How about plants ?
Grow them in Martian soil. One by product is vegetation that might be useful as food.
Researchers have figured out that direct photochemical reactions could provide the oxygen and hydrogen humans need for long-term habitation of Mars or the Moon. The International Space Station (ISS) relies on photovoltaic-driven water electrolyzers to get oxygen from water. But the two-stage process – converting sunlight into …
There is no "martian soil".
Soil is the product of biological processes, decomposing organic matter to enrich the surrounding substrate with usable nitrogen, trace elements and organic compounds.
The martian surface is covered in regolith. Dead, sterile, irradiated, dessicated, pulverized silica compounds, contaminated with toxic chemicals like perchlorate salts.
That's a lot of plants (a quick google said about 800 per person) and they take a long time to grow too.
More practically, if they swapped all the ISS astronauts out for Sherpas or other people who can live and operate at 18,000ft - where the Oxygen is only 50% of the level it is at sea-level - they could halve their energy budget. Apparently the ISS is kept at 21%, 1013hpa [edit: wikiperida tells me it's because they're worried some of the equipment would suffer at lower pressures]
Maybe -- however the Earth's magnetic field swaps poles every half million years or so and one imagines that there is probably a point in those transitions where the magnetic field is pretty weak for a while. If radiation is that much of a problem, wouldn't we expect some extinctions every time the magnetic pole swaps? Doesn't seem to happen. So MAYBE the radiation thing is less of a problem than it seems. Now the fact that Mars is cold -- mean temperature -65C apparently. Makes Winnipeg or Novosibirsk look balmy. That might be a bit of a problem.
> If radiation is that much of a problem, wouldn't we expect some extinctions every time the magnetic pole swaps?
We would, and we know that's what happend: https://en.wikipedia.org/wiki/Laschamp_event
> So MAYBE the radiation thing is less of a problem than it seems.
It's a big problem: https://en.wikipedia.org/wiki/Mars_Radiation_Environment_Experiment
A human on earth is exposed to ~0.62 rads per year. On Mars, that's 8 rads, or 80 milliievert (mSv). The occupational dose limit is 50mSv per anno. And that's just normal cosmic radiation on Mars. During solar flares, which occured twice in the 18 months of MARIE, humans on mars would be showered with up to 20mSv in a single day.
Oh, and of course, this also means that basically the entire surface of Mars has been irradiated for hundreds of millions of years, so every usage of local resources for...basically anything, has to take that into account as well.
But yes, given the other problems, the deadly radiation is just one more cherry on top of that frozen hell-cake of a planet.
Here we go with the irrational fear of radiation!
The 50mSv limit is to prevent your odds of getting cancer from increasing by like 3.5%. If we set a limit of no greater than 8% you'd be allowed a dose of like 2,500mSv. If you're willing to accept a higher cancer risk your allowed exposure goes up exponentially. (The whole inverse square thing nature does so often.)
Also unless we choose a stupid transfer window flight time to Mars should be more like 3 months. Of course once you are there you're stuck for 2 years till the window opens again. Actually there is another transfer window which involves Venus that takes around 9 months if memory serves me? I really could only see us using that in some type of emergency however.
> If you're willing to accept a higher cancer risk your allowed exposure goes up exponentially.
Well, I'm not willing to accept that risk, because cancer sucks. That's why we have these limits and why observing them makes a lot of sense.
And we're not talking about a bit of an overexposure here, we are talking about a permanent overexposure for everyone who goes there, for the rest of their, likely severely limited, lifespan, because there is no return ticket.
And ofc. radiation doesn't "just" increase the risk for cancer. It also influences the bone marrow, wound healing ability, aging, damages the meiotic cells (birth defects ain't pretty)...shall I go on?
> Also unless we choose a stupid transfer window flight time to Mars should be more like 3 months.
Yeah, using what propulsion technology exactly? For reference, here are some of the known travel times to Mars, in days. Bear in mind, these are all unmanned craft, that don't have to care about the fragility of the human physiology:
Mars Science Laboratory, 254
Mars Reconnaissance Orbiter, 210
Mars Express Orbiter, 201
Mars Pathfinder, 212
Mars Global Surveyor, 308
Viking 2, 333
Viking 1, 304
Mariner 9, 168
Mariner 7, 128
Mariner 6, 155
Mariner 4, 228
So either we are really bad at picking good transfer windows, or a 90day trip to Mars is a lot harder than it seems. And so far, I haven't seen any futuristic propulsion Technology actually working on a real mission with real astronauts, and until that changes, I think we can quite comfortably state that a trip to Mars will take the better part of a year to complete.
> Of course once you are there you're stuck for 2 years till the window opens again
No, I'd be stuck for the rest of my life, because there is no return from Mars.
Anyone who want's to prove otherwise is welcome to try the following experiment: Launch a spacecraft purely under it's own power, meaning no boosters, into low earth orbit...from a field of sand and rocks. The only support structures allowed, are those that can be built using nothing but local materials, AFTER the spacecraft has been put there. All machinery and support materials and manpower used to build them, must fit entirely into the spacecraft that is to be launched. Contact with a control tower is allowed, but only with a lag of 6 minutes.
Sure, the gravity is higher, but then again, I'm asking only for low earth orbit, not for the spacecraft being able to, after it launched, to transfer to another planet. Oh, and since I am feeling especially generous, I am also not asking for the fuel to be produced locally ;-)
"I'd be stuck for the rest of my life, because there is no return from Mars."
Mars gravity is just over twice that of the moon, and escape velocity is directly proportional. So it's absolutely not inconceivable to have a lander + orbiter setup just as the moon landings had, you need twice+ as much power on the lander rockets. Of course that setup means that you have to take the takeoff fuel with you, and you got to land on a nice flat and stable space from which you can take off again.
In any case, return from Mars is a pretty moot point since we're still very far from getting there in the first place, in part because of all the other issues you correctly list with radiation and travel time.
> just as the moon landings had,
The lunar lander had the luxury of not having to transfer back to another planet. All it had to do was getting 2 of the 3 astronauts back to the CSM which remained in orbit. It also didn't have to carry enough food, water, materials and machinery to support the crew for over 2 years until the next launch window. It was also small enough to bring it's takeoff support structure with it.
> you have to take the takeoff fuel with you
Considering that the current proposals for mars missions need to refuel the craft in LEO just to get it to Mars, I don't see that happening. Also, the fuel we are talking about is liquid oxygen and methane. Which would need to be kept cool and under pressure, especially considering the low density of the martian atmosphere. For 2 years. Question: Which spacecraft would be capable ON EARTH to sit for 2 years in it's launch cradle, with fuel in the tanks, and still be able to take off? 2 Years in which the spacecraft sits on mars, experiencing planet wide duststorms, and an average temperature of -60°C. Any technical problems have to be fixed with what the colonists brought with them. Oh, and the spacecraft will not be freshly built of course...it will already have experienced the launch from earth, the transit flight, and the landing.
And then the spacecraft would have to launch from a base of lose rocks and regolith, completely under it's own power, with no launchpad or support structures other than those the colonists could build (and position the craft on) themselves, and no guidance from external control systems.
"Question: Which spacecraft would be capable ON EARTH to sit for 2 years in it's launch cradle, with fuel in the tanks, and still be able to take off?"
Not saying you're wrong. But here on Earth, liquid fuel ICBMs have largely been replaced with solid fuel missiles. For exactly the reasons you cite I believe.
So maybe Mars spacecraft have two sets of engines -- liquid fuel for landing and solid fuel for return?
> But here on Earth, liquid fuel ICBMs have largely been replaced with solid fuel missiles.
Intercontinental Ballistic Missiles are a very different technology than spaceships, even if they are superficially similar. For one thing, we don't have to worry about turning their rocket motor off and on frequently...they are kinda a "fire and forget" deal, so to speak.
Secondly, there is a reason solid propellants were always only ever used as boosters but not primary means of propulsion in spacecraft: They are, mass for mass, less efficient than liquid fuels. That's not a problem for an ICBM or other military applications: The payload is a lot lighter than a spacecraft, and the ability to store the rockets/ammunition for a long time without fuel degradation is a lot more important than energy efficiency.
> So maybe Mars spacecraft have two sets of engines -- liquid fuel for landing and solid fuel for return?
And what mass will these solid fuel rockets have? For comparisons sake, the SRBs of the space shuttle clock in at 590 metric tons...each. So if we wanna launch something that probably has to be a lot bigger than the spaceshuttle, we are talking thousands of tons of essential deadweight during launch on earth and transfer to mars and landing on mars. How much payload are current spacecraft designed to carry? I'm not sure, but I'd guess it's a lot less than that.
"The lunar lander had the luxury of not having to transfer back to another planet. All it had to do was getting 2 of the 3 astronauts back to the CSM which remained in orbit. "
As I mentioned in the original comment, Mars mission could easily adopt a similair pattern with a lander that just needs enough fuel to cushion the landing and then get back into orbit, and a separate orbiter. Of course, the orbiter in this case would still need enough fuel for both the trip to Mars and the return, but I don't see why that amount would be so much more to the fuel requirement to get to the moon and back because you only need it for acceleration / deceleration. Minimum distance to Mars is about 160X that of moon, trip to the moon takes <3 days, similair-speed trip to Mars would take approx a year (yes I'm surely over-simplifying, but my point is that fuel requirements aren't THAT high if one is ready to arrive a bit more slowly)
"For 2 years"
Where is this value is coming from? A first mission is likely to be much shorter, maybe less than 2 weeks on the surface (longest moon landing was just under 13 days).
Also on further thought, maybe we are talking at cross-purposes? If your "not returning from Mars" refers to a colony-type mission, you're probably right, but then again it would be crewed by people who knew they were leaving Earth forever. I am referring to teh possibility of return from a brief mission, similair to Apollo missions but scaled up about 2-3X in terms of craft power and fuel requirements (although probably 100X food and water requirements, which is maybe why missions are proposing LEO resupply before trip to Mars)
> Mars mission could easily adopt a similair pattern with a lander that just needs enough fuel to cushion the landing and then get back into orbit, and a separate orbiter.
Yes it could, but all that eats into the precious payload capacity of the craft, which isn't that big to begin with, considering what it has to transport.
And at some point we really have to ask the question why, given all these hurdles and the fact that there is NOTHING of interest for humans on the red plant, we should bother jumping through all these hoops instead of doing what we have already been doing successfully, aka. sending better and better robots instead of people.
> fuel requirements aren't THAT high if one is ready to arrive a bit more slowly)
a) even if it were that simple, that longer travel time immediately translates to more required payload capacity, just to support the crew on the trip. I eat double the amount in 12 months than I do in 6 months, it's as simple as that.
b) It's a bit more complicated than that, because you need a certain thrust to make the translation into the orbital approach to mars. You can't just "go slower, use less energy", it doesn't work like that. If you burn less energy, you have to burn it for longer. The total energy required to get from one orbital (earth) to another (mars), doesn't change just because I approach it more slowly.
> Where is this value is coming from
Basic orbital mechanics. We cannot just launch spacecraft to other planets whenever we feel like it, our propulsion technology is far too weak for that. For the trip to mars, we have to wait for earth "coming up from behind" so to speak, to use the momentum given by earths orbit. A similar effect governs any return trip. The relative orbital positions of mars and earth repeat roughly every 2 years.
I completely agree that there's no point in boots on Mars before we have a much better handle on the technology, which we can develop further on Earth , LEO, or boots on the moon. Wanting to go is just dick-swinging. I was simply thinking of how possible a return trip was, not whether it was desirable.
"The relative orbital positions of mars and earth repeat roughly every 2 years."
If that's the case, that clinches the argument, and that's my new thing learnt for the day!
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"In any case, return from Mars is a pretty moot point since we're still very far from getting there in the first place, in part because of all the other issues you correctly list with radiation and travel time."
I'm an optimist. So I think our great,great, ever so great grandchildren MIGHT eventually terraform Mars and settle there. But if it happens at all, it will be centuries from now, not decades. And it'll involve technologies far beyond our current capabilities.
"Mars cannot be terraformed ..."
A not unreasonable conjecture. But probably wrong. For one thing, we're pretty sure that Mars retained a lot of its original atmosphere for hundreds of millions of years. No reason that a manmade atmosphere wouldn't last as long. Longer than mankind most likely.
The most probable decay model for Mars atmosphere looks to be exponential decay. If we assume an Earthlike atmosphere (or as close as we can come given the weaker gravity), let's a assume an atmospheric pressure of 100,000 pascals (just a smidge less than Earth today). We find that after one half life the pressure will be 50,000, two halflives = 25000, 3=12500, 4=6250, 5=3125,6=1562,7=781,8=390. The actual current Mars pressure is 610 pascals which is roughly consistent with exponential decay with seven or eight halvings
Mars is most likely about the same age as the Earth -- 4.5 billion years (give or take). 8 halvings would be a halflife around 500,000 million years. Even if mankind were to hang out on Mars for a few million years, the loss of atmosphere over that timespan would seem to be hardly detectable.
(Would an atmosphere with a partial pressure of 20,000 Pa of Oxygen and Mars gravity be OK for humans? My GUESS is yes. But I sure could be wrong).
> No reason that a manmade atmosphere wouldn't last as long.
I give you 2 good reasons:
a) We cannot make an atmosphere
I know that "in principle" it is possible to make an atmosphere. But "in principle", and "practical" are 2 VERY different things. In principle, an Albucierre drive is possible. In practice, we cannot control mass, and we don't have dark matter.
Making an atmosphere on another planet isn't within our practical technical capabilities, and won't be for at least a couple hundred years.
b) We cannot restart a planetary core
Unless we figure out how to just pop an atmosphere into existence instantaneously all at once, which would probably be a neat trick even for a Kardashev type 2 civilization, whatever atmosphere we build on mars will be eroded away probably as fast as we can build it by solar winds, because Mars has no viable magnetic field. That's how a planet that has 1/3 of earths gravity ended up with an atmosphere that is less than 1% as dense as ours.
So unless we figure out a way to either restart a planetary core or stop solar winds at a planetary scale, any terraforming effort would be like spitting at tidal wave. And if we manage to do either of these 2 things, we probably won't need to terraform a planet, because by that point we probably have figured out FTL travel, or can live comfortably in Stargat-Atlantis-like space habitats.
Your maths is looking down the wrong end. How long it took for any previous atmosphere to be lost doesn't have any bearing on terraforming. Mars has no magnetosphere, so is now subject to the full force of the Solar Wind. If that, and it's weak gravity means it would lose atmosphere at a certain rate, so you have to produce atmosphere at a far greater rate, and this is an entire planet we are talking about here. It's a massive undertaking, even without the inherent losses.
We do not have the resources or technology to tweak our own atmosphere to make it favourable for us to live on Earth, and fix the issues we have caused here, do you really think we can do that on another planet?
This is something I find so fascinating whenever anyone mentions "terraforming mars" or similar ... We don't even have a practical, workable plan, to artificially remove enough CO2 from earths atmosphere to stave of climate change.
We're not talking about a huge terraforming effort here, only a few percent, only a nudge. And we cannot do that, here on earth, where we live, where all out workforce, wealth and resources are, where we can freely move, live and breathe.
And people talk about terraforming another planet, that we have trouble putting even a car-sized robot on.
We would, and we know that's what happend: https://en.wikipedia.org/wiki/Laschamp_event
If you want to convince us, don't quote an article that lays doubts on your claim:
"However, the lack of corroborating evidence of a causal link between the Laschamp event and population bottlenecks of many megafauna species, and the relatively moderate radio-isotopic changes during the event, have cast significant doubt on the real impact of the Laschamp event on global environmental changes."
There is actually no consensus on the correlation between a reversal of polarity and extinction events, except maybe in the Doctor Who-verse.
> There is actually no consensus on the correlation
My argument isn't that there is definite proof (which is impossible anyway following Poppers principle of empiricism as everything must remain falsifieable in principle, lest it is removed from the empirical to begin with), my argument is that there is support for the idea that radiation levels caused by a weakened magnetosphere have a negative impact on life.
So far, I have not seen ANY support for a counter to that idea.
So it's really me who is waiting to be convinced here ;-)
I'm sorry, but when you say "We know it has happened", it does sound like you do more than support the idea.
So far, I have not seen ANY support for a counter to that idea.
And I've not seen any support for the idea that there isn't a teapot orbiting the Sun between Earth and Mars, therefore it must exist. After all, there is already a Tesla up there.
Who knows, it's really hard to chose which one problem is the biggest when there are so many.
No nitrogen anywhere.
Solar power effectiveness severely reduced.
Almost no accessible water.
Next to no atmosphere.
An average temperature of -60°C.
No protection from cosmic radiation or solar flares.
Planet wide duststorms of powdered regolith lasting for weeks or months making solar power generation impossible and killing equipment.
No habitats (anyone who disagrees: Show me that the proposal can be built, and works, in isolation, in a dry valley in antarctica, with nothing but local resources and no machine that doesn't fit in whatever spacecraft should go to mars).
Microgravitation related illness.
There are just so many ways in which the red planet can kill anyone actually trying to live there, it's really hard to chose which one is the worst.
Yes, a gravitational field a bit above 1/3 that of earths.
Bipedal hominids spent about 7 million years developing under 1g of gravity. That's not gonna go away, neither do the 1E8's years of mammalian evolution.
Everything in our body is tuned to that. From our circulatory system up to and including our reproductive organs.
We already know that microgravity, or significantly lowered gravity, for any prolonged period of time, wreaks havoc on humans. Bone and muscle deterioration or fluid redistribution are just 2 of the effects.
And so far, the people we sent into such conditions were Astronauts, aka. humans in excellent physical condition, well trained, and specially selected for their fitness for such missions.
The effects may be less severe, for a time, in 1/3g than in microgravity, but a) a mars mission will last a lot longer (as in, a lifetime), and b) we can swap people in orbit...there is no swapping people on Mars.
Oh, and let's not forget, that by the time these people arrive on Mars, they will have spent 9 months already under microgravity conditions, plus however long it will take to re-fuel the launch vehicle after it reaches low earth orbit, if the craft goes with that plan.
I think you are vastly underestimating the body's ability to adapt.
Microgravity has been shown to be quite hard on the body. There is nothing to backup the assertion that lower gravity is harmful at all.
The fact that teams of guys laying undersea pipeline spend weeks living and working at pressures over 4x those at sea level seem to indicate quite the opposite.
> There is nothing to backup the assertion that lower gravity is harmful at all.
There is nothing to backup the assertion that it is harmless.
Microgravity is proven to have adverse effects. Drawing the conclusion that lower gravity has similar, albeit reduced, effects is just a logical extrapolation.
> The fact that teams of guys laying undersea pipeline spend weeks living and working at pressures over 4x those at sea level seem to indicate quite the opposite.
a) Pressure and gravity are 2 very different things
b) We are not talking about weeks here. We are talking about many months of space trip in microgravity and whatever lifespan remains to the would be colonists on mars at 1/3 gravity.
Yeah. I'm wildly supportive of space exploration, but I don't see an off-world colony (where "colony" is defined as being self-sustaining) happening in my lifetime. We might get some long-term presence on the Moon, but only because regular resupplies are somewhat feasible.
As for Mars... dig underground for thermal isolation, easier containment of atmosphere, and radiation shielding; use small nuclear reactors for power; pick a site that's near the equator and near some underground water; carry nitrogen, limit its escape from the system, extract what little there is to replace the amount that inevitably escapes; hope that low-gravity doesn't screw up humans too badly long-term (we don't really have any data on that). While all of that can theorically be done with current tech, there's so many practical problems that I can't see it happening for many decades. And if it turns out that low-gravity does screw up humans badly long-term, then the whole thing is unfeasible until some major breakthroughs in medicine.
Also, I definitely agree that we should first build self-sustaining colonies on Antarctica. It would be much easier while still being pretty good science, it would be vastly cheaper, the tech and experience involved would all come up useful for space exploration eventually, and figuring out how to make life work in hostile environments sounds like an overall good idea to me. I'm not sure why we aren't doing it; I guess it just wouldn't be cool enough (heh).
Why cant we just leave Antartica alone ?
WOul dyou like it if another species decided to destroy your home just because they can ? Show some grace and set an example instead of trying to destroy and take everything thaat i not yours and leave it to the other animals on this earth.
What animals? AFAIK, and I'd be delighted to be wrong on this, the interior dry deserts of Antarctica don't have any life beyond some extremophile microorganisms. Which would probably only benefit from having a bit more energy around anyway. I would think that getting life to be in places where it wasn't is a pretty good example.
"We might get some long-term presence on the Moon, but only because regular resupplies are somewhat feasible."
I agree, and I wonder why the obsession with boots on Mars. The challenges of 'colonising' are similair between Moon and Mars - no soil, no nitrogen, no atmosphere, close to no water, reduced gravity, radiation exposure etc etc. You anyway need to live in caves to start off with at least. On the moon, you have the advantage of being closer to Earth for resupply and initial support, and an order of magnitude more solar power available. If we can't establish a moon colony there is no way we can do so on Mars (and whatever we learn on the moon would help later on Mars)
>I agree, and I wonder why the obsession with boots on Mars.
I think that boots on Mars, as in getting a few live humans there for a bit and then getting them back shortly thereafter, should be feasible. Very expensive, of course, maybe too expensive to justify it. I'd be personally fine with that just for the sheer inspiration factor, but I'd totally understand people who aren't. I do not consider objections to such a project as the usual "space is bad" inanity we sometimes hear. If we do it, I'm happy, but if we don't do it because of public opinion, I can live with that.
Mars colony - don't worry about that. Yes, there's a lot of talk about it. But it's not going to happen any time soon. Nobody is going to even try, not for a long while, so no big resources are going to be spent on that. Talk is cheap.
Moon colony - that's the interesting thing. I think it's right at the edge of feasible, but with far better returns than just getting someone on Mars for a bit. I think it's a project that would lead to solving a lot of the same problems we'd have on Mars, while also providing impetus for a whole lot of great engineering that comes up real useful on Earth, much like the early space programs did. I hope this one gets serious.
There's no need for genetic engineering. Humans with one thumb and three fingers are common in places where brothers and sisters have sex with each other: Norwich for example. Or the US redneck states. The problem is attaching those hands to a brain capable of turning the reactor on and off.
"A paper published this week said about 1.5kW out of the 4.6kW energy budget of the Environmental Control and Life Support System on the ISS is used up by the Oxygen Generator Assembly (OGA), which relies on electrolysis." so about 4 or 5 panels worth out of all those panels they have there. I'm not really seeing a problem given Wikipedia states that "Altogether, the eight solar array wings can generate about 240 kilowatts in direct sunlight, or about 84 to 120 kilowatts average power (cycling between sunlight and shade)." so they are not going to suffocate up there due to lack of power to run the electrolyzer.
Oxygen production on Mars isn't a problem either as you can manufacture it by converting CO2 to CO resulting in O2 while venting the CO and apparently the Moxie ISRU experiment on the Perseverance rover was successful https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9432831/ so again I don't see that as an issue as there is plenty of CO2 on Mars, alternatively if you can get ice from below the surface you can melt and purify it probably by way of distillation because it's the easiest method even though it's more energy intensive than reverse osmosis but the advantage is that it's simpler and doesn't require consumables like membrane filters, and then you simply electrolyse the purified water.
Oxygen production on Mars isn't a problem either as you can manufacture it by converting CO2 to CO resulting in O2
Where is that CO2 going to come from?
The maths/chemistry on that modei of oxygen production don't look good: 2 molecules of C02 produce 2 molecules of C0 and 1 molecule of O2. In effect, half the potential oxygen from CO2 gets pissed away in highly toxic CO.
You burn the carbon monoxide for energy to power the ... oh, wait.
(Frankly I think the carbon monoxide problem takes a back seat to the energy-input problem. Solar power? Significantly diminished in Mars' orbit and dust storms are a big problem. Small nuclear reactor? Great, now you either need to cart suitable fissile material with you or find it when you get there – plus the weight and volume of the reactor itself in your mission payload. Colonizing Mars is just a stupid idea.)
I agree with you that the power issue is a deal breaker but if that problem is solved then solar panels are probably the best way to make power even though they will generate less than on Earth but they are otherwise reasonably lightweight, don't forget any install will be a reasonably large array compared to what a rover or lander would have so I would expect that they would have to also include small robots that could continuously move along the panel array sweeping them of dust as they go along.
There's plenty of it available because it comprises 95% of Mars atmosphere https://www.nasa.gov/feature/goddard/2019/with-mars-methane-mystery-unsolved-curiosity-serves-scientists-a-new-one-oxygen so all you need to do is have an open pipe to the outside and a pump to compress it because atmospheric density is 1% that of earth (or about the same as that on Earth at 100,000 feet) and then you can have as much of it as you like to extract however much Oxygen you need out of it.
Well naturally it would be a hugely energy intensive process but if you're on Mars you have to do it otherwise your astronauts will run out of breathable air, besides even if it's several multiples of what it would otherwise be to get Oxygen from electrolyzing water it's still only a reasonably small amount given how much energy the ISS uses to maintain three people being there continuously. Don't forget that the ISS needs regular water deliveries whereas you'll never run out of CO2 on Mars and it's right outside your spacecraft or base-station.