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Al Stahler: Air and light … and clouds


By exploring the solar system, heading towards the sun, we come to the planet Venus; leaving, we arrive on Mars.

The more we study Venus and Mars, the better we understand Earth.

No matter what the diamond industry’s promise… no stone lasts forever. Wind, rain and ice… the gut of an earthworm… a bacterial biofilm… rocks collapse – turn to sand, turn to dust, they dissolve, they rust.

Which makes it difficult to determine whether billions of years ago volcanoes erupted, glaciers sank, lakes formed. The rocky evidence turned to mud, clay, disappeared, washed away.

With such fragile rocks, the oldest pieces in our “Earth-rock” collection… come from the moon.

Ditto for the atmosphere. The air we breathe today is not the same as it used to be. And the moon is no help: the moon is airless. But Venus and Mars hold clues.

The air does not weigh heavy … but it is not weightless. The air in a small room – say, ten by ten, by eight feet high – weighs sixty pounds.

Martian air is thin… that same room on Mars would only contain one pound of Martian air.

The air on Venus, on the other hand, is super thick: our little room on Venus would contain a good and a half tons – over 3,000 pounds – of Venusian air.

In the four and a half billion years since the planets were formed, Mars has lost most of its air, while Venus has retained its ancient air.

What happened to put Earth in between?

Despite their different thicknesses, the atmospheres of Venus and Mars are, in a way, very similar. The air on both planets is almost entirely – 95% – carbon dioxide. The Earth’s atmosphere, on the other hand, contains less – MUCH less – than one percent CO2.

Looking at Venus and Mars, we can conclude that when a planet is young, its atmosphere is almost entirely made up of carbon dioxide. The air on Venus and Mars has never changed in composition. What happened on Earth?

Liquid water.

Carbon dioxide dissolves in water (think club soda). When carbon dioxide dissolved in Earth’s ancient oceans, chemical reactions turned the gas into limestone. The rock then fell to the bottom of the sea and was buried. Almost all of Earth’s carbon dioxide is locked away, far from the surface, in rock.

(With its incredibly thick carbon dioxide atmosphere, the greenhouse effect on Venus has gone crazy … the average surface temperature of Venus is over eight hundred degrees Fahrenheit.)

Like Earth, Venus and Mars have clouds. The rarefied air of Mars can only contain thin icy clouds, like the wispy tails of mares we see high in Earth’s atmosphere.

The thick air of Venus, on the other hand, sports super thick clouds… clouds so thick that we have never seen the surface of Venus through a telescope… just clouds, from the pole. north to south.

Clouds are complicated. On a hot summer day, a cloud drifting above the sun casts shade and cools us down. But if you want to cool off at night, you have to wish for clear skies.

The clouds trap the heat. Clouds at night keep us warm.

Calculate why the Earth’s climate is as it is today… and how our climate might change in the future… clouds are a stimulus. Climatologists describe clouds as the most uncertain part of their climate calculations. No one knows what the clouds will do in the future.

So it would be interesting to know how the clouds behaved. It’s not easy, even with spacecraft.

The shining part of a crescent moon is lit by the sun. But on closer inspection, the dark part is not pitch black. The dark part of the moon is often lit by a faint ghostly light. This ashy light does not come directly from the sun… the dark part of the moon is not illuminated by the sun.

When sunlight hits the Earth, it is reflected, in particular, by clouds. By reflecting towards the moon, this light reflected by the clouds illuminates the dark part of the moon… with an “earthshine”.

A solar observatory sits above Big Bear Lake in the San Bernardino Mountains of southern California. For more than twenty years, the observatory has observed not only the sun, but also the moon… measuring the brightness of the earth.

Last August, researchers published their results: the brightness of the land reflected in the clouds off our coast seems to have faded a bit … implying that the ocean off the west coast not as cloudy as it used to be.


Sandhill cranes spend their summer north of us, but in the fall they fly south… to spend the winter with us here in Northern California in the Central Valley.

A flock of cranes – a flock of cranes – flies in a “V”, much like the “V” of a flock of geese. But sandhill cranes don’t honk… sandhill cranes buzz. If you hear buzzing above your head, look up. (A herd of cranes is also known as a “dance.” If you’re in the valley, look for sandhill cranes in the fields… dancing.)

Next Saturday night (October 9), find a spot to watch the sunset (as of this writing, the forecast is for clear skies). As soon as the sun goes down, Venus and the crescent moon will appear, close together, above the western horizon. Covered in clouds, Venus will be luminous. And the dark part of the moon will be illuminated by the glow of the earth.

Al Stahler enjoys sharing science and nature with his friends and neighbors on The Union and on KVMR-FM. He gives lessons for children and adults and can be contacted at [email protected]

The domes of Venus (seen on radar, radiated through clouds from the Magellan spacecraft) may have been formed by volcanoes.
Photo courtesy of E. De Jong et al. (JPL), MIPL, Team Magellan, NASA

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