The global ocean on Europa contains about twice the liquid water of all the Earth’s oceans combined.
The chances for life there have been uncertain, because Europa’s ocean lies beneath several miles of ice, which separates it from the production of oxygen at the surface by energetic charged particles.
Without oxygen, life could conceivably exist at hot springs in the ocean floor using exotic metabolic chemistries, based on sulfur or the production of methane.
However, it is not certain whether the ocean floor actually would provide the conditions for such life. …