I have just been reading “Human missions to Mars: enabling technologies for the red planet” by Donald Rapp (Praxis publishing 2008). According to this book one major challenge facing a future manned mars landing is transportation of hydrogen to the surface of Mars for ISRU. But this book is 10 years old. Has any significant progress been made on this issue since then?
I believe the main issues are the amount of insulation that can be applied, its weight and its effectiveness at reducing boil off, especially on the surface of Mars v the possibilities for active refrigeration and the complexity and power consumption of such a refrigeration system.