The main problem with creating imaginary worlds with alien life is that we have a grand total of one planet known to bear life. It is terribly hard to generalize from only one data point… and from a scientific perspective it’s a very bad idea. As a science fiction writer, you have much more freedom to take a possibility and run with it. While Mars may or may not contain living or formerly living microbes, its geological history can give useful insight into conditions on a planet other than Earth where life could possibly have evolved.
The Life and Death of Mars
Early Mars is believed to have had significant water for well over one billion years. It is perfectly possible that life could have evolved there. We won’t know for certain until a lot more experiments are done but it’s perfectly reasonable to use a Mars-like planet where life evolves as the setting for your science fiction.
The Death? of Mars and the Importance of Plate Tectonics
What happened to Mars is that plate tectonics stopped, reducing the cycling of carbon dioxide into the atmosphere. This caused the atmosphere to thin, Mars’ temperature to drop, and its remaining water to freeze, although new evidence (muddy splashes on the Phoenix lander) suggests some extremely briny water probably exists in certain areas for part of the year. There is speculation that the trace amounts of methane in the planet’s atmosphere may be caused by microbes living underground, but there certainly isn’t much in the way of an obvious biosphere.
The loss of internal heat and plate tectonics is something that is believed to happen more frequently with smaller planetary objects than with large ones. However, in the case of moons orbiting gas giants, the moons’ interiors may remain liquid and continue to produce magnetic fields and geological activity even on much smaller bodies than Mars. This can be clearly seen with the inner moons of Jupiter.
There may also be too much tectonic activity. This may be the case with Io, which is the most geologically active object in the solar system and keeps ejecting its volatiles into space.
Mars-like Planets as a Home for Intelligent Life
There is a problem with using a Mars-like planet as a home world for an alien race that evolved there: the time required to evolve an intelligent organism. It may be possible for an intelligent organism to evolve and create technology sufficient to leave the planet before a biosphere without plate tectonics becomes unable to support complex life, but we really don’t know enough to say. We need more data…
While it might be unlikely for intelligent life to evolve on a planet like Mars, such a planet would make a wonderful colony world for aliens that have managed to leave their home planets.
What this Means to the Writer
Mars-like planets with liquid water and brimming with life can be used by the science fiction writer and be perfectly accurate. As home worlds for space-faring alien races they should be used with caution with regard to their ability to support life for long periods of time. Since we only have one example of a Mars to study, you have plenty of room to maneuver within what is known.
More information on past and present Martian conditions can be found at Marstoday.com and Spaceref.com. Information on Io can be found at planetaryexploration.net