Explorations with impact.
Craters made by dropping two steel balls into a planter filled with salt.
The salt is covered with spices.
Craters can be made by dropping steel ball bearings into a tray of salt or sand. These craters share several properties with impact craters in the solar system: The crater is much larger than the impacting object, the crater is circular even if a non-spherical object hits at an angle, and debris is ejected a long way from the crater.
Fill the tray with salt to a depth of 5 cm (2") or more, more is better, as much as 10 cm.
Put black paper around the sides of the tray to catch salt ejecta and show its pattern.
To Do and Notice
Drop the ball bearing into the salt from a height of 25 cm
Notice that the ball bearing creates a crater and disappears from view.
(If the ball bearing doesn't disappear make a deeper layer of salt.)
Notice that the crater is circular, and larger in diameter than the ball bearing, and notice how ejecta sprays out around the crater.
Notice that there is a raised rim around the crater.
Drop an identical ball bearing from a height of 1 meter (1 yard) notice that the crater is larger and the ejecta sprays further.
Recover the ball bearings from the salt by coming through the salt
with your fingers with a rake from a cat litter box or by using the
If you have a non-spherical fishing weight, drop it into the salt and notice that the crater is circular even though the weight is not.
Roll the ball bearing down a grooved track or through the PVC tube
so that it hits the salt at an angle.
Notice that except for very low angles (measured from the surface), the crater produced by the ball bearings is circular.
Cut a hole in the construction paper larger than the craters you
Smooth the tray of salt and cover it with the black construction paper.
Drop a ball bearing into the salt through the hole in the
paper.(Guide it with the PVC tube.)
Notice that salt is ejected a long way onto the paper compared to the diameter of the crater, this salt is called the ejecta.
Launch the ball bearings into the salt through the hole at an angle, notice that the ejecta pattern is not symmetrical. More is scattered forward than backward.
Sprinkle cocoa powder or spices onto the surface of the salt.
Notice the pattern of powder after the impact. Notice that powder near the rim of the crater is covered in salt.
Whats Going On?
In cratering of a planetary surface, the impacting object delivers
a large amount of kinetic energy to the surface. This energy spreads
out like the energy from an explosion, creating a circular crater
that is larger than the impacting object.
When we drop an object into salt the kinetic energy of the impact of the object is transferred to the salt which sprays out in a circular pattern.
The ejecta is thrown a long distance from the crater.
When an object impacts the surface at a low angle from the surface, the ejecta does not spread out in a circularly symmetric pattern. Most of the ejecta continues on in the direction in which the meteoroid was traveling. Little ejecta goes back along the path from which the meteoroid arrived. The same thing happens in our model of an impact.
Craters are the most common feature on most planetary
The earth is an exception since erosion has removed traces of craters..
Large objects are completely vaporized by the energy of their impact, their kinetic energy creates an explosion that is more than 100 times greater than if they were made of TNT.
Measure the diameter of the crater, measure the circle made by the
crest of the raised rim.
For a given height of drop, computer the ratio of the diameter of the crater to the diameter of the ball bearing.
Make a plot of how the diameter of the crater depends on the height from which the ball is dropped.
Calculate the energy of the impact. E = mgh
where m is the mass of the impactor in kg, g is acceleration of gravity, 10 m/s2, h is the height of the drop.
For example a 10g ball dropping 1 meter has energy of: E = 0.01 * 10 * 1 = 0.1 Joule.
TNT releases about 1 Calorie per gram so if this this 10 gram ball were explosive then it would release 104 calories or 50,000 Joules.
As a meteorite travelling at 20 km/s it would have an energy of E = 1/2 mv2 or 0.5 * 0.01 * 4 * 108 = 2 x 106 Joules.
Record the impact with a digital video camera and play the impact back in slow motion one frame at a time.
Notice the expulsion of the ejecta, the creation of a crater, and the modification of the crater by slumping.
A meteoroid is a rock.
A meteor is a flash in the sky.
A meteorite is a rock from space which makes it to the ground
Meteoroid impacts ejected rocks from the Moon and from Mars which
eventually reached the earth.
In 1911 in Nakhla, Egypt, a rock from Mars fell to earth. It hit and killed a dog.
Scientific Explorations with Paul Doherty
28 November 2000