How did gases get trapped in the interiors of Earth and other rocky worlds?
Perhaps you’ve noticed a couple of statements that might at first sound contradictory: We said that Earth and other rocky worlds were too small to pull in hydrogen gas as they formed, but we’ve also said that they had gas trapped in their interiors that formed atmospheres through outgassing. How can both be true?
The basic answer is that the “gas” trapped in their interiors was not hydrogen but heavier molecules (such as water and carbon dioxide), and these molecules were chemically attached to some of the rock that built these worlds. In other words, these “gases” joined the rocky worlds in the form of rock, which put them in the worlds’ interiors. The gases could then become separated from the rock by the effects of the internal heat within these worlds, and then released into atmospheres through volcanic outgassing.
However, this answer leads to another question that scientists still don’t fully know how to answer. According to models of solar system formation, it was too hot in the region where the rocky worlds formed for any gases to be chemically attached to rock. Therefore, scientists assume that all the trapped gases must have been brought in from cooler regions of the solar system, where it was possible for gas molecules to be attached to rock. This is a reasonable assumption, because models tell us that at least some particles from cooler regions of the solar system should have moved inward, where they could help form the rocky worlds. Moreover, we know that both asteroids and comets that formed farther from the Sun have such gases mixed with their rock (or rock/ice in the case of comets).
The question that scientists are still trying to answer is: Which region of the solar system did most of this material come from? That is, did the gases trapped in the rocky planets come primarily from the region of the asteroid belt or from the much more distant realm of the comets? The answer is not yet known, though recent evidence appears to be favoring the asteroid belt as the source of this gas.