  Although statistical evidence is not overwhelming, possible support for an
approximately 35 million year periodicity in the crater record on Earth could
indicate a nonrandom underlying enhancement of meteorite impacts at regular
intervals. A proposed explanation in terms of tidal effects on Oort cloud comet
perturbations as the Solar System passes through the galactic midplane is
hampered by lack of an underlying cause for sufficiently enhanced gravitational
effects over a sufficiently short time interval and by the time frame between
such possible enhancements. We show that a smooth dark disk in the galactic
midplane would address both these issues and create a periodic enhancement of
the sort that has potentially been observed. Such a disk is motivated by a
novel dark matter component with dissipative cooling that we considered in
earlier work. We show how to evaluate the statistical evidence for periodicity
by input of appropriate measured priors from the galactic model, justifying or
ruling out periodic cratering with more confidence than by evaluating the data
without an underlying model. We find that, marginalizing over astrophysical
uncertainties, the likelihood ratio for such a model relative to one with a
constant cratering rate is 3.0, which moderately favors the dark disk model.
Our analysis furthermore yields a posterior distribution that, based on current
crater data, singles out a dark matter disk surface density of approximately 10
solar masses per square parsec. The geological record thereby motivates a
particular model of dark matter that will be probed in the near future.
