Warming Mars as a first step in Terraforming the Red Planet.

Warming Mars as a first step in terraforming. From the writer of The Terraformers’ Toolkit. (The Terraformers’ Toolkit: Everything you have ever wanted to know about terraforming and our future in space. The where, why, when, how and what of terraforming.) www.amazon.co.uk/dp/B09X6739P...
Temperatures
Mars is cold, very cold with temperatures which vary from -133 C (-207 F) at the poles in winter to as high as 27 C (80 F) at the equator in summer. The planet’s average temperature is -63 C (-81 F), and because it orbit further from the sun than Earth it receives just 43% of the earth’s sunlight.
Mars has a carbon dioxide atmosphere with just 0.006 bars pressure, just a tiny fraction of earth’s. It has a good deal of water ice and frozen carbon dioxide on its surface and in its polar ice caps. Both of these could serve as greenhouse gases if they were heated enough to sublimate into the atmosphere. A temperature rise of just 8 degrees might be enough to do this.
There are three potential ways Mars might be warmed at the start of a terraforming process:
Albedo
Mars polar ice caps are very bright and reflect most of the sunlight which hits them. Therefore they are said to have high albedo or reflectiveness. Changing the surface of the ice from white to black would allow more of the sun’s heat to be retained and begin to melt the ice. This could be done by impacting carbonaceous asteroids into the poles. The impact energy of these asteroids could cause more melting, increase atmospheric pressure by allowing frozen carbon dioxide to enter the atmosphere and raise temperatures even further. The downside is that Mars normal seasonal temperature swings result in candidate falling onto the poles as snow, so the warming would have to be sufficient to stop that happening. Asteroids could also add more carbon dioxide to the atmosphere if they impacted carbonate rocks. However, so far these have been hard to find.
Space mirrors
Giant mirrors in space, many miles across, have been suggested as a way to increase the amount of sunlight reaching Mars.
The mirror could be placed in a polar orbit around Mars so that it was always in the light and facing the sun. This should reflect over 70% of the sunlight it receives onto Mars surface. However, the mirrors might need to be constantly adjusted to maximise their effectiveness.
The mirror could also be placed at a stable Lagrange point on the far side of Mars where it should require less adjustment to keep it in place. However, it would have to be very large to extend beyond Mars shadow.
Currently we have little idea about how we would build such a structure. It might be possible in our far future to convert a metal rich asteroid or one of Mars small moons into a mirror, but this is just conjecture at present.
Greenhouse gases
Artificially produced greenhouse gases are far more effective than carbon dioxide at producing warming. They are transparent to incoming sunlight, but serve as a thermal blanket stopping some of the sun’s heat radiating back into space.
Although they would initially have to be produced chemically, later genetically engineered microorganisms could produce these gases, protected from low temperatures by a green house, a layer of plastic on the surface or a sprayed aerogel layer on the ground.
CFCs chlorofluorocarbons have been suggested as appropriate and effective greenhouse gases. They are 10,000 times more effective at warming than carbon dioxide, but they destroy ozone which protects the surface, and people, from dangerous radiation.
Fortunately there is another group of greenhouse gases called PFCs perflorocarbons which don’t contain ozone damaging chlorine, instead they use Florine which has been found on Mars. PFCs have very long lifetimes, as long as 50,000 years, which means that they would continue to help warm Mars for thousands of years. They are also more resistant to being broken down by ultra violet radiation than CFCs.
[Examples of appropriate PFCs are CF4, C2F6, C3F8 and SF6).]
Life
Even if all the frozen carbon dioxide on Mars surface were added to the atmosphere, this would probably only build an atmosphere with 0.15 bars surface pressure, about one seventh of earths pressure at sea level. This is not enough to build an atmosphere to allow plants to grow. Therefore more oxygen and nitrogen will be needed to be imported to make Mars ready for life.
Other approaches
Nuclear blasts have been suggested as a way to melt the polar ice caps and warm Mars. However, this might require as many as 10 million blasts and would contaminate the surface with radioactive elements. Additionally space law prohibits the use of nuclear weapons in space.
Summary
It will probably be a combination of different approaches that will be needed to produce sufficient warming to make Mars life ready. But the costs and technical complexity mean it is likely to take a long time to complete.