Planetary atmospheres are layers of gas that surround celestial bodies like planets. They influence a planet’s temperature and pressure. Atmospheres are common among planets in our solar system and can extend thousands of kilometers into space.
What Makes Up an Atmosphere
Planetary atmospheres consist of various gases, such as nitrogen, oxygen, and carbon dioxide. They can also contain trace elements and particles. The specific mixture of gases varies depending on the planet’s formation and evolution.
Atmospheres are structured into layers, where temperature and density change with altitude. For instance, Earth’s atmosphere includes layers like the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The density of gases decreases as altitude increases.
Why Atmospheres Matter
Planetary atmospheres regulate temperature, preventing extreme fluctuations between day and night. They also offer protection from harmful solar radiation and incoming meteoroids, which burn up upon entry. The presence of an atmosphere enables weather phenomena, such as winds and storms, by distributing heat and moisture across the planet.
Atmospheres also support life. They facilitate the water cycle and enable gas exchange necessary for biological processes. For example, trace gases like carbon dioxide in Earth’s atmosphere are important for maintaining a habitable temperature through the greenhouse effect.
Diverse Atmospheres Across Our Solar System
The atmospheres across our solar system exhibit significant diversity, influenced by factors like a planet’s size, distance from the sun, and geological activity. Earth’s atmosphere is primarily composed of about 78.1% nitrogen and 21.0% oxygen. It maintains an average atmospheric pressure of 1.013 bars and a surface temperature of around 15° Celsius, supporting liquid water and diverse life. It also contains water vapor and about 380 parts per million of carbon dioxide.
In contrast, Mars possesses a thin, cold atmosphere with an average atmospheric pressure of 0.007 bars, dominated by about 95% carbon dioxide. This thinness means it lacks sufficient pressure to sustain liquid water on its surface. Venus has a very dense atmosphere, with a pressure around 92 bars, and is primarily composed of 96% carbon dioxide, leading to an extreme greenhouse effect and surface temperatures of around 500° Celsius.
The gas giants, such as Jupiter, Saturn, Uranus, and Neptune, have massive atmospheres rich in lighter gases like hydrogen and helium. Jupiter’s atmosphere is approximately 89.8% hydrogen and 10.2% helium, while Saturn’s is about 96.3% hydrogen and 3.2% helium. Uranus is 82.5% hydrogen and 15.2% helium, and Neptune is 80% hydrogen and 19% helium. These thick, deep atmospheres contribute to their distinct banded appearances and powerful storm systems.
How Planetary Atmospheres Form and Evolve
Planetary atmospheres can form through various processes, including outgassing from volcanic activity, which releases gases previously trapped within the planet’s interior. Early atmospheres were also supplied by collisions with icy small bodies. The gravitational attraction of a celestial body can also capture gases from the surrounding solar nebula during its formation.
The evolution of an atmosphere is influenced by factors such as a planet’s mass, magnetic field, and temperature. Planets with greater mass and cooler atmospheres retain their gaseous envelopes more effectively. Solar wind can strip away atmospheric gases, particularly on planets lacking strong magnetic fields. For instance, Mercury, being very close to the sun and having a weak magnetic field, has essentially no atmosphere, with an atmospheric pressure of only 10^-12 millibars.