How was the Earth formed? How did the Moon get here? Are all planets formed in the same way? Deep questions like these often seem unanswerable, especially in the elementary school classroom! But as we have seen, simple models can convey concepts and ideas with a power and scope that few people appreciate.

Don’t worry, we won’t create entire worlds from scratch, but we are going to use models and activities to demonstrate how the active environment of a solar system shapes and changes the surface of planets both suddenly, and gradually over long periods of time. The theory that th

ings usually change gradually over time, but occasionally are radically transformed by titanic events is called punctuated equilibrium. There is a lot to learn about how the surface of the Earth and Moon got the way they are today, so let’s go exploring!

Activity 25: Modeling the Moon’s Surface in Clay

Modeling the lunar surface in clay seems like a very tall order for younger children. I’ve often had my education students (and experienced teachers!) scoff at this activity and claim that such an art project is much too hard for students younger than high school age. These people couldn’t be more wrong.

When making a scientific model it is important to remember that we are not striving to create great art, or even mediocre art! Instead, we are striving to create an understandable representation; something that helps show what we know about a particular part of Nature, in this case, the lunar surface.

We help students achieve this by guiding them step by step to create their own models. The idea is to get them to put into physical form something they have learned about the lunar surface, such as the large mountains that exist at the center of large craters! We do not have to produce great art in order to produce better understanding and comprehension for our students!

For the Educator

Facts you need to know

1. Planets and moons are formed by a process called accretion. Basically, small pieces collide and stick together making larger pieces. Gravity (and other forces) help speed the process and the larger a piece is, the faster it tends to grow.
2. The smaller, free orbiting pieces that haven’t become planets or moons yet are called meteoroids and asteroids^[1]. Meteoroids are anywhere from the size of a grain of dust up the size of a large car or truck. Asteroids range from the size of a small building, to hundreds of miles wide; these meteoroids and asteroids are the basic building blocks from which planets are assembled – and the building process still continues today. The word asteroid means “star-like”. When the largest of these bodies were discovered in the early 1800’s, they appeared as small drifting stars in the telescopes of astronomers.
3. When a small piece of material such as an asteroid collides with a planet or a moon, it is referred to as an impactor. These impactors strike at tens of thousands of miles per hour and can hit the surface with tremendous energy, enough energy to reshape the very surface (and interiors!) of worlds as large as the Earth.

Teaching and Pedagogy

Once the model is made, there is still quite a lot to be learned! The largest craters and maria the students made represent some of the oldest features on the Moon. These maria were formed more than three billion years ago when the Earth and Moon were quite newly formed. These huge impacts were some of the last major objects to strike the Moon, and they give us a clue as to how the entire Moon (and the rest of the planets) were formed. Smaller objects smashed together and stuck to each other, creating a new larger object. The original impacts were wiped out as one asteroid after another struck the growing moon – but some of the last major impacts were preserved because nothing larger has wiped them out in their turn… yet! The interior of the young Moon was much more molten than it is today, and the last impacts fractured the lunar crust and allowed floods of lava to reach the surface.

Just like the real Moon, our model landscape preserves a record of both the size, and timing of the impacts. Does one crater overlap another – it must have happened later in time! Are there craters on the lava flows filling a maria? This tells us the lava flow happened first. It’s not always easy to read the rugged lunar surface in real life, but your students can get an idea of how astronomers date the features of a planetary surface in chronological order. Rays tell us about time as well. These lines of powdery material are very transitory, they disappear in just a few million years. Only the newest craters on the 4-billion year old lunar surface have them. This might also be a good time to remind students of the difference between a few million and a billion – the Moon is really old!

Those lines we pressed into our model with string? This can be your student’s introduction to longitude (vertical lines marching east to west) and latitude (horizontal lines). Not only do the lines help your students draw an accurate map on paper, they can be used to find the location and document it on your map. On your clay model, choose a location to be point (0,0) You may wish to put a little toothpick with a sticky note flag there to mark the spot!

Horizontal (latitude) lines above this point are numbered +10, +20, +30, etc. The lines below this are -10, -20, -30, and so on. Vertical (longitude) lines to the right of this point are numbered +10, +20, +30, but lines to the left of this point are numbered 350, 340, 330, etc. Remind your students that longitude lines run around the whole globe – 360 degrees worth! Our piece of the lunar surface is just that – a piece and not the whole Moon!

Have your students use their system of latitude and longitude to find the location of the center of some of the major craters. You can have them record them on their maps, or just make a list of the names with the locations shown next to each name. Wait… did someone say GPS? Yes, that’s right! These latitude and longitude lines are precisely the same at the latitude and longitude measurements that help our GPS devices tell us where we are, and keep us on the correct road when we are travelling.

Student Outcomes

Conducting the Activity

Materials

1. A large block of light-colored modeling clay, enough to make a slab that is 6-inches square and ½-inch thick.
2. A smaller block of dark-colored modeling clay. (The exact color will not matter, as long as the colors contrast well.)
3. A piece of aluminum foil large enough for your slab of clay. Oil-based, non-drying clays can stain table tops, clothing, or papers with oily residue in a matter of hours if left in place.
4. Various size marbles and beads.
5. Some larger, smooth-surfaced balls such as baseballs, hard rubber handballs, etc. These should be between two and six inches in diameter.
6. One 12-inch piece of string per group
7. Construction paper and markers.

Building the Lunar Surface Model

1. Begin by flattening out the large block of clay into an even layer in the baking pan. When the layer is relatively flat, turn the pan over and tap the layer of clay out onto a sheet of construction paper. When turned upside down and dropped onto the construction paper, the surface of the clay may settle and will likely not be perfectly flat – don’t worry, that won’t affect our model at all.
2. Now take the largest ball you have and press it firmly into down into the surface, you may even want to rock it back and forth just a bit. When you take it away, you should have a nice depression, perhaps with the edges raised just a bit. This will be a maria – but we aren’t done with it yet!
3. Move to the next size smaller balls and make one or two more large craters. Be sure you press them firmly into the surface so that they are deep enough. You may notice that these depressions even overlap a bit – don’t worry, craters tend to do that!
4. Now it is time to fill in your maria. Take the dark colored clay and roll out a 2-inch ball, then flatten it out to make it nice and thin. Make sure the piece you have is pressed out large enough to cover one of your large depressions all the way to the edges; if you don’t have enough clay, start again with a larger ball!
5. Lay this thin piece of dark clay into the depression and press it in place. If it goes beyond the edges at some point, you can either trim the extra away with a plastic knife, or smooth it onto the surface – lava flows from maria do sometimes overflow their crater and flow out onto the lunar surface!
6. Now you can start with marbles and beads, pressing small craters into the surface as you like. Make lots of them and don’t worry about using them in order – just tell the kids to have fun with this. Remind the students that it is perfectly alright for craters to overlap! Does anyone notice that new craters sometimes wipe out older ones? Don’t ignore the dark maria surface! Maria have almost as many craters covering them as the rest of the Moon does!
7. Choose a few scattered craters to be “new” (no more than 100 million years old!). Use a pencil to lightly scratch ‘splatter marks’ – lines leading directly out from the edge of the crater like a sunburst. These lines are called rays and are actually made of powdered material blasted out of the crater when it was made.

Exploring the Lunar Surface Model

1. Have the students use string to mark lines of latitude and longitude on the model; this works best if students work in pairs. Have one student stretch the string horizontally across the model while the other presses it lightly into the surface. Make these latitude lines one inch apart across the model. Now make an identical series of lines running vertically, again one inch apart. When finished, you should have a grid of latitude and longitude lines on your lunar landscape!
2. Have the students use construction paper and markers to make a map of the landscape they have made. Start with a series of latitude and longitude lines drawn in pencil with a ruler, then use the lines on the lunar landscape to map out the craters and maria you have made in colorful markers. Have the students name the larger craters on their maps using a theme. Will they choose U.S. Presidents? Rock bands? Favorite cartoon characters? Have fun with this!
3. Crater diameter is a good rough indicator of impact energy. Generally speaking, when a crater doubles in size, the impact energy needed to create it is ten times as great. If you have craters 1cm, 2cm, and 4cm in size; the 2cm crater required 10 times the energy of the 1cm crater, while the 4cm crater needed 100 times the energy of the smallest crater! Rank your craters by size and make a bar graph of the impact energy needed to create them.
4. The crater we see is usually ten times larger than the asteroid that created it. Choose the largest maria on your model and create a model asteroid that would be large enough to make such an impact. Display this giant impactor with your model.
5. Dim the room lights, then try using a small flashlight to illuminate your model. Shine the light from the side and take a photo of your clay model this way. Can you see shadows filling craters? Are there long shadows from mountains reaching across the surface? Compare your model to a photo of the Moon taken near the terminator (the line separating light from darkness.) You will see many similarities between your photo of your model and the real Moon – this is one way that we know our model / hypothesis is accurate, because we use it to predict what we find in Nature!

Supplemental Materials

Going Deeper

Mapping is an important technology, but reading a map is not as easy as it seems.  Find a close-up photo of the Moon on the internet and print it out.  Now let’s take a look at a lunar atlas, you will find an excellent one online at www.fullmoonatlas.com.  Find the area that matches your photograph and see how many features you can recognize and name.  This may not be as easy as it seems, your photograph and the atlas may be different magnifications, and the photos may be taken from different angles or under various lighting conditions.

Being an Astronomer

Time for another look at the Moon?  Sure, why not, it’s always exciting!  Whether you are looking at high-resolution photos from NASA, or through the eyepiece of a telescope, you can see a lot of detail on the lunar surface.  Examine the areas near the terminator (the dividing line between light and darkness) to see the most detail.  Can you find a maria region?  These areas are distinctly darker than the surrounding highland regions of the Moon, and their smooth surfaces shows off later craters with great effect.

Can you see an area where lava has broken out of a crater and spilled across the lunar surface?  If the telescope or photo is good enough, you can sometimes even see waves and ripples in the maria surface, frozen in place as the lava solidified billions of years ago.  Small craters on the surface of the maria are also good candidates for showing off rays.  The best way to find these features is to look at the lunar surface with low power (40-60x) and try to spot a bright ‘splash mark’.  Zoom in on one of these ‘splash’ features at 80-150x and you will see a crater surrounded by rays of powdery and bright lunar dust blasted out of the crater by the enormous energy of the asteroid impact.

Another thing to look for is overlapping features.  Can you see craters on top of a lava flow?  Which came first!?  Can you see small craters inside larger ones?  This takes a good eye and some patience, but you can begin to see a timeline of events, carved out of the lunar surface by giant rocks, falling from space.

Being a Scientist

In astronomy, scientists often work from photographic evidence.  Very few scientists have traveled to the Moon, and none have gone to Mars, yet we learn new things every day from scientists who study photographic evidence gathered by distant spacecraft.

This time, we will use a high-resolution photograph of the Moon – or a portion of its surface.  Your students will construct a timeline from photographic evidence.  Think of a large number of footprints in the snow outside a busy store; which footprints were their first?  Which were placed later?

We can determine our timeline for craters (or footprints!) by looking for things that overlap.  If one crater overlaps another, it must be newer.  Have your students begin by looking at the largest craters first.  Those that are overlapped the most must be older.  Those craters which have nothing overlapping them must be newer.

Brightness is also an indication of newness.  Craters that are bright and prominent are generally very new – less than 100 million years old!  Craters that are dull and show no evidence of bright interiors or bright streaky rays around them must be older.

Erosion is another line of evidence.  Is the crater rim fresh and shows a complete circle?  This complete crater must be relatively new.  Some craters show rims that are thinner and more worn down, sometimes they are even incomplete.  These features indicate very old craters, often more than 2 billion years old.

Have your students make a timeline, showing major craters from youngest, to oldest.  Have them present their findings to the class and cite the evidence that supports their ideas!

Following Up

Geology is much more than a science that names different kinds of rocks!  Geology is a dynamic science, but it generally acts over enormous scales of time and wide geographical regions.  One advantage of looking at the Moon from so far away is that we can see the entire surface in one view and zoom down into features that interest us without losing ourselves in irrelevant details.

The lunar surface lacks many things that we would miss if we were there, like air, water, weather, plants, and oceans, just to name a few.  The Moon even lacks an active geology, there are no active volcanoes or earthquakes on the Moon.  Some people might think they wouldn’t miss earthquakes, but quakes and volcanoes are part of an active geology which recycles minerals and materials that help to make life on Earth fertile and abundant.

Even so, it is the very things we might miss that make the Moon such an excellent place to study geology.  With no air, water, weather, or active geology, the lunar service doesn’t change very much on any sort of human timescale.  Even the most casual features on the Moon such as a crater the size of a baseball, or an astronaut’s footprints, will last for millions of years.  With no wind to erase those footprints, or water to wash them away or fill them with silt, and not even an earthquake to cause a landslide to cover them up – what is left?  The only active weathering that happens on the lunar surface is the steady rain of dust and rocks from outer space.

It would take a rock the size of an egg to obliterate an astronaut’s footprint, hundreds of such rocks strike the Moon every day, but the surface of the Moon is really quite large compared to a single footprint.  If you want to see even a portion of that footprint erased, you are probably going to have to wait a very long time!  But on the day it finally happens, a future geologist will be able to say for sure that the footprint happened first.  Geology gives us timelines in stone!

Activity 26: Dynamically Modelling The Moon’s Surface in Flour

This activity is larger, messier, and a lot more fun that the clay model we made in Activity #25. In the last activity, we pressed various size balls into a clay surface to make ‘craters’, depressions that were smooth and round, but not very exciting or dynamic. We’re going to take this up a notch and let kids see the crater making process as it happens! By dropping weights into pans of flour to look at the resulting craters, plus the ejecta – material which is blasted out of the crater on impact.

For the Educator

Facts you need to know

As large and impressive as craters may be, they change the landscape in a matter of seconds. The Barringer Crater near Winslow, Arizona has as much volume at 400 professional football stadiums; even so, it was excavated in under 5 seconds.

Craters average between 10-20 times as large as the asteroid that created them. Crater Tycho on the Moon is over 90 kilometers wide, it took a 6-10 km wide rock to create it. This mountain-sized impactor is about the same size at the Chixulub impactor which killed the dinosaurs.

Craters are typically a bowl-shaped depression with a raised rim around them, they also sometimes feature a raised central mount in their centers.

Ejecta includes all the material blasted out of the crater at impact time. There is an ejecta blanket, a bright layer of material that surrounds the crater. There are also rays, streaks of material radiating away from the center of the crater. Rays can be very long, sometimes more than ten times the diameter of the crater itself.

Teaching and Pedagogy

If you don’t want to fool with dying corn meal, you can cover your flour surface with black spray paint instead. This method is quicker and you can stir the flour after the experiment and use it again immediately.

Flour covered with paint is a reasonable analog for the lunar surface. Almost waterless, the lunar rock pulverizes into dust when an asteroid of any size strikes the surface into a fine powder very similar in consistency to flour. The sunlight darkens the surface of the Moon over time in a process called radiation darkening, we’ve used paint to simulate this. When the light colored rock is blasted out of a crater, it falls back onto the surface making a sunburst or splatter shape we call rays. Can you see evidence of rays on any of your craters? Have the children draw what they see here.

How far do the rays go out from the central crater? Can you measure this? Is the ratio of crater diameter to ray length the same for all craters of any size? This is an interesting investigation to do; it takes patience and a little measuring, but the math is very simple. Make a chart showing the size of the crater, the size of the rays, and the ratio between them. You may find that your larger craters blasted flour right out of the pan and onto the floor! Don’t worry about that (you did use that plastic tarp, right!?), use the smaller craters for your investigation and see what you find!

Want another example or the ray-making process in action? Toss a water balloon high up in the air and let it splatter on the dry pavement of a parking lot or play area – you will see the same splatter pattern with ‘rays’ of water streaking away from the center of the impact!

Impact craters on the Moon also have raised crater rims. These are areas where the blast force has pushed the rock back from the center of the impact, causing it to pile up like snow in a freshly plowed parking lot. This happens because the solid rocky surface of the Moon forces the blast energy to turn 90-degrees, from straight down to horizontal. Look at your lunar-flour surface, can you see evidence of raised crater rims? See if you can measure the height of the rims above the flat surface. Is there a relationship between crater size and rim height? Another chart can help you settle this question!

Want to do it again? Just stir the flour well with a spoon, add a little more as needed and then re-smooth and repaint the surface. You can do this activity many times! If you want to keep the flour in the pan and try again tomorrow, I strongly recommend putting a cover of plastic wrap or aluminum foil over the pan to protect the flour from moisture and insects!

You can also save the flour in large zip-shut plastic bags or plastic jars and save it for salt dough art projects! Don’t forget to sift the flour to get the pebbles, marbles, and other “asteroids” out of it before you use it! Do you find that the black paint has tinted the flour a bit? Don’t worry, a little paint won’t hurt the salt dough at all – just don’t reuse it for cooking!

Student Outcomes

Conducting the Activity

Materials

1. 10 lb bag of flour.
2. A deep dish pie or cake pan
3. A 2 lb box of corn meal
4. Several bottles of dark blue/black food coloring, or a bottle of black ink
5. A large 12-ft square tarp or plastic sheet. Check the paint department of your local home improvement store for this, sometimes sold as a ‘plastic drop cloth’ for protecting floors and furniture while you paint.
6. A roll of masking tape to hold the plastic sheeting securely down on your floor and prevent tripping.
7. An assortment of small pebbles, marbles, etc. Nothing larger than 1-inch diameter.

Building the Impact Crater Model

1. Dilute two bottles of dark food coloring in ½ cup of water. Put the corn meal in a large bowl, add the colored water and stir for several minutes until well mixed (a kitchen blender works well if you have one.)
2. Spread the corn meal out on cookie sheets or aluminum foil and allow to dry. You can put the cookie sheets in a low (180 degree) oven for an hour or so if you wish to speed the process. Once dry and cool, return the corn meal to a bowl and stir thoroughly to insure all granules are separate. Store in original box or in a zip-shut plastic bag.
3. Lay the tarp or drop cloth out on the floor and tape it down securely – you’re going to need a large area for this so you may want to push the desks aside!
4. Put the large pan or box-top in the middle of the tarp and fill with flour to the top. You can overfill a bit and use a yard stick to strike off the top to make a smooth, flat surface.
5. Sprinkle a thin, even layer of dark corn meal on top of the flour.

Exploring the Impact Crater Model

1. Have everyone gather round and pick up an edge of the tarp; most should stand well back to keep splattering flour off shoes and clothes.
2. Choose a lucky student to drop a pebble or marble into the flour from a height of about 1 meter.
3. Students can now inspect, photograph, and sketch the crater they have created. Have your students look for the crater basin, crater rim, ejecta blanket, and rays.
4. Measure carefully and see how large the crater was compared to the impactor that made it. How about the size of the ejecta blanket compared to the crater? How far do the rays extend away from the crater?
5. Want to try again? (My students always wanted to do this multiple times!) Try using different size pebbles, comparing the effects of large and small weights. Don’t let children throw their pebble, as that can make a real mess!

Supplemental Materials

Going Deeper

How are craters discovered? From our work here, you might think that you just have to look for a crater to find them. On the Moon, finding craters is fairly easy, but not so on our own Earth!

Craters on the Moon are visible to anyone with a pair of binoculars; if you have access to a telescope, you can see thousands of craters. The Moon is a unique environment, there is no air, no water, and almost no erosion on the surface at all. Unlike Earth where rain, wind, and even earthquakes and volcanoes disturb and reshape the surface, our Moon is geologically dead, there are no active reshaping processes there. The only weather and erosion on the Moon comes from rocks falling from space to strike the surface.

If you could go back in time 100 million years, the Earth would look very different. Apart from dinosaurs, even the continents would be in different locations! Mountains that look old and rounded now would have looked new and rugged then; some mountains that we are familiar with would not even have been formed then!

Our active Earth wears away, buries, and destroys most craters in just a few million years. Most of the known craters on Earth have been discovered from space, either from the space shuttle (1981 – 2011) or from the International Space Station.

Being an Astronomer

Telescope time again! Now it is time to take another good look at some craters on the Moon’s surface. Can you see the features we discussed such as crater rims, ejecta, rays, and central mounts in lunar craters? Can you find overlapping craters where one impact destroyed evidence of another earlier impact?

You will find that some craters appear bright – these are relatively new, less than 100 million years old! Other craters show eroded rims indicating they must be a billion years old, or even older. You may also find maria; dark, circular basins filled with dark colored lava. These maria are impacts so tremendous that they cracked open the Moon’s crust allowing lava to flow in from deep within the interior. The Moon’s interior is all frozen solid today so no impact, no matter how large, could cause a maria to form in modern times.

Being a Scientist

Sometimes the size and scope of the damage that an impact event can create are hard for students to imagine. If you have older students (6^th grade and up), you may wish to have them investigate what an impactor could do if it struck the Earth.

Purdue University in Indiana has a wonderful website called Impact Earth! (www.purdue.edu/impactearth) which allows students to enter data on the size of an impactor, its speed, angle, and the type of terrain that is struck. Once you enter the data, your students can indicate how far away they are and see how the impact effects them.

The Impact Earth! website shows blast damage, heat damage, ejecta damage, and seismic damage from the impact and describes in detail what the impact would be like for the observer on the ground.

Following Up

Earth, Mars, Venus, Mercury, and the Moon all show substantial impact damage from asteroids striking their surfaces, but larger planets like Jupiter, Saturn, Uranus, and Neptune do not.

Investigate these planets and compare them to our own Earth and Mars. Why would our inner planets all show impact damage but these large worlds do not?

Answer: These larger worlds are Jovian planets, sometimes known as ‘Gas Giants’ – they have no solid surfaces at all. Asteroids may impact them, but they easily penetrate into the planet’s interior without leaving a mark on the gaseous surface.

Activity 27: Exploring Crater Rays in Detail

Many students are fascinated by crater rays. Once you’ve seen one of them on the Moon’s surface, you just can’t help looking for them like shamrocks among the clover. Ray systems occur in almost all craters on the airless Moon, but they are virtually unknown on the Earth – why do you think that is?

The answer has to do with our thick atmosphere – and the Moon’s complete lack of air. On Earth, if an asteroid is large enough to strike the surface and make a crater, the blast will look rather like a mushroom cloud from a nuclear test explosion. The extreme heat creates a rising column of hot air that carries pulverized rock high aloft into the stratosphere. If you look at the rising plume from a large volcanic eruption in a photo or a video, you will have an idea of the amount of energy such an impact can release.

Things are completely different on the Moon; with no air, it doesn’t matter how much heat the impact generates, there will be no plume of dust and smoke because there is no air to rise and carry it aloft. Pulverized rock dust sprays out more like water from a hose, flying in perfect parabolic curves with no wind to disturb or distort its path. Modeling a single impact on Earth in your classroom requires a little ingenuity, but we can do it easily!

For the Educator

Facts you need to know

1. Rays are made of pulverized material ejected from the crater during an impact. The reason we see streaks of material is because the irregularities in the rim alternately block and channel the flow of material flowing outward.
2. Ray material is often as fine as sand, or even flour in real life.
3. Earth’s atmosphere stops rays from forming. The dust is suspended in the air as a dust cloud which drifts away on the wind. On the airless Moon, or nearly airless Mars, rays are distinct and easy to see.
4. Rays stand out because the finely powdered material is bright and more reflective than the darker ground on which it lies.
5. Rays on the Moon are easiest to see in the days just before and after the full moon.

Teaching and Pedagogy

Rays on the Moon are made of very finely pulverized rock that is as fine as flour. Jagged edges along the irregular crater rim channel the explosive power of the impact and help create the streamers of powdered rock we call crater rays.

One of the most famous crater and ray systems on the Moon is from Crater Tycho. Tycho is almost 90 miles wide and 4 miles deep – it is a virtual twin of the impact that destroyed the dinosaurs here on Earth 65 million years ago, some scientists even hypothesize that the Crater Tycho on the Moon and the Crater Chixulub on Earth were made from two pieces from a single asteroid that broke apart and fell into the inner solar system at about the same time.

The rays from Crater Tycho run for more than a thousand miles across the surface of the Moon and are easy to see with any small telescope on a full moon night. It is likely that the rays from your crater went out much further than your students expected them to! In fact, if you were skeptical about why I asked you to put down a 5-ft wide spread of craft paper, you probably aren’t any longer!

Rays and crater volume are both a good measure of impact energy. It requires energy to excavate a crater and lift out all the rock and soil that used to be where the crater is now. The famous Meteor Crater in Arizona has a volume about 400 times larger than a football stadium, and this huge crater was excavated in just a few seconds.

Rays are also a measure of impact energy. Like excavating a crater, it takes energy to first pulverize the rock, and then to lift and throw it over great distances. The rays from great craters like Tycho are rarely more than an inch thick, but they extend over vast distances. These rays represent thousands, even millions of tons of rock that was smashed to powder and then thrown across tremendous distances! How much larger was your ejecta blanket than your actual crater? What was the size ratio between the crater and your ray systems? All of these things represent impact energy from the asteroid smash that created your crater!

Student Outcomes

Conducting the Activity

Materials

1. Flour and black spray paint (See Activity #22.)
2. Two pieces of 5-ft long x 30-inch wide black or dark blue craft paper (any color will work here as long as it is as dark as possible.)
3. A 10-inch spring form cake pan – $10 (You may get paint on this, so don’t bring a nice one from home!)

Building the Crater Ray Model

1. Tape your two pieces of black craft paper down to the floor – this should give you a nice 5-ft square area to work in.
2. Put your spring form pan ring down on the paper (don’t attach the bottom!) and carefully fill it with flour to the top. Use a ruler to strike off the excess and sweep it away carefully with a soft paint brush. Try not to leave any stray flour on the black paper.
3. Lift the ring straight up. The flour will slump a little around the edges and leave you a nice mound about 2-½ deep in the center. Spray black paint over the mound of flour keeping the can at least 18-inches away from the surface. If you would rather not work with paint in the classroom, try putting some black or dark blue food coloring into a bowl with about 2-3 cups of flour. Keep stirring the flour with a whisk and gradually add food coloring until the flour is a dark, uniform color. You can then put the dark flour in a sifter and sift a dark surface layer over your pile of white flour. The color is only for contrast, and this works just about as well as paint.

Exploring the Crater Ray Model

1. You are now ready to drop a weight into the flour pile. If you have access to some disk-shaped weights common to science labs, these work wonderfully. If not, a large marble or slightly flattened 2-inch ball of clay will work well. Drop the weight from about 2-feet up; if you are using disk weights or flattened balls of clay, be sure to drop them so they land flat against the surface!
2. The impact on your pile of flour will not only make a satisfying crater, but a very dramatic system of rays spreading out over your black paper surface. It is often advisable to photograph the crater and its rays with your smartphone camera before children start to measure and explore!
3. Measure the crater diameter from one edge of the rim to the other and record this.
4. Measure the ejecta blanket from one edge to the other and record this. The ejecta blanket is the more or less continuous circle of material thrown out of the crater at the time of impact.
5. Measure the rays spreading out from the crater from the crater’s rim out to the tip where the ray disappears. Measure enough of them so that you can get a good average. If there are enough rays, each child can measure one or two. Record the shortest and longest rays, and calculate the average length of rays for your crater.

Supplemental Materials

Going Deeper

We haven’t always discussed “impact craters” on the lunar surface. When I was young, we were taught that almost all the craters on the Moon were volcanic in nature, and that the idea of something large enough to strike the Earth or Moon and make a large crater was a ridiculous idea.

The discovery of the true nature of impact craters is tied up with two men, and one giant impact crater in northern Arizona. Daniel Barringer purchased what became known as Barringer Crater in 1903, hoping to mine the site for tons of meteoric iron he assumed must be buried there. Barringer published many articles in scientific journals claiming to prove that the crater was made by a giant meteorite striking the Earth. Although the scientific community never accepted Barringer’s work as conclusive – the Barringer family steadfastly claims that he discovered the meteoric nature of impact craters before anyone else.

Gene Shoemaker first came to Barringer Crater in the late 1950’s and continued to study the site into the early 1960’s. Shoemaker’s analysis of shocked quartz proved that the crater had to be of meteoric origin. Shoemaker was slated to be an Apollo Astronaut, but a heart ailment kept him from flying. Never the less, his work on impact craters was verified by the Apollo astronauts, and today we all know that almost every crater on the Moon was caused by the impact of asteroids from space – not volcanic explosions!

Being an Astronomer and Scientist

We combine the astronomer and scientist sections for this activity because they are so closely interwoven. If you have a telescope, so much the better, but if you do not then a high quality photograph of the full Moon will serve.

1. At or around the full moon, take your telescope an hour or so after sunset when the Moon is well above the horizon. Viewing the Moon at 80-100x, scan for craters with bright ray systems.
2. Have your students draw a crater and a ray system as accurately as they can, paying attention to the crater diameter and ray length. If you can determine the extent of the ejecta blanket, add that to your sketch!
3. After sketching, measure the size of the crater and compare it to the length of the rays and extent of the ejecta blanket.
4. Calculate the ratio of the sized of the crater compared to the ejecta, and to the rays. Compare these ratios among the students – can you find a consistent relationship between crater size and ray length?

Following Up

A class visit to Barringer Crater (also known as Meteor Crater) in Arizona might not be possible for your class – however there are many videos that will take you there without leaving the comfort of your own school room. As with all videos on the web, be sure to preview them to insure that the content is age appropriate for your students.

Activity 28: Dynamically Modeling The Lunar Surface in Plaster

This is a fascinating (and messy!) activity which always seems to delight children. The fact that the asteroid impacts which shape the worlds and moons in our solar system are violent and sudden affairs is easily brought home to everyone with this exciting activity! This activity will take a bit more preparation, and practice, than anything else we have done before. The practice involves timing, because wet plaster hardens quickly and if you start too soon, impacting rocks will simply disappear as though you’ve tossed them into a bucket of water – but wait too long and they will just bounce off the surface without affecting anything! You will need to try this on a small scale by yourself before you do the larger activity with students!

For the Educator

Facts you need to know

1. Working with Plaster of Paris takes practice. Plaster can be a messy medium and your best choice will be working outdoors. Likewise, plaster can damage clothing and shoes – children will need to wear old clothes and shoes if possible for this activity!
2. You may wish to ask your custodians for help with this project. You will be mixing and pouring heavy materials, and chances are that your custodial team has more experience working with mortar than you do! The custodial team at my school loved working with me on these projects, I’m sure yours will be happy to help too!
3. A permanent model offers many advantages over a temporary clay or flour model. Permanent models can be touched, painted, measured, photographed, and displayed for parents and administrators.

Teaching and Pedagogy

Your new plaster model of the lunar surface has quite a few features that other models lacked. The dark painted surface contrasts very well with the ejecta blanket material (white plaster) so you and your students can clearly see that material was ejected from the craters as they were formed.

You may wish to measure the size of the ejecta blanket (calculating the approximate area of such a feature can be an interesting geometry problem for older students!) Is there a correlation between the size of the crater and the size of its ejecta blanket? Modern geologists and astronomers are investigating questions like these even today!

No doubt you will also notice that later events (the small rocks) made marks on top of older features. This is exactly what happens on the lunar surface as we have discussed before. Your model shows you geological timelines forming in action! Have your students map your landscape on a piece of construction paper and name the major craters. Can they construct a timeline that shows when these craters were formed?

The maria made of dark plaster also offers areas for investigation. If you took photos before and after the maria was formed, how many features were obscured by the lava flows as the original crater filled and became a maria? How does this formation relate to our timeline? Can your students notice ripples or inconsistencies in the lava flow now that it has hardened? These features still exist on the Moon today billions of years after these lava flows hardened into stone.

You may also have noticed that our model lacks some features that the others possess. Our flour models showed beautiful rays, but our plaster model shows none. Ask your students why not? In fact, our flour model was made of powdery material that was perfect for forming rays made of streaks of fine powder grains. Our plaster model was made wet – and our little rocks could in no way strike the surface hard enough to pulverize it into a powder again!

Student Outcomes

Conducting the Activity

Materials

1. 25 lb bag of plaster of paris – (See your local home improvement store for this, the paint department usually has it!)
2. 25 – 50 lb bag of “play sand” – Play sand is finer than builder’s sand and does a better job for us with this project. The biggest problem is lugging the stuff around, but it can be used for lots of classroom projects!
3. A very large dish pan or cafeteria pan and a large metal spoon or garden trowel to mix the plaster. A wheelbarrow can also be used if your custodian has one.
4. Can of flat black spray paint (any dark color will do.)
5. The top from a case of copy paper
6. A roll of duct tape
7. A quantity of black, water-based classroom paint (about ½ cup.) Black food coloring can also be used for this if available.
8. Large trash bag or aluminum foil for lining the box top
9. Assorted rocks and pebbles from fingernail size up to egg size. Use only one of the largest size (2-inch) rocks, 5-7 of the 1-inch size, and everyone else gets a smaller size.
10. Large tarp or drop cloth, at least 12 x 12 ft. (See Activity #22)

Building the Lunar Landscape Model

1. Everyone wears old clothes for this. The plaster may splatter about a bit, and it will not really come out of clothing or off of shoes. The tarp will help, but just be aware of this issue.
2. Reinforce all the corners of the box top with strips of duct tape. Be sure you use enough, the plaster mixture will be heavy and if it bursts out of your box, the activity will be ruined!
3. Lay out the tarp and the cardboard box top from the copy paper and line the box with a large trash bag or a generous layer of aluminum foil. Have all your materials at hand, pre-shake the can of spray paint, and make sure everyone has a rock to throw.
4. In your large dish pan (even a wheel barrow works well!) mix 2 parts dry plaster to one part dry sand. It is fine if you have extra sand, but too little will not do, be sure to make enough! If you end up with more wet plaster than you need, the extra can be dumped onto a plastic trash bag to set and then thrown away when hardened. Follow the directions on the bag, but mix the plaster wet, add just a bit more water than strictly needed. The mixture will be like cake batter when mixed properly. Make sure you use the spoon to dig into the bottom and corners of the pan so that all the plaster is mixed in. If you feel you’ve made it a bit too runny, you can add another cup of plaster in – don’t worry, it will thicken up and harden!
5. When mixed, pour the plaster into your cardboard box mold, filling it to the top. Immediately spray paint the top of the plaster. This is an excellent time to have a volunteer rinse out your dish pan thoroughly with a garden hose! If you have some extra plaster, pour it into a paper cup as a tester. Poke into this mixture with a stick – if the plaster is no longer runny and the stick leaves any sort of permanent mark, you are ready to begin. This won’t take long, perhaps a not even a minute.
6. Have your students each hold the edge of the tarp and lift it up in front of themselves as an apron or splash guard. (Don’t lift up the box of wet plaster and spill it!) Begin with the student holding the largest rock, toss it vigorously into the middle of the box. After this, the students with mid-sized rocks can toss them in one at a time. Don’t drop them, you must throw them down into the plaster to make a large enough impression. Finish up with all the smaller rocks. If you have 30 students, you will have an excellent landscape – if fewer, some students can toss an extra rock or two.

Exploring the Lunar Landscape Model

1. Allow the plaster to harden for at least an hour before you move it, then carry it inside. It will be heavy, get some help with this! Be sure you display it on a sturdy table where it will not fall!
2. Now it’s time to fill in the maria! You may wish to take a photo of the landscape before and after you make the maria for comparison! Put a couple of cups of plaster (no sand this time) in a large mixing bowl, add ½ cup black paint or squirt a whole bottle of dark blue or black food coloring into the required water. Mix the plaster and make sure it is thin and runny! Pour this plaster carefully into the largest crater in your landscape – your maria is filling with lava! If some of the dark plaster-lava overflows the maria and runs out onto the surface, that is excellent – just like it happens on the Moon! You will notice that some of the craters are filled in and obliterated by the lava flow, point this out to the students as it happens!
3. For extra realism, you may wish to toss in some very small rocks (less than ¼-inch) to make small craters on the maria floor.
4. [Optional] You can use a chalk snap-line to mark lines of longitude and latitude on your model. Ask your custodial staff about this, chances are good that they may have one which you can use already; if not, one of them will probably know how to use it and be able to help you with this. If you do not have a snap line – you can use colored builder’s twine (available at any home improvement store.) Leave your model in the cardboard box and cut notches every inch along the edges of the box. Thread the twine back and forth through the notches – first lengthwise, then crosswise. The twine will mark out lines of longitude and latitude that will help your students draw and map the landscape they have made!

Supplemental Materials

Going Deeper

Map making is one of the oldest mathematical activities. Maps make visible, physical representations of sizes, distances, and spatial relationships that transcend language. This is why map making is one of the most powerful techniques a science teacher has for effectively teaching the ESL student.

Once you have put longitude and latitude lines in place on your model, have students make a grid on a piece of construction paper. Have the students map the features of your lunar model onto their own paper – this makes a great activity station for group work day.

Tell the students how many miles or kilometers each square represents, then have them use the grid to determine things like x-y location of various craters, sizes of craters and maria, and the distances between various features using the Pythagorean theorem or just by measuring with a ruler.

Being an Astronomer

Another night at the telescope looking at the Moon? Sure! The Moon is beautiful and mysterious and worthy of a lifetime of study. If you have been doing these lunar surface activities through a semester, your classes will be bringing more knowledge to the eyepiece each and every time they look.

When we come to the telescope with a mental model of the Moon, its craters and maria fresh in our minds, then we come prepared to explore and discover new things. In short, we are primed for learning – not just seeing.

If your students have another opportunity to study the Moon through a telescope, have them look for evidence of geological processes such as lava flows, landslides inside the walls of giant craters, even geological erosion of ancient crater rims.

Being a Scientist

Craters, in spite of their great age, tell us a lot about the impact energy of the asteroid that made them. Larger craters obviously indicate more energy, but how to measure this? With your plaster model, you have a fun and easy way to investigate this. By filling a plaster crater with water to the very brim, you can measure the volume of the crater quite precisely; more volume indicates that more surface material was blasted away, and hence more impact energy!

To measure the water, you will either need a graduated cylinder (a very precise measuring cup of sorts), or a scale that can weigh in grams. A graduated cylinder is measured precisely to allow you to record how many milliliters of liquid are inside. Start with a cylinder with 100 mL of water, and after you have filled a crater you have 13 mL left – then you have used 87 mL of water to fill the crater – this is the crater’s volume, and a direct measure of the energy that created the crater in the first place.

A bottle of water and a digital scale work just as well. Weigh the full bottle in grams, and weigh it again after you have filled the crater. If your bottle weighs 1000 grams full, and 835 grams after filling the crater, you have used 165 grams of water to fill the crater. Interestingly, this means your crater volume is 165 mL. This exact correlation between grams and mL of water is not a coincidence – French scientists designed the metric system with water in mind so that 1 mL of water was defined to be exactly 1 gram of mass.

One thing your students will notice is that they cannot directly measure the volume of the maria you have created because you have filled them with plaster ‘lava’. Scientists and astronomers on Earth have the same problem when studying the Moon! Have your students measure and record the diameter of the craters alongside their volumes. Can you find any correlation between energy and diameter? Try graphing your craters with energy on the vertical axis and diameter on the horizontal axis!

After naming, mapping, and measuring the volume of the craters, record the crater energy (volume in mL) on their maps. Make a list of the craters on your map and classify the size of the impacts. This little adventure into a more mathematical analysis of your lunar landscape can be both exciting and fun.

Following Up

Craters are everywhere in our solar system. Take some time on the internet to search for photos of Mars, Mercury, even Pluto, these bodies are loaded with craters! Try searching for images of ‘Moons of Saturn’, or ‘Moons of Jupiter’ – there are more than 120 of these moons for you to explore, and all of them have craters.

How large are these craters compared to the little moons themselves? Take a look at a crater named Stickney on the Martian moon Phobos. This crater covers a substantial portion of the surface of the Martian moon. How large a crater do you think a moon or planet can have without being destroyed? Scientists debate and study this issue today!