I've always said that we should for life on Mars underground...
Life needs influx of energy, and the development of life needs effortless energy source (such as the sun). This rules out life developing under the surface of present-day Mars, but life could have easily developed on the surface or moved underground or developed under the surface when the planet was more active. Life has little place on present Martian surface though, there's absolutely no water there, and life as we know it sucks without a solvent.
I always wondered why do we expect to find the possible life on the surface and easily. I guess we assume that because life on Earth is everywhere, life on other planets would be either everywhere or nowhere, so if it is underground we'd find evidence on the surface, and it will eventually spread everywhere and adapt to everything. But the truth is that some planets might have some life that's only adapted to a limited number of conditions, didn't have enough stimuli to adapt to many new ones, and is therefore limited to a specific territory, and might be very hard to find.
Let's see:
- Mars - hopes of favourable conditions and water (Sun and water) in the past, life could have migrated underground, or remained local to a specific area
- Europa, Ganymede and Callipso - the subsurface ocean is there, but it's probably relatively stale, no major energy influx that would kick-start life. Europa feels the strongest tidal forces, and probably has plate tectonics on all three of it surfaces, so there's biggest chance for hot vents that would trigger this. Our life would probably adapt to live there, but life's not that likely to start there. Still deserve checking, especially Europa.
- Titan - it has quite some energy influx, relative of course, but enough to trigger a complete weather cycle like the one on Earth; and it has abundance of chemical elements that could constitute an alternative biochemistry. We should make another trip there. With bigger probes. Alternative biochemistries are unlikely, but if they are possible, Titan is a place where it's very likely to find one of them.
- The gas giants - the conditions are turbulent enough to drive the processes to start life, but they are too erratic and unstable. Still, I wouldn't outright discount bacteria developing high in the atmosphere. We have such living on Earth, probably migrated from the surface, but the energy conditions on the gas giants are pretty damn good - I would check for life there. But than again, that would have been picked through spectral analysis... So it's very unlikely.
- Enceladus - that's the most likely spot for life other than the Earth. Liquid water, a lot of radioactive decay and tidal heating, and extremely active. It's the only place that has all the components and isn't a gas giant.
We should send landers (or ice drills) to all of those ASAP.