- One rationale for Mars is In-situ Resource Utilization (ISRU). The particular opportunity for utilization that’s most commonly cited is to collect carbon dioxide from the Martian atmosphere. Carbon dioxide can be split to obtain breathable oxygen, though it’s not the most limiting consumable. The element that atmosphere processors on the ISS most routinely need to have replenished from an outside source is hydrogen. In combination with hydrogen, carbon dioxide can also be made into methane rocket fuel. It might be said that the best use for Mars is making fuel to leave Mars. That, in and of itself, is not a compelling reason at all to start a colony.
It becomes even worse when recognizing that there are far better places from which to obtain methane. The most common type of asteroid in the solar system is the carbonaceous chondrite, so named because it’s full of carbon. Significant amounts of carbon dioxide and methane can be present on these bodies, along with other useful volatiles like water and ammonia. While it’s technically feasible to export methane from Mars, it would be like exporting water from the desert.
Mars is solid ground that is (relatively) easy to get to without too bad of a gravity well to get back into space from. Mars has water, gypsum, carbon, phosphorus. Mars has just enough air to block a good portion of the bad cosmic radiation without needing 3 foot thick stone shelters. Mars is really the only place out there that we know of that offers the hope of Humanity being a multi-planet species. Mars is not all that great, really, but it is the best we have.
An Earth/Mars economy will have nothing to do with the settlement or development of Mars. Of course, settling Mars will have benefits for the Earth economy as new technologies are developed, but a Mars/Earth economy isn't going to happen anytime soon, and it shouldn't be a requisite for going. Why would you want to export methane? Mars has resources and gravity that will be of use to those living there. That's the point.
I honestly don't understand why Mars and not the Moon. They both have the same problems: weak gravity, no atmosphere worth speaking of, and limited life-sustaining resources. But the Moon is close. It's a logical step. Something goes wrong, you jettison, and in a couple of days you are orbiting Earth. On Mars, you die. (Unless you are Mark Watney, of course.) Our isolated habitats on Earth have all failed for one reason or another, and the author makes a good point that maybe we should have completely isolated and self-sufficient outposts on Antarctica or in the ocean first. Maybe then transfer those working ideas to the Moon, with the plan to make a much easier trip from the Moon to Mars, rather than trying to carry everything off Earth in one big, fat ship. Sadly the Trumplicans will defund all of these projects and put us into a technological dark age for the next decade-plus, so none of this will happen in my lifetime. Damnit.
And yet, before that, he stated that NASA's budget was oversized and largely wasted, and he plans to cut all Earth Science programs, and he's super-excited to outsource more work to the likes of SpaceX... so that he can cut the budget further. He has talked positively about space exploration, yes, but at every turn he also speaks of cutting NASA's excessive budget and outsourcing the majority of the work to contractors. So while he says "Yay Space!", he also says, "Boo NASA budget." Rubio and Cruz are working on legislation to undermine Trump's efforts to defund NASA with their NASA Transition Authorization Act of 2016, to protect particular high-profile projects within their states, but ... as with everything Trump, it can change 180 degrees between dinner and breakfast, if he starts tweeting out bullshit about Boeing again, for example...