[meteorite-list] Dropped Balls Link Crater Depth To Meteorite Diameter

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:25:42 2004
Message-ID: <200305211614.JAA27779_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/030519/030519-4.html

Dropped balls link crater depth to meteorite diameter.
Philip Ball
Nature Science Update
21 May 2003

The perplexing relationship between craters and the size,
shape and speed of the meteorites that may have
made them has driven a team of physicists back to the sand
pit.

Having dropped lots of balls of various sizes from various
heights they conclude that, on grainy ground, at least, the
depth of a depression is a function of the diameter of the
projectile that made it[1]. It had been thought that crater depth
is linked only to crater width.

Scientists are keen to understand the forces that
pitted bodies such as the Moon, so that they can work
out what sorts of projectile turned the Solar System into a
giant coconut shy billions of years ago. But impacts in loose material
such as sand are very different from those on hard, rocky ground.
Sand absorbs the energy of an impact. It is initially very soft - a ball
sinks into it even if set down gently - but is ultimately very hard -
even a falling cannonball doesn't sink very far.

Previous studies focused on crater diameter. This is proportional to
the size of an impacting object and its speed or the height from which
it is dropped. It was assumed that a crater's depth is simply related
to this, with wider craters being deeper. For example, many
meteorite craters in rock have a depth about one-fifth of their
diameter.

Not so, say Jun Uehara and colleagues at the University of California,
Los Angeles. They made careful measurements of the depth of the
dips created when they dropped plastic balls into loose sand, and
found quite different relationships from those that describe crater
width. Specifically, the depth is proportional to the diameter of the
ball and to the height from which it falls.

This finding implies that estimates of crater depth made using the old
assumption will be consistently too large, if the impact occurs in
loose, grainy material.

Grain of truth

The team reckons that the findings might also shed light on some of
the strange, counterintuitive behaviour of granular materials.

For example, the pressure at the bottom of a tall column of sand is
the same irrespective of how high the column is, and the pressure
underneath a sand pile is lowest where the pile is highest. Sand
pouring through a funnel can become stuck even when the funnel is
several times wider than the largest grains. All of these properties
result from the way in which stresses are passed from one grain to
another as they are pressed together.

The researchers think that this same influence of grain contacts
affects how energy is dissipated in an impact. The energy might be
converted to frictional heat, for example, as the grains slide against
one another. Understanding this mechanism would help to explain the
'impact-damping' properties of sand - this effect is what makes it so
difficult to run on a beach.

References

    1.Uehara, J. S., Ambroso, M. A., Ojha, R. P. & Durian, D. J.
      Low-speed impact craters in loose granular media. Physical
      Review Letters, 90, 194301, (2003).

      http://dx.doi.org/10.1103/PhysRevLett.90.194301
Received on Wed 21 May 2003 12:14:31 PM PDT