Part 1: The atmospheres of Mercury and Earth are significantly different in composition, density, and structure. Mercury has a very thin and tenuous exosphere that consists mostly of oxygen, sodium, hydrogen, helium, and potassium, with no substantial pressure. In contrast, Earth's atmosphere is much thicker and composed primarily of nitrogen (around 78%) and oxygen (around 21%), along with trace gases such as carbon dioxide, neon, and others. Earth's atmosphere also has a well-defined structure with distinct layers, including the troposphere, stratosphere, mesosphere, thermosphere, and exosphere, which support weather and life.
Part 2: The primary reason for the difference between the atmospheres of Mercury and Earth lies in their proximity to the Sun and their respective gravitational strengths. Mercury, being the closest planet to the Sun, experiences extreme temperatures that can cause any gases released into its atmosphere to quickly escape into space due to its weak gravity. It lacks a substantial atmosphere because it cannot retain light gases effectively. In contrast, Earth has a stronger gravitational pull that enables it to retain a thicker atmosphere, allowing it to support complex life forms and weather systems. Additionally, Earth’s distance from the Sun ensures a more stable temperature range, conducive to the development and maintenance of a robust atmosphere.