Al Stahler: Air and light … and clouds


Exploring the solar system, heading in, toward the sun, we come to planet Venus; heading out, we come to Mars.

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

No matter the diamond industry’s promise … no rock lasts forever. Wind and rain and ice … an Earthworm’s gut … a bacterial biofilm … rocks fall apart – turn to sand, turn to dust, they dissolve, they rust.

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

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

The same goes for the atmosphere. The air we breathe today is not the same as the air of long ago. And the moon’s no help: The moon is airless. But Venus and Mars hold clues.

Air does not weigh much … but it’s 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 hold only a single pound of Martian air.

Air on Venus, on the other hand, is super-thick: Our little room on Venus would hold a good ton-and-a-half – over 3000 pounds – of Venusian air.

Over the four-and-a-half billion years since the planets formed, Mars has lost most of its air, while Venus has held on to its ancient air.

What happened to put Earth in-between?

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

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

Liquid water.

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

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

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

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

Clouds are complicated. On a hot summer’s day, a cloud drifting over the sun throws down shade, and cools us down. But if you want to cool off at night, you’ve got to wish for clear sky.

Clouds hold heat. Clouds, at night, keep us warm.

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

It would be interesting to know, therefore, how clouds have been behaving. That’s not easy, even with spacecraft.

The bright part of a crescent moon is lit by sunshine. But looking closely, the dark part is not pitch-black. The dark part of the moon is often lit by a dim, ghostly light. That ashen light does not come directly from the sun … the dark part of the moon is not lit by sunshine.

When sunlight strikes Earth, it is reflected, especially, by clouds. Reflecting up toward the moon, this cloud-reflected light illumines the dark part of the moon … with “earthshine.”

A solar observatory sits over Big Bear Lake, in the San Bernardino Mountains of southern California. For twenty-plus years, the observatory has been observing, not just the sun, but also the moon … measuring earthshine.

This past August, researchers published their results: Earthshine that reflects from clouds off our coast seems to have dimmed a bit … implying that the ocean off the west coast is not quite as cloudy as it used to be.

IN THE SKY

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

A swoop of cranes – a crane-flock – flies in a “V” formation, a lot like the “V” of a flock of geese. But sandhill cranes don’t honk … sandhill cranes burble. If you hear burbling overhead, look up. (A flock 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 (Oct. 9), find a spot to watch the sunset (as I write, the forecast is for clear skies). As soon as the sun is down, Venus and the crescent moon will appear, close together, above the western horizon. Covered in cloud, Venus will be bright. And the dark part of the moon will be lit by earthshine.

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at [email protected]

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

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