- 60% Xenon
- 20% Oxygen
- 15% Nitrogen
- 5% Carbon dioxide
Xenon is a noble gas. It has very few common chemical reactions, and is frequently used in situations where air is too reactive to be safe. It's safe to touch and even safe to breathe.
It's also a very effective anesthetic. Present-day doctors in Europe use it because it is remarkably side-effect free, if quite expensive. A few breaths of Xenon and you'll be out cold.
20% oxygen is around the level we have in our atmosphere. This way, fire still burns and iron still rusts in your world the same as on Earth.
You have enough carbon dioxide to grow plants, but they need N2 in the air to keep the nitrogen cycle running. 15% is too little for the nitrogen-fixing bacteria in the soil, but your crew could have genetically modified soil bacteria for farming.
Why Carbon Dioxide?
This is how you die. CO2 is very stable and inert, but at concentrations of around 40000 ppm (4%), it becomes quite toxic. At these levels, it's so far above the lethality threshold that if you breathe this, you will die.
How does it work?
An astronaut is working on a new construction for the planetary base when her breathing mask undergoes a malfunction, disabling the low-air alarms. The astronaut continues working as her mask slowly runs out of air, and loses track of time. When the tank is almost empty, the broken alarm fails to remind her to refill, and without realizing, she begins to breathe the atmospheric air.
After three or four breaths, she begins to feel unusually drowsy. As soon as she realizes what happened, she holds her breath, but the xenon has already taken effect. Two shaky steps towards base later and she collapses on the floor, unconscious. Her brain, fooled by the apparently welcoming air, resumes breathing, taking in 13 times more carbon dioxide than she can handle. Instead of carbon dioxide from her lungs breathing out into the air, CO2 diffuses from the atmosphere into her alveoli, flooding her bloodstream with carbonic acid. Her blood pH plummets out of control, and she dies of carbon dioxide poisoning in under a minute.
- No damage to skin or clothing
- Lethal after a few breaths
- Behaves effectively the same as our atmosphere in most everyday situations
- You can grow crops with minimal modifications.
- You need to explain how the hell the atmosphere became 60% Xenon. A comet, maybe, or it orbits a star with lots of Xenon?
Edit: To clear up the issue of where did all the xenon come from, here are a couple possible sources:
Presence in solar system: The planet orbits a star that burns hot enough to produce Xenon (our sun can't produce anything past iron), a nearby Xenon-rich star went supernova nearby a long time ago. This means that there's a lot of Xenon in the solar system to begin with, and its stability means that most of it will hang around. The planet either formed from a gas-and-debris cloud that contained xenon, which bubbled up to the surface during formation, or received it from xenon bubbles in porous ice and rock from asteroid and comet bombardments. This is where we got most of our atmospheric gases, because they exist in abundance in the space surrounding our planet. As long as there's xenon in the system, the planet will get a decent share of it.
Radioactive decay: Xenon is produced by nuclear decay from iodine-135 in our nuclear reactors (this actually causes problems because the gas blocks the nuclear chain reaction). The xenon produced by this is radioactively unstable, but there is another process that turns iodine-129 into xenon-129, which is very stable. The half-life of iodine-129 is 16 million years, so a planet rich in iodine-129 would steadily produce Xenon gas, bubbling up to the surface. This process could have already ended, resulting in a Xenon-rich atmosphere even after the decay has slowed to a crawl. Iodine-129 is a common product of nuclear fission, which occurs slowly in nature in underground uranium deposits. So a large amount of uranium, under immense geological pressure, slowly produces Iodine-129, which slowly decays into Xenon-129, which is stable and floats up to the atmosphere in geothermal vents, volcanoes, and hot springs.