You wouldn't hear it coming. A 100-ton extraterrestrial object hits the Earth at hypervelocity, more than 11 kilometers per second, and sometimes as fast as 20-25 km/sec. (Smaller objects slow down or are destroyed because of air resistance.) That's faster than sound (about 300 m/sec).

The object comes to a stop in about one hundredth of a second.

1. There is a rapid release of a tremendous amount of kinetic energy. What would be the effect of that rapid release of energy on the solid earth, the air, and living things? What forms might that energy take, and where might it go? Respond here:

Effects on rocks and soil: ________________________________________________________
____________________________________________________________________________
____________________________________________________________________________

Effects on the shape of the land (topography): ________________________________________
____________________________________________________________________________
____________________________________________________________________________

Effects on the air: ________________________________________________________
____________________________________________________________________________
____________________________________________________________________________

Effects on living things: ________________________________________________________
____________________________________________________________________________
____________________________________________________________________________

 

2. The energy release creates a shockwave stronger than any material it hits. What could be the effects of that shockwave on the rocks and soil, and on large bodies of water? Respond here:
____________________________________________________________________________
____________________________________________________________________________

 

3. What would be the effects of impact on the object itself? Would it remain intact (in one piece)? Respond here:
____________________________________________________________________________
____________________________________________________________________________

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Landsat 7 Educational Activity
Finding Impact Craters with Landsat
Student Name: _______________________________
Class Period: ________________________________
Date: _______________________________________

Student Reading for Step 2:
Known Effects of Impact Events

When an object from space hits the Earth…

• There's a huge explosion.
• The impact makes a big hole or crater with a raised rim and sometimes a central peak.
  The hole is many times larger than the impacting object.
• There is a rapid release of a tremendous amount of kinetic energy as the object comes to a stop in about one hundredth of a second.
• The impact releases extreme heat. Usually, the object itself is vaporized. Sometimes it melts completely and mixes with melted rocks at the site.
• If the impact occurs in water, a whole column of water is vaporized.
• The impact also produces a super-hot blastwave – a shockwave – that radiates rapidly outward from the impact point through the target rocks at velocities of a few kilometers per second.
  The shockwave is stronger than any material on Earth. It deforms rock in ways that are characteristic of an impact event. No other event on Earth deforms rock in these ways.
• Tiny glass droplets can form during the rapid cooling of molten rock that splashes into the atmosphere.
• Large impacts also crush, shatter, and/or fracture the target rocks extensively beneath and around the crater. See diagram at: http://craters.gsfc.nasa.gov/crater_diagram.html
  
• Hot debris is ejected from the target area, and falls in the area surrounding the crater. Close to the crater, the ejecta typically form a thick, continuous layer. At larger distances, the ejecta may occur as discontinuous lumps of material.
• Large impact events can blow out a hole in the atmosphere above the impact site, permitting some impact materials to be dispersed globally by the impact fireball, which rises above the atmosphere. The resulting extensive dusk and smoke clouds can cause darkness lasting for a year.
• Special carbon molecules called Buckminsterfullerene or (Bucky-balls, after Buckminster Fuller) can travel to the Earth in the impactor. They can hold special gases called "noble" gases that are indicators of extraterrestrial origin.
  
• Large impacts can trigger earthquakes and initiate volcanic eruptions.
• The heat ignites fires, and they may rage across a large region.
• Impact events can alter the chemical composition of the atmosphere. The extreme heat can generate large amounts of nitrogen oxides (NOx). NOx is easily transformed into nitric acid, resulting in acid rain.

   

More About ...

More About Impact Events in General
Impact craters are geologic structures formed when a large meteoroid, asteroid or comet smashes into a planet or a satellite.

A very large number of meteoroids enter the Earth's atmosphere each day, amounting to more than a hundred tons of material. They are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface. The average meteoroid enters the atmosphere at between 10 and 70 km/sec. All but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction, and they hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

All the inner bodies in our solar system have been heavily bombarded by meteoroids throughout their history. The surfaces of the Moon, Mars and Mercury, where other geologic processes stopped millions of years ago, record this bombardment clearly. On the Earth, however, which has been even more heavily impacted than the Moon, craters are continually erased by erosion and redeposition as well as by volcanic resurfacing and tectonic activity. Thus only about 120 terrestrial impact craters have been recognized, the majority in geologically stable areas of North America, Europe and Australia. Spacecraft imagery has helped to identify structures in more remote locations that can be explored for positive identification.

More About the Energy Released by Impact
Energies of impact are almost incomprehensibly large. They come chiefly from the kinetic energy of the impacting object. An object only a few meters across carries the kinetic energy of an atomic bomb as it strikes another object at high velocity. The impact of an object only a few kilometers across (smaller than many known asteroids and comets) can release more energy in seconds than the whole Earth releases (through volcanism, earthquakes, tectonic processes, and heat flow) in hundreds or thousands of years.

More About Extraterrestrial Objects in the Solar System
Thousands, possibly millions, of objects move throughout the solar system, orbiting the Sun. They range from microscopic dust particles to objects tens of kilometers across. Each object moves in its own orbit. We don't know how often they have hit the Earth in the past.

More About Impact Velocity
The minimum impact velocity for collisions with Earth is 11.2 km/s. This is equal to the escape velocity for an object launched into space from Earth's surface.

More About the Sizes of Craters
Objects of less than half a kilometer in diameter can make craters 10 km in diameter.

More About Crater Shapes
Nearly all impact events result in circular craters. In rare cases where the angle of impact was very low (0-10 degrees from the plane of the horizon), craters can be ovoid in shape.

More About Finding Impact Craters on the Ground
When looking for impact craters in satellite images, first pay attention to circular features in topography or bedrock geology. Look for lakes, rings of hills, or isolated circular areas.

On the ground, look for changes in the physical properties of the rocks in and around impact structures. Fractured rock is less dense than unaltered target rock around the structure. Also look ejecta and shocked rock fragments on the original ground surface outside the crater, and for fragments of the meteorite.

Download PDF of this worksheet.

Landsat 7 Educational Activity
Finding Impact Craters with Landsat
Student Name: __________________________________
Class Period: ___________________________________
Date:_________________________________________

Student Worksheet for Step 4:
Describing Satellite Images of Possible Impact Craters

Part I. Consider what effects an impact event might have, and describe those effects below. Though you are working in groups for this step of the activity, each student must complete this worksheet.

A. The object itself: Would you expect to see any evidence of the object itself in a satellite image? What evidence might you find?

_________________________________________________________________________________________

_______________________________________________________________________________________

B. Shape of the land: What kinds of changes would that impact make to the shape of the land where it hit, and all around?

_________________________________________________________________________________________

_________________________________________________________________________________________

C. Effects of Time: What kinds of changes will occur to the impact site over time? Remember that some changes are fast, and some are slow.

_________________________________________________________________________________________

_________________________________________________________________________________________

D. What else might you see in these satellite images that could help you learn about an impact crater?

________________________________________________________________________________________

_________________________________________________________________________________________

_________________________________________________________________________________________

 

Part II. As a group, study all of the satellite images. Below are their fake names (to use until you've identified them yourselves as impact craters or something else):

AOR
Latitude: N 19° 6'
Longitude: E 19° 15'
Size: 12.6 km in diameter

ELG
Latitude: N 67° 30'
Longitude: E 172° 5'
Size: 18 km in diameter

HGH
Latitude: N 75° 22'
Longitude: W 89° 41'
Size: 20 km in diameter

MAN
Latitude: N 51° 23'
Longitude: W 68° 42'
Size: 72 km in diameter

MSH
Latitude: N 46° 16'
Longitude: W 122° 12'
Size: several km in diameter

RCH
Latitude: 21°04'N
Longitude: 11°22'W
Size: 38 km in diameter

SCH
Latitude: N 37° 20' 36.1"
Longitude: W 116° 33' 59.9"
Size: About 300 m in diameter

You need to know that…

• All of these satellite images show the Earth's land surface, not another planet's surface, and not the Earth's atmosphere. No hurricanes or tornadoes appear in these images.

• The colors in these images are false colors. White isn't always snow; lakes often appear black; vegetation is sometimes red.

• All of these landforms are large. One is 300 m in diameter, and the others are 1 km in diameter or larger. Most of them 10-90 km in diameter.

• Aliens from other parts of the universe had nothing to do with creating the landforms in these images.

• If you see a letter or a face, it's just an accident of nature.

• People sometimes make large craters with explosives or large mining equipment.

• Multiple Impacts: Sometimes impacts come in twos or threes. It's rare, but it can happen when a comet or asteroid breaks into a couple of large pieces just before it strikes the Earth.

 

Part III. As a group, now choose two of the images you find most interesting, and prepare to describe them to the class as directed by your teacher.

A. Circle the name or names of the one or two landform(s) your group has chosen to describe for the class. Write next to it if you think it is an impact crater or some other kind of landform.

AOR       ELG      HGH      MAN      MSH       RCH        SCH

B. What evidence do you see in the satellite image that your landform is or is not an impact crater? Describe it here:

_______________________________________________________________________________________

_________________________________________________________________________________________

_________________________________________________________________________________________

 

Come to agreement as a group about whether or not the image you've chosen is or is not an impact crater, and why.

Download PDF of this worksheet.

Landsat 7 Educational Activity
Finding Impact Craters with Landsat
Student Name: _______________________________
Class Period: ______________________________________
Date: _______________________________________

Student Worksheet for Step 6:
Questions You Would Ask
on a Field Expedition to a Possible Impact Crater

Identifying what might be an impact crater in a satellite image is only the first step in identifying it with 100 percent certainty. That requires people making a field expedition to gather and study evidence at the site itself.

Field expeditions cost money. Getting money nearly always requires writing excellent grant proposals. You have to prove you understand the science and know what you're doing before people will give you the money to do it.

Your task is to write a series of questions you would use to guide a field expedition to determine whether or not a given landform was an impact crater.

You will do well on this learning assessment if you:

• Show evidence that you have a full and complete understanding of how an impact event can shape the land, soil, and surrounding rocks, as well as the atmosphere and living things;
• Use terminology accurately;
• Explain your ideas in ways that makes sense;
• Use complete sentences.

Download PDF of this worksheet.

Wednesday, December 29, 2004