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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.
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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.
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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
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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?
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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?
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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.
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