A few quick calculations around here gave me that after removing all the CO2 and major toxic gases (carbon monoxid and nitrous oxid), you're left with an atmosphere that's 60% N2, 35% Ar, only 4.4% O2 and 0.6% H2O.

This is clearly unbreathable, unless you increase pressure quite a bit.

That's no issue, and the comments about the disrespect to human life are ridiculous. First of all, there is almost 1% of Argon in our own atmosphere anyway, so we are pretty used to breath it, although in lower partial pressure. Second, inert gases are commonly used by deep divers. Exposure to them was tested since many decades and often for long periods of time (i.e. underwater habitats). Argon is not used for breathing in scuba just because it has high narcotic effects in depth, but it should not be an issue at low pressures.

Rationale?

Quite evident – it allows for easy and cheap breathing gas production on Mars. Note that this atmosphere processing is for the Mars stay, not for the travel, so the comments about atomic weight are irrelevant. The advantage is the composition of Mars atmosphere – after removing the CO2 (95.32%), CO (0.07%), and traces of some other gases, Oxygen (0.13%) and the inert Nitrogen (2.7%) and Argon (1.6%) remain in a composition very well suitable for breathing – with similar partial pressure of Oxygen (~29% instead of ~21%) and inert gases. So all you need is a compressor and filters for the unwanted components. No need for possibly expensive and failure-prone artificial gas mixture with inert gas recycling.

For those wondering why Argon was chosen instead of Nitrogen (which is more common on Mars), the reason may have to do with the fact that when Nitrogen is heated up with Oxygen, it may be very unhealthy for the human body.

Argon seems to be safer, thus the reason they use it.

Back on topic. After watching the video it's clear why this is being tested. It is stated that Marssian atmosphere has plenty of argon (don't know to what extent this is true). And argon being a noble (innert) gas is perfectly safe. You can't get less toxic then noble gasses really (except radon which is radioactive), and the risk is minimal, so i don't see how this test can even remotely be seen as looking down the barrel of a gun.

If they can in fact pull useful gases from the atmosphere and convert other found materials into food, fuel, and construction materials then it makes the journey more affordable. Its also a handful of steps closer to true self sufficiency.

The more things you use from local sources, the less you have to bring and the longer you can stay. Instead of being a one time stunt we can start thinking of mars as a long term outpost… maybe even a near future colony.

I'd suggest they simply do away with the filler gas altogether. I know that a pure O2 atmosphere was tried in the past, and with tragic results. The difference is that if a pure O2 atmosphere is used, but at a pressure equal to its part in the Earth's atmosphere, i.e., about 0.2atm, the fire hazard would only be the same as it is in Earth's atmosphere. This would also effectively reduce the "filler gas" mass to 0, and the cosmonauts would get the same amount in their lungs that they would at sea level on Earth.

Another advantage of this is that there would be 80% less pressure for the ship to hold in from the vacuum of space. Since Mars' atmospheric pressure is less than 0.01atm on average, this would make those structures to be made lighter due to less structural reinforcement.

Well, unsurprisingly, the report isn't clear about the rationale or the safety of the test, but one can think about it.

First: safety.

It should be OK, really. Argon is an inert gas that shouldn't react with our body's chemistry, much like helium and much like molecular nitrogen. So replacing nitrogen with argon shouldn't cause any damage at all. And regarding the reduction of oxygen, I suppose they are aiming for something like 3000 or 4000 m partial pressure, which we know that isn't harmful for humans.

As for the rationale…

I guess the problem here is that wereas a Mars mission could produce oxygen from the carbon dioxide collected in Mars, it would have a hard time finding a replacement for nitrogen because there isn't all that much N2 out there. However, there's even less argon, about half as much, so I don't really see the point. The best idea I could come up with is that this might be an attempt to minimize the atmospheric treatements needed to reuse martian gases for breething. If this experiment is successful, they'd just need to get rid of the CO2 and other noxious gases, like carbon monoxide, add O2 in adecuate proportions and pump it in until the adecuate pressure is reached, warm it up a wee bit, and voilà, the atmosphere becomes breathable. It might be it, I don't know.