Remote Sensing/Reflectance
Spectrometer
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 77058-1113, USA (Tel: 281-486-2172, Fax: 281-4862186, Email: order@usra.lpi.edu).
At the time of writing (August 2000) the cost of each spectrometer (including activity
sheets) is $129.95 + 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 020 7679 7900 for more details.
This experiment will also soon be available online. The Mars in the Classroom pages
are housed at http://www.ucl.ac.uk/GeolSci/MITC. 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
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
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
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.
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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?
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Remote Sensing (Student Notes)
1
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?
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Table 1. The Electromagnetic Spectrum
Radio
waves
Micro
waves
Infrared
Visible
Ultraviolet
Long wavelength
Radar
Mapping
Thermal
(heat)
mapping
Gammarays
Short wavelength
Low energy
Use
in
remote
sensing
X-rays
High energy
Thermal
Mapping
Visible
imaging
Compositional
mapping
reflectance spectroscopy
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
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?
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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?
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Remote Sensing (Activity Sheets)
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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.
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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 yaxis.
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?
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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.
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Remote Sensing (Activity Sheets)
2
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?
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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 1.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
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 3:
Yellow
Object 6:
White
Remote Sensing (Activity Sheets)
Object 1:
Object 4:
Red
Green
Object 2:
Object 5:
Brown
Black
3
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
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Yellow
Brown
Brown
Brown
Brown
Red
Brown
Brown
Brown
Brown
Brown
Red
Brown
Brown
Brown
Brown
Brown
Red
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Brown
Brown
Blue
Red
Red
Red
Brown
Brown
Red
Red
Red
Red
Red
Red
Red
Red
Red
Red
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Blue
Brown
Brown
Brown
Red
Red
Brown
Blue
Brown
Blue
Brown
Black
Brown
Brown
Blue
Blue
Brown
Red
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
Red
Brown
Blue
Blue
Blue
Brown
Green
Brown
Red
Blue
Red
Red
Red
Red
Red
Red
Red
Red
Figure 2. Distant Measurement
15
16
17
18
Figure 3. Low Orbit Measurement
Question 2: What differences can you see in the two images? Which is the more
detailed?
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Remote Sensing (Activity Sheets)
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Question 3: Give reasons why both kinds of resolution are valuable in some instances,
with examples.
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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?
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Question 5: Where would you land and why? Use the pixel number from Figure 1 as a
label.
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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?
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Remote Sensing (Activity Sheets)
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REMOTE SENSING / REFLECTANCE SPECTROMETER DATA SHEET
Table 1 : Reflectance values for the Earth rock material.
Colour
Blue
Green
Yellow
Amber
Orange
Crimson
Red
IR-1
IR-2
Wavelength (nm)
470
555
585
605
635
660
695
880
940
Reflectance Value
0.11
0.24
0.25
0.28
0.30
0.28
0.26
0.33
0.30
Table 2 : Reflectance values obtained from the Mr. mission.
Colour
Blue
Green
Yellow
Amber
Orange
Crimson
Red
IR-1
IR-2
Wavelength
(nm)
470
555
585
605
635
660
695
880
940
Reflectance Values
Object 1 Object 2 Object 3 Object 4 Object 5 Object 6
Remote Sensing (Data Sheet)
0.13
0.27
0.51
0.75
0.90
0.96
0.97
0.99
1.00
0.02
0.09
0.11
0.12
0.13
0.11
0.11
0.16
0.15
0.02
0.08
0.10
0.17
0.25
0.21
0.24
0.60
0.81
0.10
0.18
0.25
0.28
0.31
0.33
0.33
0.32
0.37
0.28
0.27
0.41
0.48
0.48
0.48
0.53
0.51
0.53
0.13
0.22
0.20
0.20
0.23
0.19
0.17
0.24
0.23
1
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
0.3
Reflectance Value
0.25
0.2
0.15
0.1
0.05
0
400
500
Remote Sensing (Data Sheet)
600
700
Wavelength (nm)
800
900
1000
2