The Science of Becoming "Interplanetary": How Can Humans Live on Mars?

Perhaps, someday soon, people could be traveling to Mars regularly in the same way tourists travel to different countries. Those visiting the Red Planet might even be treated to messages like this:

"Good morning, all, and welcome to Mars! The time is twelve fifty hours, Standard Martian Time, the month is Khumba, and the day is the fifteenth. Ambient temperatures on this beautiful day around the equator are a balmy twenty degrees celsius (68 deg. F). If this is your first time to the Red Planet, congratulations on booking accommodations that coincide with Martian Spring! 

"Yes, that means that the Dust Storm season is now behind us, and we can all look forward to the many sunny months ahead. It also means that those who have booked the "Red Planet Adventure Tour" will have the chance to conduct surface walks with clear and unobstructed views of the Martian surface.

"We hope you are enjoying the view of our fair world as we descend to the surface. A reminder that we offer a full suite of accommodations here aboard the MarsLift. Since you'll be with us for a few days before we touch down, we recommend you take full advantage of our facilities. Our velocity has been adjusted to ensure you enjoy the sensation of Martian gravity for the entire descent.

"In the meantime, we invite you to take a look up at the Martian landscape using the ARES app. Prominent features and the story of how they were discovered will be indicated in your heads-up display. If you cannot access this app via neural link, we provide display glasses for just this purpose.

"This includes Valles Marineris and Olympus Mons, the two largest features of their kind in the Solar System. Just think: in a few days, you could be exploring these locations in person!"

Historically, Mars has gone by many names. Auqakuh, to the Inca; Al-Qahira, to the ancient Arabs; Huǒxīng, to ancient Chinese astronomers; Horus, to the ancient Egyptians; Kasei, in Japan; Ma'adim, to the ancient Hebrew, Mangala, to the ancient Hindu astronomers, and Girunugal to the Mesopotamians (Sumerians).

Western astronomical traditions are drawn from the ancient Romans, who derived their traditions from the ancient Greeks, who adopted some of theirs from the Babylonians and Sumerians. What was Girunugal to the Sumerians was Nergal to the Babylonians, Ares to the Greeks, and Mars to Rome. While the names have changed through time, the association with war, strife, blood, and fire — thanks to Mars' red appearance — have remained fixed throughout.

Astronomers often refer to Mars as "Earth's Twin" because of the similarities this celestial neighbor has with Earth. For starters, it is the second most-habitable planet in the Solar System, at least by our standards. Like Earth, it has polar caps, evidence of surface and subsurface water, a daily cycle that lasts close to 24 hours, and a tilted axis that causes seasonal variations.

Beyond that, the contrasts between our two planets are pretty extreme. Mars is an extremely cold, desiccated body with a very thin atmosphere compared to Earth. Its surface is drier than the driest deserts on Earth, water can only exist on its surface in ice form, and the level of irradiation it receives is enough to kill off most species of terrestrial plants and animals.

Yet, humans could establish a permanent human presence on the Red Planet with the right kind of hard work and technology. And with a LOT of hard work, the planet could be ecologically transformed (aka. terraformed) to the point that it could be "Earth's Twin" in just about every sense of the word. 

Thin, cold, and toxic

Unlike Earth, Mars has no planetary magnetic field (or "magnetosphere.") On Earth, this field is believed to result from action in the planet's interior. This consists of the molten outer core revolving around a solid inner core (in the opposite direction of Earth's rotation), which creates a dynamo effect that generates a magnetic field.

According to multiple scientific investigations, Mars had a magnetic field about 4 billion years ago. However, its lower mass and density caused it to cool faster, leading the outer core to solidify while the inner core remained molten. This arrested Mars' interior's dynamo effect and its magnetic field disappeared.

Consequently, Mars' once-dense atmosphere was slowly stripped away by solar wind over the next few hundred million years. While it is replenished by outgassing, the atmospheric pressure today is less than 1% that of Earth's. And what little atmosphere it has is a toxic plume, composed predominantly of carbon dioxide, argon, and nitrogen, with traces of methane and water vapor.

The atmosphere is not only unbreathable, but also incredibly thin. Measured on the surface, the air pressure averages about 0.636 kPa, roughly 0.6% that of Earth's at sea level. And whereas Earth's atmosphere is composed of 78% nitrogen and 21% oxygen, Mars' atmosphere is a toxic plume composed of 96% carbon dioxide, 2.6% molecular nitrogen, 1.9% argon, carbon monoxide, and water vapor.

Because Mars is about 50% further from the Sun, it receives much less solar radiation and heat from our Sun. Another factor is Mars' thin, tenuous atmosphere, which cannot absorb much heat from the Sun. As a result, the average surface temperature on Mars is a frigid -82 °F (-63 °C), but this ranges considerably based on location and the time of year.

During a Martian summer, temperatures can reach as high as 95 °F (35 °C) around the equator at midday. During winter, temperatures plummet to as low as -233 °F (-135 °C) in the polar regions. But even when temperatures are at their warmest, the very thin atmosphere ensures that most of the heat is lost just a few inches from the surface.

To illustrate, if it were possible for a person to stand naked on the surface of Mars, their body would feel some severe temperature differences. While sand would feel very warm beneath their toes, everything above their ankles would be freezing. Above the waistline, temperatures could get low enough to freeze them cryogenically.

Radiation and dust storms

Then there's the small matter of all the radiation people would be exposed to. On Earth, human beings in developed nations are exposed to an average of 0.62 rads (6.2 mSv) per year. Because Mars has a very thin atmosphere and no protective magnetosphere, its surface receives about 24.45 rads (244.5 mSv) per year — more when a solar event occurs.

NASA has established an upper limit of 500 mSv per year for astronauts, and studies have shown that the human body can withstand a dose of up to 200 rads (2000 mSv) a year without permanent damage. However, prolonged exposure to the kinds of levels detected on Mars would dramatically increase the risk of acute radiation sickness, cancer, genetic damage, and even death.

Dust storms are a regular occurrence on Mars and happen whenever the lower atmosphere heats up, causing air currents to pick up dust and circulate it around the planet. This can occur when Mars is at the closest point in its orbit to the Sun (perihelion) and can also be exacerbated due to variations in temperature between the hemispheres — i.e. when one is in summer.

At times, dust storms on Mars can grow to the point that they encompass the entire planet. These are known as Martian Global Dust Storms (GDS), and they occur only in the latter half of the Martian year. Other than that, dust storms are temperamental and happen with every passing "dust storm season," which coincides with winter in each hemisphere.

Martian gravity

And then there's the matter of Martian gravity, which is roughly 38% that of Earth's (3.72 m/s² or 0.379 g). While scientists do not yet know what effects long-term exposure to this level of gravity would have on the human body, multiple studies have been conducted into the long-term effects of microgravity — and the results are not encouraging.

This includes NASA's seminal Twins Study, which investigated the health of astronauts Scott and Mark Kelly after the former spent a year aboard the International Space Station (ISS). In addition to muscle and bone density loss, these studies showed that long-duration missions to space led to diminished organ function, eyesight, and even genetic changes.

It is fair to say that long-term exposure to around 1/3rd of Earth-normal gravity would have similar effects. Like astronauts serving aboard the ISS, these effects could be mitigated with a robust exercise and health monitoring regiment. But the possibility of living under these conditions, and children being born in them, raises a whole lot of unknowns.

But enough of the bad news! When it comes right down to it, there are several ways to overcome these challenges so that people can lead comfortable lives on Mars!

Regolith and ice

An absolute must for missions destined for the Moon, Mars, and other destinations in deep-space is the ability to be as self-sufficient as possible. This is known as In-Situ Resource Utilization (ISRU), and it entails using local resources to provide the necessities — like propellant, oxygen, water, building materials, and energy.

For starters, much of Mars is covered in silicate mineral dust and sand (aka. regolith) resulting from wind and (past) water erosion. This dust could be combined with bonding agents to create "Martian concrete," or bombarded with microwaves and printed as molten ceramic. The resulting shell would provide radiation protection, while a flexible inner structure would serve as the main habitat.

Another possible method would be to harvest local ice, which is plentiful in Mars' northern lowlands near the polar ice cap. This ice could then be combined with aerogel or other bonding agents and used to create "ice houses" that would protect against the radiation and the elements while also ensuring a view of the landscape.

The availability of water ice is also crucial and requires that landing sites be selected and scouted well in advance. The Northern Lowlands, which sit just south of the northern polar ice cap, have abundant supplies of water in the form of permafrost and subterranean ice. Recent observations also indicated that Valles Marineris (Mars' massive canyon system) has lots of ice just a few feet (1 meter) from the surface.

Yet another possibility would be to build habitats inside caves and stable lava tubes, many of which have been observed in Mars' Tharsis region. Much like the lava tubes on the Moon, these tubes could be accessed through collapsed sections (aka. "skylights"). These subterranean cave systems would provide natural radiation shielding and could be sealed and pressurized.

Recent studies have shown that there may be plenty of subsurface ice in these tubes, many of which are located in the equatorial region of Mars. This region is far more amenable to human settlement than the poles because it experiences warmer temperatures. Thanks to Mars' lower gravity, these lava tubes are significantly larger than similar features on Earth.

In terms of energy, solar arrays and wind farms are the most obvious means of providing electricity in the Martian environment. However, the thin nature of the atmosphere and the fact that Mars receives less light than Earth means that these methods will not be enough on their own. During the Dust Storm season, solar arrays will become effectively useless, and may even need to be taken down to prevent damage.

Luckily, NASA and China are developing compact nuclear reactors for long-duration missions off-world. NASA's current Fission Surface Power (FSB) concept, which began as the Kilopower project, will reportedly generate 40 kilowatts of power for ten years. China is seeking to create something even more powerful, allegedly 100 times as much!

Regardless, a combination of wind, solar, nuclear, fuel cells, and even biomass reactors will ensure that habitats on Mars (and their inhabitants) will have all the electricity they need to live, work, and thrive on Mars.

"The Greening of Mars"

Terraforming Mars comes down to three major steps: warming the surface, thickening the atmosphere, and altering the environment to something Earth-like. Luckily, these three tasks are interconnected and mutually beneficial.

Thickening the atmosphere will warm the surface and reduce the amount of radiation the planet receives. Similarly, warming the surface will melt the polar ice caps, releasing frozen CO₂ and water vapor that will further thicken and warm the atmosphere. Introducing terrestrial microbes, lichens, plants, and animals will help stabilize the environment, create oxygen through photosynthesis, and establish a life cycle on the planet that will ensure long-term habitability.

With the right kind of commitment and resources, messages of welcome for people traveling to Mars could sound like this someday:

"Good morning, all, and welcome to Mars, humanity's home away from Earth. The time is oh-eight-thirty hours, Standard Martian Time, on this beautiful day of Sagittarius the third. Today's average temperature is a balmy 73 degrees Fahrenheit (23 deg. Celsius) in the northern hemisphere, while the south continues to experience a cool winter, with an average temperature of minus thirty.

"As we make our descent into the great city of Nergal, we encourage you to enjoy the view of Oceanus Borealis, the largest surface ocean in the Solar System. If you peer straight down, you'll see the great river valley of Valles Marineris and its Outflow Channels emptying into Chryse Mare. Look to your left, and you will see the Tharsis region and its prominent mountains still standing tall. From north to south, these massive features are Ascraeus, Pavonis, and Arsia Mons.

"To the left of them, you will see Olympus Mons, the tallest planetary mountain in the Solar System. Today, this feature towers at the edge of the warm water shallows of Amazonis Mare. For those who do not use ocular implants, the magnification app in the display glass will show you the dense jungles that surround the base of this behemoth and the beautiful sandy beaches and turquoise waters just beyond.

"As the Carriage comes about during our descent, you will get a chance to see the Southern Highlands. Things to be on the lookout for include the Thousand Lakes region, which is bordered on either side by the Great Lakes of Mars — Argyre and Hellas Mare. Those destined for this area are sure to enjoy the endless shorefronts, hot springs, and fishing retreats.

"We remind you that Martian flora and fauna are adapted to the local environment and that the export of local species is strictly prohibited by interplanetary law. All travels coming to and leaving from the surface will be subject to bioscans to determine if they are carrying harmful microbes or biota.

The first step is to trigger a greenhouse effect on Mars. There are several proposed methods for doing this. For one, "super greenhouse gases" like ammonia, methane, or chlorofluorocarbons (CFCs) could be introduced into the Martian atmosphere. This would thicken the atmosphere, raise global temperatures, and melt the polar ice caps.

There's also the possibility of using orbital mirrors to concentrate solar radiation and direct it toward the Martian surface. Positioned near the poles, these mirrors would sublimate the ice sheets and contribute to global warming. Once the atmosphere is thickened and warmed, conditions will be stable enough for water to flow on the surface again.

This will also require drilling into the ground to release additional pockets of methane and water. Over time, this will lead to precipitation and the gradual disappearance of dust storms. The introduction of microbes, lichens, plants, and eventually animals will help stabilize and enrich the soil with organic nutrients and create a complete life cycle on the planet.

Photosynthetic organisms will need to be introduced first (possibly with some genetic modifications), ranging from cyanobacteria to purple-pigment proteobacteria. These microbes would metabolize atmospheric CO₂ to create oxygen gas. Over time, this would lead to a "Great Oxygenation Event" similar to what happened on Earth about 2.4 billion years ago.

In time, the atmosphere will become warm and breathable to the point that humans can wander around outside without a pressure suit. To ensure long-term stability, it will also be necessary to introduce an artificial magnetic field to prevent Mars' new atmosphere from being stripped away (and offer more radiation protection).

Several options for this have been proposed over the years, some that are smaller in scale and some that involve megascale engineering. At the smaller end of things, research has shown that electromagnetic toruses could be built around surface bases that would generate artificial magnetic fields. Another idea is to use Phobos, the larger of Mars' two moons, to generate a plasma torus around Mars.

Since Phobos orbits Mars every eight hours, generating this torus would be as simple as ejecting matter from the surface and using electromagnetic and plasma waves to drive a current strong enough to create a planetary magnetic field. A similar idea (but bolder!) is to build a series of planet-encircling superconducting rings to generate an artificial magnetic field.

At the Planetary Science Vision 2050 Workshop in 2017, Dr. Jim Green (Director of NASA's Planetary Science Division) proposed placing an artificial magnetic shield at the Sun-Mars' L1 Lagrange Point. This would create an artificial magnetotail that would shield Mars from incoming solar particles and allow the atmosphere to thicken over time.

An even more ambitious suggestion is to restore geological activity so that Mars will generate its own magnetic field again. Once again, there are many potential options available. One way would be to detonate a series of thermonuclear warheads to melt the outer core. The second involves running a massive electric current through the planet to heat the metallic outer core and melt it again.

However, there are limits to how much we can alter Mars to suit our needs. With only 38% of Earth's gravity, Mars would likely retain less than the 101.325 kilopascals (kPa) of air, which we are used to on Earth. Given its distance from the Sun, Mars would always receive about 40% of the solar radiation Earth does, limiting how warm outside temperatures get. 

Then again, the magnetic shield mentioned earlier could help out with that. If the shield could polarize itself as needed, it could block out harmful radiation while enhancing the parts of the spectrum where photosynthesis occurs. This includes the yellow-red part of the spectrum (565-750 nanometers) and the yellow-green (500-590 nm) part for purple plants.

While adapting to Martian gravity will always be a challenge, there are strategies for dealing with this as well. In the long run, rotating pinwheel stations could be built in orbit that simulate Earth gravity (9.8 m/s²) and Martian gravity (3.721 m/s²). While the "Earth wheel" could offer gravity therapy, the Martian wheel could service visitors looking to adapt to Martian gravity.

Future Martian residents will need to commit to a health regimen that includes lots of exercise, healthy eating, radiation checks, and regular examinations in the short term. Future medicines and bioenhancements may also exist to help people mediate the long-term effects of living in lower gravity.

The challenges for living on Mars are certainly massive in scope! But there are solutions, and all that's required is the right kind of commitment and sense of adventure! Once we establish a foothold on Mars, the first generations of "Martians" will soon follow. On that day, the nickname "Earth's Twin" will take on a new meaning!

"Welcome to Nergal spaceport! For those departing our fair planet, please ensure that you have submitted to all pre-departure screenings. We would hate to think you would be taking more home from Mars than a few keepsakes and some precious memories! Speaking of which, be sure to check out the Duty-Free gift shop on your way out.

"If there's one thing we enjoy sending home with our visitors, it is the fine work of some of our planet's more prominent artists! And be sure to pick up a bottle of Mangala or Foche Estate late harvest ice wine made from the finest harvests in the lowlands. Or, if you're feeling more adventurous, try a bottle of Invierno Burya, Mars' premier brand of vodka!

"Those destined for Earth are reminded to include a brief stopover at Hóngsè Station, where you will spend the next few days in extreme comfort. See the 'Red Planet,' as it was once known, from space as you spent time readjusting to Earth-normal gravity. Though you may miss the feeling of having that extra spring in your step, it is important for your health and safety once your return home.

"We are sad to see you leave, but hope to see you again soon! Or as we say on Mars, 'Dukhee hai kàn dào depart, pero nous aasha kàn dào di revoir pronto¹!"

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¹Marspeak: a mishmash of English, Chinese, Hindi, Russian, French, Spanish, et al.