  We investigate the formation of a crater in a 2-D bed of granular material by
a jet of impinging gas, motivated by the problem of a retrograde rocket landing
on a planetary surface. The crater is characterized in terms of depth and shape
as it evolves, as well as by the horizontal position of the bottom of the
crater. The crater tends to grow logarithmically in time, a result which is
common in related experiments. We also observe a horizontal symmetry breaking
at certain well-defined conditions which, as we will demonstrate, could be of
considerable practical concern for lunar or planetary landers. We present data
on the evolution of these asymmetric states and attempt to give insights into
the mechanism behind the symmetry-breaking bifurcation.
