Atmosphere

An atmosphere is a layer of gases that surrounds an astronomical object. This layer is held in place by the object's gravity.

Without gravity, the gases would simply drift away into space. The atmosphere is vital because it can change the surface of a planet through chemical or photochemical interactions. For example, the Sun's light can interact with the gases to change their makeup. A planet's ability to keep its atmosphere depends on two main factors. High gravity and low temperatures help a planet retain its gases for a long time.

Most atmospheres begin forming during a planet's earliest stages. This process often starts within a rotating disk of gas and dust around a star. In this disk, dust particles collide and stick together to form planetesimals. Near the star, these bodies grow into protoplanets made of rocky, refractory materials. Further out, planetary embryos form by accumulating volatiles, which are substances like water or gases. These embryos can grow to ten times the mass of Earth. Eventually, they pull in massive amounts of gas from the surrounding disk to become gas giants.

There are two main types of atmospheres: primary and secondary. A primary atmosphere is created when a planet's gravity is strong enough to hold onto gases during its formation. For terrestrial planets, a secondary atmosphere often forms later. This happens through outgassing, where volatiles are released from inside the planet due to intense heat. This heat can come from the heavy bombardment of early collisions. The final composition depends on the chemistry of the original stellar nebula and the temperature of the system.

Atmospheres exist in a state called hydrostatic equilibrium. This is a balance between two opposing forces. First, there is the air pressure created by the constant motion of gas molecules. Second, there is the restraining force of gravity that pulls those molecules down. Because there is less gas mass above you at higher altitudes, the pressure decreases as you go up. This creates a pressure-gradient force. The temperature of an atmosphere is also managed by an energy budget. This budget balances the energy received from the Sun against the heat the planet radiates back into space.

Different planets in our Solar System have very different atmospheric compositions. The gas giants, such as Jupiter and Saturn, have massive atmospheres made mostly of hydrogen and helium. Their high gravity and low temperatures allow them to keep these light gases. In contrast, terrestrial planets like Earth have denser atmospheres. Earth's dry air is 78.08% nitrogen, 20.95% oxygen, and 0.93% argon.

Venus has a very thick atmosphere that is 96.5% carbon dioxide. This creates a pressure 80 times higher than Earth's. Mars has a very thin atmosphere with a surface pressure only 0.6% of Earth's. It has lost about 80% to 85% of its original water to space.

Space environments can be harsh on these gas layers. The solar wind constantly works to strip away a planet's outer atmosphere. This stripping happens more quickly if a planet is close to the Sun. However, a magnetosphere can act as a shield to slow this process. Earth's magnetosphere fends off incoming plasma at a distance of about 10 Earth radii. Some moons also have significant atmospheres. Saturn's moon Titan has an atmosphere made mainly of nitrogen. The dwarf planet Pluto has nitrogen and methane, though these gases freeze when it moves farther from the Sun.

Astronomers can even study atmospheres on exoplanets, which are planets orbiting other stars. One method used is transit spectroscopy. This involves watching a planet pass in front of its host star. By comparing the light at different wavelengths, scientists can detect specific gases. In 2002, researchers detected sodium in the atmosphere of HD 209458b. This is a hot gas giant with temperatures over 1,000 K. Other planets have shown signs of hydrogen, oxygen, or carbon. Some "lava planets" may have atmospheres made of vaporized rock, including silicon oxide, sodium, and potassium.