# Would scanning solar systems/planets for carbon concentrations be a viable way to search for alien life?

Working on a story about a crew of astronauts on a mission searching for evidence of alien life, either still living or long extinct. Not a biologist or chemist, but from my elementary understanding I had the idea that a viable method to conduct such a search would be to scan for carbon concentrations on planets--not how much is there exactly, but how it's dispersed.

Would this (or a similar method) work? If not, is there another way to detect the presence of extinct life (in the form of fossils, etc.) without having to actually touch down on the planet?

Thanks.

• If there were a viable method already known, we would be using it on Mars. May 14 '20 at 4:14
• @JustinThymetheSecond we dont need to, mars has already been traveled to and we have got an answer that is a solid maybe. you are right that there isnt one known, but its probably not know because we dont have the technology for it but this question isnt asking for now May 14 '20 at 14:30

First of all, fossils have most of the organic part replaced by inorganic substances.

You won't find much organic chemistry there.

Then, scanning just for carbon only you might have false positives: a planet with a high concentration of methane like Uranus for example would trigger your systems.

If you want to search for life, you have to search for life fingerprints, but keeping in mind that the only life we know is based on our model, hence it can be a biased perspective. What you might search for, among others:

• water and free oxidizing agents (since oxidizing agents are by nature highly reactive, having it in the atmosphere means some process shall replenish it. On Earth it's because of plants releasing oxygen)
• spectral lines of chlorophyll
• Oxygen -> oxidizing agent in free form. Limiting search to just one element would be a form of bias you mentioned ;]
– PTwr
May 14 '20 at 12:23
• "spectral lines of chlorophyll"; while it looks likely that any sort of wide-spread life would rely on something akin to photosynthesis (it needs to harvest energy somehow; replication is energy intensive), it would appear likely that alien life would use a somewhat different mechanism to do it, if only a different photocollector substance. May 14 '20 at 15:25
• Chlorophyll isn't even the only photosynthesis pigment on Earth. May 14 '20 at 16:22
• @PTwr Searching for oxygen is a good candidate because it usually always requires something producing it, otherwise it gets consumed by another chemical process May 14 '20 at 17:56

That's not a long way off what we currently do when looking at planets in other solar systems*. Each chemical (Oxygen, Carbon Dioxide etc.) absorbs light at a specific colour/frequency, by looking at the light that reflects of the planet we can spot the 'gaps' where it's being absorbed by the atomsphere and therefore work out what the composition of the faraway planet's atmosphere is.

However, having the "right" (earth-like) atmosphere doesn't necessarily imply there is/was life on a planet, just that it would be capable of supporting earth-like life. Using this method, you would probably find lots of planets that could have life but don't, and might miss planets which have life of a very different form. You have the great advantage that you can do this from a long way away; however to find proof of life you'd probably have to physically go there.

• I'm pretty sure that if you observed free oxygen, you'd have found life. It is highly reactive and does not stay around unless a bio-sphere is replenishing it. May 14 '20 at 13:24
• @OscarBravo - life is one source of oxygen, but not the only one. Both venus and mars have some oxygen although not as much as earth; I'm sure other planets would too? I am not an expert! May 14 '20 at 16:35
• Venus and Mars have $O_2$ at around 0.1% compared to 21% for Earth, so it's really just a trace. However, it turns out there are abiotic scenarios for an $O_2$-rich atmosphere: Strong UV breaks up water, $H_2$ escapes to space, surface chemistry doesn't react with $O_2$ (weird...). Seems a bit unlikely, though. May 15 '20 at 8:08

A better strategy is to look for Carbon dioxide in varying concentrations (Indicative of respiration) another would be to scan for water at stable temperature

• If you're scanning for carbon dioxide, you'll conclude that Venus and Mars probably both have life, while Earth doesn't -- they've both got more carbon dioxide in their atmospheres than Earth does.
– Mark
May 14 '20 at 20:58