The machines we’ve sent to Mars are great at selling real estate. They transmit hauntingly beautiful, color-balanced images of ancient and untouched hills and valleys, and the fractured, alien rocks of a world that has billions of years of history to tell. It’s easy to become infatuated with such a place and to imagine, like Elon Musk does, that Mars represents an existential backup plan and the first of our off-world colonies, where opportunity awaits conveniently free of earthly constraints—for now.
But these same exploration machines also point to a less glamorous side to settling other worlds, and to deeper questions about our responsibilities to the rest of the universe. When NASA’s Mars helicopter Ingenuity recently sent spectacular aerial pictures of the crashed backshell and discarded parachute from the Perseverance rover’s 2021 landing, it was easy to applaud the technological marvel of it all. But if you really examine those images, you’ll realize that you’re looking at the scattered, cracked, and carelessly tossed trash from another planet. It is, to be blunt, a pile of crap, and that’s something that seems to follow humans wherever we go.
Just a century ago, the moon was completely pristine. Or at least as pristine as anything in a solar system filled with asteroids and interplanetary dust can be. The lunar surface might have been pelted relentlessly by all of that stuff for four and a half billion years, but it was all-natural weathering, the kind of patina you simply can’t buy. Today, though, the moon has accumulated an estimated 180 metric tons of human garbage. That includes the remains of about 70 space-going machines, 12 pairs of boots, and 96 plastic bags of human feces, urine, and vomit.
On the Earth, humans generate an estimated 1.2 billion metric tons of solid garbage per year. About half of that is organic waste, in the form of unwanted or unusable food and other materials like horticultural waste. Paper products and plastic amount to around 27 percent of the total, and then there’s metal, glass, and “other.” Of course, exactly who produces this garbage is very unevenly distributed across the globe, with the average citizen of the United States beating out pretty much everyone else, with around 0.72 metric tons of solid waste per person per year. By comparison, Turkey generates only about half as much waste per capita.
These figures don’t include recycling, but for the most part our efforts in that direction remain woefully limited on these planetary scales. And as for liquid waste, well, that’s a whole other story. Global production of wastewater stands at an estimated 1,500 cubic kilometers a day, which adds up to enough wastewater to fill up a cube about 80 kilometers on each side every single year.
It’s pretty horrifying to think about all the garbage we make on Earth and the garbage we’ve already left on the moon, but as with plenty of other complications, the question of trash is entirely absent from plans for putting humans on Mars. What would it actually look like?
Let’s imagine (as Elon Musk has suggested) 1 million people. That’s a large enough population for us to assume that this settlement’s economy supports an amount of consumerism, as well as all the critical needs of this many souls for water, food, clothing, domestic items, and even entertainment. This is much more than a science outpost. If citizens of our million-person settlement were to be as wasteful as the average American, they would generate around 720,000 metric tons of solid waste and perhaps 68 cubic kilometers of wastewater per Earth year.
Of course, living on Mars would force us to produce less garbage. After all, a lot of terrestrial human waste comes from the large-scale industrialization of our societies and the transportation of food and goods across an entire planet, which a smaller Mars society wouldn’t face. We also, on Earth, won’t immediately die if we toss a paper cup or forget to turn off a faucet, and consequently many of us are seldom confronted directly with the downsides to waste. By contrast, survival on a world far from Earth, in a deadly (albeit picturesque) environment, leaves no room for mistakes or a lack of forward thinking. A million-person community on Mars will have to be focused on safety, efficiency, recycling, and minimizing waste at all costs.
But even the most efficient, innovative Martian society won’t be without its garbage. The fictional astronaut Mark Watney in Andy Weir’s scientifically motivated story The Martian figured out how to grow food from human waste and to, rather ingeniously, make water from rocket fuel. All shining examples of thrift and reuse. But think about the garbage he also generated (duct tape is more or less indestructible), as well as all of the hardware he eventually abandons on the surface of the planet—and that’s just one human. In reality there will be limits to how “green” a Martian settlement can ever actually be, even if that is a bit of a misnomer on a red planet.
The current situation on the International Space Station provides a reality check of sorts. When it comes to water they do well, with around 93 percent of all water being recovered from waste (and yes, astronauts already drink this purified water). But official figures indicate that four astronauts on a yearlong mission can generate around 2.5 metric tons of dry garbage. At 0.63 tons per person, that is only a little better than a typical earthbound American’s production.
A major issue is that the ISS doesn’t make its own supplies, so despite severe weight constraints due to the cost of rocket launches, everything has to be shipped in cargo craft, encased in packaging, and is not really designed for repurposing. Furthermore, life on the ISS is demanding and precarious enough that it’s awfully hard to reuse or improvise new uses of materials without compromising the function or integrity of your life-sustaining surroundings. Unlike in some sci-fi movies where spaceships or alien habitats get endlessly patched and rigged, the real universe is much less forgiving.
Of course, a functioning million-person society on Mars will have to have some manufacturing capabilities, and efficiency will demand more extensive recycling methods, and could limit things like packaging (which alone represents about 65 percent of household trash in the United States). All of which could, and should, make it much more efficient than the ISS. But even if our Martian settlement could reduce its waste production by factor of 100 (a spectacular, and perhaps wholly unrealistic accomplishment) that would still mean thousands of tons of garbage per Earth year to dispose of. What do you actually do with garbage on a planet like Mars?
Back here on Earth, we have been shoving garbage into landfills for at least 5,000 years, so we’ve gotten to know that method very well. A major factor is the enormous transformation brought about over time by microbial action, where a fantastically diverse crew of bacteria and archaea break down pretty much anything that can be broken down. The outputs of this process include gases like methane, CO2, and hydrogen, as well as other small-molecule hydrocarbons that can dissolve into liquid. This decomposition is key to shrinking landfills and to, eventually, producing relatively harmless, soil-like detritus. But we simply don’t know whether these processes will work the same, or well at all, on Mars: The microbial fauna that come along with humans and make new homes in the waste on Mars may be different, the Martian temperature is often well below zero Celsius, and there’s little to no air or liquid water. Furthermore, while placing a million people on Mars will inevitably contaminate the planet with chemicals and terrestrial organisms, dumping trash in landfills could take that to a whole new level, potentially erasing any chances for ever studying indigenous Martian life or an entirely lifeless rocky planet environment. Perhaps landfills on Mars need to be inside their own hermetically sealed environments in order to decompose and to avoid contamination, but that creates a whole new set of demands on energy and resources.
Of course, if our hypothetical settlement on Mars is indeed superefficient, most of the long-term garbage it produces will consist of stuff that is not biodegradable, such as inert composite materials like fiberglass and complex fabrications like electronics. We might even be tempted to consider brute force approaches like incineration to shrink this pile or to extract precious elements—after all, on Earth some 25 to 45 percent of solid waste is, in principle, combustible. But on Mars, without oxygen in the atmosphere, you’d have to either use some of your carefully manufactured breathable air or have some alternative chemical source of oxygen, like a metal oxide and an elaborate combustion cycle, to enable waste to be burned.
It’s very likely that, even with the best intentions, a large human presence on another world like Mars will produce substantial amounts of garbage. The best that can be done is to figure out the least burdensome and least damaging way to store this trash until, perhaps, a future population has the capacity to deal with it. Short of building enormous containment structures, this probably means burying waste after carefully surveying for areas of geological stability with the least likelihood of any subsurface water and chemistry that could spread contaminants.
In other words, while the most important jobs in a million-person Mars settlement will be in maintaining supplies of breathable air, drinkable water, and edible food, the next most important jobs could be in municipal sanitation. Anyone signing up?