  Natural icicles often exhibit ripples about their circumference which are due
to a morphological instability. We present an experimental study that explores
the origin of the instability, using laboratory-grown icicles. Contrary to
theoretical expectations, icicles grown from pure water do not exhibit growing
ripples. The addition of a non-ionic surfactant, which reduces the surface
tension, does not produce ripples. Instead, ripples emerge on icicles grown
from water with dissolved ionic impurities. We find that even very small levels
of impurity are sufficient to trigger ripples, and that the growth speed of the
ripples increases very weakly with ionic concentration.
