If they are looking merely at the atmosphere, what kind of atmosphere are they looking for? one like ours would not be very supportive of the deep submersive life one would expect under layers of ice, with i would assume, core heat, and magma to do the job of supporting life in the heat category, and to supply food.
Chemosynthesis on the other hand, on a planet with an atmosphere like venus' would be quite possible, high levels of sulphur to support a producing bacteria, one only needs to look at the mid ocean rifts and the life that survives down there, it is even possible that life started in the deep oceans, but evolution points the other way, towards the surface.