# Maximum habitability of a planet with no indigenous life

I've been thinking about terraforming and planets in general.

We can look at the planets we know that might be terraformable - Venus and Mars - and understand the technical requirements for doing so. We can even imagine other planets and what might be necessary for them.

What I'm trying to create is a planet that has no indigenous life, but yet is remarkably easy to settle; it needs little or no terraforming work at all -- the temperature is just right; it has oceans, land, rivers, soils that can support plants, maybe even a nice big moon to produce tides. Basically, it's Earth's sterile twin.

But I'm aware that a lot of the things which make Earth great for life are actually a result of it having life.

So the question is: How feasible is my scenario? How perfect can I make this planet without it ever having had any life to help make it perfect? What compromises will I need to make in order to make it realistic?

• What exactly do you mean by indigenous life ? If this planet is still at the state where water or single celled life just appeared (c.f en.wikipedia.org/wiki/…) there is life, just not really big dinosaurs roaming around. – everyone Nov 21 '17 at 15:40
• @everyone - I means no life. Nothing. Not even microbes. That initial spark of creation simply didn't happen. – Simba Nov 21 '17 at 15:43
• Is it okay if your planet once had life but then exposure to radiation from a nearby star killed everything 25 million years ago? – Muuski Nov 21 '17 at 19:16
• no life means no soil. soil is built up largely from organic matter. – Mr.Mindor Nov 21 '17 at 19:22
• If you would settle for 'no current live' a nearby supernova would help – Jeremy French Nov 22 '17 at 11:33

The biggest problem is going to be atmospheric oxygen. The main source of oxygen in Earth atmosphere is photosynthetic plant life. Oxygen is a reactive molecule. It tends to react with minerals (oxidize like rust) and not be present as a gas in the atmosphere. Without plant life, you are going to need a continuously renewing source of oxygen, and I'm not aware of anything that would work for this.

You could have a planet atmosphere with a high carbon dioxide content, that could be terraformed simply by adding plant life to convert the ${CO_2}$ to ${O_2}$. This is going to be difficult to do on land as topsoil is a complicated material requiring a number of interrelated life forms, so your best bet would be a planet with large oceans. Seed the oceans with some algae and other basic life forms and wait a few years/decades/centuries for the oxygen content to rise.

Other problems to consider: With no free oxygen, your planet will not have an ozone layer as ozone forms from oxygen in the upper atmosphere absorbing radiation from the sun. So expect high levels of ultraviolet light. Ocean life would be protected, so seeding the oceans would still work.

• Add to that a good fraction of initial oxygen production would be used to oxidize whatever can be oxidized. Original rocks are bound to contain a lot of materials (e.g.: iron) that will act as a mop for oxygen and that will (as did in our Earth) until all these materials have been completely saturated. – ZioByte Nov 21 '17 at 16:00
• If the planet was rich in O2 to begin with (whether in-atmosphere or easily extractable from wherever it is), would that not make the planet easy to settle, under the idea that humans can (unsustainably) live there right off the bat while having time to set up an ecosystem for a renewable O2 source? – Flater Nov 22 '17 at 9:52
• @Flater A planet won't be rich in O$_2$ t begin with; most oxygen will be in the form of water, carbon dioxide, or silicates and metal oxides in the planetary crust. – kingledion Nov 22 '17 at 15:55
• @kingledion: which is why I left it undecided whether it's already in the atmosphere or relatively simple to extract from wherever it is stored. My question doesn't focus on the specifics of how to extract it; but rather the viability of using a non-renewable supply temporarily, to give you time to set up a renewable supply. – Flater Nov 22 '17 at 16:17
• "wait a few years/ decades / centuries" I haven't done the math, but I'm thinking it's more like "wait a few thousand / hundred thousand / million years" in order to reach breathable levels. – Shufflepants Nov 23 '17 at 9:13

## Oxygen

It turns out that plant life isn't the only way to create oxygen. Photodissociation of carbon dioxide uses ultra-violet radiation to create oxygen. The more oxygen is produced the less efficient this process is (due to ozone's ability to absorb UV) which would create a self-regulating system.

## Soil

This is the bigger problem, I think. On earth our plants create wind breakers and reduce the amount of nutrient rich top soil we have remaining. You only have to look at the effects of deforestation on earth to imagine what might happen to a planet with no plants. I imagine some areas might remain but you won't have 'good' soil to plant.

• Photodissociation of CO2 produces too little oxygen for it to accumulate in the atmosphere, unless the planet happens to revolve around an ultraviolet laser tuned to the correct vawelength. Oxygen is too reactive to remain free unless lots of it are produced constantly; if Earth's photosynthetic life died almost all oxygen from the atmosphere would be gone in a blink of a geological eye. – AlexP Nov 21 '17 at 16:16
• You won't have any soul to plant. Soil itself is the result of broken-down plant and animal substances. Without that, at best you have sand... and even there, a lot of the sand out there is actually from broken-down mollusc shells and coral. – Ben Barden Nov 21 '17 at 19:11
• @AlexP this ... "planet happens to revolve around an ultraviolet laser" ... this is a topic which needs to be explored in depth! – akaioi Nov 21 '17 at 23:21
• @AlexP Do you think Photodissociation could produce enough oxygen to saturate the oxygen mops (free iron etc) over the course of thousands/millions of years? – DarcyThomas Nov 22 '17 at 0:41
• @DarcyThomas: Almost certainly not. After all, we know that on Earth it didn't. The problem with photodissociation of CO2 is that it produces oxygen (as ultra-reactive monoatomic oxygen!) and carbon, and those two like to combine to produce CO2 back... – AlexP Nov 22 '17 at 0:54

I think you'll find a lifeless planet needs some work.

You'll have rocks, you'll have sand, but you won't have soil. You'll get grit and powder without a lot to hold it together. To be fair, you'll also get clays which can bake together, but I'm suggesting you'll have a lot of picturesque dust storms.

And erosion. Without binding agents (roots, loam, etc) the sand will be more likely to roll into the ocean.

Next ... you'll have iron lying around unrusted. Hey, you might want to send a mining operation in first, because the major iron ores on Earth (hematite and magnetite) are basically just iron & oxygen. I'm no archaeometallurgist -- who is, these days -- but I'm wondering if there wouldn't be huge iron deposits that you could just scoop up.

Per another answer, you'll have to seed the oceans with algae. That's workable, but note that the water, sea rocks, and land rocks will soak up the released oxygen until all the reactive materials have soaked up the oxy. This can take a long time:

If I were you, I'd set up banks of hydrolysis machines to speed this up, and sell off the extra H2 to passing interstellar traders.

• Ferrous carbonate also forms directly on steel or iron surfaces exposed to solutions of carbon dioxide, forming an "iron carbonate" scale – Slarty Nov 21 '17 at 17:43

Your probably going to have to have biodomes for many decades before the planet is in any shape to do what you want.

The planets doesn't have any life on it according to you. This means no algae or photoplankton which means no oxygen. Even if you start seeding immediately, it took the earth 1 billion years to complete this process, and I am assuming that is too long to wait. You will need enough photo-planketon and/or algae to cover all the oceans of the world. Since its too much to bring with you will need something that grows like wild fire. You would have to increase oxygen creation basically a billion times what earth had at that time. Not very realistic, even if you split the oxygen from the water molecules you couldn't do it fast enough.

So I suggest bio-domes, which you can control how much outside air you bring in and how much oxygen you let out. You need top soil, and that isn't simple either especially without existing life. At least with domes you could bring top soil with you to start the growing process.

Otherwise you would need handwavium to explain having usable top soil without the presence of some life. Your only hope of having the 20% oxygen content is basically some interglatic lighting bolt or electrical current splitting 20% of the planets water into hydrogen and oxygen without blowing up the planet.

A simply battery could naturally occur, in the ocean but it would have to be in vast in size. This could generate the electricity necessary to split the water molecules and give you the oxygen you need. However, you would have to arrange something else to bond with the hydrogen or it could generate devastating explosions.

Ok, here a very wild idea not without a bit of handwave but semi plausible. Indigenous life can be not present just because the planet is just too young, maybe life there is in the "aminoacid soup" stage.

The planet atmosphere is covered in thick clouds and semi-perpetualy raining and the clouds are barring heavy UV/IR radiation, heavy enough to broke H2O in O2 and a lot of Hydrogen. Hydrogen being less dense concentrates in upper layers while oxygen in the bottom. Note: You can need to handwave massive perpetual eletric storms to produce the necessary amount of oxygen by "natural eletrolyse".

The "soil" will be mud, very easy for some crops. No need for rivers and oceans but with that amount of rain it will probably got lot's of rivers and seas. Also the constant wet surface will help prevent oxidation (IF the water is not rich in "diluted" oxygen and salt, like our oceans) Note: You do need to handwave that mud with mineral dissolution to produce a mud good enough for agriculture without excessive NaCl and poor in metals (and other things get easily oxidated).

The tricky part is to make enough IR/sun light at surface to allow agriculture (unless you can use "abyssal agriculture" and yes, there's a question about it).

• We don't know what started life on earth. It is entirely possible that a planet could exist in a potentially habitable state for 100s of millions of years before single cell life starts. – Jeremy French Nov 22 '17 at 11:32

What you want can't exist(without bacteria/plants no o2 for you and without life no soil either) but here are some workarounds:

1.Another race colonized it before , finished their thing with that solar system then left taking their infrastructure with them.

2.Life existed in primitive form but got nuked by gamma rays from a nearby supernova and your explorers just landed a few weeks after the last plant disintegrated to basic organic material absorbed by the soil.

3.Synthetic planet where in reality the planet is hollow and at the core there is a system in place to maintain the surface equilibrium of oxygen levels etc...

4.Magic + umpalumpas go wild ...

You can have it be an "artificial planet" terraformed by an alien race with requirements similar enough to humans, but never colonized by them and possibly forgotten. A sort of NOS (New Old Stock) planet.