Extraterrestrial atmosphere

The study of extraterrestrial atmospheres is an exciting area of research in astronomy. It helps scientists learn more about our own planet’s air and how other worlds work. Many objects in our Solar System have atmospheres, just like Earth does. This includes the giant planets, as well as smaller worlds like Mars, Venus, and moons such as Titan and Triton.

Other space objects, like moons, comets, and even the Sun, also have layers of gas surrounding them. Scientists also think that planets far outside our Solar System, called extrasolar planets, might have atmospheres too. By comparing all these different atmospheres, researchers can learn more about important processes such as the greenhouse effect, how clouds and tiny particles behave, and the chemistry of the air.

In September 2022, a group of scientists formed a team called “Categorizing Atmospheric Technosignatures” (CATS). This group looks at studies of faraway planet atmospheres to search for signs of life and advanced civilizations. This work helps us understand not just our own world, but what might be possible on other planets.

Planets

Inner planets

Earth is not included here because this article focuses on atmospheres beyond Earth. For details about Earth's atmosphere, see atmosphere of Earth.

Mercury

Main article: Atmosphere of Mercury

Because Mercury is small and has little gravity, it does not have a thick atmosphere. Its very thin atmosphere contains a little helium and traces of sodium, potassium, and oxygen. These gases come from the sun’s solar wind, radioactive decay, meteor impacts, and breaking down of Mercury’s surface. Mercury’s atmosphere changes constantly because its particles escape into space due to the planet’s heat.

Venus

Main article: Atmosphere of Venus

Venus’ atmosphere is mostly made of carbon dioxide, with small amounts of nitrogen and other elements like hydrogen, sulphur, carbon, and oxygen. Venus’ atmosphere is much hotter and thicker than Earth’s, but shallower. The air pressure on Venus’ surface is about 92 times that of Earth. The large amount of carbon dioxide creates a strong greenhouse effect, making Venus’ surface very hot, around 470 °C, which is hotter than any other planet in our solar system.

Mars

Main article: Atmosphere of Mars

The atmosphere of Mars is very thin, mainly made of carbon dioxide, with some nitrogen and argon. The surface pressure on Mars is much lower than Earth’s, leading to big temperature changes. Martian temperatures can range from about −140 °C in winter to around 20 °C in summer.

Between the Viking and Mars Global Surveyor missions, Mars experienced colder global atmospheric temperatures during certain periods. More recent observations show that Mars’ south pole is melting, with pits in the ice growing about 3 meters each year. Scientists are studying whether this is part of a larger climate change on Mars.

On June 7, 2018, NASA reported that the Curiosity rover found seasonal changes in atmospheric methane, along with kerogen and other complex organic compounds.

Giant Planets

The four outer planets—Jupiter, Saturn, Uranus, and Neptune—are giant planets. They all have atmospheres mostly made of hydrogen and helium, blending into their liquid interiors without a clear boundary.

Jupiter

Main article: Atmosphere of Jupiter

Jupiter’s upper atmosphere is about 75% hydrogen and 24% helium, with traces of methane, water vapor, ammonia, and silicon-based compounds. Jupiter’s clouds, made of ammonia crystals, form bands of zones and belts. The most famous feature is the Great Red Spot, a huge storm larger than Earth. In 2000, a smaller storm called Red Spot Junior formed in the southern hemisphere.

Saturn

The outer atmosphere of Saturn is about 93.2% hydrogen and 6.7% helium, with traces of ammonia, acetylene, ethane, phosphine, and methane. Saturn’s clouds are made of ammonia crystals, and it has a banded pattern similar to Jupiter’s. Saturn’s winds are very fast, and it has a warm polar vortex and hurricane-like structures.

Uranus

Main articles: Atmosphere of Uranus and Climate of Uranus

Uranus’ atmosphere is about 83% hydrogen, 15% helium, and 2% methane, with traces of acetylene. Uranus has a banded cloud layer, but it is not easily visible without enhancing images. Storm activity on Uranus is less common than on Jupiter or Saturn, possibly because of methane and acetylene hazes in its atmosphere.

Neptune

Neptune’s atmosphere is about 80% hydrogen, 19% helium, and 1.5% methane. Neptune has very active weather, with the fastest winds in the solar system, reaching speeds of up to 900 m/s in storms. Neptune has changed in brightness since 1980, which scientists think may be due to both solar and seasonal changes.

Other bodies in the Solar System

Natural satellites

Ten moons in our Solar System have atmospheres. These include Europa, Io, Callisto, Enceladus, Ganymede, Titan, Rhea, Dione, Triton, and Earth's Moon. Some of these atmospheres are very thin, while others are thicker.

Titan has the thickest atmosphere of any moon, even thicker than Earth's. Its air is mostly nitrogen, with some methane and other gases. This thick air creates a hazy layer that hides the moon's surface.

Triton, a moon of Neptune, has a very thin atmosphere made mostly of nitrogen with a little methane. Its temperature has been rising, possibly due to changes in its seasons or its surface.

Pluto

Main article: Atmosphere of Pluto

Pluto has a very thin atmosphere made of nitrogen, methane, and carbon monoxide. Scientists think Pluto's atmosphere might change as it moves around the Sun in its long, stretched-out path. Observations show Pluto's atmosphere has been getting warmer, but this is likely because Pluto was closest to the Sun in 1989 and is slowly moving away.

Brown dwarfs

Main article: Brown dwarf

Brown dwarfs have special atmospheres that change color with temperature. Their air is mostly hydrogen, and as they cool, gases like methane and water vapor become more noticeable.

The way these objects spin and their composition can change their atmosphere. Some have clouds made of iron or silicate, while others may have clouds of chromium, potassium chloride, sulfides, or even water. Brown dwarfs spin quickly, and some have very fast winds moving eastwards.

Exoplanets

Many planets outside our Solar System, called exoplanets, have been found to have atmospheres. Most of these are very hot planets called hot Jupiters or hot Neptunes that orbit very close to their stars. Scientists study these atmospheres using two main methods. First, they look at the light that passes through the planet’s atmosphere when it moves in front of its star. Second, they measure the light coming from the planet itself when it is hidden behind its star.

The first time scientists saw the atmosphere of a planet outside our Solar System was in 2001. They found sodium in the atmosphere of a planet called HD 209458 b. Later, using powerful telescopes, they discovered huge amounts of hydrogen, carbon, and oxygen around this planet, reaching temperatures of 10,000 K.

Scientists have also found water vapor in the atmospheres of other planets, such as HD 189733 b. In 2013, they detected clouds in the atmospheres of planets called Kepler-7b, Gliese 436 b, and Gliese 1214 b. In 2017, they even saw glints of light from Earth reflected by ice crystals in our own atmosphere, which helps them learn how to study distant planets.

Life

Main articles: Planetary habitability and Biosignature

Scientists are very interested in finding methane in space because it might be a clue that life exists somewhere else. Methane can also be used as fuel for future space missions.

Methane has been found in small amounts in the atmosphere of Mercury. Venus might have a lot of methane high up in its atmosphere. The Moon also has tiny bits of methane coming from its surface.

On Mars, methane has been detected in its atmosphere. Scientists are trying to figure out where it comes from. It could be from volcanoes, cracks in the ground, or maybe even tiny living things far below the surface.

The giant planets also have methane. Jupiter and Saturn have methane in their thick atmospheres. One of Saturn's moons, Titan, has lots of methane, even lakes on its surface. Titan's methane can change into other chemicals, which is very interesting for scientists.

Other planets like Uranus and Neptune also have methane. Their moons, such as Triton, have small amounts of methane too. Even faraway objects like Pluto and Eris show signs of methane on their surfaces.

Methane has also been found on comets like Halley's Comet and Comet Hyakutake, and even on a planet outside our solar system called HD 189733b. This is the first time scientists have found an organic compound on a planet beyond our solar system.