Originally Answered: Was there ever life on Mars?
The most habitable areas of Mars right now, if they exist - may possibly be about as habitable as some of the least habitable areas of Earth. But those do have microbes, and even some lichens. So Mars may have life even now. It lacks oxygen, but many microbes can manage without. It has high levels of radiation especially in solar storms, but again, many microbes can withstand far higher levels, some even live in reactor cooling ponds.
For present day life, then we haven't yet found clear evidence of water that is at the right temperature for life and also not too salty for Earth life. But it may exist there, there are possibilities for it, also life that could exist even without water using the humidity of the atmosphere. And Mars life if it uses different biochemistry may be able to reproduce in conditions that are outside the range for Earth life, e.g. too cold.
But for past life, lots of evidence suggesting habitability very strongly.
First of all many signs suggesting that in the very early days on Mars, there was a global ocean covering nearly the entire Northern hemisphere. An ocean means it must have had a much thicker atmosphere than it does now.
Then, this is a little more recent in the Mars history, Curiosity has found lots of evidence on the ground of past flowing water in Gale crater with good evidence that it was filled with water several times in the past. And what's more it found that the pH of the water was just fine for life. Not too salty, not too alkaline, not too acid. At least some organics and nitrogen from comets. But it probably had a nitrogen rich atmosphere also early on. Again this means the atmosphere must have been thicker.
So, seems like Mars was probably pretty much as habitable as Earth early on.
The big question though is, did life evolve there? Or did it get to Mars from Earth? Or even, the other way, get from Mars to Earth? Are we all Martians?
Or - was it a habitable planet, with seas, organics, sunlight, nitrates etc, everything in place for life - but not a single living thing on the entire planet for milions of years? For all we know, that might be what happened.
There is no way to find out for sure except to continue to explore Mars and search for past and present day life there.
If it had life in the past, either indigenous, or from Earth, then many think it is likely to still be there today, because life is so tenacious, on Earth anyway.
That depends how far it evolved I think. If it got as far as photosynthetic life, and hardy spores able to withstand almost anything for millennia or millions of years, is one scenario. But what if it didn't evolve that far? Maybe some fragile early form of life, which never got quite robust as Earth life? Especially if it didn't have photosynthesis, it might be very limited in distribution.
You can invent many scenarios there, life evolved there and became extinct, life never evolved there but got there from Earth, life has never been there at all, or it evolved and became extinct many times even.
The most exciting possibility is to find life there that is different from Earth life in some essential way. E.g. n different bases, or it has DNA but the cells are structured completely differently, or the way the DNA is decoded, and the way it is translated into proteins is different. Or it doesn't even use DNA at all, but some other method. Or different type of cell wall material, or different cytoskeleton, or different ways of generating energy.
All Earth life is based on a single pattern basically, it's like we only know one language, and one vocabulary and grammar for life and have never come across any other life languages at all. Any divergences from that on another planet would be very interesting, and it's impossible to guess how far reaching the results would be.
I think we'd also learn a lot even if there is no life, but we find some form of protolife. If there was an ocean of organics, habitable, for hundreds of millions of years, as we think, well - surely something complex must have happened there? Something at least intermediate between non life and life, which could tell us a lot about the early stages of evolution which we can't see at all in Earth fossils - they just don't go back far enough.
Or, if all you get are complex but uninteresting "tarry deposits" and such like, this would be very interesting as well, would make life seem likely to be rare in our galaxy. But in laboratory experiments we get things that look like cells, so surely, we'd at least get some of that, on Mars, cell like structures.
It's going to be an interesting next few decades as we find out about whether there is past or present day life there and what it is like if there is - and what it is like if there isn't, for that matter.
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About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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