EAS 105 – THE
PLANETS
Prof. Robert L. Nowack
Lecture 7
Sunlight and
Cosmic Matter Distribution
The Sun emits much of
the energy received on Earth in the form of light. Light is a type of electromagnetic radiation.
Although not obvious, James Clerk Maxwell, in the last century, inferred
that the light we see is a specialized kind of coupled electrical and magnetic
disturbance traveling through space.
Consider first an
electron in an atom. It has a negative
electrical charge and is attracted to positively charged protons and repelled
from other negative electrons.
If an electron can be made to move back and forth, it will
create "kinks" in the electrical field lines of attraction. If many electrons can be induced into motion
or forced to change motion, electromagnetic disturbances or waves will be
generated.
One of Maxwell's
greatest achievements was identifying light as a type of electromagnetic wave. Hertz investigated this phenomenon and
inferred that radio waves are also a form of electromagnetic radiation. His work led to the invention of the radio by
Marconi around the turn of the last century.
One property of electromagnetic radiation is that the speed it travels
(in a vacuum) is a constant, where
c = 2.997 x 108 m/s
Another distinctive
property is the "wavelength" of the disturbance, l .
Radiowaves
~ 1 cm to 1000 km
AM Radio ~
300 m
Short Wave ~
30 m
FM, TV ~
3 m
Radar ~
30 cm
Microwaves ~
3 cm
Infrared ~
3 - 300 x 10-6 m
Visible light ~
5 x 10-7 m
Ultraviolet 10-7
- 10-8 m
X-rays 10-9
x 10-11 m
Gamma Rays ~
< 10-12 m
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Aside:
In many instances,
frequency is used instead of wavelength.
Frequency is measured in Hertz or cycles/second. The frequency is inversely proportional to
wavelength, with f = c/
.
For example, Channel
4 broadcasts at 69 MegaHertz with the wavelength l ~ 4.2 meters.
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Visible light is only a
small band of electromagnetic radiation.
The energy that an electromagnetic wave can carry is inversely
proportional to the wavelength or 
.
X-rays and Gamma rays
have high energy that can split apart delicate molecules in the human body and
consequently be harmful. Fortunately,
gas molecules in the Earth’s upper atmosphere are hit first by X-rays and gamma
rays emitted from the Sun, shielding us a bit from harmful high-energy rays. Even ultraviolet radiation giving us a tan are
partly blocked by the upper atmosphere.
Earth’s
Atmosphere and Radiation Absorption with Altitude
Type
of Electromagnetic Radiation
The Earth’s atmosphere absorbs radiation
at most wavelengths at various altitudes above the surface. Only visible light, some infrared and radio
waves can penetrate the atmosphere and reach the surface. These wavelengths of the electromagnetic
radiation spectrum are called “atmospheric windows” because the atmosphere is
transparent to these incoming wavelengths.
Other planets with an atmosphere absorb radiation of various wavelengths
with altitude and will depend on the mass and gases that comprise its
atmosphere.
There are basic
"windows" through the atmosphere to observe planetary and stellar
objects. Hence, the importance for
viewing celestial objects above the atmosphere, such as by the Hubble Orbiting
Telescope.
The analysis of
different wavelengths of electromagnetic radiation emanating from a given
source is called spectroscopy.
Visible 'white' light is
usually a combination of a range of wavelengths and colors.
The first thing that
light can tell us is the temperature of the emitting body. Temperature of a substance is related to the
amount of internal molecular motion. An
ideal "Black Body" or perfect radiator (like the black iron on your
stove) looks black at 70°
F (293 K), but if you heat it up it glows "red hot". All bodies
naturally radiate electromagnetic radiation because of their internal molecular
and atomic vibrations.
Ex) Take
an infrared photo of the class, you'd see 120 glowing bodies (us!).
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Aside:
Use Wien’s Law to
determine the temperature of a body:
lT =
2900
where:
l is the wavelength in microns (10-6 m) of the maximum intensity
radiation
T is the temperature of the body in Kelvins.
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If we analyze light from
the Sun, we infer a temperature of ~ 5800 K. If we look at light reflected from the
planets, it is mostly reflected light from the Sun, so we see a 'hump' at about
yellow-green light. However, at much longer
wavelengths in the infrared, we find the planets own radiation.
Ex) For
Venus, ~ 1 x 10-5 m for radiated peak (or about 10
microns).
Using Wien’s law: 
= 2900, would compute cloud top
temperatures of about 290 K. (Unfortunately
below the clouds, the temperature increases to 740 K!! - not very livable!!).
If light passes thru a
gas, such as the Sun's outer atmosphere, then certain wavelengths of the light
will be blocked out by absorption from the gas. However, the specific wavelengths absorbed
will depend on what elements are present in the gas.
Gases give a series of very sharp absorption lines in the
spectrum. From the precise measurement
of the wavelengths of these spectral lines, one can deduce what elements are
present.
Absorption lines are
caused when gas molecules and atoms absorb certain wavelengths and let others
pass. The absorption spectrum for the
Sun shows a number of sharp absorption lines.
From the position of these lines, one can determine what gases are
present.
Cosmic
Distribution of Matter
The vast bulk of the
solar system (99.9%) is in the Sun; 90% of the remaining fraction is in
Jupiter. The Sun, Jupiter, and other
giant gas planets are 99% hydrogen and helium based on absorption lines in
spectrum of emitted light. Much hydrogen
on Earth is retained in the form of water, H2O. However, because of the Earth's escape
velocity of 11.2 km/s, lighter elements like hydrogen and helium gas can escape
the gravitational attraction of smaller planets like Earth. But, heavier elements are trapped near the
Earth. The cosmic distribution of matter
can be estimated by taking direct meteorite samples of solid material that have
fallen from space.
We find the following cosmic
abundances of different elements:
|
Element |
Symbol |
Atomic |
Number of Atoms per |
|
Hydrogen |
H |
1 |
1,000,000 |
|
Helium |
He |
2 |
68,000 |
|
Carbon |
C |
6 |
420 |
|
Nitrogen |
N |
7 |
87 |
|
Oxygen |
O |
8 |
690 |
|
Neon |
Ne |
10 |
98 |
|
Sodium |
Na |
11 |
2 |
|
Magnesium |
Mg |
12 |
40 |
|
Aluminum |
Al |
13 |
3 |
|
Silicon |
Si |
14 |
38 |
|
Sulfur |
S |
16 |
19 |
|
Calcium |
Ca |
20 |
2 |
|
Iron |
Fe |
26 |
34 |
|
Nickel |
Ni |
28 |
2 |
(Note
the scale of the Powers of Ten)
On Earth, the lighter
elements have been lost.
Note: Even numbered atomic elements are more
abundant than odd, which may result from part of the process of nuclear
synthesis.
Note: "Noble gases" (like He, Ne, Ar, Kr,
Xe) are more rare on Earth than the mean cosmic abundance. In fact, helium was first discovered from the
solar spectrum before actually found on Earth.
A "cosmic
chemist" would attempt to make sense of the cosmic abundances as well as
various differentiation processes, which would give rise to modified abundances
in different planetary and stellar objects.