Morning ‘star’ Venus is Earth’s twin – sort of |

Morning ‘star’ Venus is Earth’s twin – sort of

The morning star blazes in the east before dawn – Lucifer, “bringer of light.”

Or Venus; the former name predates its satanic connotation.

Venus reflects enough sunlight to cast shadows. (The best place I’ve found to see my Venusian shadow was a cave in the mountains of southern Arizona; any place shielded from other light will do).

Venus and Earth formed at the same time, four-and-a-half billion years ago. They’re virtually the same size, the same density, likely made of the same stuff. The two planets are twins.

Up to a point.

Three-fourths of Earth’s surface is inundated with water. We might presume the Venusian surface to be equally wet – a presumption abetted by the fact that our twin is wholly swathed in clouds.

The light with which Venus casts shadows is sunlight, reflected off its cloud-tops. Unlike stars, planets can only reflect light, not emit it. But that distinction holds only in the visible. In microwave wavelengths, Venus shines brightly on its own, its emissions easily detected with radio telescopes.

To emit so much microwave energy, Venus must be hot: Not quite red-hot (over 950 degrees Fahrenheit), but microwave-hot – some 860 degrees at the surface.

If our planetary twin ever had oceans, they’ve long since boiled off.

Perhaps Venus still has “oceans” – floating above the surface as clouds.

That’s assuming the Venusian clouds are composed of water.

One of the basic lessons of kindergarten is that you can mix colors to create new ones. Astronomers use spectroscopes to sort out the colors emitted by stars or reflected by planets. Every substance emits or reflects its own mix of wavelengths, allowing us to identify what stars and planets are made of.

But spectroscopy alone could not identify Venus’s cloud droplets.

Looking through a windshield, the driver’s view is compromised by glare reflecting off the glass, or off other objects on the road. Fortunately, when light reflects off glass, or water, or many other substances, it becomes polarized – the reflected light waves all wave the same way, making them simple to filter out with polarizing glasses.

Just as different substances emit or reflect different colors, they also polarize light differently.

Combining clues, we find that the clouds of Venus are not made of water. The clouds that make the morning star so bright before dawn are composed of droplets of sulfuric acid.

Ultraviolet radiation (UV) is dangerous because it carries sufficient energy to break chemical bonds – the bonds between atoms in our DNA, for instance.

Or the bonds between oxygen and hydrogen atoms in water. Boiling off the surface of Venus, molecules of water would have wafted upward, into the upper atmosphere, where they’d be exposed to UV form the sun. Absorbing that UV, they’d then break apart.

Loosed from the much heavier oxygen atom, barely held by Venus’s gravity, atoms of hydrogen would fly off into space, never again to be part of a water molecule. Once gone, Venus’s oceans were gone forever.

Assuming, of course, that Venus really did once have oceans.

On Earth, carbon dioxide (CO2) dissolves in water – in the oceans – and is transformed, by biology or by abiotic chemistry, into limestone. With no water on its surface, Venus cannot sequester its CO2 in stone.

The greenhouse gas remains in the atmosphere, absorbing and re-radiating heat energy, making the Venusian surface hotter ‘n … well, hotter than the hottest kitchen oven on Earth.


Last weekend’s sky was spectacular for atmospheric phenomena, but not for anything much above that. We’ll try again this Saturday night, 7, at the old Nevada City Airport. Wear your mud shoes.


Alan Stahler trained as a biologist and is an amateur astronomer. He teaches private enrichment classes for students of all ages. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5).

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