Introduction

Tactile 1 – The Full Moon

Let’s start our exploration by observing the overall appearance of a “Full” Moon. Study the lunar surface featured on Tactile 1. Note that the tactile number and title is centered at the top of each tactile. The scale of the first eight tactiles is found in the upper right-hand corner and represents how many kilometers each centimeter on the tactile represents. A centimeter bar found just below the scale will help remind you how big a centimeter is. It may be helpful for you to use your fingers and determine which of your fingers is closest to being a centimeter across so that you can use that finger on each tactile as a guide. But remember, each tactile has a different scale so your finger will represent a different amount of kilometers for each tactile.

On Tactile 1, move your hands around the outer perimeter of the Moon and you will feel a ridge line meant only to define the shape or outline of the Moon. The real Moon is actually a sphere, or like a ball. Here it is represented as a circle and the half you are exploring with your hands would come up “out” of the page towards you.

To give you some sense of scale, the Moon has a diameter of about 3,476 kilometers (km). This tactile is 20 centimeters (cm) in diameter, therefore, each cm = 174 km (you can arrive at that by dividing 3476 km by 20 cm). Note that this information is also given in the upper right-hand corner of the page above the centimeter bar.

Do you notice the relatively smooth regions and the rougher regions? The smooth regions are those that appear darker than the surrounding region when looking at the Moon with the naked eye. These smooth areas are called maria (mare for just one), which is Latin for “seas” or “oceans”. They once were seas of molten lava called basalt that poured from the Moon’s interior and cooled to solid rock.

Find the mare near the middle labeled “A”. (Note, if Tactile 1 does not have a Braille “A”, search for a smooth area found just to the upper right of center). Now move your finger from left to right, horizontally through the Braille “A” or smooth area, to get a better idea of the size of the mare. To the left of the “A” is a rough area and to the right of the “A” is a rough area. How many finger widths is it across? If you multiply your number of finger widths times 174 km, what is your prediction for how large that particular mare is? You probably got just over three finger widths or about 522 km (3 x 174). This is a mare called the Sea of Serenity. Since it is not perfectly round, it ranges from 522 km to 700 km across. If you measure just below the “A,” you will find it to be about four finger widths across. How did you do?

The rough regions or “highlands” are marked with many pocks or holes called craters. From samples returned to Earth by the Apollo astronauts, scientists were able to determine that the basalt is younger in age than the lighter colored rock found in the highland, or rougher, regions. If the maria are younger in age, why do you suppose they are smoother?

Explore Tactile 1 some more. From the center of the tactile graphic which is just below the “A” one finger width and to the left one finger width, trace your finger straight down to the six o-clock position. At the edge of the Moon, move two finger widths to the left and one up to find a large round crater that your fingertip will fit into. This is where our journey about craters really begins.

Craters come in two varieties. Some are formed by volcanic activity and others are formed by asteroids and comets traveling in space that collide with the Moon. Up until the late 1800’s, most scientists believed that the craters on the Moon were formed by volcanism. However, in 1892, American geologist G.K. Gilbert postulated that craters on the Moon were formed by objects in space impacting the surface of the Moon. Gilbert used experimentation and his keen observations of debris fields to arrive at his conclusions.

The crater on Tactile 1 you just explored was formed by an impact with a large bolide or meteor sometime in the past. Impact craters can be further classified as simple or complex. Simple craters have a bowl- shaped depression with raised rims that are 15 kilometers (km) across or less. Complex craters are greater than 15km in diameter and have shallow, relatively flat floors, a raised central dome, and giant terraces around their walls. Some craters larger than 300km across have concentric rings rather than central mountains and are classified as impact basins.

Once again, find the crater located at the edge of the Moon tactile at the 6 o-clock position and two finger widths to the left and one up. This crater is called Tycho and was named after the great 16th century astronomer Tycho Brahe. The crater Tycho is about 85 km in diameter and is visible to the naked eye. Would it be considered a “simple” or “complex” crater?

Tactile 2 - Tycho

Turn to Tactile 2 in the book. Be sure and touch the centimeter bar in the upper right hand corner of the tactile and note the number above it to get an idea of the scale.

Tactile 2 shows two views of the crater Tycho. The tactile towards the top of the page represents a top-view or bird’s-eye view from high above the crater. This tactile shows how the crater would appear if we were soaring high above the crater looking down on it.

The second view on the bottom of the page is a cross-section view of Tycho. Imagine this… you slice an apple into two pieces and then observe the peel, edible part and core; that would be the equivalent of a cross-sectional view of an apple. Another example of a cross-section that may help: consider a ham and cheese sandwich made with two pieces of bread. When cut in half, the bread forms the top and bottom layers with two middle layers of cheese and ham. That is the same idea illustrated in the bottom tactile … the view of a crater from the side including what’s above ground and the layers that are below ground.

Now go back and feel the top bird’s-eye view. Start from the left hand side and trace your finger around the rim of the crater. Sighted assistance may help you determine the rim of the crater. Now notice that the central uplift, right in the middle of the tactile, is very prominent in Tycho. This is evident in the top-view and is about 3 scale cm across. Using the scale for this tactile, about how wide is the central uplift?

The central uplift is also noticeable in the bottom tactile or cross-section. In the cross-section, the “surface” of the Moon trails off to the left and to the right of the hole representing the crater. Starting on the far left trace your finger along the surface of the Moon and notice that it rises up to a peak and then down into the crater floor. As you continue to sweep your fingers along the floor, you will notice the central uplift region that is found in the center of Tycho. Central peaks form by the rebound of rocks that were highly compressed at ground zero of the impact. The lines below the crater represent the fractured rock beneath the surface. Because the physical forces involved in an impact the size of Tycho and larger craters are so much more than the necessary forces to make rock fracture, the surface around an impact area behaves much like water!

Therefore, scientists can learn a great deal about impacts by studying slow motion film of water droplets hitting a surface. Most of the same features seen in water droplets hitting a larger body of water are also accounted for in impact basins, including the central uplift which is formed as debris in the middle is pushed up due to the slumping or “falling” of the sides of the crater shortly after impact.

Tactile 3 - Moltke

Tactile 3 presents a top-view and cross-section of the crater Moltke. This crater is 7km in diameter. Explore the graphic. What kind of crater is Moltke – simple or complex? Is there a central uplift? Using your “scale” finger on the cross-section view at the bottom of the page, what can be concluded about the depth of the crater with respect to its diameter, or its depth/diameter ratio? Taking into account the differences in scale, what can be said about the depth/diameter ratio of the crater in Tactile 3 compared to Tactile 2? In other words, are smaller craters deeper compared to their diameter or more shallow compared to their diameter, when compared to larger craters?

Tactile 4 – Schrodinger Basin

Tactile 4 represents a complex crater system that is so large, it is considered an impact basin which significantly alters the geology of the surrounding area. This particular basin is known as Schrodinger Basin and is 320 km in diameter! Notice that it has an uplifted ring within the outer rim of the crater itself. These concentric circles or rims can be found on the bird’s-eye view and the cross-section view.

Tactile 5 - Theophilus

Now it’s your turn. Explore Tactile 5 and the corresponding scale. This is the crater called Theophilus. What is your estimate of the diameter of Theophilus? Would it be considered a complex or simple crater? On the cross-sectional view at the bottom of Tactile 5, explore the lines emanating down beneath the crater. These represent fissures and fractures where the underlying rock is cracked. It is evident that craters come in many different sizes and that the features of craters are largely a function of the size of the object that makes the impact. But what about really large impacts? Could there be impacts that are even larger than Theophilus (Tactile 5) or even Schrodinger (Tactile 4)? Remember Schrodinger is 320 km in diameter! The answer is absolutely. In fact, the largest impact basin known on the Moon, and possibly the solar system, is referred to as SPA or the South Pole Aitken basin, or abbreviated as S.P.A.. SPA was first discovered by Soviet probes in the early 1960s. Because of SPA’s observable patterns being spread over such a large area, it was not recognized as an impact basin until years later.

Tactile 6 – South Pole Aiken Basin or SPA

Tactile 6 is a tactile creation of SPA and the far side of the Moon. The size of the basin is immense at approximately 2500 km across and about 12 km deep. That means that it stretches over nearly a quarter of the Moon, with most of it located on the far side of the Moon. Again, check out the scale under the title in the upper right hand corner of the tactile. As you explore the overall tactile, notice how different it feels than Tactile 1 which is a tactile of what we call the “near side” of the Moon.

Turn back and check out Tactile 1 again. Because the Moon makes one rotation for every revolution, or once every 27.5 days, the same side of the Moon always faces us. Therefore, observers from Earth always see the same side. It wasn’t until probes and other missions flew around the Moon that the “far side” of the Moon was photographed and mapped. Do you notice how Tactile 6 does not have nearly as many smooth areas, or maria, as Tactile 1? Most scientists agree that this is because of a difference in crustal thickness. A thinner crust on the near side allowed much more outpouring of lava into basins than on the far side.

Although it is barely visible, chances are you will not be able to feel the large circle within the outer rim of the Moon that indicates SPA. This highlights the subtleties, or hidden characteristics, that scientists must often pick out from visual images.

Tactile 7 – Raised line diagram showing size of SPA to size of Moon

Now check out Tactile 7 where we have represented the outline of the Moon with a raised line. Notice also the inner raised line that represents the size of SPA without all the other details in Tactile 6. Using Tactile 7 as a guide go back and explore Tactile 6 and see if you can discover the SPA basin. Tactilely, it is virtually impossible to find, however, with sighted assistance it may be recognizable.

All the craters explored in this book are real examples of craters found on the Moon. These impacts occurred a long time ago, and we rarely can see an impact “live.” So what really happens when a large object impacts the Moon, or any other solar system object for that matter?

Computer modeling and studies of impact craters on Earth have allowed us to better understand the dynamics of what happens during an impact. Keep in mind that objects are typically moving about 20 km/s when they impact the Moon and that the angle of impact will affect cratering also. It should likewise be noted that the object that impacts the Moon does NOT leave behind a crater the same size as the object. Estimates put the size of the crater left behind by a typical impact at 10 to 20 times larger than the object itself! That is why even a relatively small object can do a lot of damage when it impacts the Moon, or for that matter, the Earth.

To better understand impact events, we have divided impacts into 3 stages based on a book by Bevin French (1998) Traces of Catastrophe: A Handbook of Shock-Metamorphic Effects in Terrestrial Meteorite Impact Structures.

Tactile 8 – Impact Stages

The 3 stages include compression, excavation and modification. Explore Tactile 8 to get a better idea about what happens at each stage. Each of these are cross-section views. The horizontal line on each of the three stages represents the Moon’s surface. Starting on the left and moving to the right, allow your finger to trace along the Moon’s surface until you find the point of impact.

Stage 1 is the initial impact and compression of the surface. Notice the one dot in the crater that represents the object impacting the Moon. Also notice the lines below the crater representing compression cracks below the Moon’s surface.

Stage 2 represents the displacement of the material, which excavates the crater. During this stage, debris (called ejecta) is thrown out of the growing crater and falls back to the lunar surface as an ejecta blanket, including long ejecta rays. On the tactile, you will find lines and bumps above the crater floor representing the ejecta being thrown out of the crater. The lines below the crater floor represent further cracking and fracturing occurring below the surface. Eventually the debris being thrown from the crater will fall back to the surface and form ejecta rays. Ejecta rays extend out from the crater Tycho and can be seen with binoculars from Earth. Go back to Tactile 1 and see if you can discern the ejecta rays emanating outwards from Tycho. Note that sighted assistance may be necessary for this observation.

Stage 3 includes the modification of the crater shortly after impact. Once again, starting on the far left of the bottom tactile and moving to the right, notice the layer, or blanket, of material on the surface of the Moon, but you find no debris above the crater since it has all settled back down to the surface.

Although not all impacts have the same effect due to angle of incidence, the speed at which the object strikes, size, and composition of impacting object; these 3 stages are usually found to some degree or another. The outer rings and terrace-like features found in the complex craters and impact basins are due to the uplift and settling of the area following the impact.

Tactile 9 – Apollo astronaut suit

Special suits like the one illustrated in Tactile 9 were worn by the Apollo astronauts for all their missions. The crew wore these during mission-critical events such as launch, exploration of the lunar surface and re-entry into Earth’s atmosphere at the end of the mission. This special suit protected the astronauts from the vacuum of space as well as the extreme temperatures they might face. For example, while working on the surface of the Moon the temperature might get as high as 260 degrees Fahrenheit (127 degrees Celsius)! That is 160 degrees higher than a hot summer day here on Earth! When the sun goes down on the Moon, the temperatures can dip to a chilly minus 280 degrees Fahrenheit or minus 173 degrees Celsius!

The basic space suit was used for liftoff, re-entry and landing. This suit consisted of several layers of protection including a water-cooled undergarment, a multi-layered pressure suit and additional layers for heat and scrape protection, boots, gloves, a communication cap and a clear plastic helmet. Oxygen and water for cooling were provided by the Apollo capsule during lift off and landing.

When worn on the lunar surface, the astronauts added protective boots and gloves much like you would wear for winter weather, a set of filters on the helmet to block out harmful rays from the sun and a special life-support backpack. The backpack provided oxygen to breath and helped to remove the exhaled carbon-dioxide as well as water to cool the astronauts. Altogether, the space suit weighed 180 pounds on Earth, but only 30 pounds on the Moon!

Starting at the top of the astronaut’s head, notice the thick helmet and smooth visor to cover and protect the astronaut’s head. Moving down the suit, the arms and legs may seem ‘puffy’. This is due to the positive pressure provided by the suit to help protect the astronauts from the harmful space environment. The buttons on the front of the space suit are for connecting the life supporting oxygen and water as well as communication leads while being worn by the astronauts.

In addition to being a life support environment for the astronauts, the space suit needed to be flexible enough for the crew to be able to move and work on the lunar surface. In order to collect the rocks and core samples and deploy the instrument packages, the suited-up astronauts had to be able to bend their arms and work with their hands. Consider trying to pick up a golf ball from the ground while wearing 6 layers of clothing and gloves!

Tactile 10 – Apollo Saturn V rocket with astronaut to scale

The Saturn V rocket was a super heavy-lift launch vehicle used by NASA to launch the Apollo astronauts to the Moon from 1969 – 1972. The rocket stood over 360 feet tall, was 33 feet in diameter and weighed 6,540,000 pounds! The spacecraft was primarily made of aluminum with some titanium, polyurethane, cork and asbestos. Three rocket stages were needed to boost the heavy rocket, astronauts and all 100,000 plus pounds of mission-related equipment out of Earth’s gravitational pull and on to the Moon. As of 2019, the Saturn V rocket is still the only launch vehicle to carry humans beyond Earth orbit.

Let’s explore the tactile of the Saturn V rocket. This graphic illustrates the location of the command, service and lunar modules sitting on top of the first, second and third stage booster rockets. A six-foot-tall astronaut is provided for comparison to the left of the rocket, near the bottom of the page.

Starting at the bottom. Three of the five engines are shown. They are arranged with two on the outer corners and one in the center. Two additional engines on the other side of the rocket form a square. This 138 foot tall section of the rocket weighed over 280,000 pounds empty and over 5 million pounds when fully fueled with liquid oxygen! When in flight, the outer four engines could be turned or gimballed to help steer the rocket. The center engine remained fixed in place.

The third stage was shorter than the other two at nearly 59 feet and a bit narrower at 21.7 feet. Without propellant this stage weighed 23,000 pounds. When fully fueled it weighed nearly 262,000 pounds! Unlike the first two stages, the third stage had only one rocket engine. This stage was used twice during a lunar mission: once for lunar orbit insertion and the second time for the trans-lunar injection. Two smaller auxiliary units located at the bottom of the stage were used to keep the spacecraft in its proper orbit.

The Apollo lunar module, or LM, sat above the three rocket stages for its flight to the Moon. The LM was the first crewed spacecraft to operate exclusively in the airless vacuum of space and to land on the Moon. A crew of two astronauts flew the LM to the surface of the Moon from lunar orbit and back, leaving the bottom of the LM on the Moon. The LM became a lifeboat for the Apollo 13 crew when an onboard explosion disrupted their mission, preventing them from landing on the Moon. On the last three Apollo flights, the LM was upgraded to include a lunar roving vehicle or LRV on the descent stage. This LRV permitted the astronauts to explore a larger area around the landing site.

The Service module sat between the Command module and the lunar module and provided life-saving support for the astronauts and the spacecraft. This section of the Saturn V housed the systems needed to keep the spacecraft in lunar orbit and to help steer it back to Earth once the mission was complete. Also housed here were the power, environmental and communications systems used to keep the astronauts comfortable and to be able to communicate with Mission Control on Earth.

The top-most stage, or Command Module, housed the astronauts, their food and equipment and the waste-management system as well as the guidance and navigation systems. This tiny space is where the crew of three astronauts lived and worked for their 8 – 12 day journeys. More on this part of the Saturn V is shared in Tactile 11.

Tactile 11 – Apollo command module with astronaut to scale

The conical shaped Apollo command module had a diameter of nearly 13 feet and was just over 11 feet tall. The six-foot-tall astronaut to the left is provided for scale. Within this module, three astronauts lived and worked in 210 cubic feet of space. How small is this? Imagine living with three friends for more than a week in a space the size of closet!

Five windows were provided for the astronauts to witness spectacular views of the Earth, Moon and the vastness of space. The two side windows were just 13 inches x 13 inches located near the left and right astronaut couches. Two forward facing windows near the front entry hatch were just 8 inches by 13 inches. These windows were used for rendezvousing and docking the LM. The fifth window was located directly above the center astronaut couch. All of the windows were made of a special aluminum-rich glass.

Note the heat shield located at the bottom of the command module. This protected the capsule from the heat created by friction upon reentry into Earth’s atmosphere. Typically, enough heat is generated upon reentry to melt most metals, including the aluminum skin of the spacecraft. To protect the spacecraft, this bottom heat shield was composed of a special type of resin that can withstand high temperatures. The 3,000 pound heat shield varied in thickness from 2 inches on the side facing the reentry to just half an inch on the opposite side.

Tactile 12 – Apollo landing sites on lunar nearside

Take a second to explore Tactile 12. You will note that it is very similar to Tactile 1, the near side of the Moon where all 6 of the US Apollo missions landed on the nearside. The locations of the Apollo missions 11, 12, 14, 15, 16 and 17 are indicated on this tactile. Note their locations are close to the lunar equator, or middle of the Moon. These missions would not have been possible if it weren’t for President John F. Kennedy. In a special message to congress on May 25, 1961, he urged Congress to support his plans for the nation’s space program. His speech may be heard here.

Below we share just a few highlights from each of the six landed Apollo missions. Unfortunately, Apollo 13 did not reach the surface due to the unfortunate explosion of the oxygen tanks while on flight to the Moon. The astronauts did however, return safely back to Earth.

APOLLO 11 – This mission accomplished a primary objective of President Kennedy’s – to land a two person crew of American astronauts on the Moon and safely return them to Earth. On July 20, 1969, while astronaut Michael Collins orbited in the command module (Tactile 11), astronauts Neil Armstrong and Buzz Aldrin landed on the Moon. Upon safe landing, Commander Armstrong radioed his infamous confirmation: “Houston, the Eagle has landed”. Together, Armstrong and Aldrin deployed a US flag, an experiment package called the Apollo Lunar Surface Experiments Package (ALSEP), and collected 50 rocks and lunar soil or regolith samples totaling about 48 pounds during their two and a half hours of extravehicular activity, or EVA, on the lunar surface. Common rocks at Apollo 11 are volcanic basalt and breccia that formed 3.6 to 3.9 billion years ago

Apollo 11 voice transcript

APOLLO 12- The primary objective for Apollo 12 was to further explore the lunar surface and to deploy the ALSEP package which included seismic, scientific and engineering experiments. This was also a test of whether the astronauts could make a precision landing. On November 19th, Apollo 12 touched down approximately 950 miles west of Apollo 11, about 600 feet from their target. While astronaut Richard Gordon orbited the Moon, astronauts Charles Conrad and Alan Bean explored the lunar surface. They too collected lunar rock and regolith samples. The nearly 75 pounds of samples collected during the seven and a half hours were almost all basalts that formed 3.1 to 3.3 billion years ago.

Apollo 12 voice transcript

APOLLO 13- Apollo 13 successfully orbited the Moon following an onboard explosion and returned safely to Earth. No samples were returned.

APOLLO 14 – The primary mission objectives for Apollo 14 were the same as those for Apollo 13 in addition to returning a larger amount of lunar material and scientific data from ALSEP. This mission included a collapsible, two-wheeled cart called the modular equipment transponder, or MET. It carried tools, cameras, a portable magnetometer and could carry any collected lunar samples back to the lander. On February 5th, 1971 Apollo 14 landed about 110 miles east of the Apollo 12 landing site in the Fra Mauro highlands, or the rough area on Tactiles 1 and 12. Astronaut Stuart Roosa orbited the Moon while astronauts Alan Shepard and Edgar Mitchell spent thirty-three and a half hours collecting rock and soil samples and deploying or conducting ten experiments. The crew collected 93 pounds of lunar rocks and regolith. The samples are primarily breccias with some basalts. The basalts are 4.0 to 4.3 billion years old, older than the nearby mare or smooth regions.

Apollo 14 voice transcript

APOLLO 15 – Apollo 15 was the first of the longer missions to the Moon, and the first to include a rover. Mission objectives included conducting lunar surface science, conduct engineering evaluations of new Apollo equipment and conduct orbital and photographic tasks. On July 30th , while astronaut Pete Worden orbited the Moon astronauts Dave Scott and James Irwin landed next to a lunar rille or lava channel. They set up the experiments and collected more than 170 pounds of lunar samples over more than eighteen and a half hours of EVA time. With the addition of the lunar rover, the astronauts were able to explore more than 17 miles. Of note for this mission was the collection of Genesis rock, an anorthosite, or the material that makes up the mountains near Apollo 15. This and other rocks returned from Apollo 15 have been dated between 4 – 4.5 billion years old.

Apollo 15 voice transcript

Apollo 16 – Similar to the previous Apollo missions, the primary objectives for this mission were to survey and collect lunar rock and regolith samples, conduct experiments inflight and on the surface and to take photos from orbit. On April 20th, 1972 while Thomas Mattingly orbited the Moon John Young and Charlie Duke landed in the central highlands. In addition to spending more than 20 hours on EVAs, the astronauts conducted an evaluation of the lunar rover putting it through a "Grand Prix" exercise consisting of S-turns, hairpin turns and hard stops. Young and Duke collected more that 731 rock and soil samples weighing more than 210 pounds. Initially thought to be volcanic basalts, most of the Apollo 16 samples turned out to be breccias! The few basalts formed 3.79 billion years ago. Two anorthosite, or highlands samples, are between 4.44 and 4.51 billion years old, close to the age of the Moon itself!

Apollo 16 voice transcript

Apollo 17 – The last Apollo mission objectives included sampling lunar rocks and regolith, deploying and activating surface experiments, conducting in-flight experiments and taking photographs. In addition to the regular ALSEP package, the astronauts deployed a gravimeter and several biomedical experiments. On December 14th, 1972 while Ron Evans orbited the Moon, Gene Cernan and Harrison Schmitt landed in the Taurus Littrow Valley, a deep and narrow valley. Three EVAs on the lunar surface lasted more than twenty two hours during which Schmitt and Cernan collected more than 240 pounds of rocks and regolith. Of note: this mission is the first to include a scientist-astronaut, Dr. Harrison Schmitt who is a geologist. Rock and soil samples collected in the valley include 3.7 – 3.8 billion year old volcanic basalts and some highlands samples that are between 4.2 and 4.5 billion years old. Note: the solar system formed just 4.56 billion years ago! Apollo 17 was the last lunar landed mission with astronauts nearly 50 years ago!

Apollo 17 voice transcript

Websites:https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo.html

Final Impacts to Ponder

It is interesting to note that SPA basin shown in Tactiles 7 and 8 is thought to be formed from an object moving at a relatively slow speed with respect to the motion of the Moon and at a high angle of incidence, thus accounting for the relatively shallow crater formed.

What a sight that would have been to witness such an event from Earth! Aren’t you glad you weren’t on the Moon when that occurred? It is humbling to realize that the Moon has been hit so many times in the past and yet, is Earth immune? Why is it that we don’t find as many impact craters on the Earth? What factors would prevent us from finding craters on Earth? Impacts and cratering are an inevitable part of the evolution of any planetary body – it has happened in the past and will continue to mold landscapes in the future. This book has highlighted some of the most common types of craters and has allowed you to explore what these craters are like. For more information on cratering, please see Bevan French’s book, Traces of Catastrophe, which is available online http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html, or one of several web sites that may explain this process in more detail such as…

The Earth Impact Database: http://www.passc.net/EarthImpactDatabase/

and NASA’s Remote Sensing Tutorial – Impact Craters: http://geoinfo.amu.edu.pl/wpk/rst/rst/Sect18/Sect18_1.html

---