Remote Sensing/Reflectance Spectrometer

Transcription

1 Remote Sensing/Reflectance Spectrometer

2 REMOTE SENSING / REFLECTANCE SPECTROMETER TEACHER NOTES The remote sensing experiment is designed to take a full hour to complete, and can be undertaken using just the materials provided within this pack as a paper based experiment. If you wish to introduce a practical aspect, it is possible to purchase the ALTA Reflectance Spectrometer to allow the students to take their own reflectance measurements. All of the data in this module was taken using an ALTA. The ALTA is available from: Orders Department, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston TX , USA (Tel: , Fax: , order@usra.lpi.edu). At the time of writing (August 2000) the cost of each spectrometer (including activity sheets) is $ post & packing. If this cost is prohibitive to you, an ALTA is available for loan for the cost of post and packing from: Dr. Wendy Kirk. Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT. Please contact Dr. Kirk on for more details. This experiment will also soon be available online. The Mars in the Classroom pages are housed at This will follow the same structure as the pack, but will allow students to take a more hands on approach to taking and using the reflectance measurements. Activity sheet - aims and benefits To obtain accurate results from this practical and answer the questions correctly, the students must be able to: a) work methodically b) transfer data into graphical form c) interpret graphs and tabular data There is a lot of graph work in this experiment, so it is important to provide a context for the data. Stress the importance of the graphs regarding the nature of the Martian surface and make certain they understand the usefulness of transferring the information to image format for interpretation and location purposes. Hints and answers to the question in the Student Notes pack. We have given only a short introduction to the electromagnetic spectrum and remote sensing. Depending on the ability and level of your group you may wish to spend more time on this to ensure that the students have a firm grasp of wavelengths. For the questions (see answers below) it would be better for you to lead a discussion to get the students to really think about their answers. Remote Sensing (Teacher Notes) 1

3 Question 1: Some advantages are that it is less costly/dangerous to send an orbiter. The remote sensing craft is able to safely spend a lot longer in orbit around the planet, and will be able to look at a lot more of the planet than a probe would by landing in just a single place. Disadvantages are that you get less surface detail from an orbiting craft, and you can t obtain or study rock samples from remote sensing. There will be many other advantages and disadvantages that the students will think of for themselves. Question 2: We would expect the student to send a remote sensing craft first to search for the most interesting (and safe) landing sites. Question 3: Reflectance spectroscopy uses mainly ultra-violet, visible and infrared wavelengths. Again it would be useful for the teacher to take the students through the electromagnetic spectrum if they are unfamiliar with it. Notes on Activity Sheet 1 The students should be led through these activities and encouraged to think carefully about their answers with full justification. Some of the points we would expect for each question are: Question 1: Where there is water there may be life. Is there water there now? If not, where did all that water go, and will there still be any signs of life? Question 2: Red as that is the colour of the surface of Mars. The students will actually find though, that the red things plotted are brightest at the infrared wavelengths. Question 3: Anything red (i.e. paper, blood, apples). Nothing other than rock, soil or dust is likely to be found on the surface of Mars. Question 4 (from point 3): Yes, they should find that Object 2 will have a similar shape, but will not be as bright. Object 6 will also be similar but has a slight difference in shape. For your information, the actual objects used for this experiment are: Earth Rock: Play sand Object 1: Red card Object 2: Martian soil simulant Object 3: Dark red flower petals Object 4: Red house bricks Object 5: Glossy red car Object 6: Grey cement powder The Martian soil simulant is actually volcanic rock from a volcano in Hawai i and has similar spectral properties to Martian soil. Graph 1 is an adaptation of Mars Pathfinder data, and shows the shape and brightness of the Martian soil as measured by the rover on the surface of Mars. Question 5: At least one spectrum should be the same/similar to the Earth rock. From this, the students should conclude that water was present, as we have stated earlier that the Earth rock formed in the presence of water. In reality it is not this simple of course! Remote Sensing (Teacher Notes) 2

4 Notes on Activity Sheet 2 Question 1: In the higher orbit, the images will cover a larger area, but at a lower resolution (i.e. less detail). In the lower orbit, less area will be covered, but more surface details will be seen. Which is most useful depends on what they want to do if they want to find out how the composition varies across the whole planet on a large scale, then the higher orbit is best. If they want to locate a specific landing site, as they do in this experiment, then the lower orbit (higher resolution) will be better in order to fully characterise that site. Question 2: The main difference will be that they will see more blue squares in the lower orbit image. Figure 3 is the more detailed one pixel in Figure 2 is represented as 9 pixels (3 3) in Figure 3. This information should be given to the students as you lead the discussion. Question 3: This has been answered above (depends on what they want to do etc.). Try to draw out more specific examples from them using the information they gained from answering the previous questions. Question 4: Use the lower orbit data as this will give more detailed information for the small spacecraft that will land within a single pixel. Question 5: This will be up to the students. Some will go right in the middle of the blue squares to be sure of hitting an area containing the right rocks while others will want to land at the boundary between two colours to look at how the surface varies across the boundary (this will require confidence in their ability to achieve a pinpoint landing). There will be a number of responses to this question and there is of course more than one correct answer. Encourage discussion of the advantages and disadvantages of some of the suggestions. Question 6: The Mr data has only provided limited compositional information. If you are going to have a human landing, you need to make sure that the ground isn t too rocky, and that there are no other nasties like crevasses, cliffs or dunes of dust that will make the surface unstable to land on. There are a number of other remote sensing instruments that can be used to characterise the landing site in some of these ways. While we do not expect the students to know the names of such instruments you should try to get them to think of the kind of information they would want from other remote sensing instruments (i.e. topographic data, climate information etc.). Remote Sensing (Teacher Notes) 3

5 REMOTE SENSING / REFLECTANCE SPECTROMETER STUDENT NOTES In this experiment you will be using reflectance spectroscopy to learn about the surface of Mars and try to find a good landing site for your Manned mission. Reflectance spectroscopy is a form of remote sensing often used to study planets. What is remote sensing? Some spacecraft (satellites) in orbit around the Earth and visiting other planets carry different kinds of cameras to look at features on the surface of the planet, or at its atmosphere. The information gathered by these satellites is used by scientists to discover many things about the planet, without having to land on the surface. Because these satellites find things out (i.e. sense things) about the planet without ever touching the ground (i.e. remotely), this method of exploration is called remote sensing. These cameras can collect information at many different wavelengths. Looking at an object at lots of wavelengths across the electromagnetic spectrum can tell us different things about that object. But what is the electromagnetic spectrum? Most of you will have seen light split into its various colours, such as in a rainbow. You will all have heard of things such as X-rays, microwaves, and radio waves. What you might not know, is that all of these are part of the same thing, called the electromagnetic spectrum (Table 1). The only difference between them is their wavelength: at one end of the spectrum are the long wavelength, low energy radio waves and at the other are the short wavelength, high energy gamma-rays, with many other wavelengths in-between. Remote sensing uses all of these wavelengths to get information about a planet. Question 1: What are the advantages and disadvantages of using remote sensing instead of landing on the surface? Give a reason for your answers. Question 2: With these in mind, which type of mission would you send to the planet Mars first, a landing mission or a remote sensing mission? Why? Remote Sensing (Student Notes) 1

6 How does reflectance spectroscopy work? Reflectance spectroscopy is a type of remote sensing that looks at the way in which a surface reflects light at different wavelengths. The reflectance value or brightness at each wavelength can be plotted on a graph to produce a spectrum, and the shape of the spectrum can tell you things about the object you are looking at. Question 3: Look at Table 1. What parts of the electromagnetic spectrum does reflectance spectroscopy use? Table 1. The Electromagnetic Spectrum Radio waves Micro waves Visible X-rays Infrared Ultraviolet Gammarays Long wavelength Short wavelength Low energy High energy Use in remote sensing Radar Mapping Thermal (heat) mapping Thermal Mapping Visible imaging reflectance spectroscopy Compositional mapping GLOSSARY: ELECTROMAGNETIC SPECTRUM radiation of different wavelengths make up the electromagnetic spectrum. Radio waves and X-rays are examples of electromagnetic radiation that make up the electromagnetic spectrum. REFLECTANCE SPECTROSCOPY a remote sensing technique that measures reflected sunlight to get information about a planet s surface. SPECTRA plural of spectrum. SPECTRUM a plot of brightness at different wavelengths. WAVELENGTH the distance between the peaks of two waves. Remote Sensing (Student Notes) 2

7 REMOTE SENSING / REFLECTANCE SPECTROMETER ACTIVITY SHEET 1 As a part of the Mars Reconnaissance or Mr. program, the Space Agency has decided to launch a remote sensing spacecraft carrying an instrument to undertake reflectance spectroscopy and search for possible landing sites for a future mission to land humans on the surface. You are in charge of the mission, and have been told to look for areas where water may have existed when Mars was much warmer. Question 1: Why do you think these are important areas to study? Your mission is a success and the spacecraft is sending backs lots of data. Now you have to analyse it! Your team is provided with the results from a region of Mars that experts say once contained water. These scientists also have a spectrum of an Earth rock that they think is the same as the rocks in this part of Mars and which proves that water once existed there. They want you to find a place in your region that has a spectrum similar to the Earth rock, as this will confirm the presence of water in the past. It is up to you to locate these rocks (if they exist) and decide whether this really is a good candidate for a human landing site. Your task: Your data is contained in Tables 2 and 3 with brightness (reflectance) values versus wavelength for a number of objects. You will be using these values to plot a graph for each object. The graph you plot is known as a spectrum and provides information about the object (i.e. brightness etc.). Before you plot your spectra, take a look at Graph 1. This shows two spectra as an example and guide for your graphs. Look carefully at the example spectra in Graph 1. The measurements were taken at very similar wavelengths that the cameras on-board your spacecraft work at. Using the information provided on Graph 1 and the colour that relates to wavelength in Table 2, answer the following questions. Question 2: At what colour (i.e. wavelength region) would you expect the surface of Mars to be brightest? Remote Sensing (Activity Sheets) 1

8 Question 3: What other objects can you think of that are this colour, and are these likely to be present on Mars? Try to think of at least four. Now you have to determine whether the scientists were correct when they said that rocks similar to the sample from Earth are to be found in your region, and hence that water once existed there. Do the following: 1. Using the data table for the Earth rock in Table 2, plot out the spectrum using Graph 1 as a guide, with wavelength on the x-axis and brightness (reflectance) on the y- axis. 2. For each object in Table 3, plot a separate graph as you did for the Earth rock. You should have 6 more graphs. 3. Look carefully at all of the graphs you have drawn. Question 4: Do any of the spectra from the Mr. mission look like the Earth rock? Which one is most similar? Question 5: From your answer to Question 4, do you conclude that water once existed in this particular region of Mars? Give a reason for your answer. Remote Sensing (Activity Sheets) 2

9 REMOTE SENSING / REFLECTANCE SPECTROMETER ACTIVITY SHEET 2 The remote sensing instrument of Mr. passed over the region of Mars you have been given twice during the mission, once from a very high orbit, far from the planet and once from a lower orbit closer to the planet s surface. Question 1: How do you think the results from the two orbits will differ? Which will provide the most useful information to you and why? You learned a lot from the spectra that you plotted in the first exercise. But now you want to find out exactly where those spectra are located on the surface. To do this, you decide to create a picture from your data. The Mr. sensor uses an electronic camera to take measurements, consisting of a grid of 36 picture elements or pixels. Each pixel has a number from 1 to 36, as shown below in Figure Figure 1. A grid of pixels, as used by the Mr. mission. The number of pixels covered by a given area or spectrum depends on the camera s resolution i.e. how much of the surface detail can be seen in the data. To make the data easier to visualise, we will assign a colour to each of the spectra plotted in the previous exercise. The colours assigned are: Earth Rock: Blue Object 1: Red Object 2: Brown Object 3: Yellow Object 4: Green Object 5: Black Object 6: White Remote Sensing (Activity Sheets) 3

10 Each pixel will contain a single spectrum that represents the strongest signal from the area the pixel covers. Tables 4 and 5 reveal which spectrum was recorded at each pixel. Colour in the pixels accordingly in the two blank camera displays provided. Table 4. Distant Measurement Table 5. Low Orbit Measurement Pixel No. Spectrum Pixel No. Spectrum Pixel No. Spectrum Pixel No. Spectrum 1 Yellow 19 Brown 1 Blue 19 Red 2 Brown 20 Brown 2 Brown 20 Brown 3 Brown 21 Blue 3 Brown 21 Blue 4 Brown 22 Red 4 Brown 22 Blue 5 Brown 23 Red 5 Red 23 Blue 6 Red 24 Red 6 Red 24 Brown 7 Brown 25 Brown 7 Brown 25 Green 8 Brown 26 Brown 8 Blue 26 Brown 9 Brown 27 Red 9 Brown 27 Red 10 Brown 28 Red 10 Blue 28 Blue 11 Brown 29 Red 11 Brown 29 Red 12 Red 30 Red 12 Black 30 Red 13 Brown 31 Red 13 Brown 31 Red 14 Brown 32 Red 14 Brown 32 Red 15 Brown 33 Red 15 Blue 33 Red 16 Brown 34 Red 16 Blue 34 Red 17 Brown 35 Red 17 Brown 35 Red 18 Red 36 Red 18 Red 36 Red Figure 2. Distant Measurement Figure 3. Low Orbit Measurement Question 2: What differences can you see in the two images? Which is the more detailed? Remote Sensing (Activity Sheets) 4

11 Question 3: Give reasons why both kinds of resolution are valuable in some instances, with examples. Question 4: Which of the two would you use to locate a landing site, given that the scientists want to send the astronauts to a site that once held water? Question 5: Where would you land and why? Use the pixel number from Figure 1 as a label. Question 6: Are you certain that the site seen by Mr. is suitable for a Manned landing? What other information do you think you need to be sure? Can remote sensing be used to obtain any of this information? If not, what else can you use? Remote Sensing (Activity Sheets) 5

12 REMOTE SENSING / REFLECTANCE SPECTROMETER DATA SHEET Table 1 : Reflectance values for the Earth rock material. Colour Wavelength (nm) Reflectance Value Blue Green Yellow Amber Orange Crimson Red IR IR Table 2 : Reflectance values obtained from the Mr. mission. Reflectance Values Colour Wavelength (nm) Object 1 Object 2 Object 3 Object 4 Object 5 Object 6 Blue Green Yellow Amber Orange Crimson Red IR IR Remote Sensing (Data Sheet) 1

13 REMOTE SENSING / REFLECTANCE SPECTROMETER EXAMPLE SPECTRA Graph 1 has wavelength on the x-axis and reflectance value (brightness) on the y-axis. Two spectra are plotted on the graph one bright soil and one dark soil. You can see that the bright soil has much higher reflectance values, because it reflects more light. Graph 1: Example spectra Reflectance Value Wavelength (nm) Remote Sensing (Data Sheet) 2