Alan Stahler
Special to The Union

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July 5, 2014
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Mars tonight, Saturn Monday

The moon acts tonight as a skymark, drawing the eye to the ruddy planet Mars, very close by. Next Monday night, the moon draws us to the silvery planet Saturn.

It takes energy to make water evaporate, to go from liquid to gas. In the liquid phase, water molecules move about slowly, and thus have no problem clinging together.

But in the gas phase, the molecules fly about fast, and they can’t hold together. It takes energy to make molecules fly fast, so evaporation sucks up energy, cools things down: Evaporation cools sweaty skin.

Same trick backwards: To make water condense, to go from gas to liquid, energy must be sucked out of water molecules — they must be slowed down, if they are to cling together, to link up.

On a warm day, a cold beverage sucks energy from the molecules in the air around it, slowing water molecules, allowing them to link up, to form droplets on the glass.

Same trick forwards, backwards, and big-time: Bathing in the sun, the topmost waters of the ocean grow warm. Warm (speedy) water molecules evaporate into the air above.

Like a stovetop, warm ocean water warms the air sitting just above it. Warm air rises. As it rises, the warm, moist air expands, and cools.

Blow into a freezer. The cold air of the freezer sucks energy from the molecules in your breath, slowing them down.

Slow water molecules cling to one another – they condense - forming tiny cloud droplets.

When water evaporates, it cools whatever it is evaporating from. When water condenses, it warms its environment. Condensing cloud droplets warm the air around themselves, goosing it to rise yet more … whereupon, it cools yet more … more cloud droplets condense … warming the air around themselves … which rises yet more.

Rising higher and higher, growing thicker and thicker, a storm cloud develops. Drawing moisture from the tiny cloud droplets, large raindrops form and fall …

… even as the energy they carried, rises higher and higher. The energy carried upward by water molecules in rising air – the “latent heat” of those water molecules – eventually rises high enough that it can escape into space.

All planets, billions of years ago, were born hot. They’ve been cooling ever since. Knowing a planet’s mass – what it would weigh, were we able to weigh it – and knowing what it’s made of, we can calculate how warm that planet should be today – how much heat it should now be giving off.

Saturn – find it near the moon, Monday night - gives off heat faster than expected.

Also unexpected: Saturn’s atmosphere contains less helium – the gas we put in balloons – than expected. Which perhaps explains where that extra heat is coming from.

Beneath the surface of Saturn – beneath the clouds we see in our scopes – helium gas may be condensing into helium raindrops.

When water vapor condenses to form droplets, it releases latent heat, some of which escapes from Earth.

When helium gas condenses, it, too, releases latent heat, which our spacecraft detect, rising upward, as helium raindrops fall downward, thousands of miles, onto Saturn’s rock-and-metal core.

Al Stahler’s science programs can be heard on alternate Tuesdays at noon on KVMR-FM (89.5 MHz). He teaches students of all ages, visits classrooms, and may be reached at

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The Union Updated Jul 5, 2014 01:01AM Published Jul 5, 2014 01:01AM Copyright 2014 The Union. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.