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Ice Ice Baby!

By Linda | February 18, 2008

Just the other day I was talking to my brother, Lou and the topic of global warming came up. Specifically, we were talking about how the snow at the summit of Mt. Kilimanjaro in Africa is vanishing. Being quite the science junkie himself, my brother cited some recent studies indicating that the Snows of Kilimanjaro are not melting, but are in fact sublimating.

When I confessed that I was unfamiliar with the phenomenon of sublimation he explained that sublimation is the process by which a substance (in this case water) changes from a solid directly into a gas, completely skipping the liquid state.

Sublimation is what sometimes makes the ice cubes in your freezer appear to shrink. A water molecule, when sufficiently energized (such as by solar radiation on a mountaintop) can break free from the frozen molecules surrounding it and instantaneously transition to a gaseous state. This is what dry ice (frozen CO2) does at room temperature. Water ice does not melt in space. It sublimates.

A comet is essentially a mass of dirty water ice. It only becomes visible to us after it has entered the sublimation zone. This is the point in the comet’s orbit that brings it close enough to our sun (about 280 million miles away) for solar energy to begin to sublimate the comet’s icy mass. Only then does the comet develop its characteristic water vapor “tail” (called a coma) which appears to glow when the sun’s light reflects off of it . Before a comet reaches the zone of sublimation, it is no more than a dark icy lump hurtling through space.

Make your own Comet!

You can make a miniature comet and watch as it sublimates–just like a real comet being heated by the Sun!

Materials:

3 lbs of Dry Ice

A Large Bowl

Plastic Garbage Bag

Several Smaller Plastic Bags

Insulated Gloves

A Hammer

2 Cups Water

Sand

A few drops of Ammonia.

Directions:

Use the plastic garbage bag to line a bowl big enough to hold a quart of water.
Put two cups of water into the lined bowl.
Add a couple of spoonfuls of sand
Sprinkle in a few drops of ammonia and stir the mixture well.
Wearing gloves, wrap the dry ice in several layers of plastic bags. Use a hammer to pound the dry ice into small pieces. When the dry ice is crushed add about two cups of it to the bowl while stirring your comet “soup”. Keep stirring while the dry ice freezes the water.
When the mixture is almost completely frozen, lift it up using the plastic liner of your bowl and shape the wrapped mixture into a ball. When the “comet” is frozen and can hold its shape on its own, unwrap it and set it somewhere you can watch it.
The dry ice will sublimate into a gas. You may see jets of carbon dioxide shoot from your comet. After a while, your comet will shrink and become pitted, like a comet that has been eroded by the Sun.