Much of the public interest in space has always revolved around the idea of life on other planets. We know that there are other planets, even around other stars, some of them even “Earth-like”, in some optimistic definitions of the word. But the current lack of detailed information about such places makes fertile ground for imaginative speculation. I will try to refrain from speculation here, and stick to the factual.
We have many inborn instincts that deceive our minds about the matter. For one, we think that recognizing life and intelligence is easy. Just bring it before us and we will categorize it as either:
- alive and intelligent (example: elephants, some whales, great apes)
- alive and non-intelligent (example: bacteria, lichen, viruses)
- non-living and intelligent (example: ?)
- non-living and non-intelligent (example: vacuum, atoms, radiation).
It is of course our generalist diet and dependence on social behavior that make recognizing life and intelligence so important and instinctive to us. It takes a lot of effort not to anthropize everything that we encounter, not to react to what our brain thinks is a face for example.
Our ancestors walked for thousands of kilometers, encountered new environments and ecosystems, and survived. We are all descendants of surviving colonists. Our species has traveled just about everywhere on the surface of this planet, and marked all the habitable places. That is, habitable for humans. As we have expanded the reach of our scientific equipment, we have found life in places where we never though there would be life: at the bottom of the ocean at thousands of atmospheres of pressure, in scalding heat or acidity, living with no direct access to sunlight. We call these kind of beings “extremophiles”, because they live in environments that are extreme compared to our human views of habitability.
What makes life, and indeed intelligence, so interesting is that it breaks molds, defies definition, and jumps from host to another, forever in between destinations. Life, as we understand it, exists in the interstitials, in the sweet spots between states and phases, conversing in an endless dialogue between solid and fluid. As much as we humans like categorizing things, life itself cannot be contained in any box (or if it can, it will die inside it).
Life, as we understand it, exists in the interstitials, in the sweet spots between states and phases, conversing in an endless dialogue between solid and fluid.
As a species, we have traveled thousands of kilometers vertically, but move just ten kilometers straight up or down, and the Earth becomes hostile; with either too much or too little pressure for human habitation. It is not the planet as a whole that is habitable, just certain zones within a thin layer of it are. Our senses evolved to this thin layer between the earth and the sky, and until modern science did not even become aware of the many invisible layers and processes above and below this one, that our lives depend on: the ozone layer, the magnetosphere, ground water, deep ocean currents.
The circulatory systems of the Earth, the water cycles, the carbon cycles, and various mineral cycles, all must align at sweet spots for life to flourish. Such fertile locations include of course the alluvial plains, where human city-cultures arose some ten thousand years ago. But there are also two especially active locations on Earth: the Amazon, and the Great Barrier Reef.
Gravity presses tons of bioavailable minerals to the bottom of the oceans, where the lack of sunlight prevents plants from making use of them. Only deep ocean currents, or continental drift can bring these nutrients back to the surface. By the rotation of the Earth, nutrient dust is regularly carried in the air from Africa to Southern America, where it meets water vapor coming from the Pacific (both wind directions driven, paradoxically, by the very same rotation of the planet). Where they meet, the Amazon.
The Great Barrier Reef, the largest structure created by life on Earth, was born when a large slice of coastal plain became flooded about ten thousand years ago. The edge of the continental shelf stays close enough to the surface to receive plenty of sunlight, creating large lagoon-like areas between it and the current coastline. Like they do for ships that sink, corals started to metabolize the trees and other matter from the moment they became submerged, slowly covering them in an accumulation of rock-like deposit. In addition to material flowing in from land, nearby ocean currents such as the Capricorn Eddy help sustain the ecosystem by bringing up nutrient-rich waters from the seabed.
When we now look to the Solar system, our colonist intuition tells us to look for solid surface, terra firma, somewhere to raise a flag and stake a claim. But to truly make humans a multiplanetary species, we need to build, grow, or transport an entire ecosystem capable of sustaining both itself and us humans at the top of the food chain. Quite a few species will have to become multiplanetary in order for that to happen.
As it happens, one of the most promising location for humans discovered outside of Earth exists about 50 km above the surface of Venus, where the levels of temperature, pressure, gravity and radiation are all comparable to the thin layer of Earth we call home. There is of course nothing solid or liquid at that altitude, nowhere to plant anything, not even the intrepid explorer’s flag. A conceptual project (HAVOC) has been proposed to study the conditions in that altitude in Venus (and possibly to invent a flagpole adapted for clouds). But to make that layer into a permanent second home for humans requires designing the ecosystem from scratch. This idea is daunting, but also liberating. I for one am excited to imagine the steps needed to grow our own “Great Reef” to float in the sky of another planet. Most of the building materials should already be present; if you think about it, trees on Earth create solid wood almost entirely out of air, out of thin air. On Venus, CO2 and sunlight are in abundance.
In the same way as when life arose from the seas and moved onto land, moving life into space will have to be at least as much adaptation as conquest. Climbing the formidable gravity wells accommodate travelers best when packed into the smallest mass possible. Ideally, just the instructions how to grow life could be packed into small “seeds” that could then adapt to the local conditions when they arrive (or it is thinkable that this has already happened long ago, and we are the result of panspermia).
All currently known life forms, even extremophiles, have evolved and adapted into the wonderful thin layer of our planet, this “region of interaction” first named biosphere by Eduard Suess. What are the necessary characteristics of such a layer of interaction, and how do they contribute to life as we know it?
On first approximation, the surface of the planet is where the different phases of matter separate, the solid earth, the liquid water, and the gaseous air, like the concentric spheres of classical cosmology. But the solidness and fluidness of a substance is relative, and our senses experience them as such because we have evolved into this layer. It is thinkable that a lifeform adapted to a different layer, or with different mass and strength would see things differently. For example, a bird might sense air currents like a fish senses water, or an elephant senses vibrations in the ground.
The point of view also changes with timescale, and density. The smallest flying insects experience the air viscosity differently, and their flight is more like swimming than gliding. If you are made of gossamer you experience more things as hard and solid than if you were made of diamond. The slower your perception is, the more foggy movement appears, and so on.
The spherical shape of a planet’s surface is the result of the opposing forces that act on its mass: Gravity pulls everything together, while pressure pushes outward in all directions. The total sum of countless trillions of small collisions eventually separate the mass of the forming planet into layers, of which the separation between “surface” and “atmosphere” is just one.
The end result of the separation process could be just a lifeless set of perfect concentric spheres, like the rings of Saturn. But on Earth, the separation is not complete, there are continuous cycles of matter, interacting over the layer boundaries. An example is the water cycle, continuously evaporating, condensing, raining, sublimating, diluting and conveying all over the biosphere.
This spontaneous layering to spheres follows density, so it does not necessarily result in ordering by phases of matter. In addition, increasing pressure towards the center can melt material that would otherwise solidify. The current theory about the internal structure of our planet is that it has a solid inner core, surrounded by liquid outer core, surrounded by viscous mantle, with a mostly solid crust on top
And of course at planetary scales, solidity is relative. Also gases, when they are thick and viscous enough, can behave more like liquids.
Chain reactions must triggered by something. Just like a snowflake cannot form without a seeding speck of dust, the complex biochemistry of life cannot appear if all base materials are just cleanly separated. Some flaws must be present in the interface, it cannot be a perfect mirror. Crystallization cannot start without a seed, and crystals do not naturally grow into perfect spheres. Exterior solid crusts inevitably erode into uneven rocks and sands, due to to tidal forces, winds and waves, even meteor collisions if nothing else.
The concentric spheres of matter must interact, even interpermeate to some extent, to make them more fertile places for life to evolve. Reservoirs, niches, potentials and flows should be present, with local variations in temperature, flow speed, density and such. Growth can then slowly adapt to different situations, as long as the overall conditions are stable enough.
Such interactive layering does not happen only on planets. The surface of the Sun is also wildly active and complex, but due to its heat cannot sustain the kind of complex biochemistry that we associate with life. The valencies of the chemical bonds in an organism need to be compatible with the ambient energy levels (including radiation) of the environment, so that macromolecules can be both synthesized and broken down near each other.
By a big stretch of the imagination, we might theorize a system of life not based on macromolecule synthesis, or even molecules. For example, we don’t really know what happens in the layers of quark soup in the pressures of a rotating neutron star. But for now, such things are beyond what is known, clearly in the realm of speculation which I said I would try to avoid here.
Even if we stay within the realm of chemistry, we should be looking for life in the shapes and forms that are expressed through it, rather than reducing it to any kind of quantitative process. Otherwise there would be no other purpose to life than “to hydrogenate carbon dioxide”.