> But here on Earth, liquid fuel ICBMs have largely been replaced with solid fuel missiles.
Intercontinental Ballistic Missiles are a very different technology than spaceships, even if they are superficially similar. For one thing, we don't have to worry about turning their rocket motor off and on frequently...they are kinda a "fire and forget" deal, so to speak.
Secondly, there is a reason solid propellants were always only ever used as boosters but not primary means of propulsion in spacecraft: They are, mass for mass, less efficient than liquid fuels. That's not a problem for an ICBM or other military applications: The payload is a lot lighter than a spacecraft, and the ability to store the rockets/ammunition for a long time without fuel degradation is a lot more important than energy efficiency.
> So maybe Mars spacecraft have two sets of engines -- liquid fuel for landing and solid fuel for return?
And what mass will these solid fuel rockets have? For comparisons sake, the SRBs of the space shuttle clock in at 590 metric tons...each. So if we wanna launch something that probably has to be a lot bigger than the spaceshuttle, we are talking thousands of tons of essential deadweight during launch on earth and transfer to mars and landing on mars. How much payload are current spacecraft designed to carry? I'm not sure, but I'd guess it's a lot less than that.