Life on Earth has co-evolved with our planet’s atmosphere over the past four billion years, during which time the atmospheric pressure seems to have remained relatively constant. Most forms of life are finely adapted to the pressure found at Earth’s surface where the atmosphere is thicker than on the moon or Mars, but thinner than on neighboring Venus. A pressurized environment is one of our requirements for life, and astronauts take careful precautions to stay pressurized in the vacuum of space. Even so, life on Earth has spread to nearly all ecological niches and occupies even low-pressure environments that would otherwise be uninhabited.
Piezophiles (or barophiles) are microorganisms that have evolved to tolerate extremely high pressure environments. They typically live on the ocean floor or near deep-sea hydrothermal vents and survive at pressures several hundred times that at Earth’s surface. Most piezophiles also seem to be psychrophiles, meaning they are also tolerant to cold environments, a helpful adaptation for residing at the ocean’s floor. At high pressures and cold temperatures, the membrane around most cells loses its fluidity and becomes somewhat gelatinous, which inhibits the transfer of nutrients into the cell. Piezophiles, however, have developed the ability to release a disproportionate amount of unsaturated fats, compared to saturated fats, allowing a permeable membrane to persist even at these extreme conditions. This ability of piezophiles to regulate their cellular membrane under high pressure conditions is an area of active research to understand the underlying genetic mechanism.
Other microorganisms have developed strategies for surviving in the low pressures of Earth’s upper atmosphere or beyond. Bacterial species have been discovered on hailstones in storms as well as in clouds. In fact, some microbial species are suspected to attach themselves to dust particles that are carried aloft by wind to induce the nucleation of icy cirrus clouds in the upper atmosphere, suggesting that these lofty microorganisms may regularly influence Earth’s weather. Some microbial species have even managed to survive the extremes of space by taking the form of a dormant spore. Bacillus spores, for example, are known to have survived sterilization processes and the journey into space. We can even claim with near certainty that life exists on the moon and Mars, due to the bacillus spores that reside within the machinery of spacecraft built on Earth. The risk of contamination is extremely low, as these spores could not survive the dangerous radiation at the surface. Nevertheless, life is resilient and manages to survive in some of the most unlikely places.
Astronaut suits provide a technological solution for humans to survive in otherwise inhospitable conditions, and even the modern incarnation of space suits weigh upwards of ninety pounds apiece. The suit can take thirty minutes or longer to don, and it contains material on the inside to keep the astronaut cool and comfortable. The helmet and suit are inflated to the pressure at 30,000 feet above Earth’s surface--an altitude slightly higher than most commercial airplanes fly. This particular pressure was chosen in the event of a shuttle bailout, likely to occur at such an altitude, and the space suit is designed in general not only to keep an astronaut safe during routine spacewalks, but it is also optimized to help an astronaut avoid a deadly scenario. Space suits are also reusable, and NASA keeps several sizes on hand that can be used by different astronauts and for different missions.
Future human space colonists will need to depend on their pressurized suits and ship cabins even more than human explorers today. In particular, any colony on Saturn’s satellite Rhea will require particular care to the extremely low pressures on this icy moon. Pressurized domes for cities as well as resilient space suits for astronauts will be necessary for human habitation on such worlds. Humans have shown the ability to adapt to a wide range of environments, and space colonization likely will be no exception.