Water. It?s essential for life as best we know it. Almost three-fourths of the Earth is covered with water. We live on the pale blue dot, and our lives depend fundamentally on water. Yet, just after Earth formed about 4.5 billion years ago, the surface was mostly dry. ?So, where did the water come from?? asked a high school teacher this morning at the Astrobiology Summer Science Institute for Teachers (ASSET) here at San Francisco State University. It?s a good question that his students are very likely to ask as they study the evolution of our planetary system.
The quick answer is comets. Some water was also released the rocky materials that coalesced to become planet Earth. About 4 billion years ago, the Earth was bombarded by ice, rock and dust, which today we call comets and asteroids. The cratered surface of the Moon records this bombardment.
Comets are great icy snowballs of water, carbon dioxide and ammonia ices, organics (hydrocarbons), and dust. Roughly, they are 75% ices. We know this from studying comets that invade the inner solar system. We?ve studied them with spectroscopes on ground-based, airplane and spaced-based telescope to determine the chemical composition. More recently, spacecraft have swept up particles and returned them to Earth for analysis. We?ve also studied meteor showers?the delightful lightshows put on by nature when Earth plows through the debris shed by a comet.
When I stand on the edge of the ocean, or fly across the Pacific, it seems incredible that snowballs from space could have filled the oceans. Given several hundred million years, it works. For most of us, it?s difficult to imagine the immense amount of time involved. It?s simply beyond human scale. So, how can teachers make planetary evolution and comets ?real? in the classroom?
This morning, the ASSET teachers made comets. For the remainder of the day, we?ve watched them sublimate, hissing as gas escapes from their surface. They?ve evolved from icy pristine snowballs flecked with dust, to dark cratered masses as the carbon dioxide escaped, and left behind the husk of the comet: dirt and organics. This models what happens to comets that repeatedly visit the Sun. Ultimately, all that?s left is the dust and metals that generate meteor showers.
With a nod of credit to Dr. Dennis Schatz, the original cometary chef, and Vice President for Education at the Pacific Science Center, here?s how to make a comet at home or in your classroom.
Ingredients for one comet:
- 2 lbs dry ice
- 2 cups water
- 1-2 tablespoons of dirt
- 1-2 tablespoons of ammonia (plain, not sudsy)
- 1-2 tablespoons of hydrocarbons
- 1 large bowl
- 1 measuring cup?2-cup size is useful
- 3 garbage bags: large size
- leather work gloves
- safety glasses
- pie plate
- apron and chef?s hat (optional)
- hair dryer (optional)
There are some tricks to making a good comet.
- Work in a well-ventilated area. Dry ice releases carbon dioxide, which can accumulate in closed spaces. Plants are OK with excess carbon dioxide; people are not.
- Wear leather work gloves to handle dry ice (to avoid frost bite).
- Wear safety glasses when making the comet.
- Display the comet in a well-ventilated area.
Preparation: You?ll need to break up the dry ice into walnut-sized pieces. It?s easier if the dry ice has ?rested? a while in an ice chest. Overnight is OK if you?re making multiple comets and have a large amount of dry ice. Otherwise, simply store the dry ice in a small ice chest for a few hours.? Don?t latch the lid as the sublimation of carbon dioxide will blow open the lid. Don?t put the dry ice in the freezer?it?s too warm and the dry ice will sublimate away. Put dry ice into a double plastic bag, and break up shortly before making the comet with a meat tenderizer or other hammer. Do this outside on the ground, not a floor or cabinet top as the combination of frozen dry ice and hammering will damage surfaces.
Put on safety glasses and gloves. Line the large bowl with one garbage bag. Don?t open the bag. Pour in 2 cups of water; mix in the dirt, ammonia, and hydrocarbons. Any cola drink is a good source of ?hydrocarbons.? Mix with a long handled spoon until the water looks dirty, and all of the materials are wet. Add 2 cups of dry ice, and stir until the mixture seems stiff?1 or 2 minutes. Lots of ?smoke? will crawl out of the bowl and sink toward the floor?carbon dioxide is heavier than air.
Gather up the plastic bag around the comet, compressing the mass together. Leave the plastic bag open a little so that carbon dioxide continues to escape. (Don?t twist it shut.) After a couple of minutes, open up the plastic bag and retrieve your comet. Place the comet in the pie plate, and observe. You can use a hair dryer to blow a ?tail? away from the comet. The comet will change dramatically over the next few hours.
Troubleshooting: There?s a bit of artistry to making comets, and it?s good to practice before going public. Here are a couple of common problems:
The comet doesn?t stick together. Usually, this means that you need a bit more water. Add water to the mixture stir, and compress.
The comet turned into a mass of suds. You used ?sudsy? ammonia, and the carbon dioxide is making bubbles out of every bit of soap in the ammonia. I did this once, and while it was very entertaining, my comet did stick together. My comet turned into many small comets, like Shoemaker Levy 9.? Use plain ammonia.
Why do this? It?s a reasonable model for the nucleus of a comet, and it?s a fun project that brings space down to Earth. As to the difficulty of comprehending age of the Earth, we work with the scientific evidence and display the results on timelines. The early bombardment of Earth is at 4 billion years ago.
ASSET is supported by the NASA Astrobiology Institute, the SETI Institute, and a Lockheed Martin Missiles and Space Systems in Sunnyvale, Sunnyvale. ?Voyages Through Time? was originally developed with the support of the National Science Foundation, NASA, Hewlett Packard Company, Educate America and the SETI Institute in partnership with NASA, California Academy of Sciences and San Francisco State University.
- Video: Comets: Bright Tails, Black Hearts
- All About SETI
- Image Gallery: Great Comets
- Comets Through Time: Myth and Mystery
- Video: Take One Asteroid: A Recipe for Space Civilization
- Catastrophe, Mother of Evolution: How Life Survived Early Bombardment of Asteroids and Comets