IMPACT CRATERING TEACHER NOTES AND HANDY HINTS

Transcription

1 Impact Cratering

2 IMPACT CRATERING TEACHER NOTES AND HANDY HINTS The Impact Cratering module can take one of many forms, depending on time available, equipment, and the nature of the students. The lesson could be entirely non-practical, with the teacher leading a discussion using slides and pictures from books or the Internet, and encouraging participation from the students. If the teacher wished, this could be interspersed with demonstrations from the activity sheet. These could be used to demonstrate to the students a) the effect of larger projectiles on the final crater, b) ejecta patterns and rays, c) the effect of oblique impacts, d) the effect of velocity on the final crater form and e) the effect of target material on the crater. Alternatively, the teacher could opt to have the students carry out the full activity themselves. In this case, it is recommended to use two lessons (depending on the working speed of the students). The choice is yours. Activity sheet - aims and benefits This activity has been taken from the Hawai i Space Grant College who should be acknowledged if your results are placed in public access areas (i.e. displays, web pages etc). They also have many more classroom activities that you can try (see for details). In its full form, this experiment requires many scientific skills from a student. Good experimental practices are needed, and to obtain accurate results the students must: a) work methodically b) take good notes c) record information accurately d) transfer written data into graphical form In addition, the students will find some of the measurements difficult to take, and will need to think carefully about how to get the most accurate readings. Hints for these problems are below. The students will also be encouraged to think about the experimental setup and consider where possible errors may occur. Hints 1. This practical really can be messy - ensure that precautions are taken to stop the flour ending up all over the floor and students. Certain coloured powders may stain clothes - if this type of material is chosen, aprons are strongly recommended. 2. When carrying out the experiment, the target (i.e. flour and paint) may have to be reprepared several times. There is no need to start out with fresh materials each time; as long as the top layer contrasts the lower layer strongly enough, the materials can be used again and again. 3. Measuring the diameter of the craters can be difficult, particularly if the bowl is smaller than the ruler. Use a compass instead to span the diameter and then measure the compass separation. The same method can be used to measure the diameter of the projectiles. If you have an able class, increase the complexity by Impact Craters (Teacher Notes) 1

3 using two or three different projectiles of different size/mass and get them to repeat the experiment and compare the results of each. 4. (Q2) The graph should show that the diameter of the crater and length of rays increases with increasing velocity (i.e. increasing projectile height). 5. (Q4) The compaction of the flour is very important. You will get very different results from flour that has been sieved into the bowl and flour that has simply been dumped into the bowl from the bag. Results will be better if the flour is sieved, or at least "fluffed up" with your fingers/hands. Discrepancies may result if a particularly thick layer of coloured powder is put on top. Also, it may sometimes be difficult to determine where the rays end, particularly if the rays of one crater overlap with another. You should encourage the students to discuss these points. 6. (Q5) The students may or may not have a raised rim at the edge of their crater. All fresh impact craters in reality have this feature. The student version will also lack a central peak or terraced walls. The main reasons for this is the lower velocity of the impactor in the experiment and the small size of the impactor. In real impacts, the velocity of the projectile will be of the order of 10km/s or more, and the projectile is vaporised on impact. The student's projectile of course will still be there. Extensions 1. Discuss the possibility of asteroids/projectiles impacting the Earth. There has been a lot of interest in this recently. Get the students to predict what would happen if an asteroid were to hit the Earth - what difference would an impact into water make compared to an impact onto solid ground? 2. Look at a succession of craters that have been weathered to different degrees on Mars/Moon/Earth. This can lead to discussions of weather patterns on the different planets, superposition of craters, and the effects of surface material on the degradation of craters. 3. Take the discussion of the effect of water on Martian craters a bit further. Get the students to experiment with different kinds of wet material - flour/sand/plaster of paris and see if they can reproduce any of the Martian features. Water is not the only liquid that can be used for this - perhaps a liquid like sunflower oil would also work (the difficulty in this experiment is to get the final shape to hold). UNTESTED BY US SO TRY THIS OUT BEFORE THE LESSON!! Please let us know if it works! 4. Try multiple layering. In a real impact event, the material from below the surface ends up on top in the ejecta blanket - this is shown nicely in the experiment here. Further to this try adding another colour on top of the current experimental setup and see if the reverse layering still holds. UNTESTED BY US please let us know if it works! 5. Try oblique impacts. Get the students to predict what will happen to the ejecta before they carry out the experiment. The ejecta will be thrown preferentially downrange. We haven't worked out a way of measuring the angle yet, but 90 vs. non-90 is good enough to demonstrate the effect. If you can find a way of Impact Craters (Teacher Notes) 2

4 measuring the angle, you should be able to undertake a more scientific study by plotting graphs of angle vs. ray length. Answers to questions from the background information sheet and "What can impact craters tell us?" 1. The picture with the larger number of craters is older. The students should try to give a reason why the age of one part of Mars would be younger than another. The answer to look for is something covering over the older impact craters and "resetting" the crater count to zero. Lava flows can cause this on Mars, for example. 2. The main difference between the two craters is the appearance of the ejecta blanket. The reason for the difference is that the ejecta in a) was emplaced ballistically, i.e. it was thrown out, and it stayed where it landed, while that in b) flowed along the ground once it landed. The flow resulted from the presence of melted ice in the ground, which created a mud-like flow. Therefore, by studying this crater we can tell that water was once present beneath the surface in this area. 3. Again, there could be many answers to this question. The obvious answer is to go to a place that had water (and hence where life may once have existed). Make a list of the answers you get for future reference - they may be useful during mission planning! Impact Craters (Teacher Notes) 3

5 IMPACT CRATERING STUDENT NOTES What is an impact crater and how do they form? An impact crater is basically a large hole in the ground caused by a meteorite smashing into the surface of a planet or moon. Every solid body in the Solar System suffers from the effects of impact cratering, and you can see this when you look at our own Moon through binoculars. The projectiles (i.e. meteorites) that cause impact craters travel at very high speed, faster than anything we see in our everyday lives. They travel so fast that they are vaporised upon impact, and very little is left of the projectile after the impact event has taken place. What do impact craters look like? Impact craters are normally circular features, and range in size from the microscopic to the very large (>2000km diameter). The size and shape of an impact crater depend on the size of the projectile, its velocity and the type of material that it hits. Small impact craters (like Meteor Crater, Arizona) have simple shapes like a bowl. Larger craters have a different shape (see Figure 1). They have mountains in the centre of the crater called a central peak and huge landslides occur on the inside of the crater to form what we call terraces. When an impact occurs, lots of material is thrown out around the crater, and this material is called ejecta. Ejecta is made up of rock and soil from the surface, and sometimes forms bright rays (you can see this around the bright crater Tycho on the Moon). Impact Craters (Student Notes) 1

6 Figure 1: Some features of an impact crater. What can impact craters tell us? 1. The age of a surface. The only way to be certain about the age of the surface of another planet is to get a sample of rock from that surface and date it in a laboratory on Earth. Unfortunately, this is not yet possible for most of the planets and moons in our Solar System. To date, it has only been achieved with rocks from the Earth, Moon and a bit of Mars, so we have to use other techniques to work out the age of most planetary surfaces. Impact craters can give us an idea of whether a surface is old or not simply by counting the number of craters in a certain area. The older a surface is, the more impact craters it will have. Question 1: Look at Figures 2a and 2b, which show two different regions on Mars. Which area is older and why? Why would two areas on Mars have different numbers of craters - what could have caused their difference in age? 2. The geology of an area. The shapes of impact craters can vary due to the type of material they form in. A solid surface will have different looking impact craters to those in a softer, powdery surface. The presence of water can also affect the way a crater looks and by studying the shape and appearance of impact craters from photographs, scientists can get some idea of what the area is like. Question 2: Look at Figure 3, which shows pictures of two different Martian craters. Label the different features you see on each, using Figure 1 as a guide. Name some of the differences between two craters. Can you suggest what may be present under the surface of crater 3b that is not present under crater 3a? What happens to a crater after it forms can also tell us something about the area it lies in. For example, if a crater has formed on very loose material, then the winds on Mars will erode the crater quite quickly. Other craters on Mars look as though they have been in the middle of a river of flowing water (they have a streamlined appearance, similar to that of small islands in rivers on Earth). Impact Craters (Student Notes) 2

7 3. Rocks below the surface When an impact crater forms, it throws out material in a particular way. The ground is flipped upside down so that the layers from deep in the ground end up being on top (you will see this in your experiment). This means that by looking at the ejecta of craters, you are looking at the material that is present below the surface. This is interesting for scientists who want to know about the history of Mars. Digging down into the ground is a little like going back in time. Events that happened long ago may have been covered over by later events such as lava flows. Only by probing into the surface will we be able to see what the surface was like billions of years ago. Perhaps some rocks will be able to tell us what the atmosphere was like, or contain fossils of early life. But to find these things out we need rock and soil samples from that time, and sometimes that can only come from deep down. Impact craters give us a chance of finding out about the rocks deep below the surface without the need to land there. This is one of the reasons impact craters are so important to study. Question 3: Write a summary of how impact craters can help you understand more about an area. If you were selecting a site on Mars for a human landing, what kind of area would you like to go to and how could impact craters help you decide where to go? GLOSSARY CENTRAL PEAK - a mountain formed in the centre of large impact craters. These do not occur in small impact craters. EJECTA - a sheet of material (rocks, soil) surrounding the crater that was thrown out during the impact event. ERODE - the process by which something is worn away over time. FLOOR - the floor of an impact crater (usually either flat or bowl-shaped), lying below the surrounding ground level. IMPACT CRATER - a hole in the ground (usually circular) caused by the impact of a meteorite onto the surface of a solid body. METEORITE - a lump of rock or ice that hits the surface of a planetary body. PROJECTILE - an object that strikes a surface to produce an impact crater. These can include dust particles, pebbles, rocks, comets and asteroids. RAISED RIM - this is the edge of the impact crater and is composed of rocks and material piled up on top of one another. RAYS - bright lines of ejecta coming from an impact crater. These can extend for vast distances in the case of large impacts. TERRACED WALLS - these only occur in large impact craters. After the crater forms, the walls of the crater slip, creating huge landslides inside the crater. Impact Craters (Student Notes) 3

8 IMPACT CRATERING Figure 2a Figure 2b Impact Craters (Student Notes) 4

9 IMPACT CRATERING Figure 3a Figure 3b Impact Craters (Student Notes) 5

10 IMPACT CRATERING ACTIVITY SHEETS This experiment is a slightly modified transcription of the web pages of the Hawai i Space Grant College. You may find more information about the experiment from their pages ( Aims and objectives To investigate the way the diameter of a projectile affects the final form of an impact crater. To compare the results from this activity with real craters observed on the Moon and Mars. Apparatus (for each group) 1 large high-sided bowl (~30cm diameter) 1 ruler 2 bags of flour 1 compass 1 pot of dry powder paint (not white!) 1 pair of tweezers 1 flour sifter 1 spoon 1 sieve (optional) data tables/paper/pens/graph paper projectile (i.e. large gobstopper or projectile) Aprons and old newspaper/table covering Experiment, observations and measurement 1. Preparation - Put on your apron and roll up your sleeves - this could get messy! Put down some old newspaper or a table covering to catch any overspill from the bowl. - Put an even layer of flour in the high-sided bowl to a depth of approximately 6cm. Cover with a thin layer of dry coloured powder paint (any colour will do as long as it contrasts well with the flour underneath. A flour sifter often helps at this point to get an even layering). - Measure the diameter of your projectile, and record this on your data chart. 2. The Experiment - Take the projectile and drop it onto the prepared surface from a height of 30cm. - Make a note of the diameter of the crater, the number of rays and their length. The rays will be visible as white streaks radiating from the crater. Measure the length each ray and take an average value. Record all information in your data table. - Record any other observations you make about the crater. - Repeat the above, this time dropping your projectile from 60cm, 90cm and 2 metres. Remember to record all information in your data tables. Data analysis 1. Plot graphs using the data you recorded in your tables. Plot Graph 1 as the crater diameter vs. projectile height and Graph 2 as average ray length vs. projectile height. Impact Craters (Activity Sheets) 1

11 2. Explain what each graph tells you about the relationship between the projectile and the crater. 3. If the projectile was dropped from a height of 6m, estimate the size of the final crater from your experimental data. 4. Describe and discuss where possible errors could occur in your experiment. In particular, how could your experimental set-up and procedure differ from those of another group carrying out the same experiment with the same instructions? 5. From your experimental observations, describe the appearance of an impact crater. Give reasons why your craters differ from the real craters observed on the Moon and Mars. Impact Craters (Activity Sheets) 2

12 IMPACT CRATERING DATA TABLE Sheet No. of Name: Other members of group: Class: Date: Projectile diameter (mm): Trial No: Height (cm) Diameter of crater (mm) No. of rays Length of Ray 1 Length of Ray 2 Length of Ray 3 Length of Ray 4 Length of Ray 5 Length of Ray 6 Average length Notes Impact Craters (Activity Sheets) 3