Calcium, Potassium, and water vapor. Mercuryś atmospheric pressure is 10 -14 power. That is 1 quadrillionth of Earthś atmospheric pressure. Research shows that the exosphere is made of sun particles, and debris from meter collisions. Venus atmosphere is so dense that the mass of venus atmosphere 92 times than the Earth. Even though the thick clouds made of sulfur dioxide, and sulfuric acid droplets reflect at least 90% of the sunlight back into space, Venus still reaches a temperature of 863.6 F-degrees.