Is Mars Habitable?

Notes on the Martian housing crisis

Is there any place on Mars where a terrestrial organism could make a home for itself?

The answer matters because it affects our behavior as explorers. If no part of Mars is hospitable to Earth life, then it’s fine to start landing Starships full of bulldozers and grizzled miners at the scientifically most interesting spots on the planet and get to digging. More modestly, it means that long-stay explorers can hide from radiation in natural features like caves or lava tubes without worrying that their presence will compromise the science they came to Mars to do.

Conversely, if Mars has niche environments that a microbe could make a life in, then exploration has to be more circumspect. In the most sensitive places, we may need to adopt the kind of cleanroom-type precautions scientists use when drilling into pristine subglacial lakes. But it’s hard to sterilize space probes to this standard, and impossible to do it with entire spacecraft like Starship. At a minimum, the attempt to limit contamination while digging will add layers of difficulty to what is an already exceptionally hard endeavor.

So it’s in everyone’s interest for Mars to be uninhabitable. But on Mars, at least, we believe in science! So the only thing to do is look at the facts as we now understand them.

Centering Martian voices

I want to stress that whether Mars is habitable is a different question than whether Mars has life. It’s possible that Mars is a paradise, an Eden for a class of organisms with a fundamentally different biochemistry than ours. It’s also possible that Martian and Earth life are related, but that organisms there found some novel evolutionary pathways to living in conditions that our own biota can’t survive in.

While it’s hard to imagine a life form that could thrive on such a dry, cold world, that’s only because imagining any kind of biochemistry is difficult. No human mind could have come up with the DNA/RNA + protein template for life if we didn’t have our own example to crib from. With that one example in hand, it’s tempting to treat it as universal template for life. But that’s the kind of question we are going to Mars to answer, not something that we should assume going in.

So the point of talking about habitability is not to prejudge the search for Martian life, but just to avoid muddying the question with our own exploration activities.

Surviving vs. thriving

It is important to make a distinction between survival and reproduction. There are plenty of places (like North Dakota) where it’s technically possible to stay alive, but you would never dream of raising a family.

Similarly, we already know Mars is inhabited by a few thousand hardy microbes clinging to the inside of our various rovers and probes. NASA stopped trying to fully sterilize equipment after Viking, and experiments on the space station have taught us that microbes can live for years in harsher environments than Mars, gritting their teeth out on the hull of the ISS until some kind soul swabs them back onto a petri dish.

But what we’re really interested in are not these castaways, but scenarios where hitchhiking life gets a foothold in an environment where it can reproduce and grow on a reasonable time scale. And the requirements for reproduction and growth are much stricter.

Barriers to growth

The few known examples of lifeless habitats on Earth allow us to define some absolute requirements for terrestrial life.

All known organisms require six chemical elements to grow: phosphorous, oxygen, nitrogen, carbon, hydrogen, and sulfur (I like to remember them with the acronym PONCHOS). If any were lacking on Mars, it would give us an easy out, but they are abundant in Martian soil and in the atmosphere. Our best bet on an elemental veto might be nitrogen, which could be scarce in its bioavailable form. But there’s still enough of it that we can’t count on its absence as a prophylactic.

A second requirement for life on Earth is water. Here the story used to be uncomplicated: Mars was the OG desert planet. But rovers after Viking showed us that you can hardly lift a rock on Mars without finding water ice. There are deposits of ice just under the surface across much of the planet, thick caps at the poles, and unknown but vast quantities sequestered deep underground. Whether liquid water or brine flows anywhere on Mars remains an open question. But in its solid form, we know water is everywhere.

The lack of oxygen on Mars might seem like a showstopper, but that’s only an expression of our own oxycentrism. You and I are eukaryotes, dependent on things like sandwiches and breathable air to get through the day. But microbes take a far more expansive view of what is edible, and have a suite of different metabolic pathways to suit different circumstances. Many organisms on Earth would be happy to dine on Martian rock¹, or on the hydrogen gas that occurs naturally in the planet’s crust. So we can’t count on starvation or anoxia to serve as a natural barrier to the one-celled set.

Two hard limits

The strongest constraints on Martian habitability turn out to be temperature and a measure of available moisture called water activity.