Download Unit 1 Ch. 4 - Observational Astronomy

Lesson: Observational Astronomy
Field Museum Extensions
a. Related Exhibitions.
1. Rocks and Minerals Exhibit—Meteors and Meteorites. In this exhibit,
students will understand how a study of meteors and meteorites
contributes to scientists’ understanding of the universe. Students should
be able to locate the asteroid belt in a diagram of the solar system.
Students should understand the difference between an asteroid, comet,
meteor, and a meteorite and how these differ from planets and moons.
Students should also be able to explain how an asteroid might become a
meteor and then a meteorite. Students should discuss the composition of
meteorites and how they are similar to other rocks seen in this exhibit and
how they are different. Students should see the effects of certain meteorite
impacts on the earth and explain the energy transformations. Also,
students should explain how meteors and meteorites change their shape as
they move through the earth’s atmosphere.
Asteroids are rocks of varying size (but, by definition, less than 1500 km
maximum diameter) that orbit the sun. Larger asteroids might be called
planetoids or minor planets. An asteroid differs from a planet because of
its size (it is much smaller) and its inability to have an atmosphere (there is
not enough gravitational pull). Some moons are actually asteroids that
have been trapped by the gravity of a planet and are in orbit around those
planets. The moons of Mars, Phobos and Deimos, are examples.
However, moons need not be trapped asteroids. They are any satellite of a
planet. A large collection of asteroids exists in the asteroid belt between
the orbits of Mars and Saturn. Some of these have orbits that come close
to, or overlap with, the earth’s orbit. If the asteroids pass close enough to
the earth, they may be pulled by gravity into the earth’s atmosphere.
Once an asteroid moves into the upper atmosphere of the earth, it is called
a meteor. Most of these rocks vaporize because of friction, forming a
“shooting star” that can be seen in the night sky. The exhibit describes
how the friction of atmospheric gases against the outer surface of a meteor
wears the meteor down to a cone shaped rock. The reduction in net force
as the atmosphere abrades the meteor to a conical shape can be used to
demonstrate force vector components, as shown in the diagram below.
When the meteor first enters the atmosphere, the abrading forces are equal
to the forces perpendicular to the meteor surface. Once the meteor has
been worn down, the abrading forces are only a component of the
atmospheric forces. Thus, the rate of abrasion decreases as the meteor
continues to fall.
Some meteors are large enough that a portion survives the descent through
the atmosphere as well as the impact with the earth’s surface. The rock
that remains from a meteor after it has hit the earth’s surface is called a
meteorite. Meteorites with interesting impact results are shown in this
exhibit. For example, Benld meteorite, which hit the Southwestern Illinois
town in 1938, demonstrates the kinetic energy of a falling meteor.
The composition of an asteroid can vary from a high percentage of carbon
and tar-like material (classified as a C-type, or carbonaceous, asteroid) to a
high percentage of nickel and iron (classified as a M-type, or metallic,
asteroid) to a high percentage of silicate minerals (classified as a S-type,
or stony, asteroid). Students may notice that some of the meteor examples
look like dark rocks found elsewhere in the exhibit. Although the
materials of asteroids are found in rocks on the earth, the percentage
composition is unique to these celestial bodies. Similar to asteroids are
comets. Comets are composed of materials similar to asteroids, but also
contain a large percentage of ice. When the orbits of comets approach the
sun, a “tail” of dust and gas pointing away from the sun is visible, as light
is reflected and refracted in the debris being ejected from the comet’s
surface.
2. The Evolving Planet. At the beginning of the exhibit, the early years of the
earth’s existence are described, along with the beginning of life. How did
celestial objects play a role in the earth’s early formation and possibly in
the beginnings of life? (You may want students to continue their research
in this area using one of the web resources below and then present their
findings to the class.) As students move through the exhibit, they should
take note of the six major extinction events that have occurred (one is
underway presently) and the cause of these events. When they reach the
part of the exhibit that mentions the end of the Cretaceous, they should
take note of the extinction event. Although this is not one of the major
extinction events in the exhibit, it is one of the most well known as it
marked the end of the dinosaurs and scientists have debated its cause for
years. There is strong evidence that a meteorite impact caused a chain of
events that wiped out much life on earth, including the dinosaurs that had
ruled the earth during the Cretaceous.
There are several theories about what happened toward the end of the
Cretaceous period that resulted in the extinction of nearly 75% of species
at the time, and wiped out the dinosaurs. Some scientists believe that
several events occurred over a long period of time that resulted in the
extinctions, which appeared to have occurred in stages. One explanation
for the large loss of species (which has strong scientific support in the rock
record) is an extended meteor shower accompanied by major meteor
impacts. One huge impact crater is in Mexico, buried below the Yucatan
Peninsula, near the town of Chicxulub. The Chicxulub crater contains
shocked quartz and tektites, the evidence of a meteor impact. (These
rocks are on display and described in the exhibit.) The impact displaced
an enormous amount of rock, being nearly 2 million times more powerful
than the most powerful nuclear explosive device. Many organisms were
burned by the intense wave of infrared radiation as ejected rock mass
returned to earth. Mega tsunamis, thousands of feet high, and huge
earthquakes that occurred around the globe would have been created by
the impact. Also, the explosion created a blanket of super-heated rock
dust, much of which remained in the atmosphere long after cooling. This
blanket reduced the amount of sunlight reaching the earth and disrupted
photosynthesis. Adding to the smoky atmosphere were global wildfires
that had been started by red-hot boulders returning to the earth’s surface.
The fires and vaporized limestone rock created high levels of carbon
dioxide and started a period of rapid global warming. Obviously, many
species of terrestrial and marine organisms could not survive this
succession of disasters.
That these events may have occurred is supported by the large amount of
the element iridium, rare on earth but more common in asteroids, found in
the rock layer that was created during this time (commonly known as the
Cretaceous-Tertiary, or k-t, boundary).
Students can calculate the energy change involved in the impact by using
the estimated characteristics of the Chicxulub meteor:
Mass, m, of 1 x 1018 kg, speed, v, of 1 x 105 m/s
The kinetic energy of this meteor during impact: ½ mv2 = 5 x 1027 J
b. Harris Educational Loan Center.
1. Eyewitness: Planets Audio/Visual. Journey through space and time,
witnessing the birth of the sun and traveling to the outer limits of the solar
system.
2. Fossils from Paleozoic Seas Experience Box. Learn how fossils are used
to determine when new life forms appeared on the planet and when large
numbers of species became extinct, as happened after the meteor impact at
the end of the Cretaceous.
c. Field Museum Science/Website Resources.
1. Dinosaurs: Ancient Fossils, New Discoveries Website:
http://www.fieldmuseum.org/dinosaurs/allabout_extinction_11.asp
This page describes the meteor impact at Chicxulub and the scientific
evidence for what might have led to the end of the dinosaurs. The web
page also presents the impact of comet Shoemaker-Levy 9 into Jupiter as
an example of the destructive capability of an event of this magnitude.
2. Evolving Planet Website:
http://www.fieldmuseum.org/evolvingplanet/precambrian_2.asp
Shows an example of a meteorite and describes a theory that life
originated when a carbonaceous meteorite struck the earth.