Posted by KeighleyAstro on Apr 9, 2019 in Main |

Every time you take a breath of fresh air, it’s easy to forget you can safely do so because of Earth’s atmosphere. Guest speaker Mr Keith Berrington presented to another packed monthly meeting his lecture on ‘Planetary Atmospheres’.

Mr Berrington explained that life on Earth could not exist without that protective cover that keeps us warm, allows us to breathe, and protects us from harmful radiation; among other things.

He pointed out what makes Earth’s atmosphere special, and how do other planets’ atmospheres compare?
Here on planet 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’s 5 to 9 miles thick, depending on where you are on Earth, and it’s the densest layer of atmosphere. When we breathe, we’re taking in an air mixture of about 78 percent nitrogen, 21 percent oxygen and 1 percent argon, water vapour and carbon dioxide.

The red planet Mars has a very thin atmosphere, nearly all carbon dioxide. Because of its 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 -82 degrees.

The atmosphere of Venus, like Mars’, is 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. Mr Berrington explained; 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.

The gas giant Jupiter has three distinct cloud layers (composed of ammonia, ammonium hydrosulfide and water) in its “skies” that, taken together, span an altitude range of about 44 miles. 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.

The atmosphere of Saturn, (where NASA’s Cassini spacecraft ended its 13 extraordinary years of exploration of the planet) has a few unusual features. Its winds are among the fastest in the solar system, reaching speeds of 1,118 miles 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 megastorm (a great storm that can last many months).

Uranus get 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 49 Kelvin (minus 224.2 degrees Celsius), 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 is the windiest planet in our solar system. Despite its great distance and low energy input from the Sun, wind speeds at Neptune surpass 1,200 miles 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 250 miles per hour. Also, Neptune’s atmosphere is blue for the very same reasons as Uranus’ atmosphere.

Mr Berrington moved onto planets beyond our solar system. WASP-39B foe example. A hot, bloated, Saturn-like exoplanet some 700 light-years away, apparently has a lot of water in its atmosphere. In fact, scientists estimate that it has about three times as much water as Saturn does.

A weather forecast on “hot Jupiters” (blistering, Jupiter-like exoplanets that orbit very close to their stars) might mention cloudy nights and sunny days, with highs of 2,400 degrees Fahrenheit (about 1,300 degrees Celsius, or 1,600 Kelvin). Their cloud composition depends on their temperature, and studies suggest that the clouds are unevenly distributed.

An exoplanet called 55 Cancri e, (a planet outside of our solar system with a diameter between Earth’s and Neptune’s) that may be covered in lava, likely has an atmosphere containing nitrogen, water and even oxygen–molecules found in our atmosphere–but with much higher temperatures throughout. Orbiting so close to its host star, the planet could not maintain liquid water and likely would not be able to support life.