[bonzini]

At least if you get to Antartica you don't need a pressurized suit and an airlock to go outside. It's difficult to imagine a less hospitable world than one without an atmosphere and a magnetic field (well, not talking about Venus-level inhospitable).

[Houshalter]

Also the biggest barrier to colonization is transportation. It's much easier to get to Antarctica than it is to Mars. Lots of humans have been to Antarctica. It's unlikely anyone will ever set foot on Mars for the foreseeable future.