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GEOL104: Exploring the Planets
LAB 4: IMPACT CRATERING
Name:
OBJECTIVES
: I.
Learn about impact crater morphology and the mechanisms of impact crater formation.
II.
Learn how impact craters are used as age-dating tools
III.
Investigate a scientific problem using the scientific methods
BACKGROUND
:
Impact craters are formed when two bodies collide. Impact craters are produced on all
bodies in the Solar System that have a solid surface, and have been observed on the four
terrestrial planets, many moons, asteroids, and comets. The larger of the planetary bodies,
usually the “survivor” of the impact, is called the target
, while the smaller body which will likely
be destroyed is called the impactor or projectile. Typical impactors are meteoroids (called
meteorites after touchdown) and comets. These objects vary greatly in size, but impactor size has
generally been reduced over time. A crater is not formed by an impactor physically pushing material aside: it is formed by
an explosion similar to a nuclear bomb! Earth-bound impactors travel at speeds of 10 – 70
kilometers per second
. These hypersonic speeds create a shock wave, deforming the target
material and melting or vaporizing the impactor. This shock wave fractures the rock and
excavates a large cavity (much larger than the impactor). This activity occurs within the first
tenths of the first second
of the impact event. The most cratered bodies in the solar system are rocky, airless bodies such as the Earth’s
Moon and Mercury, where surface processes such as erosion by wind, water, ice, and tectonic
activity are not present. However, all solar system bodies are capable of being scarred by
impacts, even gaseous planets. In 1994, Jupiter was struck by the comet Shoemaker-Levy 9. The
comet was broken into several pieces by Jupiter’s gravity before finally penetrating into the
planet’s clouds. The Galileo spacecraft and the
Hubble Space Telescope witnessed the comet’s
break-up and collision, and showed scarring in the
clouds at the impact site (
Figure 0
). These scars
have since disappeared. Figure 0:
Scars from collision of comet
Shoemaker-Levy 9 with Jupiter as photographed
by the Hubble Space Telescope.
Image credit:
Hubble Space Telescope Comet
Team and NASA.
INTRODUCTION:
History of Impact Crater Studies
Page 1
GEOL104: Exploring the Planets
In earlier studies of planetary surfaces, scientists were uncertain how the large circular
pits, seen on many solar system bodies formed. In the early 1600s, Galileo Galilei recognized the
existence of these landforms on the Earth’s moon. He called these features “spots” and described
them as being circular depressions surrounded by a raised rim. Galileo did not suggest an origin
for these “spots”. In the mid 1600’s Robert Hooke initially considered that these features may
have formed from impacts, but he could not think of a source for the impactors. He later
suggested that these features were volcanic calderas. This interpretation was accepted by
scientists for nearly 300 years. In 1893 G. K. Gilbert proposed that these lunar features occurred
from impacts
.
Crater Anatomy
Not all craters are created equal. Although sharing a common process, craters vary in
their size, shape, and overall appearance. The most basic distinguishing features of an impact
crater are the crater rim
, crater walls
, crater floors
, and ejecta blankets
. A variety of landforms
can also be present, including central peaks
, multiple rings
, peak rings
, terraces
, and concentric
rings/rays
surrounding the crater.
Craters can be grouped into two major categories according to their morphology: simple
craters and complex craters. Simple craters
are small bowl-shaped, smooth-walled carters and
are generally small (diameters up to several kilometers across). Two examples of simple craters
on Earth are Nördlinger, Germany (
Figure 1)
and Barringer Crater (aka; Meteor Crater) in
Arizona (
Figure 2
). Complex craters
are larger (diameters up to a few tens up to a few hundred
kilometers), typically have flatter floors and a central peak, and can possess one or more of the
landforms listed above. Complex craters form from impacts of larger objects traveling at greater
speeds, which means more explosive energy. Most craters are circular with radial, roughly
symmetric ejecta, but there are also elongate/oblique craters (
Figure 7
) as well as craters with
lobate ejecta, called “rampart” or “splat” craters. In addition to these major categories, impact
craters can also be grouped into microcraters
or pits
(subcentimeter craters caused by impacts of
micrometeoroids or high-velocity cosmic dust), and multiring basins
(large impact craters
covering much larger areas than complex craters characterized by multiple concentric “rings”).
Figure 3
shows an example of a rampart crater, Yuty Crater, on Mars. Yuty Crater is a
complex crater and features the basic rim, floor, walls, and ejecta blanket, but also has a central
peak. Other features of complex craters include ejecta rays, as seen extending from Tycho Crater
on the Moon (
Figure 4
), concentric rings, as seen in Callisto’s Valhalla impact basin (
Figure 5
),
and secondary impact craters produced by excavated debris, as seen surrounding Warhol Crater
on Mercury (
Figure 6
). The walls of complex craters can also become terraced after the impact
event (
Figure 6, 8, and 9
). Because they excavate the target material largely by vaporizing it,
impact craters almost always produce circular structures. Elongated impact craters (
Figure 7
)
form only at very low impact angles of about <10-15 degrees above the surface.
Earth has experienced impacts and formation of complex craters, but over time erosion has muted their appearance. The Earth is so good at eroding material and covering up the damage
of impacts that some have unknowingly made their home inside craters. The city and farmland of
Nördlinger, Germany are completely located inside of an ancient crater (
Figure 1
). If you look at the previous image you can see a ring of mountains encircling the town and farms. These mountains are what is left of the impact crater rims. Other features of this complex crater have been eroded away.
Page 2
GEOL104: Exploring the Planets
Figure 1:
The city of Nördlinger, Germany, located within a heavily eroded complex crater. Image credit:
Created with NASA WorldWind by User: Vesta using Landsat 7 (Visible Color) satellite image., Public Domain, https://commons.wikimedia.org/w/index.php?curid=496218
Figure 2:
Barringer Crater (aka Meteor Crater), Arizona, an example of a simple crater
. This
simple crater was created when a 50-meter-wide iron-rich meteoroid struck Earth’s surface about
50,000 year ago. The crater is about 1.2 km wide and 200 m deep. Interestingly, due to its
relatively young age, the ejecta blanket beyond its rim is well preserved. Image credit:
National Map Seamless Server - NASA Earth Observatory, Public Domain,
https://commons.wikimedia.org/w/index.php?curid=7549781
.
Page 3
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GEOL104: Exploring the Planets
Figure 3:
Yuty Crater, a complex
rampart crater. This diagram shows the major distinguishing
features of a typical complex crater. Rampart craters form when in impact hits a surface that
contains a lot of water (or ice), causing the ejecta blanket to produce a distinctive pattern
resembling a splash or mudflow. Rampart craters exist in many places on Mars. Image credit:
Labels: Calvin Hamilton, Image: NASA/JPL/Arizona State University.
Page 4
Figure 4:
Tycho Crater, a complex
crater on the Moon, displaying
prominent ejecta rays
.
Image credit:
Howard Mooers,
East-View Observatory.
Figure 5:
Multi-Ring Basin
Valhalla of Callisto, a moon of
Jupiter.
Image credit:
NASA/JPL/California Institute of Technology
GEOL104: Exploring the Planets
Figure 6:
Warhol
Crater, a crater on
Mercury, surrounded by
secondary impact craters
.
Secondary craters are
produced when the
debris ejected at high
velocities during the
initial shock wave strikes
the ground and produces
a secondary impact,
analogous to the
formation of a primary
crater. Image: NASA/
MESSENGER.
Figure 7.
Orcus Patera, an elongate crater in Tyrrhena region on Mars formed by a very oblique (low angle) impact. Note the range of crater sizes visible in the image. Image credit: NASA/JPL/Arizona State University.
Figure 8:
Cross-section view through a complex crater
showing central peak and terraced crater walls.
Page 5
Secondary crater
rays
GEOL104: Exploring the Planets
Page 6
Figure 9: Copernicus Crater, a crater on the Moon with a diameter of approximately 93 km.
Figure 10: Lichtenberg B Crater, a crater on The Moon with a diameter of approximately 5 km.
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GEOL104: Exploring the Planets
PART 1: IDENTIFICATION AND CLASSIFICATION OF IMPACT CRATERS
Questions about Crater Features and Formation
Question 1:
Examine the crater in Figure 9
. Is this a simple or complex crater? Question 2:
What features associated with the crater in Figure 9
lead you to your answer for Question 1? (Hint: Select all that apply based on the description of simple/complex craters on page 1 of the lab)
Question 3:
Examine the crater in Figure 10
. Is this a simple or complex crater? Question 4:
What features associated with the crater in Figure 10
lead you to your answer for Question 3? (Hint: Select all that apply based on the description of simple/complex craters on page 1 of the lab)
Question 5:
While the meteors that created both of these craters were traveling at similar speeds upon impact (12,000 m/s), the meteor that created the crater in Figure 9
was much larger (9.87x10
10 kg) where the meteor that created the crater in Figure 10
was much smaller (3.56x10
5
kg). Calculate the kinetic energy
for the impacts that created the craters in Figures 9
and 10
. KE = 1
2
∗
m
∗
v
2
; unit of KE is in Joules (J), m = mass in kg, and v = velocity in m/s. make sure only v is squared
Page 7
GEOL104: Exploring the Planets
Question 6:
A 1 megaton atomic bomb explosion produces a kinetic energy
of approximately 4.18 x 10
15
joules. Did the energy produced by either of the impacts which formed the craters in Figures 9
and 10
exceed the kinetic energy produced by a 1 megaton atomic bomb? Which one? Questions about Modern and Future Encounters
Thankfully, modern civilization has not experienced a major asteroid impact. Air blasts, however, are more common. Air blasts happen when smaller meteors explode in the Earth’s atmosphere. The Earth’s thick atmosphere causes friction that heats the asteroid. At a critical point, the asteroid experiences a massive explosion. This is the same process that forms the light
of shooting stars though these fireballs are much smaller. If the piece of meteor is big enough it can travel nearer to the Earth and the resulting explosion causes more damage.
Tunguska Event – Siberia, Russia
Figure 11:
Credit: the Leonid Kulik Expedition
Page 8
GEOL104: Exploring the Planets
On June 30
th
, 1908, an asteroid ~50 meters wide entered Earth’s atmosphere and exploded over Siberia, Russia
. Thankfully the area was not densely inhabited because the resulting explosion caused a blast of air pressure that flattened trees over an area of ~760 sq. miles (Nashville is about 500 sq. miles). More recently
, an air blast was observed over Chelyabinsk, Russia
. Watch this video to learn more about why air blasts are so deadly.
Recent Chelyabinsk, Russia Meteor https://youtu.be/IGDDulsVGtk
Question 7:
Watch the video above
, what date did the Chelyabinsk
meteor explode? Question 8:
According to Dr. Dan Durda’s research (video above
), how does impact angle into Earth’s atmosphere affect the amount of damage on the ground? Explain.
Planetary Defense Programs
Examples like Tunguska and Chelyabinsk serve as warnings for possible asteroid-related disasters in the future. Many countries now have planetary defense programs to assess the threats that asteroids pose to Earth
. These programs focus on observations that attempt to find
and track new asteroids, assessments concerning the level of danger, and methods for diverting dangerous asteroids:
Page 9
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GEOL104: Exploring the Planets
https://youtu.be/uqHXiJ5pGLY
Question 9:
According to the video above
, what two
events contributed to public awareness of possible asteroid impacts? Question 10:
According to the video above
, what are the potential problems with nuking an asteroid? Explain.
Question 11:
According to the video above
, what solution is the DART mission attempting to investigate? PART 2: IMPACT CRATERS AS AGE-DATING TOOLS
The rate at which asteroids and comets impact planetary surfaces across the solar system is relatively consistent. This information allows impact crater density on a given planetary surface to be used as a way to establish a relative age for that surface. Older planetary surfaces have had more time to accumulate more impact craters (
Fig. 12a
) whereas younger surfaces have
had less time to accumulate impact craters (
Fig 12b
). Using this basic principle, scientists are able to confidently determine which surfaces and areas on a given planet are young or old. Page 10
GEOL104: Exploring the Planets
Figure 12:
Images of two separate areas of the Martian surface with very different impact crater densities
In addition to using crater density to establish ages for a given planetary surface, impact crater degradation state can also be used as a rough metric of relative age. Over time, fresh, recently formed impact craters will go from having sharp rim crests and distinct bowl shapes (
Figure 13a
) to very muted depressions in the ground that hardly resemble an impact crater at all
(
Figure 13b
). This evolution of pristine to degraded craters is a result of weathering processes and is similar to the way that mountains on Earth erode over time. The weathering state of an impact crater can be used as a rough metric of age for that particular crater (i.e. a young crater will have a distinct bowl shape, a sharp rim crest, and an obvious ejecta deposit whereas those features will be heavily eroded and not obvious for an older crater). This method of relative age dating is not perfect, however as the processes which cause the degradation of impact craters are not fully understood on all planets. Figure 13:
Images of two separate areas of the lunar surface with very different impact crater densities
Questions about crater age dating
Page 11
GEOL104: Exploring the Planets
Question 12:
Look at Figure 1
of the city and farmland of Nördlinger, Germany. If erosion removes the prominent features of craters on Earth, why are we able to see the features of craters on Mars
? Example: Yuty crater. Explain your reasoning.
Figure 14:
Crater chain on the Moon.
Question 13:
Occasionally, groups of craters can be found together in neat linear chains across the surface of a planetary body as shown in Figure 13
. Based on what you have learned about impact craters so far, how do you think a feature like this formed? Hint: Figures 4 and 6 and their associated text might give you a clue as to what processes might have formed this
.
Question 14:
Search online to find a particularly unusual crater and an article discussing it. The crater can be on any planetary body. Copy and paste a picture of the crater below and describe how it might have formed according to the article. Make sure it’s an impact crater and not a volcanic crater/caldera.
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GEOL104: Exploring the Planets
Page 13