#Frozen Methane

You can mine it from Earth's oceans (in the form of methane clathrate) and transport them to Mars etc. Or you can simply collect frozen/molten methane from the outer reaches of our solar system (or other solar systems, beyond the freeze line).

It is also highly likely that 500 years in the future, we would have devised a method to produce methane from an endothermic reaction between carbon and hydrogen. The reaction can be carried out quite slowly with current technology. With passing time, it would get easier and easier to speed the process up.

In this case, you would have huge factories running on nuclear power, producing and freezing methane, which is then sold on their chilled outlets, where chunks would be directly inserted into the vehicles' fuel tank, where parts of them would be gradually sublimated into gas form and burned as fuel.

#Frozen Acetylene

Acetylene ($C_2H_2$) has an even higher energy yield than methane. Freezing acetylene and then using it, instead of methane, would be even better and energy-efficient choice. However, production of large quantities of acetylene is difficult unless exhaustible supplies (like Calcium Carbide) are used.

Frozen Methane

You can mine it from Earth's oceans (in the form of methane clathrate) and transport them to Mars etc. Or you can simply collect frozen/molten methane from the outer reaches of our solar system (or other solar systems, beyond the freeze line).

It is also highly likely that 500 years in the future, we would have devised a method to produce methane from an endothermic reaction between carbon and hydrogen. The reaction can be carried out quite slowly with current technology. With passing time, it would get easier and easier to speed the process up.

In this case, you would have huge factories running on nuclear power, producing and freezing methane, which is then sold on their chilled outlets, where chunks would be directly inserted into the vehicles' fuel tank, where parts of them would be gradually sublimated into gas form and burned as fuel.

Frozen Acetylene

Acetylene ($C_2H_2$) has an even higher energy yield than methane. Freezing acetylene and then using it, instead of methane, would be even better and energy-efficient choice. However, production of large quantities of acetylene is difficult unless exhaustible supplies (like Calcium Carbide) are used.