Activity: Possible Collisions on Earth due to Asteroids and Comets

Materials:

About this Activity:

Procedure:

1. Discuss the hazards due to asteroids and comets using the background information. Ask students how this event could affect our society today.

2. Have students do the activity on constructing a geologic time line. This will give them the background information they will need for the debate on dinosaur extinction based on the asteroid- impact theory. This will probably take a class period.

3. Divide students into groups or have them work in their lab groups. Their task will be to research different theories as to the extinction of the dinosaurs. Have each group research the various theories so they can take a defend a position before their peers. If you do not have resources in room or in their textbook, take the class to the library to complete the research.

4. Below are some of the more popular theories that students will most likely find in their research.

   Overview of Geologic Time:

   The earth's history is divided into periods of time based on the types of fossils present in sedimentary rocks formed during those times. The time before 570 million years ago, the Pre-Cambrian era, lacked life-forms with hard parts that formed fossils easily. All large organisms were soft-bodied and lived in the ocean. During the Paleozoic era, from 570 to 245 million years ago, hard-bodied plants and animals exploded in abundance, and colonized the land. The first amphibians occurred, and ferns and conifers dominated the forests. The Mesozoic era, from 245 to 65 million years ago, was the time when dinosaurs were abundant land animals. Mammals also existed, but were of lesser importance. Later in this era, the first flowering plants occurred. The Cenozoic era, since the extinction of the dinosaurs 65 million years ago, is the age dominated by mammals. The first grasses developed, and the landscape took on its present appearance.

   In comparison, human civilizations have developed only during the last six thousand years.

   The Mesozoic era of the dinosaurs is in turn divided into three periods based on the dominance of different species: the Triassic (245-208 million years ago), the Jurassic (208-144 million years ago), and the Cretaceous (144-66 million years ago). The earth's continents began to assume a recognizable configuration only during the Cretaceous period.

   Theories of Dinosaur Extinction:

   • Luis and Walter Alvarez and their colleagues proposed that approximately 65 million years ago there was a large impact on Earth by either an asteroid or a comet. The heat from the impact caused forest fires, and lofted huge amount of dust from the impact crater into the atmosphere. The smoke and ash from the fires and the dust particles together blocked the sun for at least several months. The temperature decreased since the sunlight was partially blocked from reaching the Earth's surface. Without sunlight and in the cold, the plants began to die since they could not make food. This in turn affected the plant-eating dinosaurs since they lacked their source of food so they too began to die. Since the plant-eating dinosaurs were dying, there was a lack of food for the meat-eating dinosaurs, and they perished as well. In other words, the effects of the impact caused the ecosystem to collapse. It is speculated that this impact led to the extinction of 75 percent of the species alive on Earth during that time.

     One of the most important pieces of evidence for this theory is a clay layer deposited in the oceans during the end of the Cretaceous Period and the beginning of the Tertiary Period. The clay is rich in iridium and other trace elements which are rare in the Earth's crust, but are very common in meteorites. In fact, the iridium was found to be 160 times greater than normal.

   • Volcanic eruptions are another theory for the extinction of the dinosaurs. Some explosive volcanic eruptions emit large amounts of ash or dust into the atmosphere so that the Earth's temperature got either too hot or too cold for the dinosaurs. If the ash particles were really small, less than two microns (1 x 10^-6 meters), then sunlight would be blocked out making the Earth's temperature too cold for the dinosaurs. The smaller particles scatter the short-wavelength incoming radiation which would cool off the Earth. On the other hand, if the ash particles were larger than two microns, which is still small, sunlight would be able to reach the Earth's surface, but the heat radiation emitted from the surface would not be able to escape into the atmosphere. These larger particles do not scatter incoming short-wavelength radiation. Some of them get blocked by the particles but do not interact to scatter back into space. The radiation emitted from the Earth's surface is mostly heat radiation which has longer wavelengths. It can be scattered back to the surface by the larger particles. This would cause the Earth's temperature to increase, making it too warm for the dinosaurs to survive.

     One major problem with the volcanic hypothesis is that volcanoes, especially the explosive ones, do not produce much iridium. Iridium was found in the clay layer that was present during the time of the dinosaurs. Basaltic volcanoes produce more iridium, however they are not very explosive.

     A more recent theory states that instead of a huge explosive eruption, a long-term basaltic eruption took place which erupted sulfur dioxide gas into the troposphere. The gas then was converted into small droplets of sulfuric acid which blocks incoming sunlight. Since it is in the troposphere much of the acid will be removed by precipitation, but if the eruption continued for a long enough period of time, it could have kept up with the rain to produce an Earth-covering haze. This idea provides a long-term source for lots of sulfur dioxide, and a possible explanation for the iridium that was found in the clay during the time of the dinosaurs since iridium is present in basalt lava. (Source: Scott Rowland, volcanologist)

   • Another theory is based on the plant life that spread during the Cretaceous Period. Many of the flowering plants were poisonous. These caused diseases or illnesses in the life forms that ate these plants. When the dinosaurs ate these plants they became ill and died. This carried over to the meat-eating dinosaurs since they ate the plant-eating dinosaurs with the diseases or illnesses.

   • Another theory involves the development of mammals. These mammals came about around the same time as the dinosaurs in the Triassic Period. It was likely that the small furry mammals were more intelligent and faster than the dinosaurs. For food, these small mammals fed on the dinosaur eggs. This meant that there would be fewer dinosaurs being born. Since there were very few dinosaurs born to replace the older ones when they died, this led to the extinction of the dinosaurs.

5. To help students with organization, have them make a chart which includes the following items:

   Description of Theory	Supporting Evidence	Arguments Against		Interesting Points

   Have each student create their own chart for each theory. This will help them organize the information so they can take a position relative to the cause of the extinction of the dinosaurs.

6. Using the chart as their planning sheet, students should use this information to complete the writing prompt.

7. Now that students have researched and evaluated the different theories, have students participate in a debate on the most recent theory. Hold a class debate on the following issue: "The extinction of the dinosaurs was caused by an asteroid or a comet hitting the Earth. This resulted in the production of smoke, ash, and dust that blocked out the sun." Allow students with preparation time before the debate.

Constructing a Geologic Time Line: Student Pages

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Contact Karen Krupinsky (kgurley@gsfc.nasa.gov) or Tammy Seergae (tseergae@umd.edu) for further information.