system
Given the following post, respond as if you're commenting on Reddit:
human
Elon musk talks about using Water and Carbon Dioxide on mars to refuel rockets using a process called "Sabateir" why don't we do this on earth instead of using fossil fuels?

He mentions changing H2O & CO2 into CH4 & O2, which can then be used to refuel rockets. Why aren't we doing this on earth to save using fossil fuels?
gpt
Because the full process isn't particularly efficient. Note that the Sabatier reaction requires hydrogen (H2) as input alongside the CO2, not water (H2O).

To obtain hydrogen, one approach is electrolysis. This process, when applied to water yields hydrogen and oxygen. However, it's a rather inefficient process and would create even more need for power. In addition, water isn't as abundant on Mars as it is here on Earth.

A second approach is to simply ship in the hydrogen. One of the main issues with a manned mission to Mars is packing enough fuel for the return trip. The more you bring along, the more fuel you need to even get out of the Earth's atmosphere. So reducing the amount of mass that the rocket has to lift is very important. Using a Sabatier reaction to produce a methane/oxygen propellant while bringing in the hydrogen means that for every kg of hydrogen you bring along, you can produce 20 kg of propellant mix on Mars. And that greatly reduces the amount of mass you need to stuff onto your rocket for the return trip.

On Earth, creating fuels through processes like the Sabatier reaction will only become interesting once we have an abundant of clean, renewable energy to power these relatively inefficient production processes. Until then, we'd be creating more pollution by producing the fuels using dirty power than would be prevented.