• I love this question. The answer is a bit long, but it boils down to hot gases having too much energy to be overcome by gravity. Let me explain. The primordial Earth was a very hot place. Possibly more than 2000 K hot. The Sun, at this time, was ejecting a lot more hot gas than it is now. This gas was composed almost entirely of hydrogen and helium, but would also have significant traces of carbon, nitrogen, and oxygen. Since the thermal energy of the Earth and of the gases was higher than the gravitational energy forming a well around the Earth, these gases would have never formed into an atmosphere. As the Earth cooled, heavier gases would be captured and form into an atmosphere, which is why we now have an atmosphere almost entirely made of nitrogen, oxygen, and carbon dioxide. Another way to think of the situation is to recall the adage "what goes up, must come down." It is not so, if the object's upward velocity is greater than the escape velocity determined by gravity. Gravity on primordial Earth was very similar to the gravity we feel today. You can calculate the escape velocity depending on elevation, but lets just say that, as a rule of thumb, gases with a molecular RMS (root-mean-square, think "average")_velocity of above about 2000 m/s freely escape the atmosphere. If you calculate vRMS for Hydrogen gas or Helium gas at room temperature, it's 2700 and 2200 m/s, respectively, so hydrogen and helium escape the atmosphere of the Earth today. Oxygen has a vRMS of 675 m/s, Nitrogen 722 m/s, and CO2 576 m/s. So those stay in. If you bump up the temperature to 2000 K, like primordial Earth, and then account for monatomic hot gases from the sun, the slowest gas is monatomic oxygen at 2500 m/s, so they'd all escape (or, more properly, never be captured in the first place). You can also think of the analogy of a billiards table. There is a restriction keeping the balls on the table (the bumper around the outside), but, if the balls are given too much energy, they shoot off of the table. Hot primordial Earth would have the same bumper size as modern Earth, but the billiard balls would have 5 times as much kinetic energy, so they'd be flying off of the table everywhere.
    • Satyam Agrawal
      thats very much helpful thanks

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