Despite its hostile, cold surface Europa is one of the most intriguing places to look for life. It has a crust made from ice and an ocean underneath the ice, and so there are two places where life could have established itself.
First, photosynthetic microbes could live in the diurnal tidal cracks, taking advantage of the liquid water there. A few centimetres down in a crack, they would be sheltered from radiation by the water or slush above them, but still able to use sunlight as an energy source. They would be relatively easy to detect because their remnants might be present where slush from a closing crack is pushed out onto the surface.
Second, the ocean beneath the ice has potential as a habitat because it is shielded from the adversities of space such as cosmic radiation and (small) impacts. Most importantly, it is made up of the key ingredient for a habitable environment: liquid water. Volcanoes or hot springs on the ocean floor might deliver the two other key ingredients: energy and nutrients – very similar to what has been observed at the mid-ocean ridges on Earth. To explore the possibility of life near Europa’s ocean floor is much more complicated than looking at the tidal cracks, since a space probe would have to melt or drill through the ice, dive deep into the ocean and then transmit its findings back to Earth.