The special material is
Aluminum and Composite Fibre Reinforced Plastic - or,
Carbon Fiber Reinforced Polymer - CFPR
CFPR
- has a very high strength for a given weight
- reflects well heat
- does not rust
- durable
It may not be the best. But it is practically the most fitting substance that can be used to optimized on multiple criteria.
The material has to be very strong to be able to withstand the vibrations and pressure/stress due to atmosphere and high velocity of the rocket through the atmosphere.
The material has to be as light as possible, as heavier spacecraft is more difficult to carry and needs more energy to maintain in space.
The material needs to withstand high temperatures as Sunlight falls on it directly. Also, the radiation present in that location should be tolerable by the material. The material should be able to manage the dust and the atmospheric conditions around Mars in its orbit.
