Cratering

Cratering

Explorations with impact.

Craters made by dropping two steel balls into a planter filled with salt.
The salt is covered with spices.

Introduction

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.

Material

A tray at least 5 cm (2 " ) deep, and 20 cm (8") in diameter, e.g. a plastic tray used under planters to catch drips.
table salt, enough to fill the tray 5 cm (2") or more deep (about 8 pounds, 4 kg, of salt). (Dry sand with angular grains will also work but table salt is best)
2 steel ball bearings at least 6 mm (1/4") in diameter or roughly spherical fishing sinkers.
black construction paper (letter size paper or larger)
a pair of scissors
a meter stick (yard stick)
- optional a meter long PVC tube larger in diameter than the ball bearings.
- optional, a magnet

Assembly

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

Crater made in salt.

Drop the ball bearing into the salt from a height of 25 cm (10").
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 magnet.
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 have produced.
Smooth the tray of salt and cover it with the black construction paper.

salt tray covered with paper to catch crater ejecta

Black construction paper covers the tray of salt, ready to catch ejecta.

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.

Notice radial lines of powder inside the crater resulting from debris sliding down into the crater after it has formed.

What’s 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.

So What?

Craters are the most common feature on most planetary surfaces.
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.

Math Root

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.

Going Further

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.

Etc.

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.