Spacecraft exploration of the planets has virtually proven that there are no advanced forms of life. These same space probes have almost ruled out even simple extraterrestrial life. Currently, there is no evidence that any of the terrestrial planets, except Earth, are habited by life. And at first glance the outer planets appear too cold to sustain life. However, there are some environments that, either in the past or currently, have some potential for fostering life forms. These environments include those like Mars, Venus, the middle atmospheres of the giant planets, Titan, and the oceans of Europa.
Of all the environments listed above, the most heavily studied is that of Mars. Over a dozen missions have involved the collection of data about our red neighbor. Although much has been learned about Mars and its history, we are still left wondering if life does, or ever did, inhabit the red planet. Due to a much thinner atmosphere, Mars is subject to a much greater intensity of UV radiation than Earth. As a result, organic material on the Martian surface will most likely have been destroyed due to irradiation. Despite the lack of organic material that would be indicative of life on Mars, scientists running the Viking missions could not absolutely rule out the possibility of primitive life. For instance, as famous scientist and communicator Carl Sagan pointed out, the Viking probes could have landed at a number of arid sites on Earth and failed to detect life! Just a few years later, American biologist E.I. Freidmann reported his discovery of tiny organisms thriving in the pores of rocks in seemingly lifeless, Mars-like dry valleys in Antarctica. Since the Viking probes merely scratched the surface at only two places, we cannot rule out the possibility of Martian microbes below the surface. The current set of rovers has cemented the evidence for water on Mars in the past, but none of them has been equipped to detect life or fossilized remains of life. In addition, many planetary scientists believe that Mars had a wetter, warmer past — an environment more conducive to life. Recent evidence makes it even clearer that running water once existed on Mars, and it may still exist under the surface.
The prospects for life on present day Venus are very slim. With an amazingly thick atmosphere comprised primarily of carbon dioxide, Venus is the victim of a runaway greenhouse effect. Surface temperatures of well over 500 degrees Celsius make the likelihood of finding life on the planet's surface highly unlikely. Despite the overwhelmingly harsh conditions on Venus, some scientists have posed a hypothesis that life could have formed on Venus during its youth, when conditions would have been more accommodating to life. These scientists contend that if life did indeed exist on Venus at one time, it could have taken up residence in the Venusian atmosphere. They have even gone so far as to suggest that microbial phototrophic life might be found today in the dense cloud layers that shroud Venus' surface from our view. There's no evidence to support or refute these claims. And due to its dense atmosphere and scorching temperatures, Venus may not be at the top of the list for research destinations in the Solar System to search for signs of life.
Beyond our local neighborhood of terrestrial planets, there are some interesting places that could provide insight into life in the Solar System. The asteroid belt separates the terrestrial planets from the larger gas giants. These planets' atmospheres are huge compared to Earth and are hydrogen dominated. If you were to try and parachute onto the surface of one of these planets, the pressure and temperature would increase with depth. However, some scientists have surmised that there may be some atmospheric layers with temperatures and pressures not too different from the atmosphere of the Earth. Carl Sagan and others have speculated about the evolution of organisms that could float in such atmospheres. However, most biologists consider it very unlikely that replicating molecules could ever form in such a diffuse environment.
Similar to Venus, Titan, the largest of moon of Saturn, also has a dense atmosphere. Unlike Venus, however, Titan has an atmosphere made of hydrocarbons, ethane and methane. The resulting chemistry of such an atmosphere makes an intriguing laboratory for life. The surface temperature on Titan is much colder than Earth, well over one hundred degrees below the freezing point of water. Despite the extreme cold, scientists believe that liquid oceans are likely to be present at the surface. These oceans will be made of ethane, not water like on Earth. Although we wouldn't expect the chemistry on Titan to be like that of Earth, it may give us some clues about the chemistry of early Earth. And although it is too cold for Earth-like life, this hydrocarbon chemistry could, using the diffuse energy from the Sun and a little extra nitrogen, result in some of the important building blocks of life.
As we extend further and further away from our Sun, the intensity of light drops off exponentially. For organisms dependent on light, this is a recipe for doom. However, the discovery of Sun-independent organisms deep within rocks and near deep-sea vents on Earth has reminded us that entire ecological systems can thrive on sources of energy other than sunlight. For satellites orbiting large planets like Jupiter and Saturn, this energy can be a result of tidal heating. The energy from tidal heating is sufficient to melt water ice beneath the surface of Jupiter's moon Europa, and has piqued interest in the possibility of organisms that might have evolved in these sub-surface oceans (a theme developed in Arthur C. Clarke's novel 2010). It might be interesting in the future if we can drill through the icy crust of this moon and study the "buried ocean" believed to exist there.
The biggest surprise in the past decade is the number of moons in the outer Solar System that appear to have all the ingredients for life — liquid water, an energy source, and carbon-rich material. Ganymede is barely the size of Rhode Island but it has a sub-surface ocean and geysers shooting tiny crystals of ice into space. Modeling suggests that a dozen or more large moons or moons with tidal heating are habitable in places under the surface. This large amount of real estate for potential biology far from the Sun is called the "cryogenic biosphere." Although we don't expect to meet creatures of any great intellect in our Solar System, there are clearly places left to explore that may shed light on biochemical and biological evolution.