The Unbearable Lightness of Martian Gravity: Health, Evolution, and Colonization

SpaceX has repeatedly taken on the so-called impossible and done it. But Elon Musk may have taken a step too far when he presented in September 2016 a vision of long-term Mars colonization. Going well beyond his proposed Mars transport system, he spoke of a self-sustaining Martian city of a million inhabitants. Humanity, he said, could gain the advantage of a second home at which to avoid extinction if Earth became uninhabitable.

He discussed engineering, propulsion, efficiency, and finance, but the toughest limiter on colonization could be something far harder to engineer around. A critical factor that could well limit long-term Mars stays was missing from Musk’s narrative entirely: the relationship between Earth-evolved biology and Earth gravity.

Musk discussed gravity in terms of transport and even mentioned it as a source of fun — travelers and residents would be able to jump high and lift heavy things with ease. But gravity, never to be neglected in rocketry, may also have to be weighed far more soberly in health terms. Not only could low Martian gravity, at just 38% of Earth’s, have detrimental long-term effects on all major physiological systems, it could also impair reproductive success. If so, colonists could end up struggling to produce offspring, a defining factor for successful colonization anywhere.

As Musk showed, many technical issues can be addressed through innovation and trial and error. Radiation can be shielded against (mostly, hopefully). Chemical plants can be built, and water, air, and heat can be manufactured. In addition, psychological and other factors in ship and hab confinement are already under study and may be manageable. But the most recalcitrant problems of all — with health — are likely to take shape and become clear only over time.

Zero g health research not promising

Existing research on the health effects of zero g indicate potential for serious and pervasive negative health effects from gravity changes. Long-term exposure to Mars g might well produce at least some of the same types of effects as zero g, just more slowly and less severely.

Zero g has been found to produce not only muscle atrophy in astronauts, but a host of other health issues, which isometrics and exercise bikes can offset only partially. Falling bone mineral density and circulatory issues, including impaired heart health, have been found in returnees. These issues do not just vanish after landing. Following only limited stints of zero g, indicators of negative cardio-vascular effects in astronauts have remained for years.

Going to Mars and returning are also starkly asymmetrical health challenges. Adapting to lower gravity is much easier than adapting to higher. Under lower g, muscles are suddenly stronger and bones more resilient relative to environmental demands, but under higher g — say, upon return to Earth — muscles are suddenly weaker and bones more brittle.

Limited existing zero g research suggests negative effects on three major physiological systems: muscular, skeletal, and circulatory. These are hardly mere footnotes to the task of transporting Silicon Valley brains across the expanse. And there is no reason to expect neurological and reproductive systems to get free passes either.

Studies of zero-g reproduction and embryonic development in lab animals suggest potential issues for Mars colonization that are far more profound than individual health challenges. Producing offspring is mission-critical for any attempt at colonization. However, reproduction among spacefaring rodents has gone quite badly thus far. Mice in an earthside control group produced normal embryos, but their rodent colleagues loaded on a Space Shuttle mission produced no embryos at all. A group of rats sent into orbit to engage in weightless hanky-panky managed to produce some pregnancies, but no births. Every pregnancy spontaneously terminated.

These were just a few rodent studies, but in the worst case, the results could represent the first available illustrations of a much deeper structural issue.

Earthly life evolved in 1g. What if it also depends on it?

The “plans” encoded in DNA for growing an organism are completely unlike engineering plans for a rocket or hab. They are sets of highly contextualized interacting decentralized procedures. Each cell continually responds to its immediate environment. It takes cues, for example, from the type of cell it has become, from the types of cells around it and their proximity, and from the hormonal, mineral, nutritional, and other signals in the blood supply.

Now consider that each embryonic process for each earthly animal has evolved in and taken place under 1g. It would be extraordinary if these processes had not evolved to depend to some degree on 1g. To not so adapt, were there any advantages to doing so, would have been a pure loss. In the absence of any case of non-1g, there would never have been any evolutionary cost to optimizing for 1g. Optimizing — so far — would have always been a case of all reward and no risk. Biological evolution is an impressive adaptive system, but even it has no way to develop risk hedges against conditions that never faced any organisms, even over several billion years.

Gravity changes would probably be least likely to bother simple organisms such as bacteria. However, the more complex the developmental process, the more likely that at least some critical elements would have been fine-tuned over evolutionary time to the context of always having taken place under 1g.

Consider the contrasting case of temperature. Temperature is an ever-fluctuating variable to which earthly life is widely adapted over various ranges of tolerance, both across species and to a lesser degree within each living organism. People can become accustomed to a wide range of temperature demands, such as swimming in icy water or running marathons in hot weather, provided they have time to acclimate.

Temperature has varied widely over the eons and also does so on many shorter time cycles, from day to night and from week to week, and by season, ice age cycle, altitude, and geography. Temperature adaptation both by individuals and across generations therefore enjoys a vast body of evolutionary precedent. Other variables such as atmospheric gas composition, pressure, and ambient and solar radiation have also all changed substantially with time.

Gravity has not. Instead, it has been the closest thing to a constant for all life. Despite the many adaptations to aquatic, aerial, and land life, with their differing structural support and movement resistance characteristics, low g itself is something that biological evolution has had no direct opportunity to tackle.

Mars g is likely to be better than zero g for development because it would at least supply some vertical orientation and relative weights for embryonic development to reference, albeit with a much weaker signal. Still, this potentially deal-breaking issue for colonization remains and it cannot be clarified without substantial Mars g specific research. Understanding the effects of low g on reproduction and embryonic and childhood development is also mission-critical for any colonization proposal. These topics deserve a high priority for their role in refining the scope of Mars habitation visions.

Another Earthexit option

Even if long-term Mars habitation does prove too destructive of human health, or makes reproduction impossible, the Earthexit story would not necessarily be over. Intermittent habitation on Mars would still be feasible, while spacecraft and stations capable of approximating 1g could provide environments for healthier longer-term living.

Large cylindrical space habitats with axial rotations calibrated to the craft’s radius can approximate 1g over a large interior surface area. In addition, the strength of artificial gravity varies with distance from the axis of rotation. For example, such a habitat might have three stacked cylindrical levels offering 0.9g, 1.0g, and 1.1g, respectively. Dancing is on the 0.9g level and strength training on the 1.1g level.

Even if Mars habitations themselves might not end up being able to responsibly support longer-term stays, temporary residents could rotate out to 1g stations in Mars orbit to get their gravity fix — and to reproduce.

Biological perspectives and research also critical to Mars planning

Musk’s vision of Mars colonization kicked off by emphasizing the engineering and economics of getting there. It is true that without overcoming these challenges, long-term health on Mars would never become an issue. Yet for visions of colonization, mission-critical issues to consider must also extend to the length of time that hopeful Martian arrivals can expect to stay, in what state of health, with what likelihood of producing any offspring (healthy or otherwise), and with what prospects of ever functioning normally again under 1g, either on Earth or in Mars-orbiting 1g stations.

These considerations strongly support the idea that lab rodents probably ought to be the first serious Mars colonists — and that for quite some time. Their mission: to try to live (and breed) where no earthly life has lived before.