How could a colony of machines/robots affect a planet's atmosphere?

[Netiger]

I'm currently writing about a solar system which humans have just reached but they find that there is a colony of nanobots already 'living' on the planets. Initially from space the crew can see evidence of life but no actual lifeforms - if nanbots had landed on the planet hundreds of thousands of years ago and had developed before humans got there - what circumstances would mean that they emit chemicals which would affect the composition of the atmosphere, similar to how a living thing would, and how might that come about?

 

[berkeman]

Welcome to PF.

In order to self-replicate, they would need to process natural substances (minerals, flora?, etc.) in order to make the materials (metals, glass, silicon for ICs, lubricants) and energy infrastructure to do the replication. Depending on what materials and energy sources are available, that may or may not result in some level of pollution.

If they can do all this mining and self-replication with "green" energy sources like wind/hydro/solar/etc., then the level of pollution for the planet could be pretty low.

OTOH, if they need to do some terraforming to make the planet more "comfortable/hospitable" for them, they may have done some atmospheric engineering toward that end...

 

[BillTre]

Similar to what @berkeman said:
I would think of the nano-entities as if it were a form of life. @berkeman was basically describing a nanobot ecosystem. Resources, energy use and wastes.
The relative abundances of different chemicals would slowly change if an extensive life were presence.
The 21% oxygen atmosphere on earth is an example of that. It was made by photosynthesis not by non-living processes.

An important NASA approach to finding life is to look for "signs of life". Since life is chemical, uses environmental resources and produces chemical wastes. Some chemicals can be identified by telescope, so it is potentially useful. Determining the nano-dude equivalents of the biological changes should lead in the right direction.
At a more general level, looking for things out of equilibrium would indicate something s going on, either alive or requiring of further explanation. This is another level of indicators NASA might look for. This is a lot less specific for identifying life approach however.

Another approach is to identify planets with artificial satellites. This would seem to require a living thing of some kind to put them into orbit.

 

[Hornbein]

You need to decide how the boys get their energy. That will guide the rest.

 

[DaveC426913]

Tail wagging the dog.

It seems to me that what they observe will be entirely dependent on what the creators of the nanobots put them there to do.

It could range from

• lets clear out all the asteroids and make all the planets into green (or purple) paradises (paradices?)

to

• lets strip entire the system of all useful metals and leave behind a blasted corpse of a dust cloud made of nothing bigger or more interesting than grains of sand

and everything in between.

 

[snorkack]

You need to decide how the boys get their energy. That will guide the rest.

I think composition is somewhat more restraining.
A green plant does mining and self-replication with green energy, namely solar. However, there are reasons why a green plant is visible:
Chlorophyll has a specific reflection spectrum, similar for many plants. There are stones that look green for human eye; but few naturally occurring green stones are indistinguishable from green plant for a spectroscope.

Compare a nano/small robot that uses photoelements.
What does a dark stone do? Absorb sunlight and convert all absorbed sunlight into heat right at the surface where the light falls.

What does a photoelement do? Absorb sunlight, convert some of it into heat at the surface, and convert the rest into electricity... which eventually will be converted into heat where consumed (unless it is stored as chemical energy).

In principle, since you have more freedom with the chemical/spectral basis of photoelement than plants have with chlorophyll, you could camouflage a photoelement to look just like a common natural stone for a spectrometre. With constraints - light coloured stones leave you little absorbed energy to play with. If your robots are small, the better. Because if the energy is to be transmitted far from the photoelement, the spectrometre could see mismatch between absorbed light and emitted infrared; but if the robot is too small for the spectrometre to resolve then the total heat emitted by the robot equals total absorbed light.

And the other is structural materials.

Plants on modern Earth are visible because they need reduced carbon for their structural materials - proteins and polysaccharides - and the most commonly available form of carbon on earth is carbon dioxide. Reducing carbon dioxide to wood leaves surplus oxygen - lots of it, it is a gas with poor solubility and hard to store, so it is best dumped into air.
On a planet where most available form of carbon is atmospheric methane rather than carbon dioxide, plants might oxidize methane into polysaccharides and dump hydrogen into air.

And on a planet where tholins are abundant, with roughly the correct composition for structural polymers except they have the wrong internal structure, a form of life might use solar energy to convert tholins into biomass without leaving waste to detect.
So what do nanorobots consist of?

 

[BillTre]

the total heat emitted by the robot equals total absorbed light.

Not if some of the absorbed light is used to increase nanobot mass (for growth or reproduction). Energy used to generate more nanobot mass will not show up as heat. It will be stored as the new chemical mass of the nanobots.