Future astronauts exploring Mars will face a danger that may be much harder to avoid than extreme cold or radiation: dust.
NASA has released a report recommending an initial safety limit for the amount of Martian dust astronauts should be allowed to breathe.
Although human missions to Mars are still years away, engineers need the guidance now so they can design spacecraft, air filters, spacesuits and living areas that protect future crews.
The proposed limit is 0.1 milligram of fine Martian dust per cubic meter of air, averaged over 24 hours, for exposure periods lasting up to 30 days.
That number is mainly intended as an engineering target. Future Mars habitats will need to keep airborne dust below this level through filtration, cleaning and careful control of airflow.
Martian dust could be particularly troublesome because its particles are extremely small. They can also become electrically charged and stick to spacesuits, boots, tools and equipment.
Every time astronauts return from working outside, they could carry some of this material into their living areas. Once inside, tiny particles could become airborne and be inhaled.
Very fine particles can travel deep into the lungs, potentially causing irritation and inflammation, especially after repeated exposure.
There are also concerns about what the dust contains.
Mars is covered by material largely derived from volcanic rocks. Its dust contains minerals such as plagioclase, pyroxene and olivine, along with iron-rich substances and sulfates.
Scientists are particularly interested in chemicals called perchlorates, as well as manganese, chromium and extremely small iron-containing particles. Some of these substances could pose health risks if enough enters the lungs.
One major problem is that scientists don’t have samples of actual airborne Martian dust on Earth.
NASA therefore had to estimate the risk using several sources of information, including measurements from Mars rovers and landers, laboratory-made Martian soil substitutes, studies of Martian rocks and research into lunar dust.
The agency already has a 30-day exposure limit for moon dust of 0.4 milligram per cubic meter. Because scientists know less about Martian dust and its possible toxicity, NASA proposed a limit four times lower as an additional safety precaution.
Keeping a Mars habitat clean will require more than simply installing an air filter.
Astronauts may need systems for removing dust from their suits before entering living spaces. Habitats will also need to monitor particles in the air and carefully manage humidity and airflow. Very dry conditions can allow tiny particles to remain suspended longer and make them more likely to stick to surfaces through static electricity.
HEPA filters are expected to be capable of capturing Martian dust, but future filtration systems will need to remove it quickly enough after astronauts return from outside.
Many questions remain.
Scientists still need better information about the size, shape and chemistry of the particles astronauts would actually inhale. The finest dust floating through the Martian atmosphere may be different from the rocks and soil examined by spacecraft.
Researchers also need to understand how these particles interact with lung tissue and how easily the body can remove them.
For future Mars explorers, keeping the red planet outside their lungs may become one of the most important parts of staying healthy millions of kilometers from Earth.
