Providing construction material will be one way. Finding enough suitable material to construct human settlements, whether in space or on the surface of a moon or planet, is a difficult and awkward task. It would not be long before businesses are created that specialise in mining and distributing such material. The most successful will enjoy an ever-increasing demand for their materials as colonies expand and new colonies are started.
Another profitable service would be transportation. There will be a constant need to transport people and cargo of all kinds around the Solar-System. Businesses will be set up to provide reliable and regular transport services. Eventually huge spacecraft with the capability to move millions of tonnes of cargo, and hundreds of passengers, will be making journeys between the mining facilities and the colonies and outposts. Leaving Earth to work at one of the colonies for a few years, and then returning home or moving on to a different colony will become a relatively routine, if still lengthy, process. The large interplanetary ships will at least provide a high level of comfort and simulated gravity, which will be much healthier and safer than what we could provide travellers at the moment.
But there is one product that will create the most riches for the people that set up businesses to mine and deliver it. That product is ice, and especially water ice.
There is a relative abundance of water ice in the Solar-System. Even Mercury, the closest planet to the sun, has water ice preserved in craters that are in permanent shadow. There are many ice moons around Jupiter, Saturn, Uranus and Neptune, and Saturn's rings are 90 percent water ice. And then there are the Kuiper Belt objects beyond Neptune, which are mainly composed of ices, including water ice. And beyond that in the far reaches of the Solar-System, on the border with interstellar space, there is the Oort Cloud which is the source of many of the comets that periodically make their way into the inner Solar-System. It's likely to contain enough ice equivalent to several times the mass of Earth.
I expect the ice industry will be split into two: with one part specialising in ice mined on a planet or moon's surface and with it's customer base restricted to the body on which it was mined (due to the cost of transporting it out of the local gravity well), and the other part dealing with ice mined on asteroids, comets and other small bodies. Such ice will be easily transported to space-born colonies and orbital facilities, and to the small rocky worlds such as the inner Solar-System asteroids, and the moons of Mars: Phobos and Deimos. Those bodies will certainly have human activity on them as materials for construction are mined. Their demand for water ice will be high.
Surface-Bound Ice MiningIn the higher latitudes of Mars, close to the planet's north polar ice cap, lies the Korolev Crater: an almost 82 kilometre-wide impact crater filled with water ice. The base of the crater is more than two kilometres below the rim, creating a cold air trap that's allowed the crater to fill up with the ice to a depth of 1.8 kilometres.
The crater's location close to the polar ice cap, and just south of the expansive dune-filled region known as Olympia Undae, and it's abundance of water ice, makes it an ideal location for a large human colony, and the best example of a massive and conveniently located source of water. The area surrounding the crater will be relatively easy to traverse, making the construction of roads towards the north pole, and south towards the equatorial regions, straightforward. It is the most likely location for the first of the planet-bound ice mining businesses to be set up.
The person who will become the very first ice billionaire may well be living on the rim of the Korolev crater in several decades time just as material self-sufficiency from Earth is achieved. His or her vision and drive will enable the rapid expansion of the colonies on Mars, and become the inspiration for others elsewhere on the planet and far beyond.
The crater could well become the site of several sizeable towns, even cities, whose inhabitants are specialised in mining water ice. The towns would be located on the rim of the crater. Access in to the crater will be easy as the slopes are gentle and natural routes for roads would be easy to find.
The ice mining business's first contracts will be with Earth governments as they hand over the extraction of the ice to a private venture, but within decades, as the colonies themselves separate from direct Earth control and become truly independent, the contracts will be with Mars governments, and with other business sectors that have developed on the planet. By that time ice mining will have expanded to the north and south poles. It will be an efficient global industry that will allow millions of humans to live and thrive on the planet.
The same is likely to happen on other worlds that are suitable for human colonisation, with Saturn's moon Titan a prime example. There will be differences, of course. Mining anywhere on Titan, and the other ice moons will result in plenty of water ice being found. Many more smaller competing businesses are likely, which will ultimately merge to become a handful of large corporations.
Interplanetary Ice MiningSpace-bound colonies, either in orbit around planets, moons or the sun, will become a significant presence in the Solar-System once human colonisation away from Earth becomes established. They will be constructed from material mined from asteroids and most likely constructed in the asteroid belt (or from the Trojan asteroids that share Jupiter's orbit) and transported to their ultimate destination once complete. Such structures, each built to house thousands of people and to be as independent and self-sufficient as possible, will still need regular deliveries of water ice (and other volatiles). There will be no shortage of people ready to exploit that need, and the earning potential it represents.
Transporting water up from the surface of planets and the larger moons will be difficult and very expensive, due mainly to the effort and energy required to get millions of tonnes of ice out of such deep gravity wells. Such efforts would be foolish to attempt, and could never be considered as a viable business plan.
For such space-bound colonies the efficient solution is to mine the ice from small objects with a negligible gravity well, such as Kuiper Belt and Oort Cloud objects.
The objects in the Kuiper Belt, which lies just beyond the orbit of Neptune (and includes dwarf planets such as Pluto, Orcus and Eris), is the most conveniently located of the two. Once there is a human presence in that region mining operations with transport infrastructures will be set up to exploit the abundant ice riches that are available. There will be a regular fleet of cargo ships, largely crew-less, making their way inwards to the large space-bound colonies that are likely to exist around the major planets and moons. Their trajectories will be slow, but very efficient, and their regularity will ensure a constant supply of ice volatiles to a hungry and highly populated inner Solar-System.
Very large space-bound colonies have a distinct advantage over planet-bound colonies: those living there can live with Earth-level gravity, due to the ability to rotate the colony. With a large enough diameter any unpleasant coriolis effects can be eliminated (as the rotations per minute can be kept very low - less than one per minute if the colony's rotating section is 1,000 metres or more in diameter). It is likely that the ice billionaires would chose to live in such facilities due to the obvious space and luxury they could provide.
The motivation of the ice billionaires (and those in other industries) to expand their businesses further to increase their own wealth will be an important factor in the speed at which human colonisation spreads further and further away from Earth. Such activity needs to be encouraged. It is one of the ways to ensure that our species will endure if a catastrophe - either natural or of our own making - occurs on our home planet.
This kind of insurance against our extinction is essential. Governments on Earth are too slow, fickle and bureaucratic to provide that insurance any time soon. We need to embrace the commercialisation of space colonisation.
The future entrepreneurs that can exploit the business opportunities in the Solar-System are quite possibly alive as children today. They will one day leave Earth and create the most far-reaching corporations in human history. They will be motivated by profit, but inspired by the innate desire in all of us to survive as individuals and as a species.