Elon Musk has said several times that he doesn't think there will be anything material from Mars that would be worth transporting back to Earth.

"I don't think it's going to be economical to mine things on Mars and then transport them back to Earth because the transport costs would overwhelm the value of whatever you mined, but there will likely be a lot of mining on Mars that's useful for a Mars base, but it's unlikely to be transferred back to Earth. I think the economic exchange between a Mars base and Earth would be mostly in the form of intellectual property"

Elon Musk interview on the future of energy and transport - and more quotes like this.

However he is skeptical about space mining generally thinking it probably won't be possible to export from the asteroids - "I'm not convinced there's a case for taking something, say, platinum, that is found in an asteroid and bringing it back to Earth." Of course many think that this will be possible. Myself I just don't know, I've heard the arguments on both sides and remain on the fence here.

But anyway, for Mars, let's look at this a bit more closely, is there anything physical that could be worth exporting, (apart from the science value for search for life and the information returned). I can't find much by way of papers on this. So, here are a few thoughts based on online discussions and just thinking it through:

• Samples of Mars dust and rock . The first samples could be worth billions of dollars per kilogram to start with, at least that's how much they plan to spend on sample return for the Curiosity's successor sample return

However, the price would go down quickly as we get more samples from Mars of the order of tons of material. You'd only return as much as was needed for the scientific research you need to do due to the high price of return of material from Mars, once it becomes something you can do routinely.

Also, individuals might also want to buy Mars rocks, but only for as long as they are rare. This would be like supporting a lunar mission by returning and selling Moon rock. The first few rocks could be valuable to collectors, and if they were issued with a certificate of authenticity as the first rocks to be returned from Mars or the Moon maybe the first few rocks would retain their value. But longer term, how many people would want to buy into something of continually reducing value?

• There are also the products of past life. On the surface - all of that has been pretty much completely destroyed by cosmic radiation. But - could there be deposits set down by ancient life below the surface, like our oil and gas and oil shale deposits.

You'd think they must be rare or we would have spotted them on the surface. There's no sign at all of outcrops of oil shale. But on the other hand - cosmic radiation is very damaging. Would there be anything left of a surface oil shale deposit after billions of years?

It's an exponential process so you get very rapid reductions. Every 650 million years you get a 1000 fold reduction in the concentrations of small organic molecules such as amino acids on the surface because of cosmic radiation. So that's a million fold reduction every 1.3 billion years.

Cosmic radiation has little effect over time periods of years, decades, centuries or millennia. But over time periods of hundreds of millions of years the effects are huge. After 1.3 billion years, a thousand tons of amino acids gets reduced to a kilogram, with the rest converted mainly to gases like carbon dioxide, water vapour, methane and ammonia. After 2.6 billion years it's down to a microgram (millionth of a gram) and after 3.9 billion years you are down to less than a picogram (a millionth of a microgram) of your original thousand tons deposit.

So, I don't think absence of these deposits on the surface, at least not easy to see from satellites, really shows that they don't exist below the surface. There could be millions of tons of organics from past life ten meters below the surface, and our rovers so far would probably not spot a thing. The organics of course also have to be there in the first place (surely likely to be patchy, in some places more than in others) and buried quickly - if it took several hundred million years to bury them, much of the organics would be gone also.

Oil itself is surely not worth the trouble of mining to return to Earth. But if there was some unique biological product on Mars that we don't have on Earth - which you could mine to find there, maybe that could be worth returning to Earth.

• Ordinary Earth life on Mars, e.g. vegetables, fruit, decorative flowers or whatever. Perhaps Mars could be a "garden planet" to export food to orbit and space colonies.

Mars would be competitive with Earth for export of food due to the much lower launch cost. But what about greenhouses in space.

This would require it to be much easier to build a greenhouse on the surface than in space, otherwise you'd grow them in space. Since it's a near vacuum and also with the large diurnal swings in temperature, I'm not sure that it has much by way of advantages over, say, Phobos or Deimos, or indeed the Moon which has much less delta v than Mars. Even for Mars orbit, it could be as economical or more so to export from the Moon. See section above: Greenhouse construction - comparison of the Moon and Mars

It could be economical to export from Mars to Mars orbit perhaps for food that can spoil quickly. Another thought, if the natural Mars gravity was an advantage though, and easier to use than artificial gravity, perhaps it could be worthwhile.

It could also be worth doing if conditions on Mars let you produce unusual food or decorative plants more easily. E.g. rare flowers that are very expensive to grow elsewhere, or unusual and rare newfood stuffs that grow best on Mars for some reason, perhaps genetically designed for Mars conditions. This is related to the next topic:

Continuing to

• Products of present day life. If Mars has interestingly different biology, maybe RNA based, maybe XNA, or not a DNA type chemical basis at all - you might find it worthwhile to grow Mars micro-organisms in Mars greenhouses or special habitats on Mars designed to make conditions conducive for them - to make products useful for Earth.
• Or genetically engineered biology that grows best in Mars conditions for some reason (actually responds well to the near vacuum, and extreme swings of temperature for instance)

Products you could export could include

• Medicines, if Mars life produces products of value for human health
• Spices and special foods - if the extraterrestrial biology is especially tasty and is safe to eat but can't be grown on Earth, e.g. safe once processed but not safe to return to Earth and grow here while alive.
• Chemicals, e.g. if Mars life consists of XNA and the XNA is valuable, you could make large quantities on Mars to export to Earth.
• Nano structures - makes products that are unusual and useful on the nanoscale.

For this to work there must be some reason they can't be grown on Earth

• Needs Mars conditions of near vacuum, huge temperature differences from day to night, and it's easier to grow on Mars than simulate Mars conditions on Earth
• Can't be grown on Earth at all for safety reasons - e.g. photosynthetic life that's more efficient than any Earth based photosynthetic life, or depends on symbiotic microbes that would be harmful to the environment of Earth if returned here, or is the product of such microbes,
• Can grow on Earth but easier to grow on Mars.

Especially if the life is so different that it's potentially hazardous to return the life itself to Earth - or if it depends on conditions that occur naturally on Mars or are easier to create on Mars than on Earth. This case might be another reason to be really careful not to contaminate Mars with Earth life, so that you can grow the native Mars life without interference from Earth life to make the unique products that result from Mars life.

Even if you can't grow the products safely on Earth, at some point you'd have the capability to grow them in Stanford Torus type habitats, biologically isolated from Earth and designed to mimic Mars conditions. Still, by the time that's feasible, export costs from Mars could go down as prices of such habitats go down, so keeping Mars competitive.

• Geological deposits. With the dry ice, low atmospheric pressure, cosmic radiation, things will be different from Earth in some respects. For one clear example, its salt deposits are made up of sulfates and perchlorates rather than chlorides as on Earth. Not that those are worth returning, but could it have other more valuable deposits that we don't have on Earth, or rarely so? Could it have unique rare gemstone?
• Opals from Mars. In 2013, the Mars Reconnaissance orbiter found evidence of large deposits of opals (hydrated silica) on Mars. Now most of that won't be gemstones, just deposits of silica modified by water. But could it have valuable gemstones there? Might the opals have markings unique to the way they formed on Mars? This orbital discovery was backed up by discovery of trace amounts of opal in a Mars meteorite in 2015.

Raw opal found in Andamooka South Australia - photo credit CR Peters

It's different from asteroids also - which don't have those ancient seas deposits you have on Mars or the climate.

But right now, I don’t think we have anything that would be worth returning from Mars except scientific understanding of course.

What about:

• Gold from Mars (or substitute platinum, or titanium, or whatever you think is especially valuable that you could find on Mars). For gold, perhaps the geological processes on Mars involving water in its past have concentrated deposits of precious metals just as they do on Earth? It's also surely had many iron / nickel asteroids hit the planet so may have deposits of platinum, gold etc for similar reasoning to Dennis Wingo's reasoning for the Moon. Indeed more so, because it is closer to the asteroid belt so gets hit by them more often.

Remember, that

• you have to do all the work to actually run the gold mine on Mars, and then send it into orbit.
• the price of gold is going to go down as it becomes available from space - or else the amount you can sell to Earth gets regulated to keep prices artificially high.
• if it is viable from Mars, it’s likely to be viable from other places too, particularly, robotic mining of asteroids may undercut you, and then you’d get less for the price of your gold than you spent out mining it, if robot mining of asteroids costs less.
• there may be much easier of access sources of platinum, gold etc from the Moon if Dennis Wingo is right. If you can get the transport costs from the Moon down to almost zero by using Hoyt's cislunar transport system or similar, it would be very hard to compete with that from Mars.
• if the mines are operated by humans on Mars, you have to pay for all the supplies to the miners on Mars which could amount to trillions of dollars a year, before you can turn a profit. For telerobotic or robotic mining you have to pay for the telerobot replacement, maintenance and repair and you also have to pay for all the equipment needed, drilling machinery etc.

So, in short, it has to be competitive with platinum, gold etc mined elsewhere in the solar system, and you have to bear in mind that either, the prices you can get from Earth will surely go down. On the other hand if the material you are mining is very valuable, and launch costs are low, perhaps the margin due to cost of export from Mars doesn't make such a big difference. E.g. suppose the launch costs a few hundred million dollars but you are returning tons of material, worth billions of dollars, perhaps it doesn't matter so much that a few percent of your product's price is due to transport. Maybe other elements of the price such as mining are somewhat less expensive than they are for asteroids?

However for this to work, there has to be a reason why other elements of the cost of mining are low. Asteroids and the Moon have the advantages of:

• Iron rich asteroids consist of pure metal, not oxidized.
• There may be easy robotic ways to extract it e.g. using gas carbonyls, no need for drilling as this turns the metal directly into gas
• Much lower delta v requirements than Mars for some of the NEOs, and in case of the Moon could even be zero delta v with Hoyt's cislunar tether transport system.

Seems unlikely that the thin Mars atmosphere would help. Would the Mars gravity help, or be a hindrance? And the large temperature swings from day to night, could they help in any way?

• Getting colonists to pay from their fee for the passage out to Mars.

If you get colonists who pay in advance for their flight out to Mars - and they use the Mars Colonial Transporter - a 100 people at a time, if SpaceX succeed in producing that spaceship - then the spacecraft has to come back to Earth after every run to transport colonists to Mars, and would be able to take exports with it, which is essentially free transport. So there would be a multiplier effect there of the original passage fee.

However unless the products are already worth returning for one of the other reasons, then at most they could get back their original passage fee by selling the material. Otherwise you'd have a case for sending empty colonial transporter ships to Mars just to return the products.

If this happened, you'd get exports, yes, for as long as the colony continues to expand rapidly. However, that's not a businss case, as it's not going to be sustainable, as a way of supporting a colony there. Even if they can get their money for the flight back from the goods returned from Mars, they then have to support themselves on Mars indefinitely, not just pay for the flight out. And with increasing numbers of colonists on Mars, you'd need exponentially increasing numbers of colonists going out there to support them with the passage fees. If you get increasing numbers of spaceships sent there to send them their supplies, again you need to pay for that somehow.

So, I don't think relying on the nearly empty transporter as it returns to Earth as a way to support the colony is likely to work long term. It works only as long as you have exponentially increasing numbers of colonists going to Mars and nobody coming back or few people coming back.

• Export of water from Deimos for use as fuel in LEO if there is water ice on Deimos.

This is the premise of the Deimos Water Company by David Kuck. Delta v back to Earth is much less and you can produce your own fuel for the journey.

This is a closely related question - looks into what goods might exist on Mars already worthwhile for Earth - where in my answer I suggest possibility of biologically produced materials on Mars from the past, deep millions of tons deposits - wouldn't expect any trace of them on the surface because of cosmic radiation.

Robert Walker's answer to Is Mars worth mining for Earth purposes?

For more background to this, see

• Why Elon Musk's Colony on Mars in 2020s is Unfeasible - What Could We Do - Really?
• Trouble With Terraforming Mars
• Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
• Would Microbes From This Astronaut Make It Impossible For Anyone To Terraform Mars - Ever?
• How Valuable is Pristine Mars for Humanity - Opinion Piece?
• Why Mars is NOT a Great Place to Live - Amazing to Explore From Orbit - with RC Rovers, and Nature Inspired Avatars

This is now one of the chapters of my Case For Moon First - an update of m previous answer here.