Mars has a space elevator. Earth doesn't.

Space elevators are fantastic pieces of technology that make the trip from surface to orbit incredibly cheap and easy. They are unfortunately also incredibly dangerous as Earth learned when a terrorist group managed to sever the Curacao elevator in 2591. The resulting cataclysm killed hundreds of millions outright as the ~50,000 km long structure fell to Earth wrapping around the globe nearly 2 full times. A global ban on these inherently vulnerable structures was put into place shortly thereafter making any travel out of Earth’s gravity well using conventional chemical rockets extremely expensive. The fledgling Martian colony was cut off from support from Earth but soon found that as the sole power in the solar system in possession of a space elevator it wielded immense economic power. Immigration and trade from Earth to Mars slowed to a trickle, however, exports from Mars to Earth remained economically viable.

Honestly, I don't see that you have a problem

How many people own yachts? And yet the cheapest form of transport on Earth is an ocean-going super freighter. So, from the perspective of people just jumping in their cars and moving, there's already the problem of the cost of the car spaceship, cost of fuel, and travel time. Your average Joe Schmoe could never afford the trip.

How to keep the wealthy off? That's easy, too. Mars property (somewhat like Hawaii) is 100% leased to corporate ventures under strict laws and oversight by a panel of United Earth nations who manage the resource. There isn't a square inch available for non-purpose habitation and the consequences are, well, let's call them breathtaking in their scope of civil punishment.

Note that you'll never have 100% compliance. You'll have the corporations creating resorts for their people, friends, and "associates." All well within the UE rules, of course. But that kind of nearly meaninless population isn't, I think, what you're talking about.

Finally, just in case you decide to make space travel cheap. If it is cheap, then so is the policing force needed to beat away the unwashed barbarians who don't want to live within spitting distance of their Terran neighbors.

No technobabble required. Earth has a deep gravity well and a huge, thick atmosphere. Mars has a shallow gravity well, and presumably even after terraforming still has a thin atmosphere.

Going from Mars to Earth is cheap (if a bit slow). A surface-mounted mass driver can throw packages of logs directly into an Earth-intercept trajectory. The thin atmosphere means you don't need much shielding on the way out (particularly if you're launching from the top of Olympus Mons), and Earth's thick atmosphere provides easy aerocapture and landing six months later.

Going from Earth to Mars is much harder. The accelerations involved in a mass-driver launch make it unsuitable for anything but durable freight; humans and fragile goods need to be launched on chemical rockets. Either way, you're spending far more energy fighting gravity and drag than you would leaving Mars. At the other end of the six-month trip (which itself would tend to discourage human travel), the thin Martian atmosphere makes aerocapture and parachute/glider landing tricky. You'll need to haul engines and fuel with you, which further drives up the cost of travel.

Your antigravity drives or whatever you are using work only when the body has a bulk density comparable to wood, about $0.8 g/cm^3$. Anything denser than that will have to follow the chemical rocket path, with its related costs, efficiencies and limitations.

Casually enough this will allow you to cheaply ship wood, but nothing more than that.

Material limits. While we can't currently build a space elevator on Mars we can do the next best thing--elevators on Deimos and Phobos. (See here) Now, we don't need much more strength to get the low end of the Phobos elevator into the range that aircraft can intercept it.

Presto, travel from Mars to Earth becomes cheap, the hardest part of the whole mission is protecting them from their trip through the fire coming down. However, the cables aren't strong enough to build an Earth-based elevator. Thus to go from Earth to Mars still has to be done the hard way on a tail of fire.

You're not going to get it down to loggers with hand tools as the launch from Mars still needs high performance aircraft--but that's still an awful lot cheaper than rockets.

Terraforming without lots of tech.

Later this year, astronomers discover a very large icy comet inbound from the Kuiper belt, destined to make impact with an Asteroid so that it scrubs most of the velocity relative to Mars. The remnants from this collision impact Mars 2 months later delivering many megatons of water, and other interesting volatiles such as carbon dioxide, carbon monoxide, and ammonia. Won't be pleasant on Mars for quite a while, but once things settle, you have the basic materials you need in place.

Taking advantage of this event, a seed ship is sent to Mars after the fact to seed the new atmosphere with various designed to transform the atmosphere into something suitable for forestry.

Still, Mars has trouble retaining an atmosphere, so NASA also prepares a big magnetic shield for the atmosphere to help prevent atmosphere from being stripped away by the solar wind.

Making it comparatively cheap to send wood back to Earth requires non-fanciful technologies. A space elevator would be much easier to construct for Mars than Earth because of its lower gravity. Still won't be cheap to build, but you don't have to start construction for a long-time still -- waiting for better materials, construction techniques, etc.

Significant improvements in orbital transfer (between Mars and Earth) are also needed. Careful use of light sails should make this reasonably priced in either direction as long as you are not in a hurry.

Dropping wood from orbit to the Earth is not that difficult if you are willing to use the wood itself as ablative shielding.

The wood itself is the fuel for the rockets.

In combination with jedmeyer's answer suggesting that the trees grow much larger than they would on Earth, perhaps there is a real chemical process or, failing that, something catalyzed by handwavium (mined only on Mars) that allows the wood itself to function as rocket fuel. Hollow out a large tree, fill it with oxidizer, and launch it. It could fire retrorockets on its approach to Earth and splash down in the ocean where it would be recovered by a fleet of logging ships.

(I know this answer is absurd, but it amuses me greatly.)

The power of Economics!

The solution you have here is that Mars has less gravity than Earth (3.7m/s^2 vs. (9.8 m/s^2). Because of this massive difference, the trees that grew up on Earth are able to grow MUCH taller on Mars, solving the scenario in two ways.

1. The trees grow faster and taller due to less gravity. Thus, a frontier-like world could develop shortly after being seeded and "terraformed".


2. Big Trees = More Wood. If you grow trees easier on Mars, Martian wood becomes more affordable: supply and demand. The shipping is just an overhead cost.

Where the scenario gets more challenging is the "downstream" metaphor. It would be possible if you consider aerobraking, since Mars has a less dense atmosphere than Earth. Launching from Mars requires a little bit less to land on Earth thanks to its thick atmosphere than the reverse path. However, that makes it unlikely that megaflora could flourish due to the insufficient pressure.

I recommend that you consider another cause for the unindustrialized world:

-There isn't an established mining base on Mars for enough metal for automated machinery, and no one wants to ship that out when you get enough wood from the locals!

-Overpopulation on Earth means more affordable labor for companies, and there's no room to plant large forests!

I like @Mark's mass driver for logs - bare logs flying thru space.

My idea for the cheap low tech terraforming - sometime in the mid 21st century the popular prophet inheriting the American government (and its nuclear stock) is told by God that nuking the Martian polar icecaps will bring great rewards to humanity. The Americans proceed to do exactly that, launching hundreds of ICBMs to detonate on Mars. The nukes are followed with arklike rockets of seeds and spores.

The scheme works like a charm, liberating immense stores of water and CO2 which cause a runaway greenhouse effect and melt the rest of the immense subsurface stores of these gases. Certain seeds among those sent up give rise to the forests which are thriving 1000 years later.

Mars has lower gravity than Earth, so it's already easier to achieve escape velocity.

However, with all of Mars as a giant forest, the atmosphere is now much more oxygen-rich than Earth. This means that producing liquid oxygen fuel is quicker and cheaper, and any craft with an air-intake can burn fuel much more efficiently than one on Earth.

In that circumstance, Earth really would be downstream. The freighter's easily lift into Mars orbit, burn the rest of their fuel to escape orbit at just the right spot...and free-fall into a precise Earth orbit where "local" spacecraft can retrieve them and bring them down.

You could even add a touch of danger to the retrieval: if the freight isn't caught and slowed on the first pass, it'll be going too fast, and will slingshot into empty space. If a corp misses a shipment at the prime capture point, nearby opportunists could turn a hefty profit by claiming the freight later in its trajectory--when it's much more dangerous to make the attempt.

Monsanto got chased off of Earth

Monsanto is one of the most hated big corporations on this planet presently. Not because their products are actually bad; genetically modified crops are safe and pretty much ubiquitous by now. But Monsanto's business practices has every ideologically green party and ditto non governmental organisation getting their knickers in a big itchy twist. And somewhat rightfully so because Monsanto is really pushing the limits when it comes to copyrights, trying to control what people can do with their products long after the product has been bought and used by the client.

So Monsanto — or its 3018 equivalent — have set up shop on Mars instead. And they are doing quite well for themselves! Their radiation hardened planktons and tree products have been instrumental in terraforming Mars without having to bother with restoring the planet's magnetic field. And not only that but the wood is amazing, being (insert qualities that are great for wood to have here). There is no-one that has those species of trees growing anywhere, thanks to Monsanto's quite excellent gene-hackers/engineers.

Of course there is the slight issue on how to protect such a valuable product and not have it spread like weed all over the solar system and neighbouring stars. So the genetic code is written such that it requires Mars conditions — the right gravity, the right radiation levels, the right atmosphere — or the seeds / seedlings simply will not catch. And Monsanto are annoyingly good at this sort of genetic copyright protection.

Too bad for them then, when an ideologically green terrorist organisation infiltrated the Ice Miner's Guild and in a brilliant move bot-net'd an entire flotilla of harvest drones to annihilate every Monsanto office on Mars by dropping asteroid ice bergs on them. (Side plot hook: what happens when you take a genetic engineering corporation, physically smash it to bits and let out all their lab samples and secret projects on a young booming forest planet)

So there you have it: a — now defunct — genetics engineering corporation created a marvellous but very Mars specific species of wood... and then they got wiped out. Now their products keep growing on Mars, and are ripe for harvest.

Terraforming Mars: solar-powered artificial magnetosphere, water from Ceres (Mars is downstream from Ceres and ceres lower gravity makes it easy to fly water to Mars), nitrogen from Titan's atmosphere to start the Nitrogen cycle. In the long term, some fertilizers in mineral form (and additional Carbon Dioxide) must be replenished in one way or another.

Environmental issues: too much damage to ecosystems on Earth is a good motive to turn Mars into a huge nature reserve with plants, wildlife and other organisms. Migration to Mars is restricted to park rangers, loggers and other related jobs, so that population is minimal. Some area is designated for logging.

Self-government: the new Martians take control of their business. Machinery is restricted to less sophisticated tools, but cargo spaceships are still indispensable. Park rangers may double as loggers in the spare time and further reduce the need for more workers. The "martians" may actually make lots of profits from managing this business, and controlling who & how others join is a way to keep much of the profit to themselves. They may windowdress their management decisions with environmentalist ideology.