What Would We Look Like If We Lived on Mars?

Humans living on Mars would gradually become taller, thinner, puffier in the face, and weaker in the bones than their Earth-bound relatives. Mars has only about 38 percent of Earth’s gravitational pull, its atmosphere offers almost no protection from cosmic radiation, and its surface temperature averages around minus 60 degrees Celsius. Each of those conditions would tug the human body in a recognizable direction, and the research from space stations, bed-rest simulations, and animal flights gives us a surprisingly detailed sketch of what that direction looks like.

Taller, but Not in the Way You Might Expect

Astronauts on the International Space Station commonly gain a couple of centimeters in height, and the mechanism is straightforward: without full gravitational compression, the fluid-filled discs between your vertebrae expand. A numerical modeling study found that in zero gravity, individual lumbar disc heights increased by roughly two to three percent, and disc volumes swelled by as much as nine percent compared to their loaded state on Earth.¹ Mars is not zero gravity, so the effect would be less dramatic, but with only 38 percent of Earth’s pull pressing down on your spine all day, the discs would still decompress enough to add noticeable height over time. Imagine a population that is consistently a few centimeters taller than genetically similar people on Earth, without any change in actual bone length.

The height gain comes with a catch. Bones that no longer bear their usual load start to shed mineral density. A modeling study of a conjunction-class Mars mission predicted that astronauts would lose roughly 33 to 37 percent of their femoral neck bone mineral density over the course of the trip, pushing many into the clinical range for osteoporosis or its precursor, osteopenia.² That study modeled the transit period in near-zero gravity, and living on the Martian surface with partial gravity would slow the loss. But it would not stop it. Over years and decades, Mars residents would develop a lighter, more fragile skeleton, particularly in the hips, legs, and lower spine, the bones that do the heavy lifting on Earth.

Lean Limbs and Shifting Muscle

If the skeleton gets lighter, the muscles wrapping it shrink to match. On Earth, your postural muscles, particularly the soleus in your calf, work constantly just to keep you upright. In reduced gravity, that workload drops and the body responds by paring back what it does not need. Research on spaceflight and simulated microgravity has shown that these slow-twitch postural muscles begin converting into fast-twitch fibers, changing the expression pattern of the genes that build the contractile proteins.³ The overall profile of a slow, endurance-oriented muscle shifts toward one that is more suited to quick bursts but tires easily.⁴

The visual result is straightforward: thinner calves, thinner thighs, and a generally leaner lower body. Astronauts on the ISS exercise roughly two hours a day to combat this, and they still come home with measurably reduced leg muscle mass. On Mars, with partial gravity and no two-hour daily gym mandate enforced by mission control, leg muscles would waste even further. The upper body would fare somewhat better, since arms do not depend as heavily on gravitational loading, but over time a Mars resident’s overall silhouette would skew noticeably lighter and more narrow-limbed than an Earthling of the same genetic stock.

The Puffy Face and Bird Legs

One of the most immediately visible changes astronauts notice is what they call “puffy face, bird legs.” On Earth, gravity pulls blood and other fluids toward your feet. Reduce that pull and the fluids redistribute upward, pooling in your head, neck, and chest. A systematic review and meta-analysis of cardiovascular effects in weightlessness confirmed that this fluid shift leads to cardiac changes: heart rate tends to rise, stroke volume and cardiac output fall, and the jugular veins in the neck expand as they fill with extra fluid.⁵

For appearance, the upshot is a visibly swollen face. Cheeks fill out, eyelids puff, and the nasal passages congest, giving astronauts a perpetually stuffy look. The legs, meanwhile, lose fluid volume and look thinner. On Mars, with 38 percent gravity still tugging fluid downward, the effect would be less extreme than on the ISS but would not disappear. A Mars settler would walk around looking a bit moon-faced compared to someone on Earth, especially in the first months before the body partially adjusts. The cardiovascular system itself would also remodel: blood vessels in the head and neck would gradually widen, while the heart, with less work to do pumping against gravity, could lose some of its muscular bulk.

Changing Eyes

The fluid shift does not just puff up your face. It raises pressure inside the skull, and the eyes are the first structure to feel it. Spaceflight-Associated Neuro-Ocular Syndrome, known as SANS, is now considered one of the major health threats for long-duration space travelers. It involves swelling of the optic disc, flattening of the back of the eyeball, folds in the choroid layer behind the retina, thickening of the retina itself, and a hyperopic shift, meaning affected astronauts become more farsighted.⁶ The connection to fluid dynamics is well-established: the cephalad fluid shift raises intracranial pressure, and that pressure acts on the eye from behind.⁷

Would Mars gravity be enough to prevent SANS entirely? Nobody knows yet, because no human has spent extended time in partial gravity. The ISS offers zero-g data, and Earth offers one-g data, and 0.38 g is uncharted territory. But given that the syndrome appears even in astronauts who exercise rigorously and use lower-body negative pressure devices to force fluid back toward their feet, it is reasonable to expect at least a mild version in Mars colonists. Over time, a population living there might tend toward slightly flatter eyeball geometry and a gradual shift toward farsightedness, requiring corrective lenses earlier in life than people on Earth typically do.

A Thicker Skull and a Thinner Jaw

One of the more counterintuitive findings from spaceflight research involves the skull. While load-bearing bones like the femur lose density in microgravity, the cranial vault, the dome of bone protecting the brain, appears to do the opposite. A systematic review and meta-analysis of craniofacial bone changes in spacefaring animals found that the calvaria showed a trend toward increasing bone volume, while the mandible, your jawbone, showed a significant decrease.⁸ A separate study of mouse mandibles from two shuttle missions confirmed this split: the skull bones gained mineral while the jawbone lost volume and density, at least on one of the two missions studied.⁹

The likely reason for the divergence is that the cranial vault and the mandible remodel through different biological pathways. The skull vault thickens in response to the increased intracranial fluid pressure discussed earlier, while the jaw, which depends on mechanical loading from chewing against gravity, loses stimulation and atrophies. If this pattern held over generations, the visual effect would be subtle but real: a slightly more domed forehead and a somewhat finer, less prominent jawline. Pair that with the puffy soft tissue overlying the face, and the overall impression is a rounder, softer facial structure than we are used to seeing on Earth.

Thinner, More Sensitive Skin

Mars settlers would have to contend with a radiation environment that makes Earth’s look gentle. The planet’s thin atmosphere and absent magnetic field offer virtually no shielding from galactic cosmic rays or solar particle events. But even inside a shielded habitat, the lower gravity itself appears to affect skin. Long-duration spaceflight data, though still limited, point toward increased skin sensitivity and signs of dermal atrophy in astronauts.¹⁰ A study of mice that spent three months aboard the ISS found that their dermal layer, the thick structural layer beneath the surface, had thinned by about 15 percent compared to ground controls.¹¹

Thinner skin bruises more easily, wrinkles more readily, and heals more slowly. Combined with constant low-level radiation exposure, which accelerates collagen breakdown, a long-term Mars resident might look older than their biological age. The skin could take on a somewhat papery, more translucent quality, especially in fair-skinned individuals. Mars settlers living behind habitat walls and spacesuits would also receive far less ultraviolet light than people on Earth, which raises its own set of questions about pigmentation over longer timescales.

What Happens to Skin Color Over Generations

Human skin pigmentation evolved largely as a response to ultraviolet radiation. Populations near the equator developed dark, eumelanin-rich skin to protect against intense UV, while populations at higher latitudes lightened over thousands of years to allow more UV penetration for vitamin D synthesis.¹² Skin color itself is shaped by melanin, hemoglobin, and carotenoid levels, all modulated by a complex web of signaling pathways with a strongly polygenic genetic architecture.¹³

On Mars, behind shielding that blocks most radiation, there would be no selective advantage to dark pigmentation, because UV exposure would be minimal. There would also be no strong selective pressure toward very light skin for vitamin D production, since supplementation would presumably be part of the food supply. Without strong selection in either direction, skin color in a Mars colony would largely reflect the founding population’s genetic mix. But if a colony persisted for thousands of years, the mild drift that occurs in any small, isolated population could move the average in unpredictable ways. The scenario most commonly floated in popular science, that Martians would inevitably become pale, oversimplifies the genetics. Skin color is influenced by hundreds of genetic loci, and evolution moves slowly on traits controlled by that many genes unless the selection pressure is intense. For at least the first several centuries, Mars colonists would look like whatever combination of people went there.

Paler Complexion for a Different Reason

Even if skin pigmentation genes stayed the same, Mars residents might look paler anyway, for a circulatory reason. Within the first few days of spaceflight, newly produced red blood cells begin disappearing from the blood, dropping total red blood cell mass by roughly 10 to 15 percent. Older red cells survive normally, and production continues at close to normal rates, but the body maintains this reduced red cell mass as a new steady state for as long as the low-gravity environment persists.¹⁴ Less hemoglobin circulating through the capillaries just beneath the skin means less of the rosy hue that gives healthy complexions their warmth. The result would be a somewhat washed-out, paler appearance regardless of melanin levels. Combined with the puffiness from fluid redistribution, the overall facial look would be a kind of softened pallor.

Sweating, Flushing, and Thermoregulation

Mars is bitterly cold outside, but inside a pressurized habitat the real thermoregulatory challenge comes from the body itself. Research on bed-rest analogs, which simulate the fluid shift and deconditioning of reduced gravity, has shown that prolonged unloading changes how efficiently the body manages heat. The temperature threshold at which sweating kicks in rises, meaning you tolerate more heat buildup before your cooling system activates. The sensitivity of sweating itself drops, so when it does start, it is less vigorous. Similarly, the threshold for skin blood vessel dilation during exercise goes up, and the overall increase in skin blood flow during exertion decreases.¹⁵

The visible consequence is that Mars residents would flush less easily during physical activity and might appear drier-skinned. On Earth, a hard workout produces obvious reddening and visible perspiration. On Mars, with a blunted sweating response and reduced cutaneous blood flow, the same effort would produce a less dramatic visible reaction. This is a minor cosmetic detail, but it adds to the overall picture of a subtly different-looking human: paler, less flushed, with drier and thinner skin.

Growing Up on Mars

All of the changes discussed so far describe what happens to an adult body transplanted from Earth. What about children who develop from birth under Martian gravity? The answer involves a fair amount of speculation, since no human has ever developed in reduced gravity, but embryonic animal studies give some clues. When fetal mouse bone precursors were cultured during space shuttle flights, the cartilage still formed and grew, and the basic process of mineralization still occurred. But the cartilage grew less in length than it did on the ground, and the amount of calcium incorporated into the mineralized areas was significantly lower in microgravity, suggesting weaker or less dense bone even from the earliest stages of development.¹⁶ A more recent study confirmed this, finding that mineralization of fetal mouse long bones was significantly retarded in microgravity compared to controls exposed to one-g conditions.¹⁷

These are zero-gravity results, and Martian gravity would provide some mechanical stimulation. But even partial unloading during the critical windows of skeletal development could produce children with longer, more slender limbs and less dense bones than Earth-raised children of the same genetic background. The spine, freed from full gravitational compression during growth, might develop with taller vertebral bodies and thicker intervertebral discs as a baseline rather than as a reversible decompression effect. In other words, the height advantage seen in adult astronauts as a temporary stretch might become a permanent feature in Mars-born people, built into the skeleton from infancy.

Could Radiation Reshape Our Biology?

Mars colonists would face chronic radiation exposure well above anything experienced on Earth’s surface. The planet has no global magnetic field and only a wisp of atmosphere, meaning galactic cosmic rays reach the surface largely unimpeded. Habitats would provide shielding, but some exposure would be unavoidable, especially during surface excursions. Over many generations, this persistent radiation environment could exert selective pressure on DNA repair mechanisms.

There is a precedent for this in the laboratory. Researchers evolved populations of the bacterium E. coli under intense ionizing radiation and found that resistance increased dramatically through mutations in existing, well-known DNA repair genes. The bacteria did not need exotic new repair systems; functional enhancement of the enzymes they already possessed was enough to confer substantial resistance, with as few as three mutations accounting for most of the improvement in some lineages.¹⁸ Humans are vastly more complex, and our generation times are measured in decades rather than hours, so comparable adaptation would take an extraordinarily long time. But if a Mars population persisted for tens of thousands of years under chronic radiation, selection favoring more efficient DNA repair is the kind of subtle molecular change that could accumulate without being visible on the outside, even as it reshaped cancer risk and aging rates.

The Aging Puzzle

What would all these stresses do to how quickly Mars residents age? The answer is surprisingly mixed. One study of astronaut blood samples found that telomeres, the protective caps on chromosomes that typically shorten with age, actually lengthened during spaceflight. But the effect was temporary and potentially harmful: upon return to Earth, telomeres shortened rapidly, and astronauts ended up with more critically short telomeres than they had before flight.¹⁹ The researchers proposed that the elongation was not a sign of rejuvenation but rather a stress response involving alternative maintenance pathways that are activated by chronic oxidative damage.

Meanwhile, a study of the Mars-500 isolation mission, a 520-day ground simulation in which participants lived in confined conditions mimicking a Mars trip, found that epigenetic aging clocks actually ticked backward during the mission. At the midpoint, participants showed an average decrease of about four and a half years on one epigenetic age measure and nearly six years on another, along with a slowdown in estimated stem cell division rates.²⁰ Some of these changes persisted after the mission ended, while others reverted. The meaning of these findings is still debated, but they suggest that prolonged isolation and altered conditions can push biological aging markers in unexpected directions. Whether Mars residents would look younger or older than their Earth age is genuinely unclear. The skin thinning, radiation exposure, and bone loss would push toward a prematurely aged appearance, but the underlying molecular clocks might tell a more complicated story.

Walking on Mars

Even how a Mars resident moved would look different. Studies simulating Martian gravity by suspending participants on body-weight-unloading treadmills at 65 percent unloading (roughly 0.35 g) found that gait patterns shifted noticeably. Stride mechanics changed, with participants adapting to a bouncier, more loping stride rather than the efficient heel-to-toe roll we use on Earth.²¹ Over a lifetime, this difference in gait loading would reinforce the musculoskeletal changes: less impact on the legs, less stimulus for bone and muscle maintenance, and a walking style that Earth visitors would immediately recognize as distinctly Martian.

Put together, the portrait of a long-term Mars resident is someone taller, thinner-limbed, with a rounder face, slightly domed skull, finer jawline, paler and drier skin, and a loping, bouncy walk. Their eyes might be slightly farsighted, their bones lighter, their muscles tuned more for quick movements than sustained endurance. A child born and raised on Mars would carry these traits as developmental defaults rather than acquired changes, making the differences more pronounced with each generation. None of this would make them unrecognizably alien, but anyone who had spent time around Earth humans would notice the differences immediately.