Earth's atmosphere contains oxygen, which can be used as a rocket
propellant, burning with kerosene or hydrogen. Mars' atmosphere on
the contrary contains only carbon dioxide (CO2), that
will not burn with kerosene or hydrogen.
But, metals can burn with CO2, like magnesium does in
So, a spaceship landing on Mars, could in principle fill a reservoir
with liquid CO2 harvested from the atmosphere then use
that CO2 to feed a hybrid rocket motor burning metal. Or
use a second reservoir containing molten metal and feed a liquid
rocket motor. Or freeze the CO2 and mix it with powdered
metal. A hybrid motor could be the simplest way to constitute the
first stage of a rocket to get back into Mars orbit.
The reaction products of magnesium and CO2 have a very
high temperature and become a powder at lower temperatures, which is
both destructive and useless in a rocket motor. To cope with this an
excess of CO2 has to be used, to lower to temperature and
maintain a flow in the nozzle.
Beryllium oxide and lithium oxide having about the same formation
enthalpy as magnesium oxide, beryllium and lithium should burn too.
I found no data to confirm this. Having a much lower atomic weight
than magnesium they would be more appropriate for a rocket motor.
A solid rocket motor that uses frozen water and aluminum powder has
Another way to use CO2 as propellant is to rely on the
ease to liquefy it and get it back to expand, like in CO2airsoft
guns. The idea here would for example be a drone that makes
hops and short flights on Mars. It would use the energy from solar
panels to slowly fill a reservoir with liquid CO2, either
by pressurizing CO2 pumped from the atmosphere or by
cooling the reservoir and let the CO2 from the atmosphere
just enter the reservoir and liquefy. One way to help refrigerate
could be a thermal panel that is directed towards the sky at night,
to radiate heath away. Once the reservoir is filled, it can be
heated up to increase the pressure. The CO2 can then be
ducted towards nozzles to lift the drone. If the drone has a
powerful battery, it can contribute to heat and expand the CO2
further inside the nozzles and control the flight. The flight would
last only a few minutes but that would be enough to explore a site
or travel some distance over rough terrain. Once the drone lands
back, it blows the dust away from the solar panels and starts again
replenishing its CO2 reservoir and loading its battery.
Apart from a few valves, this system can be a solid state device;
requiring no moving parts.
The specific impulse of a steam rocket
is quite low but still enough to constitute a first stage for a
rocket that can rise from the Martian surface towards orbit. If CO2
can replace water effectively, that first stage would have the big
advantage of being many times reusable.