For all the drama around getting to Mars, the harder question starts after landing.

A person can step onto the planet with food, water and shelter plans in place, but none of that matters for long without something far more basic: oxygen. On Earth, breathing feels automatic because the atmosphere does the work for you. On Mars, it does not.

That difference starts with the air itself. Here on Earth, oxygen makes up about 21% of the atmosphere. Nitrogen takes up most of the rest, about 78%. When you breathe in, you take in all of it, but your body uses the oxygen and gets rid of the rest when you exhale.

Mars offers a much harsher version of air. Its atmosphere is thin, with only about 1% of the volume of Earth’s atmosphere. Put another way, Mars has 99% less air than Earth. The planet is about half Earth’s size, and its weaker gravity cannot hold atmospheric gases as effectively, allowing them to escape into space.

That alone would make breathing difficult. The composition of the Martian atmosphere makes it deadly.

The most abundant gas on Mars is carbon dioxide. On Earth, carbon dioxide becomes poisonous at high concentrations, though it makes up far less than 1% of the atmosphere. On Mars, it makes up 96% of the air.

Oxygen, by contrast, is almost absent. It accounts for only one-tenth of one percent of the Martian atmosphere, nowhere near enough for a human being to survive. Without a spacesuit supplying oxygen, a person on the surface would die almost immediately. The source material describes two things happening at nearly the same time: suffocation and blood boiling because of the low atmospheric pressure.

It is a brutal reminder that Mars is not just distant. It is actively hostile to human life.

And the air is only part of the problem.

Very little liquid water exists on the Martian surface. Temperatures plunge hard, especially at night, when they can fall below minus 100 degrees Fahrenheit, or minus 73 degrees Celsius. Even before questions of exploration or settlement come up, the basic challenge is survival in a place that offers almost none of the conditions people depend on.

That does not mean Mars is beyond scientific hope. So far, researchers have found no evidence of life there. But the search remains in its early stages, and robotic probes have only begun to investigate the planet.

That matters because extreme conditions do not automatically rule out life. On Earth, organisms survive in places that once seemed impossible. Life exists in Antarctic ice, at the bottom of the ocean and miles beneath Earth’s surface. Many of those environments bring severe cold or heat, almost no water, and little to no oxygen.

So Mars keeps scientists interested for two reasons at once. It looks forbidding now, yet life on Earth has already shown that biology can persist in difficult places.

There is also the deeper possibility that Mars was once very different. The source material notes that billions of years ago, the planet may have had a thicker atmosphere, more oxygen, warmer temperatures and significant amounts of liquid water on its surface. If that was the case, Mars may once have been much more favorable to life than it is today.

One goal of the Perseverance rover is to search for signs of ancient Martian life. Not living animals or plants, and not anything dramatic from science fiction, but evidence of primitive organisms, most likely microbes, preserved in rock.

That is why the rover examines Martian rocks for traces that could point to fossils of organisms that once lived there. The work is slow and careful. Mars does not easily give up information, and any sign of past life would need to come from physical evidence embedded in the landscape.

The images sent back by Perseverance only sharpen that contrast. They show a bleak, barren world, a place that seems empty at first glance. Yet that same terrain could still hold the record of a very different planet from long ago.

The search, then, is not only about what Mars is now. It is about whether the planet once had the conditions needed for life, and whether those traces survived long enough to be found.

At the same time, Perseverance is doing something else that could shape Mars exploration in a more immediate way.

Among the rover’s seven instruments is MOXIE, a device designed to take carbon dioxide from the Martian atmosphere and convert it into oxygen. On a planet where carbon dioxide dominates the air, that is a striking idea: use the planet’s most abundant gas to help solve one of its most dangerous problems.

If MOXIE works the way scientists hope, the benefits would go beyond breathing. Future astronauts could make their own oxygen on Mars instead of carrying all of it from Earth. The source material also notes that oxygen could serve as a component in the rocket fuel needed for a return trip home.

That changes the equation. The more oxygen astronauts can produce on Mars, the less they need to launch from Earth. And the less mass a mission has to carry across space, the easier it becomes to send people there.

Still, even “homegrown” oxygen would not make Mars comfortable. Astronauts would still need spacesuits. The planet’s thin atmosphere, low pressure, cold temperatures and lack of breathable air would not disappear just because some oxygen could be manufactured on site.

NASA is now working on the technologies needed for human missions to Mars. According to the source material, such a mission could happen in the next decade, perhaps sometime in the late 2030s.

That timeline leaves Mars in a curious place. It remains unreachable for ordinary people and unforgiving for anyone who gets there. Yet each rover, each test, each scrap of atmospheric data pushes it a little closer to something else, not a second Earth, but a world humans may one day visit and survive for a while through machines, planning and a careful supply of oxygen.

Research findings are available online in the journal Cell.