I hardly know anyone who fails to recognise the smell of the first rain on dry ground. It arrives before thought does. It smells of earth, of summer, of something recovering. Two Australian researchers gave it a name: petrichor. Today we know its chemical core precisely. It is a molecule called geosmin.

Our noses respond to geosmin more sensitively than to almost any other substance. A few molecules among trillions are enough. No perfumer taught us this. The sensitivity is older than any culture that sings about rain. Our bodies acquired it on a planet where it paid to notice wet ground from a distance.

I am writing about this scent because it reveals something missing from every plan for a settlement beyond Earth. It is not a mood. It is a signal.

The message behind the scent

Much of the geosmin we smell comes from soil bacteria of the genus Streptomyces. Millions of them live in every handful of garden soil. When the ground dries out and gets wet again, they release the compound. Tiny soil animals called springtails follow exactly this signal. They eat the bacteria and carry their spores onward. The scent we love is part of a trade that began long before us.

The same genus has given medicine more than almost any other. Streptomycin, the first effective drug against tuberculosis, comes from it. So do the tetracyclines. A large share of all antibiotics of natural origin traces back to these inconspicuous bacteria. Whoever breathes in deeply after a thunderstorm is smelling the pharmacy from which modern medicine drew its most important drugs.

We usually treat the feeling of home and the ability to survive an infection as separate things. They come from the same soil.

What a space station does to the body

On the International Space Station, nothing smells of rain. The air is filtered. The water is recycled. The surfaces are cleaned on a schedule. The station is among the cleanest places humans have ever lived for months at a time. One might call that an advantage.

NASA's data tell a different story. In more than half of all astronauts, herpes viruses that the body had kept dormant for years wake up during flight. On short Shuttle missions the figure was 53 percent. On long stays aboard the station it was 61 percent. Most crew members noticed nothing. Yet frequency and quantity rose with the length of the flight. The researchers attribute this mainly to stress and weakened immune defences.

I do not want to claim more than the numbers carry. The station is cramped and demanding. Stress alone explains a great deal. A second line of research, however, points in the same direction. Our immune system learns from the microbes it encounters. It needs constant contact with a diverse environment to work properly. A sterile environment is no sanctuary for the body. It is a classroom without a teacher.

The cargo manifest missing its most important item

Anyone reading the plans for a settlement on Mars reads tonnes. SpaceX calculates Starship in payload, propellant and landing sites. The space agencies calculate oxygen plants, water recycling and radiation shielding. All of it is necessary. None of these lists includes soil as a living system.

Martian soil is dead. It contains perchlorates, salts that are toxic to many organisms. It lacks fungal networks, worms and the bacterial communities that turn rock into fertile earth. Whoever wants to grow plants there grows them in nutrient solution or brings substrate along. The plants themselves travel easily as seeds. The life that accompanies them on Earth cannot be bagged so simply.

Healthy soil is not an ingredient. It is a community of thousands of species that keep one another in check. Whoever takes only the useful ones loses those that keep the whole stable. This is where a biological question becomes a question of design.

The soil that took their breath away

One experiment has taught this lesson before. In the early 1990s, eight people sealed themselves inside Biosphere 2 in the Arizona desert. The vast glass structure held a rainforest, a small ocean and farmland. For two years it was meant to sustain itself.

It did not. The oxygen content of the air fell slowly from almost 21 to around 14 percent. The residents suffered from breathlessness and exhaustion. Eventually oxygen had to be pumped in from outside. The prime suspect was the soil. It had been made especially rich in organic matter so the plants would thrive. The soil microbes broke this matter down. In doing so they consumed more oxygen than the plants produced. Part of the resulting carbon dioxide also disappeared into the facility's fresh concrete.

I find this story more instructive than any success. In a closed world, microbes are no side issue. They are part of life support, whether anyone plans for them or not. Without them, a settlement loses its pharmacy. With the wrong measure of them, it loses its air.

An ark still standing on Earth

For seeds, humanity has made provision. In the permafrost of Svalbard lies a backup of the world's crop plants. For microbes, this work is only beginning. The Microbiota Vault Initiative, founded by the microbiologist Maria Gloria Dominguez-Bello, keeps frozen samples at the University of Zurich. In its first phase it gathered more than 1,200 samples of human gut flora and 190 samples of fermented food from seven countries. Its next goals are ten thousand samples and a permanent site in a cold country.

This is valuable work, and it is underrated. It also shows how far there is still to go. The people involved say themselves that we barely understand how microbes interact. We know even less about how such communities can be rebuilt. The initiative protects what might be lost on Earth. What must leave Earth is not its question.

What has to travel

I draw a conclusion from this that is still missing from the plans for Mars. A settlement beyond Earth needs living soil as infrastructure. It belongs in the same category as the reactor and the water plant. It must be planned, maintained and measured. It must be allowed to grow without consuming the air.

Freezers are not enough for this. A frozen sample is the memory of a community, not the community itself. What travels must already be alive on the way. I am thinking of small, running cultures that are fed and watched. Of a garden that is already soil before it lands.

This raises a question nobody is asking yet. Who chooses which microbes humanity takes along? Soil from the Black Forest is different from soil from the Amazon basin. Each carries its own defensive compounds. Each holds medicines nobody has found yet. The selection determines which biology our descendants beyond Earth will inherit. Once a settlement stands, it cannot be taken back. If we do not make the choice deliberately, it will be made anyway. Then whatever happens to cling to boots, hands and cargo crates decides.

Such a choice calls for an order that reaches beyond any single mission. It calls for rules on origin, on the share of the countries whose soils travel, and on access to whatever grows from them. The Microbiota Vault has already shown for its own samples that rights can remain with the communities they come from. For the microbes that leave the planet, nothing comparable exists.

A handful of Earth

I consider the smell of rain the most honest test of any future settlement. If water falls on the soil of a greenhouse on Mars for the first time and nothing smells, then we have relocated people. We have left their world behind.

If it does smell, then something has come along that no cargo manifest lists. Then a community lives there that someone took the trouble to understand. It supplies medicines. It keeps the body alert. It leaves the settlement its air. That would be the real proof that humanity can leave Earth without losing itself.

Today, nobody decides which earth travels. If you had to make that choice: which soil would your handful come from?

Homepage: https://planet-futures.org