Atmospheres of Planets

Venus. The atmosphere of Venus is, like Mars, nearly all carbon dioxide. However, Venus has about 154 000 times more carbon dioxide in its atmosphere than Earth (and about 19 000 times more than Mars does), producing a runaway greenhouse effect and a surface temperature hot enough to melt lead. A runaway greenhouse effect is when a planet’s atmosphere and surface temperature keep increasing until the surface gets so hot that its oceans boil away.

Mars. Mars has a very thin atmosphere, nearly all carbon dioxide. Because of the Red Planet’s low atmospheric pressure, and with little methane or water vapour to reinforce the weak greenhouse effect (warming that results when the atmosphere traps heat radiating from the planet toward space), Mars’ surface remains quite cold, the average surface temperature being about -63°C.

Earth. On Earth we live in the troposphere, the closest atmospheric layer to Earth’s surface. Tropos means “change,” and the name reflects our constantly changing weather and mixture of gases. It is eight to 14 kilometres thick, depending on where you are on Earth, and it is the densest layer of atmosphere. When we breathe, we are taking in an air mixture of about 78.09% nitrogen, 20.95% oxygen, plus traces of argon 0.93%, carbon dioxide 0.0407%, carbon 0.039%, and water vapour.

Jupiter. Jupiter likely has three distinct cloud layers (composed of ammonia, ammonium hydrosulphide and water) in its “skies” that, taken together, span an altitude range of about 71 kilometres. The planet’s fast rotation—spinning once every 10 hours—creates strong jet streams, separating its clouds into dark belts and bright zones wrapping around the circumference of the planet.

Saturn. Saturn’s atmosphere—where NASA’s Cassini spacecraft ended its 13 years of exploration of the planet—has a few unusual features. Its winds are among the fastest in the solar system, reaching speeds of 1 800 kilometres per hour. Saturn may be the only planet in our solar system with a warm polar vortex (a mass of swirling atmospheric gas around the pole) at both the North and South poles. Also, the vortices have “eye-wall clouds,” making them hurricane-like systems like those on Earth. Another uniquely striking feature is a hexagon-shaped jet stream encircling the North Pole. In addition, about every 20 to 30 Earth years, Saturn hosts a mega storm (a great storm that can last many months).

Uranus. Uranus gets its signature blue-green colour from the cold methane gas in its atmosphere and a lack of high clouds. The planet’s minimum troposphere temperature is -224.2°C, making it even colder than Neptune in some places. Its winds move backward at the equator, blowing against the planet’s rotation. Closer to the poles, winds shift forward and flow with the planet’s rotation.

Neptune. Neptune is the windiest planet in our solar system. Despite its great distance and low energy input from the Sun, wind speeds on Neptune surpass 2 000 kilometres per hour, making them three times stronger than Jupiter’s and nine times stronger than Earth’s. Even Earth’s most powerful winds hit only about 400 kilometres per hour. Also, Neptune’s atmosphere is blue for the very same reasons as the atmosphere of Uranus.

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From a feature by Holly Shaftel, Nasa Science – Solar System Exploration.

Edited by Nigel Benetton, science fiction author of Red Moon and The Sands of Rotar.

Last update: Monday, 27th April 2020