## Physics Question #383

Jeana Willis, a 13 year old female from the Internet asks on May 31, 1998,

How long would it take for the Earth to freeze if the Sun burnt out?

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### Donald J. Barry answered on May 31, 1998

Different parts of the Earth would freeze at different rates if the Sun were to suddenly go out. We can guess some of the rates just from considering what happens at night. Over the course of 12 hours without sunlight, temperatures tend to drop between 10 and 30 degrees Fahrenheit, depending on whether you're in a desert or near the ocean. So you might expect that things would start to freeze in only two or three days with no Sun. The ocean acts like a big reservoir of heat, since its layers can mix and bring warm material easily to the surface. Areas near the ocean would remain warmer for a while longer, but the freezing would probably start in only a few months. After the time of an arctic winter, the ocean would be filled with pack ice. Since the pack ice of the arctic ocean averages about a dozen feet thick during the winter, one might guess the oceans would freeze at about the rate of a 50 feet a year. This rate would slow as a thick blanket of insulating ice was accumulated, but the effect would be the same, the warm water would be so far down as to be useless to people on the surface unlucky enough to be looking for it. Eventually, the atmosphere would liquefy and then freeze. That would probably take a few decades to begin.

We don't expect the Sun to shut off anytime within the next five billion years, but the same effects would occur if a lot of dust blocks light from reaching the surface of our planet. This happened on a big scale 60 million years ago when an asteroid hit our world. The resulting ice age killed the dinosaurs (and many other plants and animals). On a smaller scale, volcanoes affect weather every few years. In April 1815, the volcano Tambora filled the atmosphere with dust. The result was that Europe really didn't have a summer the next year and it frightened many people. The poet Byron wrote these lines to open his poem "Darkness" of that year:

I had a dream, which was not all a dream
The bright sun was extinguish'd,
and the stars Did wander darkling in the eternal space,
Rayless and pathless,
and the icy earth Swung blind and blackening in the moonless air;
Morn came and went-and came, and brought no day,
And men forgot their passion in the dread of this their desolation;
And all hearts were chill'd into a selfish prayer for light.

Mary Shelley was similarly inspired by the chill and frightening weather to write the book "Frankenstein".

Now here is some good news. The middle of the Earth generates its heat from the decay of radioactive materials left over from its formation. These decay very slowly, so the center of the planet would remain molten for billions of years to come. Geothermal energy, therefore, could sustain us almost indefinitely, but only in the few places around the planet where it can be easily tapped.  We are just now beginning to develop the technological skills to imagine doing this.

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