Because building a base on Mars is going to require quite a high amount of mass shipped to the planet's surface, much of which will be simple supplies and materials, like food, solar panels, methane generators, and so forth. These things aren't expensive on Earth - they're just very heavy.

And a base on Mars will not be a closed system. It will continuously draw in raw materials from the surrounding environment. Mars is enticing for this because carbon dioxide and water are abundant, giving you the three most important elements for fuel and food production.

Source? AFAIK NASA is performing successful experiments of closed isolated colonies. http://hi-seas.org/

Hi-Seas is not closed loop. You're thinking of something more like Biosphere 2[1] Which had it's flaws.

We've never done this on any significant scale with success.

[1]https://en.wikipedia.org/wiki/Biosphere_2

A Mars colony doesn't have to be closed loop either. If you're bringing a large power source, you can make oxygen and fix carbon from the martian atmosphere. This is already part of the plan for the purposes of making fuel and oxidizer for the return trip.

Think of it more like a nuclear submarine with a greenhouse full of potatoes attached -- we can most likely do that.

Nobody's going to try to launch an entire mangrove forest on the first go. Biosphere2 is a failure of experimental design, not a failed experiment.

>Think of it more like a nuclear submarine with a greenhouse full of potatoes attached -

Maybe, but if we want to make Mars economically self sustaining the correct analogy is nuclear submarine that never surfaces and can totally self repair, including building new nuclear reactors and eventually replace all parts in itself.

They are not experimenting with sustainable biosphere. They eat prepackaged foods and are not completely sealed from their environment. They just simulate trip to Mars psychologically and test foods.

There was Biosphere 2 experiment few decades ago, but they could not keep the biosphere stable.

I'm skeptical of how effective a simulation of a trip to Mars would be.

There's just a huge psychological difference between knowing that you're in space, millions of miles from Earth or any kind of help and knowing you're in a simulation on Earth.

I suppose it's possible to fool people that they're in the former while actually being in the latter. But the ethical issues surrounding such an experiment would make it out of bounds for NASA. It might make for a fun scifi story, though.

Hi-seas isn't self-sustaining in food, water, or atmosphere.