QUESTION:
martian craters

I was assigned a lab to do where a 
simulated surface (flour,nestle quik powder etc) is bombarded by several 
different objects; the objects i used were a small 67g metal ball, a 150g 
field hockey ball, a 45.5g golf ball, and a 168g softball. The experiment 
consisted of me dropping the objects from different heights ( 30 cm, 60 cm, 
90 cm, and eventually the ceiling of the classroom down to a table 170cm) 
After the object hit, i then set about measuring the width, depth, and 
 diameter of the crater; along with that i was also charged with measuring 
the length, and number of ejecta rays emitted, and then averaging the 
totals.
    Now i've come to some conclusions (and this is where i need some help); 
my 
first conclusion was that the thicker the surface and the higher you drop 
an object, the deeper the object will smash into the surface. I noticed 
this when a plastic marble dropped into a "hilly" area went deeper than the 
huge softball at an equal height but less thickness.
    The second conclusion i came to was that the more weight and size an 
object has the more ejecta rays it should emit. Though the field hockey 
ball often caused double craters by bouncing it did emit the most rays.
    Now i'm not asking you to do my homework for me but i want to ask this: 
Was I correct in my above conclusions? What factors do real meteorites have 
that change the shape of craters? Does weight and mass matter, or is it 
velocity and force that make bigger craters?

    I really appreciate you taking the time to answer this and I hope that 
you 
can give me some insight as to what real meteorites do when they hit the 
surface of a body.


ANSWER from Jeff Plescia on June 8, 2000:

Basically, the size of the impact crater is determined by the kinetic
energy (1/2 mvv) of the event.  The more kinetic energy, the bigger the
crater.  Obviously, since the energy goes as the velocity squared,
increasing the velocity will do more for you than increasing the mass, but
as both get bigger the crater gets bigger.

The thickness of the material should have no first order effect, from the
point of view of a meteorite, the earth is infinitely thick.  The physical
properties of the surface (e.g., the strength of the material, the number
of layers of different composition, etc) do affect the shape and size of a
crater.  For example, a meteor impacting sedimentary rock will make a
bigger crater than in igneous rock because the sedimentary rock is weaker.
If you have a very weak layer overlying a strong layer then you will be a
crater that is wider within the weak layer and narrower in the strong layer
(a sombrero shape).

Real meteors (for large objects not slowed by the atmosphere) hit the
surface at very high speeds (15-50 kilometers per second (33,750-112,500
miles per hour)), so dropping things from a few feet does not really
simulate the energy of an event.  Also, in a real impact the meteor is
destroyed, your meteors survive and bounce around so the results are not
quite real.

The more energy of an event, the more material will be removed from the
crater and so the ejecta around the crater will be thicker and extend
farther.  Rays are formed by small pieces of material through out of the
primary crater and making little craters farther from the main crater. I'm
not sure if you observed rays or just ejecta.

Regarding your conclusions:

1.  The higher you drop the object from the deeper and wider the crater
should be - the higher you drop the object the more kinetic energy it will
have.

2.  The thickness of the layer should have no effect - assuming it was
thick enough that the object did not just knock all of the material out of
the way.

3.  The more energy - the bigger the crater so the more ejecta (rays?)
there will be.