WHY WE STUDY THE GEOLOGICAL RECORD FOR EVIDENCE OF THE SOLAR OSCILLATION ABOUT THE GALACTIC MIDPLANE

The Solar System oscillates about the plane defined by the disk of mat ter in our Galaxy. This oscillatory motion gives rise to a substantial modulation in the tidally induced flux of Oort cloud comets. An obser vational determination of the quasi-periodicity of this motion carries with it significant information about the population, distributions, dynamics and origins of short-period and long-period comets. An additi onal incentive for emphasizing such a study is the information about d ark disk matter that a period can yield. If dark disk matter is comple tely negligible, the amplitude of the solar motion will be sufficientl y large that the peak-to-trough flux ratio will be approximate to 2.5 and the plane-crossing period will exceed 40 Myr. Dark disk matter com parable in mass to bright disk matter and distributed in any manner is inconsistent with K-dwarf distributions and can be rejected as a work ing hypothesis. But if a modest fraction of the disk matter is dark an d distributed Like the interstellar medium, as is consistent with limi ts deduced from K-giant and K-dwarf velocity distributions, the peak-t o-trough flux ratio can increase to a factor of 4 even though the sola r z amplitude is decreased. In that case the period can be as Little a s 30 Myr and the implied Oort population is smaller by a factor of 3. We should carefully reconsider the geological record as a potential di scriminator of these options.