What is the most effective way to build road on Mars and Moon?

Question

It is simply a matter of time before human colonized our natural satellite and the red planet, so how to lay roads that is similar to ours on lunar and Martian soil? (For now suppose we are planning to build mega cities.)

Answer 1

Roads on Mars and the Moon won't need to be built the same way that they are on Earth because gravity on those bodies is so much less.

Intuitively, the shear load when braking on a road is much smaller for a Mini Cooper compared to a fully loaded 18 wheel truck. The larger truck is pressing down on the road much harder than the small car, so the shear loads it exerts on the concrete or asphalt are much higher. So if we take the same two vehicles and put them on the moon, automatically, they will press down on the road much less. Lunar gravity is 1.622 $\text{m/s}^2$ and Mars 3.711 $\text{m/s}^2$.

Roads in Vacuum

Dirt roads may suffice for low traffic roads or for low weight vehicles. Thin concrete roads may be needed for high load or high traffic roads. Dust control may not be the prime issue for making roads but though roads would be a good approach for maintaining a pristine landscape. If the regolith particulate size is small enough then a heavily loaded truck may get stuck. A road will prevent that kind of sticking-in.

Roads in an artificial atmosphere

Regolith dust must be kept to a minimum in an artificial atmosphere under a bubble. Concrete dust is an inhalant hazard and very likely Lunar and Martian regolith has similar health hazards. Without roads, the humidity in the bubble's atmosphere will naturally keep the dust down to a certain extent though on heavily trafficked roads, this won't be enough. Because of the decreased gravity, the maximum size of dust particles that stay aloft for long periods will be larger than on Earth.

Road Laying Process

Asphalt and concrete as we know it on Earth won't work on Mars according to this forum thread. Basically, a lot of the ingredients that we are used to here on earth don't exist on Mars or the temperature ranges required for the terrestrial concrete curing process don't happen on Mars. A new process and new materials will need to be devised for road building on Mars.

Concrete on Luna should be possible though experimentation on actual lunar regolith will need to be done before large scale production can happen.

Answer 2

As mentioned the big problem with roads on the Moon and Mars would be dust. Lunar dust is especially abrasive, since it has never been weathered, so you can imagine each speck of dust as being a razor. Over time, driving through clouds of lunar dust would be very hard on machinery.

The other issue with road building is the foundation of the road. Looking at Roman roads or modern ones you see there isn't just a layer of paving over top of the soil, but the soil has been dug down and the paving sits on top of a complex layer of engineered materials to distribute the loads of moving vehicles and maintain stability. On Earth other considerations like drainage are also in play (and if you plan to terraform Mars some day you should keep this in mind as well).

Since we are thinking in a SFNal way, one possibility to quickly build roads on the Moon and Mars would be to focus concentrated heat on the surface and literally "melt" the roads into the surface. The roads will be a sort of lava trail with the basalt forming the foundation and providing stability, while the upper surface will be a smooth layer of glass. The issue here is that the actual material of the surface is not constant throughout the length of the road, so an engineering team would pretty much have to dig a ditch and fill it with the proper materials for the basalt road before the laser or giant orbiting mirror fused it into the road you want.

Frankly, since the environments are so different from Earth, you might want to think of different methods than roads. The Moon is in a hard vacuum, so magnetically levitated "trains" might make more sense. There is no air drag, so the trains can move at hundreds of kilometres per hour without penalty. Mars has an atmosphere, so perhaps a variation of Elon Musk's Hyperloop running between Martian cities would be more beneficial for the Martian colonists.