Download Asteroid Composition by Spectral Analysis

Asteroid Composition by Spectral Analysis
Objectives
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Students will understand and be able to use the classification scheme of
asteroids.
Students will construct a simple spectroscope for use in light interrogation.
Students will identify composition of emitted and reflected light of various
sources through use of a spectroscope.
Students will record observations and detail how this procedure can
identify asteroid composition.
Suggested Grade Level
9th -12th
Subject Areas
Integrated Science
Physics
Astronomy
Timeline
One to two class periods
National Science Content Standards
Science as Inquiry
NS.9-12.1
As a result of activities in grades 9-12, all students should
develop
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Abilities necessary to do scientific inquiry
Understandings about scientific inquiry
Physical Science
NS.9-12.2
As a result of their activities in grades 9-12, all students
should develop an understanding of
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Structure of atoms
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Structure and properties of matter
Chemical reactions
Motions and forces
Conservation of energy and increase in disorder
Interactions of energy and matter
Earth and Space Standards
NS.9-12.4
As a result of their activities in grades 9-12, all students
should develop an understanding of
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Energy in the Earth system
Geochemical cycles
Origin and evolution of the Earth system
Origin and evolution of the universe
Science and Technology
NS.9-12.5
As a result of activities in grades 9-12, all students should
develop
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Abilities of technological design
Understandings about science and technology
National Math Content Standards
NM-MEA.9-12.1
Understand measurable attributes of objects and the units,
systems, and processes of measurement
Make decisions about units and scales that are appropriate
for problem situations involving measurement
National Language Arts Content Standards
NL-ENG.K-12.12 Applying Language Skills
Students use spoken, written, and visual language to accomplish
their own purposes (e.g., for learning, enjoyment, persuasion, and
the exchange of information).
Background
This lesson is appropriate for students studying science, specifically physics
and/or astronomy. The students should have prior knowledge of astronomy and
physics of light as a form of energy. Additionally, some understanding of matter,
the periodic table, and the transformation of energy will help students connect
content areas and activity together. The student should also have a basic
understanding of the properties of light. Students should understand that while all
light moves at the same speed it is contained in different wavelengths and that
science is able to separate into its various wavelength components using
physical means.
Materials
Pencil, pen, note/binder paper, graphing paper, (optional: colored pencils or
markers for graphs), discarded compact disk (CD), small box – 4 x 6 x 12 inches
(shoebox), ruler, box cutter, single-sided razor blades
Lesson
1. Have students read and review articles on asteroid classification scheme,
comparing research with background knowledge and textbook support.
2. Have the students review the information on spectral analysis,
spectroscopy, and building a spectroscope.
3. Have students construct a spectroscope using the provided materials and
following the instructions below.
4. Have students simulate viewing asteroids reflected/emitted light by using
available light sources. The optimum sources include lamps of varying
energized elements such as neon, sodium, argon, halogen, etc., however,
if these are unavailable use common available light sources including
incandescent lamps, fluorescent lamps, ultraviolet lamps, infrared lamps,
sunlight, etc.
5. Students record and analyze the light medium they have available then
identify the elemental components of their light source and compare this
data to that of expected asteroid spectrographs.
6. Have students make a critical analysis of the above data and write a
report detailing the potential use of this technique to identify the
composition of the Apophis asteroid.
Extensions
Students can expound and enrich their experience in a number of ways. Start by
repeating the spectral analysis of all available light sources, creating a record of
data, and compare, and identify commonalities in light source elemental
components. Students could use their spectroscopes combined with a telescope
to identify elemental components of celestial bodies including the moon and
visible planets.
Evaluation
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Students successfully identify emission spectra of asteroids and other
celestial bodies.
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Students construct workable spectroscope, observe, and collect spectral
data from provided/identified sources.
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Students measure, record, and analyze their findings from the
spectroscopy activity.
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Students construct reasonable connections between simulations and
research of real-world incidents, specifically how we could use this
technology/technique to correctly identify the components of the Apophis
asteroid.
SPECTROSCOPY FOR THE SCHOOL
Adapted from
Simón García
I.E.S. Aljada. Murcia (Spain)
Abstract
The spectroscopy is one of the branches of science that better information has given us on
the structure and composition of the stars, nebulae, galaxies and of the Universe in
general. Nevertheless, it is accustomed to be absentee of the classrooms perhaps because
it demands instruments normally expensive and sophisticated.
This workshop tries to provide the needed material that allows students to build a simple
spectroscope of no cost, using a big box of matches and a piece of a CD.
In this workshop, we are also going to build another spectroscope and spectrometer more
elaborated that will allow us to carry out typical measures of the laboratory easily
comparable to astronomy.
We, in the workshop, will offer and they will explain didactic materials with practices
directed to our students.
What is Spectroscopy?
Spectroscopy pertains to the dispersion of an object's light into its component colors (i.e.
energies). By performing this dissection and analysis of an object's light, astronomers can
infer the physical properties of that object (such as temperature, mass, luminosity and
composition). But before we hurtle headlong into the wild and woolly field of
spectroscopy, we need to try to answer some seemingly simple questions, such as what is
light? And how does it behave? These questions may seem simple to you, but they have
presented some of the most difficult conceptual challenges in the long history of physics.
It has only been in this century, with the creation of quantum mechanics, that we have
gained a quantitative understanding of how light and atoms work.
What is a Spectrum?
How is it that we know so much about the chemical compositions, temperatures,
pressures and motions of stars and galaxies which are so very distant that we would never
dream of trying to travel to them? In order to answer this question, we first have to ask
how we know that these bodies exist at all. Well, quite simply, we know they exist
because we can see them; that is, they are emitting energy in the form of waves light and
also infrared, ultraviolet and often radio waves and x-rays as well. This energy travels
over those vast distances and provides us with an extremely rich source of information
about their make-up. A spectrum is the result of splitting up this light into its constituent
colors and it is by studying spectra that astrophysicists have been able to make their most
important discoveries.
The most familiar spectrum in nature is that splendid spectacle, the rainbow, which is
produced when light from the sun bounces around inside millions of raindrops and gets
sorted out into its constituent colors in the process. When a chemist, physicist or
astronomer wishes to examine a source of light, he may use a triangular glass prism, or
more common nowadays, a device called a diffraction grating, to disperse the light into a
spectrum. The whole apparatus for doing this job is called a spectroscope (if you look
through it), or a spectrograph (if the spectrum is recorded photographically or by some
means other than the eye). All modern spectrographs use diffraction gratings; the end
result however, is rather similar to that produced by a prism whose action may be more
familiar.
What Does a Spectrum Tell Us?
Isaac Newton, in about 1666, while he was engaged in those experiments which were to
lead to his construction of the first reflecting telescope, was the first to realize that the
colors produced when white light is passed through a prism are a property of the light
itself, rather than something introduced by the glass. This realization was to have
extremely far-reaching consequences for the whole of physics and for our understanding
of the universe in particular. The great revolution in physics, which took place during the
first few decades of this century, led to a thorough understanding of the way in which
atoms and molecules can absorb and emit light and other radiations. It was known long
before that different chemical elements emitted their own characteristic colored radiations
or lines when heated in their gaseous state, but it was the understanding of the
relationship between those lines and the structure of the atom or molecule that proved to
be so important for the development of astrophysics.
A familiar characteristic radiation from a common element is the yellow-orange light
emitted by sodium vapor. Almost all of the light from a sodium vapor street lamp comes
out in two very close lines in the yellow--orange part of the spectrum; this same element
is also responsible for the yellow color produced when, for example, salted water
(common salt is sodium chloride) used in cooking is allowed to boil over into the flame
of a gas burner.
If we look at an astronomical spectrum and see the lines characteristic of a particular
element, then we can immediately say that element is present either in the star or galaxy
itself or, in some special cases, in the space between a star and our telescope. This is
important enough in itself, but, so powerful are the techniques of spectroscopy that we
can do much more than just detect the presence of a chemical element or molecule. The
obvious next question to ask is how much of each element is present in a particular star?
In fact, this is not a very easy question to answer but it can be done, and indeed has been
done for several hundreds of the brightest stars in the sky and for quite a number of other
astronomical objects.
Classification of spectrum
When a narrow beam of white light is passed through a transparent prism, it emerges as a
band of colors, which change from red at one end to violet at the other through the colors
of the rainbow. This band of color is called a continuous spectrum.
We get much of the same sort of spectrum from the prism if the light entering it comes
from a hot, glowing body, irrespective of what the hot body is made. If, however, the hot
body is heated until it vaporizes and the light from the hot vapor is passed trough the
prism, the band of colors is weakened but is crossed by a series of bright lines. Each
element in the vapor provides one or more of the bright lines, each in a definite position
relative to the others. The same is true of glowing rarefied gas. Such a spectrum is known
as an emission spectrum.
If the light from a glowing gas or vapor passes though a similar gas at a lower
temperature before entering the prism, the bright lines are replaced by dark lines. In this
case, we obtain an absorption spectrum.
The latter is just the situation which exists in stars since the outer layers of the
atmosphere are at a low pressure and cooler than the inner layers. Thus we can expect the
continuous spectrum of a star to be crossed by a number of dark lines, although some of
the hottest stars exhibit emission lines as well.
The stars can be classified according to the spectra, which they produce. About 95
percent of the stars can be put into classes, which are labeled O, B, A, F, G, K and M,
according to special characteristics of the spectra. For example, in type A stars the lines
representing hydrogen are prominent, whereas in type G stars the lines representing
calcium are strong and the hydrogen lines are considerably weaker.
The order of the classes given above is also the order of the surface temperature of the
stars, the ones of type O having the highest temperature. It is also generally the order of
absolute magnitude of the star.
The Diffraction Grating
Light from the star comes to us in a succession of waves, as in Figure1. The wavelength
of the light is also defined in Figure 1.
If we have two slits emitting monochromatic light, that is light of one wavelength only,
such that the narrow slits A and B are the same distance from an observer O, then the
crests and troughs from A and B will reach O at the same time, and we say that the waves
are "in phase." In this case, the illumination at O will be bright due to reinforcement of
one wave by the other as in Figure 2.
If the distance which the light has to travel from B to a second point P is greater by half
the wavelength of the light (λ/2) than the distance from A to P, we shall get from A
coinciding with crest from B. The waves are completely "out of phase" and darkness
results at P. Similarly, if the distances traveled to a point Q differ by λ, then we shall
once more be in phase and brightness ensues. Thus, we get a series of dark and bright
lines on the line OPQ (Figure 3).
The distance OP and PQ will, of course, depend on λ, the wavelength of the light used.
Thus, for red light Q will be further from O than for violet light because λred is greater
than λviolet. A white light should produce a spectrum in the general region Q. Due to the
low number of sources, this would be very faint, and we can intensify the spectrum by
using many more slits very close together so that the effect from each slit can add. Such a
large number of slits may be obtained from a diffraction grating, on which can be ruled
about 600 lines per millimeter (Figure 4).
The size of the slits on the diffraction grating have to be about the same order as the
wavelength of the light used, so that the whole of the diffraction grating is very compact
indeed. The formation of several spectra from one diffraction grating can be seen in the
next section, and indeed the diffraction grating can profitably replace the prism
mentioned in section spectrum.
Formation of Multiple Spectra
Figure 5 shows how first order and other orders of spectra are formed by rays of light
whose paths differ by λ, 2λ, 3λ and so on. It should be appreciated that the wave length
of light is of the order of 6 x 10-7 meters whereas the distance AO might be of the order
of 0.2 meters so that the diagrams are, of necessity, well out of scale.
A simple spectroscope
In this experiment, we are going to show how we can build a very simple and economic
spectroscope, which has an unequal able relationship quality/ price (measure for the
power separator of the colours). Their separator power is based on the phenomenon of
diffraction, produced in this case by microscopic "mirrors" for the reading of the laser in
a compact-disc (CD). In a CD, there are 1000 points of diffraction for each millimeter of
disk, that it allows to separate the elementary colors very well.
Materials that you are going to need:
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A box of big wax match
An unusable CD
How will we build the spectroscope?
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In the first place, you are going to break the CD into pieces carefully so that don't
cut yourself. We need a piece of CD approximately 1/8 of the disk.
Next you are going to prepare a window in the superior part of the box of wax
match as it is shown in the picture.
Shorten and bend the piece of cardboard so that you can open and close the
window.
Put the piece of CD in the centre of the interior locker of the box of wax match. By
creating such a form upon opening a crack in the end of the box the reflected and
diffracted light shine on the mirror and incise in the window.
How Could We Use the Spectroscope?
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Take your spectroscope and guide toward a light, for example, a bulb. What can
you see?
You prove now with the light of a fluorescent tube. Can you see any difference?
Try to see the stellar spectrum of the sun (spectrum of absorption). Be careful and
do not focus it directly to the sun. Identify the most characteristic lines carefully.
You could also observe the spectra of emission of some lamps of public lighting
(white, of mercury; yellow, of sodium; etc.) and of any luminous announcement
displayed on a window (for example, of gas neon, red).
A More Elaborated Spectroscope
We will try to mount the spectroscope that J.Waxman (1984) described in Astronomy.
Cambridge University Oress, pp 320-323.
We can see the general aspect and the model of the window and wavelength graduation
in Figures 10-11.
Exploring Spectra Using a Spectrometer
The spectroscope can be used to look at the spectra of many different sources.
Incandescent light
An incandescent light has a continuous spectrum with all visible colors present. There are
no bright lines and no dark lines in the spectrum. This is one of the most important
spectra, a blackbody spectrum emitted by a hot object. The blackbody spectrum is a
function of temperature; cooler objects emit redder light, hotter objects white or even
bluish light.
Fluorescent light
The spectrum of a fluorescent light has bright lines and a continuous spectrum. The
bright lines come from mercury gas inside the tube while the continuous spectrum comes
from the phosphor coating lining the interior of the tube.
Neon light
The simplest source of a neon light is a night light which says 1/4 watt on the package.
These night lights have neon lights inside them. You can also find neon lights in the
windows of businesses.
Warning: Even though they are called neon lights the lights do not necessarily contain
neon gas. Some contain argon or other gasses to produce different colors. The red ones
contain neon.
The spectrum of the neon light has several bright lines. The red lines are bright. The line
used by helium neon lasers, 632.8 nm wavelength, does not appear in the spectrum of a
neon tube. It is too dim relative to the other lines.
The lines of light are produced when electrons in an excited state decay into a lower
energy state. The change in energy of the electron between these two states is precise and
results in the emission of light with a narrow range of energies, a spectral line. DO NOT
LOOK AT THE SUN!, even with a spectrometer.
Sunlight
Look at sunlight by looking at a white surface in the sun. White paper works well. The
solar spectrum is a continuous spectrum of an incandescent gas. Look closely and you
will see fine dark lines crossing the solar spectrum. These fine lines are Fraunhofer lines.
The dark lines are produced by gas above the surface of the sun which absorbs some of
the incandescent light from the sun below. Each of these lines is produced by one atom or
ion. However, several lines may be produced by one atom. Two lines close together in
the yellow are a famous pair of sodium lines.
Light emitting diodes, LEDs
These come in many colors from red, orange, yellow and green to blue. In light emitting
diodes electrons in a higher energy conduction band drop into holes in a lower energy
band. The energy lost by the electrons is emitted as light. Thus there is usually one
brightest color of light that appears as a line in the spectrum of the LED. In addition to
the bright line, there is usually a dimmer, continuous emission of lower energy light. This
lower energy light is produced when electrons decay to or from impurity states between
the main energy bands. In a solid, the well defined energy states of electrons that would
appear in atoms of a gas are spread into energy bands.
Street lights
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Mercury Vapor. Looks bluish.
Has many bright lines of mercury, like those that appear in fluorescents.
Low Pressure sodium looks orange. Has yellow lines of sodium vapor.
High pressure sodium looks yellow. Has broad bands of light.
Computer Screen
Look at a white screen on a computer. Notice the bright spectral emission bands.
Compare the spectral bands on a liquid crystal display screen to those on a cathode ray
tube display.
You can also look at Candles and Aurora
Diffraction Grating
You can also look at lights through a diffraction grating without using a spectrometer.
Just hold the grating in front of your eyes and look through it at a light.
This only works for lights which appear to be small points of light or narrow lines of light
that line up with the lines in the diffraction grating. The diffraction grating spreads the
light right and left when its lines are vertical. So look at a vertical line of light with the
diffraction grating lines also vertical, i.e. the spectrum to the right and left. Look at
horizontal lines with the diffraction grating horizontal, i.e. with the spectra above and
below the light. I usually place the diffraction grating in a plastic page protector to protect
it from scratches and fingerprints.
Candle
A candle across the room works well. You will see the continuous spectrum of the
incandescent carbon particles in the flame.
A linear filament incandescent light bulb or a distant light bulb
The continuous incandescent blackbody spectrum will appear.
Stars
Few stars are bright enough to trigger the color-sensitive cones of your eyes. However,
those such as Sirius in the winter and Vega in the summer will have a continuous
incandescent spectrum. If you look at stars through a telescope, you will gather more
light and be able to see their colors better. Hold the diffraction grating in front of a small
telescope or behind the eyepiece of a large one.
Lightning
Lightning usually makes bright vertical lines, so hold the lines of the diffraction grating
vertical to spread the spectra to the sides. Look at lightning and you will see the
continuous spectrum from hot incandescent gas plus spectral lines from excited
atmospheric gasses.
Light emitting diodes, (LEDs)
These come in many colors from red, orange, yellow and green to blue. They can be
viewed at a large enough distance so they are small. You will see a bright, narrow band
of light plus a broader dimmer band.
Sunlight
To see the solar spectrum never look at the sun. Make a large black region using black
paper or cloth. Put a bright white line down this blackness. Look at the line through the
diffraction grating. You will see a continuous spectrum. It is difficult to see the
Fraunhofer lines.
Lasers
Never shine a laser beam into your eye! However, you can project a laser dot on a wall
and look at the dot through a diffraction grating. You will see just one dot of light spread
to either side of the original dot representing the single color of light produced by the
laser. You can also shine the laser through the diffraction grating at a distant white screen
or wall. Once again, a single dot of light will be diffracted to each side. Each single dot
represents the single color produced by the laser.
Other
Some sources will not work well with a diffraction grating. Auroras are too broad and
diffuse to produce a good spectrum through a diffraction grating. Fluorescents in fixtures
are too wide also.
References
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H. Robert Mills.Practical Astronomy A User Friendly Handbook for Sky
Watchers- Albion Publishing, Observing Guide
D.TASTERSFIELD: Projects and Demonstrations in Astronomy. Stanley
Thornes (Publishers) Ltd.1979J.Waxman. A worbook for Astronomy. CUP.1984,
pp 320-323
http://www.exploratorium.educa/.html
Resources
http://Apophis Asteroid\Spectroscopy for the school.htm
http://speclab.cr.usgs.gov/
http://articles.adsabs.harvard.edu/cgi-bin/nphiarticle_query?1993Metic..28..161G
http://en.wikipedia.org/wiki/Asteroid_spectral_types