Impact gardening on Europa and repercussions for possible biosignatures

Owing to its internal ocean, Jupiter's moon Europa can potentially host extant life. However, because Europa's orbit is within Jupiter's magnetosphere, chemical biosignatures that are exposed to space may be destroyed by high-energy electron radiation. It has been suggested that biosignatures may be preserved below the radiation-penetration depth of the top few centimetres. Impact gardening, the process by which small impacts mechanically churn the uppermost surface of airless bodies, is known to disrupt near-surface stratigraphy; however, no comprehensive estimate of the effect of gardening has yet been determined for Europa. Here we use an impact gardening model to show that gardening is a global process on Europa, and has churned, on average, the top 30 cm over the last several tens of millions of years, thus, exposing all material within the top 30 cm to surface radiation. We suggest that morphologically immature craters and regions of mass wasting at mid-to-high latitudes could be weakly impacted by both gardening and radiation, and should be preferred locations for the search for life on Europa. Modelling shows that impact gardening on Europa has the potential to churn the shallow subsurface material down to 30 cm very efficiently and globally, thus destroying potential habitable niches just below the surface. Some areas where both gardening and radiation are relatively weak are, however, identified.