Some have proposed a worldhouse or para-terraforming. Richard L. S. Taylor outlined the technology some years ago.
There is a cheaper more achievable option. We could roof a smaller area at a hight of 15 to 100 meters relatively cheaply. A low world house or valley house is a normal feat of civil engineering on a par with normal urban or suburban civil-engineering. Like a modern cities roads, drainage or the reticulation of power, water, sewerage and communications. Roofing an area is a doable land development task, that would be operational at the private corporate level and not the government megaproject level. It is therefore more likely to happen.
Such a low roofed environment is also more easily explained to those that see the problems first and don’t grasp the opportunities.
The roof has vertical composite supports every few hundred meters and rafters that can support a light vehicle loaded with robots and replacement plastic. If the upper plastic is to be replaced. Cables laid from rafter to rafter allow the sheets to be drawn over the old sheet. Sealed in place and then the old sheet removed from below.
There would be several sheets bedded in the wall at two levels. The upper sheet is called the canopy. It is designed to take the damage of the wind, dust and UV. It is designed to be replaced regularly and is unpressurized. This upper layer is designed to support small robots and soft "snowshoed" astronauts. The robots clean and check the canopy. They may patch small holes and may even be able to strip off a few micrometers of dust or UV damaged plastic and lay down a new smooth layer in its place. The astronauts would be used for major repairs, emergency retrieval of a disabled robot, etc.
The lower sheet is the envelope. It is pressurised and ideally it should also be able to at least support the cleaning and patch robots. It is pushed up against the rafters by the internal atmospheric pressure and is locked in place against the rafters lower edge. Vibration, Curvature and pressure sensors are used to detect leaks and stretching.
A roof of less than 100 meters allows us another option. A fire engine like vehicle with a telescoping boom capable of reaching the envelope with a bed sized adhesive patch. This would patch a small hole in minutes. Additional poles could be assembled to lock the patch in place freeing the truck to race to the next hole if there is more than one. With a higher roof this becomes much more challenging; very fast airborne patches would be required. We are experienced with the use of emergency services and have learned to trust fire services. On the earth the danger of things like fire and automotive accident are handled each day without panic. The danger of a micrometeorite hits on Mars can be coped with and thus reduced in a similar way.
The buildings under the roof would be pressurised and alarms would get everyone in side in an emergency and seal the buildings. Airlocks added to such structures would allow trained emergency workers to pick up stragglers and support the patch teams. Both teams would be in protective suits. Livestock barns and other facilities such as farm pump houses would also be pressurisable to protect the farmers and fauna. Some species would be lost but not people or key live stock. A pressure breach would produce a rapid drop in pressure but the volume under a 60 meter roof is 0.06 cubic km or 60000000 cubic meters of air per square kilometer. It would take several dozen minutes to vent through a 10 cm hole.
A second expedient sealing technique would be set of hydrogen/ helium balloons tethered in each area. If a pressure breach occurs the resulting wind triggers the balloons release. It is drawn by the wind to the hole but can’t pass through. Adhesive patches activated by pressure and wind effects seal the balloon over the breach creating a temporary patch. Balloons stuck to the “sky” due to accidental activation will be a minor inconvenience.
· Top left on the hill. Communication towers. Local positioning systems (GPS without the satellites.)
· Left in the hill. Mines and storage tunnels; some pressurised. Back in second hill an early tunnelled complex. A base during the construction of the cities roof.
· Left foreground. A mars vehicle construction building. For trucks, mining vehicles and heavy equipment. This structure is earth covered for cosmic ray protection.
· Next door is laboratory complex that is vehicle access only. This provides quarantine protections allowing experiments you don’t what to do in the roofed area. I.e. Fruit fly and lab rat work, testing toxic ore samples, working with dangerous chemicals or making explosives for the miners.
· The larger structure on the right of this is a reactor complex if required or a large volume chemical processor and gas liquefier. Storage is underground out side the dome.
· Centre left foreground is a cluster of interconnecting structures that serve as the main base entry and exit point. It includes vehicle and robot garages. Light vehicle cleaning and maintenance. EVA training. General laboratories. Etc.
· The small tower outside on the right is a back up command centre and roof maintenance control.
· The smaller towers are just simple support structures with perhaps some lighting, emergency sirens and communication relays.
· The fat blue towers are accommodation and office space.
· The main mall is open air [no rain] near the three fat towers serves as the main recreational and commercial section o the city.
· There is extensive earth roofed housing near the mall.(dotted lines). The earth on the roof is used to block cosmic rays so that most indoor activities are radiation free. Plants can be grown on these rooves; this maximises growing space for plants even if it is only grass. All the larger buildings are similarly roofed, even the towers.
· The yellow area above the mall is incomplete housing.
· The rest is farming: crops, orchids, gardens, coppiced trees, and fish ponds.
The colonisation of Mars must ultimately be reduced to a simple problem of civil engineering. On achieving this the colony will quickly become self sufficient assuming each finds a product or service to sell to the others or to mother earth.
In this case the valley is deeper allowing a more natural sylvian [forested] landscape. The roof is supported on Fullarine cables anchored deep in the hills.
There is the option for a lake and recreations such as hang gliding. Birds and other fauna are possible. Repairs are by truck near the edges and airship inside the domes main valley over the lake. A surface repair car wide enough to spread its weight across three or more cables should be possible. This can support another vehicle via magnetic fields below the canopy. A magnetic repair vehicle slung under the canopy may also be an option if the cables are steel or have a magnetised track under them. A set of vacuum pumps, an inverted hovercraft skirt could create an inverted hovercraft.
If all else fails and a set of cables to three points just under the canopies edge can reach anywhere under the canopy defined by the three points. Absailing anchor rings added to the underside of the cables would be mandatory. Base jumping from the inside of the canopy would be fun. Insane but fun.