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NAME: ________________________ STUDENT ID: ___________________ Individual Homework #2, Due Thursday, February 23rd at beginning of class. (8 pages) PART A: IDENTIFYING THE LINES OF LIFE We have seen that when light interacts with an object some of the light is absorbed and some of the light is reflected or re-emitted by the object. By examining how different wavelengths of light are absorbed and reflected (or re-emitted) we can determine information about the composition of the atmosphere or surface of an unknown planet or moon. In the graph shown below in Figure 3 (called an absorption spectrum), the percentage of light absorbed by water in Earth’s atmosphere (as measured between 0.7 µm and 10 µm) has been plotted over a range of wavelengths. Recall that a micrometer (µm) is equal to 10-6 meters (m). Absorption (%) Note: The distinction between emission and absorption, and the physical situations that lead to them, can be confusing. Basically any hot object emits a smooth continuous spectrum, mostly visible light if it’s a star at thousands of degrees and infrared radiation if it’s a planet at hundreds of degrees. Very hot gases have emission lines superimposed by atomic transitions of the elements within the gas—a spectral fingerprint. When we do remote sensing we look at a cool object like a planet in reflected light from a star. The reflected radiation has regions removed or “eaten away” at just those wavelengths corresponding to the major atomic or molecular transitions of the surface or atmospheric material of the planets. That’s how we’ll one day figure out what distant extrasolar planets and their atmospheres are made of. Rather than showing emission, or the amount radiated by the illuminating star, the graph below shows absorption, the amount or percentage of radiation taken away by atoms or molecules. So high values of absorption means we see less radiation at those wavelengths. Water (H20) 100 50 0.1 0.2 0.5 2.0 1.0 Wavelength (µm) Figure 1 1 5.0 10 In the table below we have identified the approximate range of wavelengths and corresponding names (types) for the entire spectrum of electromagnetic radiation (light). TYPE OF ELECTROMAGNETIC RADIATION (LIGHT) WAVELENGTH (METERS) Gamma ray < 10-11 m X-ray 10-11 m – 10-8 m Ultraviolet 10-8 m – 4 x 10-7 m Visible (VIBGYOR) 4 x 10-7 m – 7 x 10-7 m Infrared 7 x 10-7 m – 10-4 m Microwaves 10-4 m – 10-2 m Radio 10-2 m – > 102m 1. Identify the range of wavelengths of light that were observed to make the absorption spectrum for water shown in the graph in Figure 1. What is the name (or names) of the type of light that was used to collect this data? 2. Using the graph in Figure 1, list the value for each peak wavelength of light that is strongly absorbed by water. 2 The specific wavelengths of light that are absorbed or reflected from an object together make up a signature (like a fingerprint) that can be used to identify the specific composition of the observed object. Furthermore, there are specific conditions and indicators that are used by scientists to identify the presence of life. For instance, life as we know it on Earth requires the presence of liquid water, and the majority of liquid water here on Earth is found between the temperatures of 0oC and 100oC. Therefore, in our search for life in the solar system, it is essential to look for signs that liquid water is present or has been present in the past. Scientists have also identified other chemical signatures that can be used to indicate the presence of life. For instance, we might look for oxygen (O2), carbon dioxide (CO2), or methane (CH4) that has been given off by living organisms. At the end of this activity we have provided a Spectra Catalog that identifies a set of key wavelengths that correspond to many of the molecules that are central to the search for life on Earth, in our solar system, and beyond our solar system. 3. Over what range of wavelengths would you need to observe if you were planning on identifying all the molecules listed in the Spectra Catalog in your search for life? PART B: LOOKING AT THE DISTANT EARTH To begin our search for life using remote sensing, we will consider what it would be like if our home were a distant planet from Earth and we had sent out a probe from our home planet that had reached the “mysterious” Earth. 1. What type of light would your probe need to be able to detect in order to search for life on Earth? Explain your reasoning. 2. During your survey you determine that water (H2O), methane (CH4), and carbon dioxide (CO2) are all abundant. List the wavelengths that may have been detected by your probe that would allow you to identify the presence of these molecules. 3 PART C: DO WE SEE THE SIGNS OF LIFE IN OUR SOLAR SYSTEM? In this part of the activity will examine the other planets or moons in our solar system and consider what information we might be able to gather about these objects through remote sensing. In each case, you should consider whether the information provided suggests that life is or is not likely to occur on the planet or moon being investigated. Imagine that your probe has left its orbit above Earth and has established a new orbit above the planet Mars. The grayscale image shown in Figure 5 below provides two pieces of information. On the left is an image of Mars in the visible part of the spectrum. The right image was taken of the surface of Mars with an infrared camera. The different shades of gray in the IR image are used to indicate the range of temperatures (from -120oC to 0oC) observed across the Martian surface. Note the dark shade of gray indicating -120oC that has been provided in the temperature scale to the right of the IR image. It comes from the THEMIS Public Data Releases, Mars Space Flight Facility, Arizona State University. (September 26, 2006) <http://themis-data.asu.edu>. ` Figure 2 4 1. In this grayscale image of the Mars shown in Figure 5, what type of information is being displayed in the expanded section between the full planet and the temperature scale bar? 2. In the IR image of Mars (the expanded section), circle the region you think corresponds to the polar ice cap. MARS INFORMATION Atmosphere Very thin; consists of 95% carbon dioxide, 3% nitrogen, 1.5% argon, and trace amounts of water. Average Surface Temperature -53oC (-63.4oF) Temperature occasionally rises above the freezing point in equatorial regions, although the pressure is too low for liquid water to exist on the surface. Surface Info Current evidence points to subsurface water (dirty ice) at latitudes at south and north between 60o and 90o. Polar Ice Caps Consist of CO2 ice, water ice, and dust. 3. If your probe was to analyze the Martian atmosphere, which wavelength(s) (from the Spectra Catalog) would dominate the absorption spectra from the data you collect? Explain your reasoning. 4. Does the information presented in the IR image, or in the table, suggest that liquid water could be located on the surface of Mars? If so, in what region(s) of the planet is it most likely to be found? If not, why not? 5 The table below provides data on Europa (a moon of Jupiter). EUROPA INFORMATION Atmosphere Extremely thin; major constituent is oxygen. Average Surface Temperature -150oC (-238oF) Surface Info Primarily water ice with strong evidence of water oceans beneath thick water ice crust. Water ice crust may be subject to partial melting due to tidal heating. 5. If your probe was in orbit above Europa, which if any wavelengths (from the Spectra Catalog) would dominate the absorption spectra from the data you collect? The table below provides data on Titan (a moon of Saturn). TITAN INFORMATION Atmosphere Very thick; consists of 90% nitrogen and smaller amounts of methane, ethane, and argon. Average Surface Temperature -180oC (-292oF) Surface Info Composed of rock and ice (ice is primarily a combination of water, methane, and ammonia), oceans that are likely a mixture of ethane and methane, and possibly deep underground oceans of ammonia and water. 6. If your probe was in orbit above Titan, which if any of the wavelengths (from the Spectral Catalog) would be present in the absorption spectra from the data you collect? 6 7. In Parts C and D you considered data collected by a probe that was sent to Earth, Mars, Europa, and Titan. Based on the remote sensing information gathered from these different locations, which planet or moon would you first select for a mission to further search for life? Which location would be your second choice? Third? Fourth? Be sure to include any necessary scientific findings that the initial probe mission found that influenced your ranking. What do you think is the likelihood of finding life in any of these environments? 7 Spectral Catalog Oxygen (O2) Spectrum peak wavelengths at 0.69 µm, 0.760 µm, and 1.28 µm. Oxygen is a by-product of oxygenic photosynthesis, a metabolic process performed by plants as well as certain types of bacteria, such as the bacterium Cyanobacteria. It is also produced by the photodissociation of water. Carbon Dioxide (CO2) Spectrum peak wavelengths at 2.0 µm, 2.06 µm, 2.7 µm, 4.3 µm, and 15 µm. Carbon dioxide is a by-product of several biological processes including respiration, a metabolic process performed by many organisms. These organisms include animals and many types of bacteria, such as the Sulfolobus bacteria. Sulfolobus are found in Yellowstone National Park at temperatures up to 90oC. Methane (CH4) Spectrum peak wavelengths at 3.3 µm and 7.65 µm. Methane is another by-product of metabolism. An example of a living organism that produces methane is the type of bacteria called Methanopyrus (capable of growth in temperatures of up to 110oC). Methane is also formed in planetary atmospheres as a result of photochemistry. Methane can be converted chemically into many other organic compounds. Water (H2O) Spectrum peak wavelengths at 0.820 µm, 0.950 µm, 1.13 µm, 1.38 µm, and 1.87 µm. Water can be found as a gas, solid, or liquid. It can be photo-dissociated into hydrogen and oxygen. In its liquid form, it is a solvent for life. Water is the by-product of several biologic processes, including respiration. 8