They talk about Life on other planets, but have we even seen the limits of life here on Earth?

Planet Mars is ice cold and desert dry. Except in the frozen ice-caps, there’s not a drop of water to be found anywhere on the surface. The sky is equally dry, except for thin clouds of water-ice crystals that sometimes form near the highest peaks.

With an average temperature of -60 ℃, it’s enough to make any unprotected human freeze to death.

And that’s not all. The atmosphere of Mars is very thin, and has nothing like an ozone layer to block deadly UV — ultraviolet — radiation from the Sun. These high-energy rays slice their way through the atmosphere — and anything, or anyone, that gets in the way.

UV radiation can cause damage to DNA, the coded instructions that tell your body how to grow. That makes you grow the wrong way, which means you could get skin cancer, or even die.

Cold, dry, and covered with radiation: who would ever live in such a place?

Turns out, some creatures do. Not Mars, exactly, but some place very similar. Right here on Earth, as it turns out.

You never realise how the stratosphere feels, unless you’ve actually been there. Imagine yourself thirty-odd kilometres above the ground, floating in thin air. And by “thin” I mean much thinner than what you’re used to. The pressures are so low here that if animal macrobes — large animals such as mice and polar-bears — end up here, their blood would boil away, instantly.

That’s because macrobe bodies don’t hold in air or water or blood by themselves: they also rely on outside pressure. On the combined weight of all those kilometres of air above them.

Less air also means less chance for heat to travel. Heat is basically molecules bumping into you, and thinner air means less air-molecules for you to get bumped by. That’s why the stratosphere is very cold. The highest and warmest part is zero degrees: the freezing point. And it only goes down from there.

For a long time, nobody thought there would be life so high up in the sky. How would it get there, in the first place? And why would it want to go, when there are so many nice comfortable places closer to the ground?

But some people got thinking. What if there are creatures living up there? Everyone likes to talk about life on other, stranger, planets, but what if we haven’t yet seen the limits of life on Earth?

Scientists already knew species that would be able to survive. A certain micro-animal, the tardigrade, can stay for ten full days in the pure emptiness of space. It can also spend time in poison or pure alcohol, and still come out alive. So there were creatures who could survive in harsh conditions, if not live a full and happy life.

The question of where life can survive is important for exobiologists, too. If things can live in extreme conditions here on Earth, it widens the horizons for finding life on other planets as well. If something lives in the stratosphere, there could well be life in stratosphere-like planets as well.

As you’re frigidly floating in this wispy sky, you’ll also be faced with lack of water. Everyone thinks of water in the sky, and clouds, and rain. But you’re above all that. All the raindrops and thunderstorms are down in the troposphere, and none of it reaches up here at all. Unless you have proper defences, the little water in your own little body will be quickly soaked dry like an un-sponge.

And finally, there’s the radiation. You may have heard of the “ozone layer” — a layer of ozone gas in the atmosphere, which helps block out harmful solar radiation. That’s good because the folks living down on Earth don’t have to worry too much about it: everything is already taken care of from them.

Up here, it’s different. You see, the ozone layer is in the stratosphere — and you happen to be above it.

There have been reports of sky-high microbes as early as the 1930s. Helium balloons were sent floating up, and brought back with air-samples from above. The problem is, it’s difficult to collect such samples and make sure they’re accurate. You need to pull in a huge amount of air just to catch one microbe, and there’s always the question of whether the creature is actually from the sky, or simply a hitch-hiker from when the balloon was launching or landing.

Over the decades, however, more and better studies have been carried out, and it’s pretty certain there are microbes in the sky. The question now is: how did they get there, and why?

Under the stratosphere, about ten kilometres from sea-level and below, is the troposphere. And that’s where things actually happen.

There are all the different air currents: local wind cells, larger cyclones, and the great Trade Winds speeding their way across the face of the planet. Get up there and you can, potentially, get to almost anywhere in the whole world.

On your own, down in land or water, you can’t move around much. You’re restricted to your own little habitat, and wherever you manage to float around within it. But if you want to travel, if you want to get to new continents and spread your people all over the world, then get on the wind.

All it takes is a slight updraught of air. That’s easy to find on land: there’s always warm air rising from the ground, or evaporating out of grass or leaves. If you’re in the sea, perhaps the salty spray will help you up instead. Some people have waxy surfaces to help them escape the water, or send spores shooting up, or take a ride on a passing jet-plane, or use other such fancy tricks.

If you’re lucky, you’ll be taken up by the air and carried to some fresh new comfortable place to live. You could hitch a ride on a cloud, staying near the moist water droplets. If you’re the type with the special protein, you can even make it rain at will and ride down on a falling raindrop.

How did microbes get up to the stratosphere, and why? One likely explanation for the “how” is: by chance. Microbes are tiny things, and it doesn’t take much to get them lifted up. They could have been swept up by cyclones, dust-storms, and general air-currents in the sky. By chance, a few of them kept getting higher and higher — and here they are.

But did they want to get here? That’s a different question.

Many observed bacteria don’t survive the stratosphere at all. They’re already dead on arrival. Others get by, but they don’t thrive. In fact, they don’t grow at all.They’re just in ‘survival mode’, with only one way to cope with the harsh environment: sporulation.

Like the snail which carries its home on its back, these bacteria harden to form space-suits out of their own bodies. And there they stay, tightly curled up, all systems put on hold until they arrive at safer spaces.

Some bacteria are more advanced. They have special proteins, to hold onto what little water they can while enduring the dryness. They have armies of enzymes on the job, repairing their DNA even as it’s being broken apart. Of course, even that cannot continue forever.

Microbes in the stratosphere aren’t happy. They’re just bearing with the situation and hoping things get better. Or are they?

What if you rise too high, and go on to the stratosphere? Let me tell you one thing: don’t despair. There’s still a purpose. While you’re out of your comfort zone, you’ll also be pushing the boundaries on where your species can live. These are the experiences which help us conquer new places, new environments.

Right now, the stratosphere is a dead-end. It doesn’t lead anywhere, except perhaps to death.

But if we keep at it, if we as a species continue visiting the stratosphere, the strongest of us will survive and pass genes on to the next generation. The stratosphere is a dead end right now, but the day may well come when a rocket flies by, on its way to a new, habitable planet.

And then, we’ll be ready.

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