IO AND THE XENOLITHS OF EUROPA

The known sources of dark matter on the surface of Europa are exogenous, either by direct accretion of comets and asteroids and their dust, or by incidental accretion of impact debris kicked up by comets and asteroids making craters in Ganymede or Io. Io is especially interesting because it is the most plausible source of rick volcanic soil. The mass of basaltic impact ejecta from Io that reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa’s ice crust. We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa’s surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady– Kipp fragments originally developed by Melosh. The model can reproduce the observed size-number distributions of small craters on either Mars or Europa. However, the model predicts that a significant fraction of the 200–500 m diameter craters on Europa are sesquinary craters caused by impact ejecta from Io; this prediction is not supported by observation, which implies that high speed spalls usually break up into swarms of smaller fragments and shards that would fall as a thin blanket over Europa at a single moment in time. Iogenic basaltic stones and dust therefore provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa’s surface.