What are the characteristics of our solar system that lead to the origins of life?The possibility of finding life elsewhere is for many people the most compelling reason for humankind to explore beyond Earth. We believe that liquid water and carbon are required for life to arise and thrive, as well as a source of energy. Many places in our solar system provide these, at least for a time; not only planets, but also some moons and even certain comets. But for life to arise we presume that a hospitable environment must be more than just transient.
Earth is in the continuously habitable zone, meaning at our size and at our distance from the Sun water has been stable at the surface even though the brightness of the Sun has varied.
Not all planets are so lucky.
We now know that there once was liquid water on the surface of Mars, but was it there long enough for life to develop? We are not sure, but its possible and if so then life might still linger beneath the surface, perhaps in a place where sub-surface heat meets the surface permafrost. Venus too shows signs it lost the equivalent of Earth's oceans into space. Did life have a chance to evolve before the planet became the dry, superheated world we know today?
There are other places where there has been liquid water for as long as on Earth. Jupiter's icy moon Europa almost certainly has a liquid water ocean beneath the surface even though its five times further from the Sun than we are. If there are hydrothermal vents at the bottom of Europa's ocean, then that would seem a very hospitable place for life, but that doesn't mean its there. The only way we are going to find out is by going there. Other moons that may have liquid water deep below the surface include Jupiter's moons Callisto and Ganymede and perhaps Saturn's moons Titan and Enceladus.
How did life begin and evolve on Earth and has it evolved elsewhere?
Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea-floor hydrothermal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in our solar system.
Understanding the processes that lead to life, however, is complicated by the actions of biology itself. Earth's atmosphere today bears little resemblance to the atmosphere of the early Earth, in which life developed; it has been nearly reconstituted by the bacteria, vegetation, and other life forms that have acted upon it over the eons.
Fortunately, our solar system has preserved for us an array of natural laboratories in which we can study life's raw ingredients - volatiles and organics - as well as their delivery mechanisms and the prebiotic chemical processes that lead to life. Comets, for example, are believed to have delivered many of life ingredients to Earth after the planet cooled. The ones we see now in our night sky contain a record of the earliest days of our solar system, which makes them an important target for robotic explorers.
Mars and Venus -- now so different from Earth even though they appear to share a prime, hospitable location in our solar system -- also provide platforms to hunt for signs of life and clarify how planets evolve. Did life evolve on those worlds earliy in their devopment? If so, what happened?
In the outer solar system, the moons Europa, Ganymede and Callisto at Jupiter and Titan and Enceladus at Saturn all harbor some of the key ingredients for life and have been targeted for detailed study by robotic spacecraft.
We can also find on Earth direct evidence of the interactions of life with its environments, and the dramatic changes that life has undergone as the planet evolved. This can tell us much about the adaptability of life and the prospects that it might survive upheavals on other planets.