Silica aerogel is a technology that's been proposed for colonisation of Mars. Basically, it's a very good insulator that's also transparent to visible light, and could be used to warm up parts of Mars to Earth-like temperatures through the greenhouse effect simply by placing it on the soil. Moreover, it would also block the more damaging wavelengths of UV. However, it wouldn't be a practical way of terraforming the entire planet.
Genetically modified microbes have been proposed for the terraforming of Mars. However, the current Martian environment is very harsh for life, being a cold near-vacuum with no liquid water and intense radiation (among other hazards). While microbes might be able to survive this environment, their growth and reproduction would probably be very slow.
I had the idea of combining these two approaches: using aerogel on the icecaps of Mars to create pockets of liquid water, then adding to the water microbes that are genetically modified to produce perfluorocarbons (PFCs). The PFCs would warm up Mars due to being very strong greenhouse gases. The benefit of using the aerogel is that the microbes wouldn't need to withstand the harsh conditions of Mars directly, and this is achieved with a relatively simple technology that has no moving parts.
Once the entire planet is warmed to Earth-like temperatures, the atmospheric pressure would also increase somewhat (due to sublimation from the icecaps and release of gases adsorbed in soil), the thicker atmosphere would reduce the radiation levels reaching the surface, and a water cycle would now be possible. Even if the atmosphere were to remain thinner than Earth's, Mars would still be much more hospitable than before, assisting colonisation and further terraforming efforts (which could very well involve more GM microbes).
What are the challenges with this approach? The ones I can think of are:
- Whether or not microbes could be genetically modified to produce PFCs. For the purposes of this question, assume that it is possible.
- The PFC-producing microbes undergoing unexpected mutations that cause them to either die off or produce unwanted chemicals. To minimise the chances of this happening, extensive testing would need to be done first (culturing the microbes in samples of melted Martian ice, for example).
- The PFC-producing microbes producing too much PFCs, warming up Mars to an uninhabitable extent. This could be dealt with by careful monitoring and (when the time is right) introducing other microbes to the water, where they would outcompete the PFC-producers.