Unlikely that any physical resources such as ice, metals, minerals etc is worth extracting from Mars as we could get them far more easily from the asteroids, because of need to launch rockets from a planet with 40% of the gravity of the Earth.
So, in near future anyway, has to be things we discover that are less tangible, such as expansion of knowledge.
There are many things we can discover and are already discovering that excite planetary geologists. But there is one potentially huge discovery from Mars.
That's the discovery of living organisms with different biochemistry from Earth, past or present. And filling in the huge missing gap in our understanding of evolution of the tremendously complex earliest known cells (a million different chemicals including DNA, transcriptase, proteins, cell wall etc etc in a complex dance) from basic chemicals.
The understanding gained from that could make it one of the biggest discoveries in science in a century.
Reason for expecting that to happen is because Mars had an early ocean at the same time as Earth for a few hundred million years, increasing evidence that Mars was habitable like Earth in the past (see for instance Oregon geologist says Curiosity's images show Earth-like soils on Mars) - and several lines of evidence suggesting present day habitats on Mars also. All of that was discovered in the six years or so - the early oceans suspected before then but only recently proved really conclusively.
However as a planet in a "snowball phase" (even though it is not covered in ice - that's because it is so dry not because it is warm) - then it's going to be hard to spot present day life there - as hard as spotting it in the cold dry Atacama desert and McMurdo dry valleys where it hides beneath the surface of rocks and just below the surface of the soil, in very low concentrations and with lifetimes, sometimes, of a thousand years for an individual micro-organism living life in the ultra slow lane.
And for various reasons, past life on Mars would also be hard to spot - if close to the surface - then deteriorated rather rapidly in geological timescales by the cosmic radiation - and deeper very ancient deposits - then any organics again deteriorate over hundreds of millions and billions of years so that there is hardly anything left.
So we need very sensitive detectors of biosignatures to have a chance. Curiosity probably doesn't have much chance of actually discovering life even if it finds past or present day deposits with clear biosignatures in them - because it just doesn't have the tools to analyse them. ExoMars, due to be launched in 2018 will be the first - hopefully of many - missions to Mars able to spot past or present day life there. It's targeting past life - later missions hopefully will also target present day life (more challenging because tend to be places such as steep slopes, high latitudes, and spacecraft needs to be sterilized to far higher levels if any part of it could be in contact with a present day life habitat on Mars).
I think myself that there is an excellent chance that within say a couple of decades, that we would discover past or present day life on Mars. Could even happen this year if Curiosity is very lucky - but that's unlikely. Could take longer, several decades, if it is hard to spot. For present day life - not all potential habitats may be in inhabited in the harsh Mars conditions - and for past life - past life may not be as robust as Earth life and may have occurred only in special locations - and fossilization and preservation of organics is a chancy thing, on Mars as for Earth.
Unlike other answers here, I don't see Mars as of any value for preventing human extinction. Disasters that could make Earth as uninhabitable as Mars would need to be pretty extreme - and there is nothing at all likely to happen to make it worth going to Mars in less than 500 million to a billion years.
It may be a useful lifeboat for whatever creatures evolve on Earth half a billion years into the future - but that is not our concern - as that is so far into the future, it's enough time to evolve to humans from the first ever multi-cellular organisms in the Cambrian explosion. And if we do terraform it now, then it would almost certainly have unterrraformed again long before then. Probably within a few thousand, or at most a few million years. That's because there is a reason why Mars is not like Earth - it has no continental drift (which on Earth returns limestone to the atmosphere as CO2), no magnetic field (which helps Earth to keep its water which would otherwise get lost from the upper atmosphere dissociated into hydrogen and oxygen), and orbits too far from the sun (Earth would be a snowball planet if it was magically moved to Mars).
If we could somehow transfer all the Earth's ecosystems to Mars without making any other changes, and full atmosphere of Earth and oceans and everything - then within a short time geologically - it would revert to something similar to its present state (though whiter because of the extra ice - so also far colder than it is now).
Or might change to some other state - but one way or another - it couldn't continue to resemble Earth for any long period of time.
Short of fixing those differences in some way (we don't know how) - then any terraforming attempt obviously depends on either setting up a radically different set of planetary cycles (we don't know how to do that either) or else continuous mega-engineering into the future as long as we want it to stay habitable (and we find it hard enough currently to keep the Earth levels of CO2 adjusted at the 0.01% level and not sure of its effects either - so - (British understatement here) a bit away from planetary mega-engineering capabilities).
See also Trouble With Terraforming Mars
So, in near future anyway, has to be things we discover that are less tangible, such as expansion of knowledge.
There are many things we can discover and are already discovering that excite planetary geologists. But there is one potentially huge discovery from Mars.
That's the discovery of living organisms with different biochemistry from Earth, past or present. And filling in the huge missing gap in our understanding of evolution of the tremendously complex earliest known cells (a million different chemicals including DNA, transcriptase, proteins, cell wall etc etc in a complex dance) from basic chemicals.
The understanding gained from that could make it one of the biggest discoveries in science in a century.
Reason for expecting that to happen is because Mars had an early ocean at the same time as Earth for a few hundred million years, increasing evidence that Mars was habitable like Earth in the past (see for instance Oregon geologist says Curiosity's images show Earth-like soils on Mars) - and several lines of evidence suggesting present day habitats on Mars also. All of that was discovered in the six years or so - the early oceans suspected before then but only recently proved really conclusively.
However as a planet in a "snowball phase" (even though it is not covered in ice - that's because it is so dry not because it is warm) - then it's going to be hard to spot present day life there - as hard as spotting it in the cold dry Atacama desert and McMurdo dry valleys where it hides beneath the surface of rocks and just below the surface of the soil, in very low concentrations and with lifetimes, sometimes, of a thousand years for an individual micro-organism living life in the ultra slow lane.
And for various reasons, past life on Mars would also be hard to spot - if close to the surface - then deteriorated rather rapidly in geological timescales by the cosmic radiation - and deeper very ancient deposits - then any organics again deteriorate over hundreds of millions and billions of years so that there is hardly anything left.
So we need very sensitive detectors of biosignatures to have a chance. Curiosity probably doesn't have much chance of actually discovering life even if it finds past or present day deposits with clear biosignatures in them - because it just doesn't have the tools to analyse them. ExoMars, due to be launched in 2018 will be the first - hopefully of many - missions to Mars able to spot past or present day life there. It's targeting past life - later missions hopefully will also target present day life (more challenging because tend to be places such as steep slopes, high latitudes, and spacecraft needs to be sterilized to far higher levels if any part of it could be in contact with a present day life habitat on Mars).
I think myself that there is an excellent chance that within say a couple of decades, that we would discover past or present day life on Mars. Could even happen this year if Curiosity is very lucky - but that's unlikely. Could take longer, several decades, if it is hard to spot. For present day life - not all potential habitats may be in inhabited in the harsh Mars conditions - and for past life - past life may not be as robust as Earth life and may have occurred only in special locations - and fossilization and preservation of organics is a chancy thing, on Mars as for Earth.
Unlike other answers here, I don't see Mars as of any value for preventing human extinction. Disasters that could make Earth as uninhabitable as Mars would need to be pretty extreme - and there is nothing at all likely to happen to make it worth going to Mars in less than 500 million to a billion years.
It may be a useful lifeboat for whatever creatures evolve on Earth half a billion years into the future - but that is not our concern - as that is so far into the future, it's enough time to evolve to humans from the first ever multi-cellular organisms in the Cambrian explosion. And if we do terraform it now, then it would almost certainly have unterrraformed again long before then. Probably within a few thousand, or at most a few million years. That's because there is a reason why Mars is not like Earth - it has no continental drift (which on Earth returns limestone to the atmosphere as CO2), no magnetic field (which helps Earth to keep its water which would otherwise get lost from the upper atmosphere dissociated into hydrogen and oxygen), and orbits too far from the sun (Earth would be a snowball planet if it was magically moved to Mars).
If we could somehow transfer all the Earth's ecosystems to Mars without making any other changes, and full atmosphere of Earth and oceans and everything - then within a short time geologically - it would revert to something similar to its present state (though whiter because of the extra ice - so also far colder than it is now).
Or might change to some other state - but one way or another - it couldn't continue to resemble Earth for any long period of time.
Short of fixing those differences in some way (we don't know how) - then any terraforming attempt obviously depends on either setting up a radically different set of planetary cycles (we don't know how to do that either) or else continuous mega-engineering into the future as long as we want it to stay habitable (and we find it hard enough currently to keep the Earth levels of CO2 adjusted at the 0.01% level and not sure of its effects either - so - (British understatement here) a bit away from planetary mega-engineering capabilities).
See also Trouble With Terraforming Mars
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.4k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0