How the Human Body Will Change on Mars: Bones, Muscles, DNA and Life in 0.38 g

Mars Will Change the Human Body. The Question Is How Much.

Mars would not simply be a new place to live. It would be a new physiological environment: 0.38 g, deep-space radiation, isolation and years away from Earth. We know those forces can reshape bones, muscles, circulation, immunity and gene activity. What nobody yet knows is what happens when humans live — and eventually grow up — there.

Mars may be a place humans can reach with machines built on Earth. Living there for years would be a biological experiment with no true precedent.

Imagine the first few minutes after landing on Mars. After six or seven months in near-weightlessness, your legs have to support you again. Your inner ear has a reliable “down” again. Blood once more has a reason to pool toward your feet. Meanwhile, your muscles have lost some of their workload, your bones have been remodeling under reduced mechanical stress, and your immune system has spent months dealing with confinement, disrupted rhythms and microgravity.

Then the airlock opens, and the next experiment begins.

You are not back on Earth. You are on Mars, where gravity is only about 38 percent as strong.

At first glance, 0.38 g sounds reassuring. You can stand, walk and drop a tool instead of watching it float away. A Mars habitat can have floors, chairs and stairs. But having gravity is not the same as having enough gravity.

For human biology, 0.38 g is a much stranger number.

Our species evolved under one gravity. The human skeleton, cardiovascular system, inner ear, muscles and even the way cells sense mechanical forces have spent millions of years adapting to Earth’s constant pull. We have accumulated decades of data about what happens near 0 g in orbit. We also know what healthy humans look like at 1 g.

What we do not know is whether 0.38 g is enough.

That uncertainty is the core of the problem. The usual shorthand — “Mars has 38 percent of Earth’s gravity, so the body will simply experience 38 percent of the load” — is misleading. Different tissues respond to mechanical forces in different ways. A gravity level that is sufficient for one muscle may still be too weak for bone. A body that becomes efficient on Mars may also become less capable of tolerating Earth.

And if people are eventually born there, the questions become much larger.

Mars begins changing the body before you even arrive

A crewed Mars mission is not one environment. It is a sequence of radically different ones.

First comes Earth at 1 g. Then months of near-weightlessness during the interplanetary cruise. Then Mars at 0.38 g, potentially for hundreds of days. Finally, for crews that return, another period of weightlessness followed by the sudden restoration of full Earth gravity.

For a Mars mission, the body would have to cross several gravity regimes in sequence. In weightlessness, the cardiovascular system no longer works against the same head-to-foot pressure gradient. Fluids shift upward, postural muscles lose part of their normal workload, and weight-bearing bones receive weaker mechanical signals. Landing on Mars does not reset those changes instantly; it asks the crew to adapt again.

By the time astronauts reach Mars, they may therefore be asked to perform one of the hardest parts of the mission — landing, unloading equipment, dealing with emergencies and beginning surface operations — while their bodies are still adapting from months of microgravity.

Mars gravity should help, but nobody has lived for months — let alone years — at 0.38 g. Any confident prediction about long-term human health on Mars therefore needs a caveat: most of our evidence comes from near-weightlessness, short partial-gravity experiments, animal studies, cell models and simulations. We are trying to interpolate across a biological gap that has never been tested directly in a human population.

The key scientific question is not whether Mars has gravity. It is whether Mars has enough gravity for each system in the human body.

Bones: Mars may slow the damage without stopping it

Bone looks permanent, but it is constantly being rebuilt. Specialized cells remove old tissue while other cells lay down new material. On Earth, walking, running and simply standing provide mechanical signals that help keep this remodeling system in balance.

Take away gravity and the message changes.

Long-duration spaceflight has repeatedly produced mineral loss in weight-bearing parts of the skeleton. Historical astronaut data often show losses on the order of about 1 percent per month in vulnerable regions, although the rate varies by person, skeletal site and countermeasure program. Modern resistance exercise helps substantially, but it does not make the underlying problem irrelevant.

And bone health is not just a density-scan number. Microarchitecture, strength, fracture risk and the ability to recover after flight all matter. A skeleton can regain mineral while still taking longer to recover its original structure and performance.

Mars gives bones something the ISS does not — continuous loading. The question is how much that helps.

A systematic review of partial-gravity research highlighted the central problem: there are no long-term human data that reveal a safe gravity threshold. The available evidence suggested that levels below roughly 0.4 g may not, on their own, provide enough musculoskeletal and cardiopulmonary stimulus for long-term health. Mars, at about 0.38 g, sits uncomfortably close to that line — but the line itself is still uncertain.

Cell experiments add another clue. Osteoblasts — cells involved in building bone — have shown impaired function under simulated Martian gravity. Animal work suggests partial gravity can protect against some changes seen in microgravity, but the response is neither simple nor proportional.

In other words, 38 percent of Earth gravity does not necessarily mean 38 percent of the problem.

A future Mars resident may need resistance exercise not as a fitness hobby but as daily medical maintenance. Habitats may contain heavy-load exercise systems, compact centrifuges or wearable resistance devices designed to create forces that the planet itself does not provide.

A Mars mission forces the human body through very different gravity environments: 1 g on Earth, near-weightlessness during transit and 0.38 g on the Martian surface.

Muscles: you might feel powerful while becoming weaker

Mars creates a strange psychological trap: everything weighs less.

A 30-kilogram object on Earth still has the same mass on Mars, but its weight is only about 38 percent as large. Lifting equipment could feel easier. Jumping would carry you higher and keep you in the air longer. Walking mechanics would change.

That does not mean the muscles are safe.

In microgravity, the antigravity muscles of the legs and trunk lose size and strength because they are no longer required to support the body in the usual way. Reviews of long-duration spaceflight consistently find major declines in lower-body muscle size and force production even when astronauts exercise.

Partial gravity may protect some aspects of muscle better than others. In one mouse experiment, lunar gravity (about one-sixth of Earth’s) prevented atrophy of the soleus muscle but did not prevent its fibers from shifting toward a different functional type. The result is a useful warning: “muscle health” is not one variable, and different adaptations may have different gravity thresholds.

A Mars resident could therefore perform ordinary work competently while their physiology slowly tunes itself to a lower mechanical demand. The legs, back and postural muscles that quietly work all day on Earth may no longer need to maintain the same reserve. That could be perfectly adequate for life on Mars — and a liability on the day someone tries to live under 1 g again.

For a permanent settler, that adaptation might not be a problem. For someone who plans to return to Earth, it could be.

The heart and blood vessels will have to learn a new planet

Gravity shapes circulation every second of our lives.

On Earth, blood tends to pool in the lower body when we stand. The cardiovascular system compensates by tightening blood vessels, adjusting heart rate and maintaining pressure to the brain.

In weightlessness, that gravitational column largely disappears. Fluids move upward. The body initially interprets the central fluid shift as excess volume and begins adjusting. Over time, cardiovascular structure and regulation adapt to the new environment.

That is why astronauts returning from orbit can become light-headed when they stand. The problem is not that the heart has “forgotten” gravity; the whole pressure-regulation system has adapted to an environment where gravity was no longer pulling blood toward the legs in the usual way.

Mars is easier than Earth in that respect, but it is not zero gravity. The first hours and days after landing could be especially important. A crew that has been floating for months must suddenly walk in a gravity field, operate a pressure suit and respond to emergencies.

Years at 0.38 g could eventually produce a cardiovascular system that is entirely adequate for Mars yet poorly prepared for Earth. We do not know whether long exposure to partial gravity would alter blood volume regulation, vascular function, cardiac structure or aerobic capacity in ways that are fully reversible.

This is one reason a Mars settlement might eventually need a “gravity prescription” just as modern medicine prescribes exercise: a daily dose of mechanical loading large enough to keep the cardiovascular and musculoskeletal systems capable of tolerating higher gravity when necessary.

Blood and kidneys: the less glamorous risks may matter a lot

Spaceflight also changes systems that rarely appear in cinematic visions of Mars. One is blood. Research from the MARROW experiment found that astronauts destroy red blood cells faster in microgravity than on Earth, a phenomenon often described as space anemia. The effect becomes especially important when a crew arrives somewhere that suddenly requires physical work again.

The kidneys face a different problem. As bone is broken down, more calcium can enter the urine. Combine that with relative dehydration, altered urine chemistry and limited medical options, and kidney stones become much more than a painful inconvenience. NASA treats renal stones as a serious exploration-class medical risk because a blocked or infected urinary tract is hard enough to manage on Earth and far harder when the nearest hospital is another planet.

Mars gravity may reduce some of these effects compared with weightlessness, but again there are no long-duration human data. That recurring phrase — “we do not know yet” — is not a weakness of the science. It is the central fact of partial-gravity medicine.

Your eyes and brain are part of the gravity experiment too

Some of the strangest effects of spaceflight happen in the head.

When fluids shift upward in microgravity, changes can occur around the eye and brain. Astronauts can develop a cluster of findings known as spaceflight-associated neuro-ocular syndrome, or SANS, which can include changes in the shape of the eye and structures near the optic nerve.

The vestibular system — the sensory machinery of the inner ear that helps the brain understand motion and orientation — also has to adapt. In microgravity, the familiar relationship between head movement and “down” vanishes. When gravity returns, balance and coordination need recalibration.

Mars will restore a gravitational reference, which should help. But again, the unknown is whether 0.38 g produces a stable human baseline or an intermediate adaptation with its own long-term consequences.

The brain faces another challenge that gravity cannot solve: radiation.

Animal studies using charged particles relevant to galactic cosmic rays have raised concerns about neural structure and cognition. The translation to humans is uncertain: laboratory exposures cannot perfectly reproduce years of low-dose, mixed-particle radiation in deep space. The evidence therefore does not justify a neat prediction such as “Mars radiation will cause cognitive decline.” It does justify treating the central nervous system as an unresolved risk that still needs better human and long-duration data.

Radiation may matter more than low gravity

On Earth, the atmosphere and magnetic field do an extraordinary amount of invisible work for us. Mars has neither a thick atmosphere nor a global magnetic shield comparable with Earth’s.

That changes the radiation environment completely.

The two major threats are galactic cosmic rays — highly energetic particles arriving continuously from beyond the Solar System — and solar particle events, which can produce intense bursts of radiation during solar activity.

The Radiation Assessment Detector that traveled to Mars with Curiosity gave researchers something invaluable: direct measurements of the radiation field during transit and on the Martian surface. Those data show why mission design matters. Depending on solar activity, transit time, shielding and time on the surface, total exposure can reach the range of fractions of a sievert and, in some modeled scenarios, more than a sievert. One published analysis estimated galactic-cosmic-ray dose equivalents of roughly 0.65 Sv near solar maximum and 1.59 Sv near solar minimum for a return-mission scenario. Those figures are scenario-dependent, not a universal Mars dose.

Even then, the number does not translate cleanly into a single “percent chance of cancer.” Galactic cosmic radiation contains high-energy particles that differ from the exposures behind most terrestrial risk data, and biological uncertainty remains large. For settlers, the practical lesson is clearer than the risk percentage: radiation protection has to be designed into the habitat from the beginning.

Early habitats are likely to place water, food stores and hydrogen-rich materials around occupied areas. Martian regolith could be piled over living spaces. Some settlements may place their most heavily occupied rooms underground or inside natural terrain features. Storm shelters would provide extra protection during major solar events.

A transparent glass city exposed directly to the Martian sky is one of the least realistic images of early colonization.

The most dangerous radiation exposure may occur during the months in deep space, while a Mars settlement could use regolith, water and protected shelters to reduce exposure on the surface.

DNA will change — but not in the way science fiction usually means

“Will Mars change human DNA?” sounds like one question, but it mixes together three different ideas: physical damage to DNA, changes in how genes are switched on and off, and inherited genetic change across generations.

First, radiation can damage DNA. Ionizing particles can break strands and create other molecular lesions. Cells repair much of that damage, but repair is not perfect — one reason radiation exposure can increase long-term cancer risk.

Second, spaceflight can change how genes are used. The NASA Twins Study and later space-omics work found broad shifts in gene expression, epigenetic regulation, immune signaling and oxidative-stress pathways. These are biological responses to an unusual environment; they are not equivalent to rewriting the genetic code.

Third, mutations in reproductive cells can in principle be inherited. That is the mechanism by which radiation could contribute to genetic change across generations. But there is no evidence that Mars settlers would rapidly become a genetically distinct population, and there is certainly no timetable for such a transformation.

A useful analogy is an instruction manual: gene expression changes which pages are being read and how strongly; a mutation changes some of the letters in the manual itself. Both matter, but they are not the same biological event.

The 2024 Space Omics and Medical Atlas brought together molecular data from several spaceflight cohorts and showed just how broad those responses can be. In the short-duration Inspiration4 cohort, most measured molecular changes trended back toward baseline during recovery. The larger lesson is that the body is remarkably plastic — but not every change disappears on the same schedule.

The NASA Twins Study reached a similarly nuanced conclusion after a 340-day mission: many physiological and molecular changes returned toward baseline, while a smaller number of findings — including some gene-expression changes and signs of DNA damage — persisted after return.

What the evidence does not support is the familiar science-fiction shortcut in which a few years on Mars create a new kind of human. The real biology is slower, messier and more interesting: damage, repair, adaptation and regulation are happening at the same time.

The immune system may become less predictable

The immune system is one of the systems known to shift during spaceflight, but the pattern is not a simple story of “weaker immunity.” Some responses are dampened, others are altered, and stress biology is mixed into the picture.

Long-duration astronaut studies have documented changes in white blood cell distributions, T-cell function and cytokine signaling. Latent viruses — viruses that remain quietly inside the body after earlier infection — can reactivate during spaceflight. Stress, disrupted sleep, radiation, altered gravity and the closed habitat environment may all contribute.

On Mars, those changes matter for a very practical reason: evacuation is not an option. Depending on planetary geometry, even a radio conversation with Earth can involve many minutes of delay each way. A serious infection, allergic reaction or wound complication has to be diagnosed and managed locally.

A settlement therefore needs more than a first-aid kit. It needs autonomous diagnostics, a carefully monitored microbial environment, vaccines and medications selected for long shelf life, and medical systems capable of working when Earth is only a delayed consultant.

The immune system will also interact with the habitat microbiome. Humans bring trillions of microorganisms with them. Mars settlers will not live in a sterile box; they will create a miniature ecosystem made of people, surfaces, water systems, food production and microbes. Managing that ecology may become part of routine medicine.

Mars could change development more than it changes adults

For the first explorers, the scientific problem is adaptation.

For the first children born away from Earth, the problem is development.

An adult skeleton has already been built under 1 g. An adult cardiovascular system learned to stand, walk and run under Earth gravity. A fetus and child would be constructing those systems from the beginning under 0.38 g.

That is a completely different experiment.

There has never been a documented human pregnancy during spaceflight, so we have no direct human data for conception, gestation or fetal development in microgravity or partial gravity. For a permanent settlement, this is one of the largest unanswered biological questions.

Animal and cell studies offer hints, not a verdict. Mouse preimplantation embryos have developed in space, but one experiment found lower blastocyst quality together with DNA damage and epigenetic abnormalities; low-dose radiation appeared to explain much of the effect better than simulated microgravity alone. Other stages of reproduction and development may respond differently.

Research also suggests that ovarian follicles and sperm-producing cells can be sensitive to radiation. Reviews of space reproductive biology repeatedly reach the same conclusion: the evidence base is too small to predict safe human conception, pregnancy and childhood development beyond Earth.

This is not a side question for some distant generation. It determines whether Mars can ever progress from an outpost that rotates crews from Earth to a genuinely self-sustaining human settlement.

A research outpost can depend on Earth for every new crew. A self-sustaining settlement, by definition, cannot.

We have decades of data on adult astronauts in microgravity, but almost no direct evidence about human pregnancy, childhood or multigenerational life in partial gravity.

Would people born on Mars look different?

Possibly — but this is the point where evidence becomes thin and speculation has to be labeled clearly.

The fastest differences would not necessarily be genetic. They could be developmental.

A child who grows up under lower gravity might build a different skeleton and muscle system simply because the mechanical forces acting on the body are different. Human bodies already respond to mechanical loading on Earth: athletes, sedentary people and people confined to bed develop different bone and muscle characteristics without becoming genetically different populations.

Mars could amplify that principle.

A Mars-born adult might, in theory, develop lower bone mass, different muscle distribution or altered cardiovascular regulation compared with an Earth-born person. The body might become exceptionally well adapted to living and moving at 0.38 g while becoming poorly suited to 1 g.

Would Martians become taller? It is plausible that lower axial loading could influence posture and spinal geometry, but confident predictions about tall, thin “Martian humans” are science fiction at this point.

Would natural selection eventually change the population genetically? If a large, reproductively isolated population lived on Mars for many generations, evolutionary pressures could act. Radiation exposure, low gravity, habitat conditions, diet and reproductive patterns could all influence selection.

But natural evolution is slow, especially in a technological civilization.

Humans might also intervene medically long before natural selection produced large population-level differences. Exercise systems, drugs, radiation shielding, reproductive medicine, gene therapies or future forms of genetic engineering could shape biology more strongly than natural selection.

If that happens, much of what we call “Martian adaptation” may be managed or engineered rather than simply left to evolution.

The body may adapt to Mars and become less compatible with Earth

The deeper question is whether a body optimized for Mars should always be described as damaged.

We usually think of health as a universal concept: stronger bones are healthier bones, better cardiovascular tolerance is better health.

But health is partly environment-specific.

A body optimized for Mars does not need to carry Earth-level skeletal mass if it will never experience Earth gravity. Maintaining extra tissue costs energy and nutrients. From a purely biological perspective, losing some capacity that is unnecessary on Mars could be adaptation rather than disease.

The problem appears when the person changes environments.

An Earth-born settler who spends ten years on Mars might need a long rehabilitation period before safely returning to Earth. A Mars-born adult could face a much more serious challenge because their body might never have developed under 1 g in the first place.

The result could be a biologically asymmetric form of interplanetary travel. An Earth-born adult at least begins with a skeleton, cardiovascular system and motor system built under 1 g. A Mars-born adult might never have had that baseline.

For visitors from Earth, Mars would be a lower-gravity environment to adapt to.

For someone developed entirely on Mars, Earth could be the harsher destination.

Could artificial gravity become part of everyday Martian medicine?

If 0.38 g turns out to be too low for some systems, the most direct solution is to manufacture more gravity.

Rotation can create acceleration that the body experiences as gravity. Spinning an entire settlement is an enormous engineering challenge, but compact centrifuges for exercise or short daily exposure are much simpler than building a whole rotating city.

A future Mars habitat might therefore include a rotating exercise module. Residents could cycle or perform resistance work while rotation increases loading on the legs. Crews preparing to return to Earth might train at progressively higher effective gravity. If future research ever showed that controlled gravity exposure benefits development, the same principle could eventually become relevant to children — but that remains a much more speculative and ethically sensitive possibility.

We do not yet know the correct dose.

Is one hour per day at 1 g enough? Two hours? Would short periods at 2 g be better? Does the body need continuous gravity, or only repeated strong mechanical signals?

Those are answerable experimental questions, and they may become some of the most important questions in human spaceflight research.

The same is true for drugs that reduce bone loss, nutritional strategies, advanced resistance training and personalized monitoring. Future settlers may wear continuous sensors that track cardiovascular function, sleep, radiation dose, muscle performance and biochemical markers, allowing countermeasures to be adjusted person by person.

A successful Mars settlement may have to recreate some of the protection Earth gives the human body for free — mechanical loading, radiation shielding and continuous medical monitoring.

So what would the first real Martian human be like?

Probably much more familiar than science fiction suggests.

The first long-term settlers would still be unmistakably human. What would distinguish them is not a new anatomy but a new physiological history: years of altered bone remodeling, different muscle loading, cardiovascular adaptation and accumulated radiation exposure.

Many spaceflight changes are at least partly reversible after return to Earth, but recovery is uneven. Some systems recover quickly, others take months, and some measurements can remain altered longer. We do not yet know what a decade of 0.38 g would add to that picture.

The truly unfamiliar biology begins when children are conceived, born and raised on another planet. We do not know whether 0.38 g is enough for normal skeletal development. We do not know how deep-space radiation alters lifetime reproductive risk. We do not know whether a Mars-born person can safely tolerate Earth gravity as an adult.

None of this makes Mars impossible.

It does mean that settlement cannot be treated as a transportation problem with a habitat attached.

Rockets can solve the distance. A settlement has to solve the biology.

The most important experiment on Mars may be us

For decades, the hardest question about Mars seemed to be whether we could land people there.

That is no longer the only hard question.

A civilization on Mars would have to learn the minimum gravity needed by each organ system, how to shield people from a radiation environment unlike Earth’s, how to maintain immunity and microbiomes inside closed habitats, how to manage pregnancy and development, and how much biological change is acceptable before “adaptation” becomes a one-way door.

The answer will not arrive in a single mission.

It will emerge from astronauts, animal studies, organoids, partial-gravity experiments, lunar missions, centrifuge research and eventually the first people who live for years under a Martian sky.

Mars may never turn humans into a new species.

But if people live there long enough, Mars will force us to answer something we have never had to ask before:

how much of the human body is truly human — and how much of it is Earth.

FAQ

How strong is gravity on Mars?

Mars surface gravity is about 38% of Earth’s. A person’s mass would not change, but their weight would be much lower.

Would humans lose bone on Mars?

Possibly. Continuous 0.38 g is likely to provide more loading than microgravity, but long-term human data do not exist. Current partial-gravity research does not show that Mars gravity alone is enough to preserve normal bone.

Does spaceflight change DNA?

Space radiation can damage DNA, and spaceflight changes gene expression and epigenetic regulation. Those are not the same thing as humans rapidly evolving new inherited genes.

Could children be born on Mars?

We do not know. There has been no documented human pregnancy in space, and no human data show whether conception, pregnancy and development can proceed safely under Martian gravity and deep-space radiation.

Would Mars-born humans be able to live on Earth?

This is unknown. If development at 0.38 g produces lower bone strength or different cardiovascular conditioning, adapting to 1 g could be difficult. The question has never been tested in humans.

Could artificial gravity solve the problem?

It may help, especially during transit or as a daily countermeasure, but scientists do not yet know how much artificial gravity or how much daily exposure is needed to protect every body system.