That's what many scientists used to think in the 1970s and is probably the main reason we had a lull with few expeditions to Mars.
Now though, it's pretty clear that Mars had a broad, shallow ocean in the early solar system. We've traced its shoreline all the way around the Northern hemisphere of Mars - and found traces of ancient deltas emptying into it. I think no longer questioned.
It's the prime objective for current and planned missions to Mars - to search for past and present life.
It probably was ice covered a fair bit of the time, but in the early solar system, Mars and Earth and Venus were all very similar with a dense atmosphere, plenty of nitrogen, atmospheric pressure tens of bars, and with oceans at high temperatures, probably into the hundreds of degrees centigrade.
As they cooled down, they must have all entered a phase which was hospitable for life. So the big question is - did any of them evolve life? Or was there life in the solar system already - there were chances for it to evolve in watery planetary embryos, which cooled down first of all.
If they did - was that life transferred from planet to planet during the huge impacts in the "late heavy bombardment"?
Now of those three planets, Venus is far too hot - and also has evidence of global resurfacing of the entire planet by volcanism. So it's not an easy place to search for life, and unlikely to have ancient traces of life there.
Earth has life of course, but this obscures the search for early life. Also because of continental drift, turns out that there isn't a single piece of rock on Earth that was here during the first few hundred million years when life probably evolved. All we have are a few mm sized zircons - which retain trace amounts of the early atmosphere, but not much else.
Mars though - had no continental drift and not much volcanism (because it is so much smaller than the other two planets) - and went into a deep freeze "Snowball Mars" state within a few hundred million years.
So - it's the perfect place to look for traces of early life and maybe find out how life evolved. It's a major challenge - but it might have, not just micro-fossils (hard to conclusively identify and prove to be microbes especially if smaller than present day life as early life surely would be) - but also - organics from that time.
It's a major challenge to preserve organics for billions of years. But not impossible - we find organics in some ancient stromatolites on Earth. On Mars, we have additional challenge of the cosmic radiation - and over billions of years - radiation from the rocks also even if deeply buried - and so that adds to the challenge - that's why Curiosity is looking for recently excavated rocks in hope to find richer deposits of organics than the ones it found so far.
Organics don't prove life either - because meteorites and comets have organics - so to find life conclusively you need to detect biosignatures. Curiosity can't do this (it does have one experiment able to detect chirality - but meteorite organics can be chirally biased - not likely to give conclusive results in an ancient heavily degraded organic deposit).
So - that's why there is such interest in looking for past life on Mars.
Then - if it did have life in the past - well it went into a "snowball phases" early on. It should be white - except - that since it lost all its water it looks red instead.
But that doesn't mean there is no life left. Earth went through snowball phases when there was almost no life on the surface - if you looked at Snowball Earth from Mars and even if you sent Curiosity rovers to the surface - you probably wouldn't detect any life there. But it was there in low concentrations - either underground - or - also quite probably on the surface - just below the surface, or beneath the ice, or similar places.
Similarly - though Mars has no obvious signs of life right now - still it might very well have life there in its "snowball phase". First of all - deep underground. But - perhaps surprisingly given its near vacuum for an atmosphere - there are also several potential places where life might survive on the surface also - crypto organisms - hidden just below the surface of the rocks, or a cm or so below the surface of the soil, similarly to microbes in Antarctica's McMurdo valleys and the Atacama high cold desert.
This is cutting edge research. It's only since 2008, and gradually more and more evidence gathered - that we have found reasonable evidence that these habitats exist. And - they remain controversial. But the warm seasonal flows, particularly - have not yet had any other explanation not involving liquid water. This is a rare phenomenon that you only get on sun facing slopes above OC, seasonal, and not correlated with the dust storms - and can't possibly be CO2 at those temperatures - narrow dark streaks on steep sun facing slopes, that gradually grow as the season progresses, and then fade away in autumn.
So - there may well be habitats on present day Mars for life. That's the most promising in a way - but there are several other possibilities with indirect evidence for them.
This is not at all confirmed. The streaks are too narrow, and too thin - probably more of a slight dampening of the dusty regolith with salty water. So - haven't yet been able to detect the signature of water there from orbit. That's the next thing to do for those - to try to get confirmation that it is water - but that might require a lander to do it.
Then - for present day life and also for past life - you have the problem that quite probably not all habitats will be inhabited. It may take millions of years for life to find a new habitat, who knows. And in past - especially in very early solar system - life may not have been very robust, and only occur in special places - such as for instance hydrothermal vents (we've detected hydrothermal vents on Mars) - but there are various other hypotheses for where ancient life might occur on Mars.
Now - Europa and Encladus are prime sites to look for life in the solar system. They would be like the early embryo protoplanets - if it is really easy for life to evolve it might have evolved there also - maybe even independently from Earth life. Titan - if it has life - can't be life as we know it on Earth as it is far too cold. Io if it has life - would be sulfur based - not impossible - but again would be very different from Earth life.
For Europa and Encladus - as for Mars - we have to be very very careful not to contaminate with Earth life. So - probably no humans sent there until we are able to study it in its pristine state and find out what is there.
The big discoveries we might make on Mars include
On Europa, and Encladus, might find any of those.
On Titan and Io, and a few other places, then we also have the possibility of finding life that is based on totally different principles from Earth life. E.g. not using water as a solvent.
And, together with study of comets, all this may give insight into what kind of chemicals were delivered to early Earth, Mars and Venus.
For Mars past life, see
Where Should we Send our Rovers to Mars to Unravel Mystery of Origin of First Living Cells?
For the ExoMars mission objectives
Call for ExoMars 2018 Landing Site Selection
For Curiosity biological objectives:
Objectives - Mars Science Laboratory
Now though, it's pretty clear that Mars had a broad, shallow ocean in the early solar system. We've traced its shoreline all the way around the Northern hemisphere of Mars - and found traces of ancient deltas emptying into it. I think no longer questioned.
It's the prime objective for current and planned missions to Mars - to search for past and present life.
It probably was ice covered a fair bit of the time, but in the early solar system, Mars and Earth and Venus were all very similar with a dense atmosphere, plenty of nitrogen, atmospheric pressure tens of bars, and with oceans at high temperatures, probably into the hundreds of degrees centigrade.
As they cooled down, they must have all entered a phase which was hospitable for life. So the big question is - did any of them evolve life? Or was there life in the solar system already - there were chances for it to evolve in watery planetary embryos, which cooled down first of all.
If they did - was that life transferred from planet to planet during the huge impacts in the "late heavy bombardment"?
Now of those three planets, Venus is far too hot - and also has evidence of global resurfacing of the entire planet by volcanism. So it's not an easy place to search for life, and unlikely to have ancient traces of life there.
Earth has life of course, but this obscures the search for early life. Also because of continental drift, turns out that there isn't a single piece of rock on Earth that was here during the first few hundred million years when life probably evolved. All we have are a few mm sized zircons - which retain trace amounts of the early atmosphere, but not much else.
Mars though - had no continental drift and not much volcanism (because it is so much smaller than the other two planets) - and went into a deep freeze "Snowball Mars" state within a few hundred million years.
So - it's the perfect place to look for traces of early life and maybe find out how life evolved. It's a major challenge - but it might have, not just micro-fossils (hard to conclusively identify and prove to be microbes especially if smaller than present day life as early life surely would be) - but also - organics from that time.
It's a major challenge to preserve organics for billions of years. But not impossible - we find organics in some ancient stromatolites on Earth. On Mars, we have additional challenge of the cosmic radiation - and over billions of years - radiation from the rocks also even if deeply buried - and so that adds to the challenge - that's why Curiosity is looking for recently excavated rocks in hope to find richer deposits of organics than the ones it found so far.
Organics don't prove life either - because meteorites and comets have organics - so to find life conclusively you need to detect biosignatures. Curiosity can't do this (it does have one experiment able to detect chirality - but meteorite organics can be chirally biased - not likely to give conclusive results in an ancient heavily degraded organic deposit).
So - that's why there is such interest in looking for past life on Mars.
Then - if it did have life in the past - well it went into a "snowball phases" early on. It should be white - except - that since it lost all its water it looks red instead.
But that doesn't mean there is no life left. Earth went through snowball phases when there was almost no life on the surface - if you looked at Snowball Earth from Mars and even if you sent Curiosity rovers to the surface - you probably wouldn't detect any life there. But it was there in low concentrations - either underground - or - also quite probably on the surface - just below the surface, or beneath the ice, or similar places.
Similarly - though Mars has no obvious signs of life right now - still it might very well have life there in its "snowball phase". First of all - deep underground. But - perhaps surprisingly given its near vacuum for an atmosphere - there are also several potential places where life might survive on the surface also - crypto organisms - hidden just below the surface of the rocks, or a cm or so below the surface of the soil, similarly to microbes in Antarctica's McMurdo valleys and the Atacama high cold desert.
This is cutting edge research. It's only since 2008, and gradually more and more evidence gathered - that we have found reasonable evidence that these habitats exist. And - they remain controversial. But the warm seasonal flows, particularly - have not yet had any other explanation not involving liquid water. This is a rare phenomenon that you only get on sun facing slopes above OC, seasonal, and not correlated with the dust storms - and can't possibly be CO2 at those temperatures - narrow dark streaks on steep sun facing slopes, that gradually grow as the season progresses, and then fade away in autumn.
So - there may well be habitats on present day Mars for life. That's the most promising in a way - but there are several other possibilities with indirect evidence for them.
This is not at all confirmed. The streaks are too narrow, and too thin - probably more of a slight dampening of the dusty regolith with salty water. So - haven't yet been able to detect the signature of water there from orbit. That's the next thing to do for those - to try to get confirmation that it is water - but that might require a lander to do it.
Then - for present day life and also for past life - you have the problem that quite probably not all habitats will be inhabited. It may take millions of years for life to find a new habitat, who knows. And in past - especially in very early solar system - life may not have been very robust, and only occur in special places - such as for instance hydrothermal vents (we've detected hydrothermal vents on Mars) - but there are various other hypotheses for where ancient life might occur on Mars.
Now - Europa and Encladus are prime sites to look for life in the solar system. They would be like the early embryo protoplanets - if it is really easy for life to evolve it might have evolved there also - maybe even independently from Earth life. Titan - if it has life - can't be life as we know it on Earth as it is far too cold. Io if it has life - would be sulfur based - not impossible - but again would be very different from Earth life.
For Europa and Encladus - as for Mars - we have to be very very careful not to contaminate with Earth life. So - probably no humans sent there until we are able to study it in its pristine state and find out what is there.
The big discoveries we might make on Mars include
- Life based on fundamentally different biochemistry - e.g. fewer or more bases, different backbone (XNA instead of DNA), not based on a DNA type sequence at all but some other approach, and life that is between living and non living as we know it, e.g. replicates, but not as exactly as Earth life.
- Pre-biotic chemistry and reactions - complex interactions that are far more complex than most chemical reactions - maybe even with "metabolisms" and quite probably with cell-like structures - but not replicating at all exactly so not life as we know it.
- Life that is different way of encoding proteins and transcribing DNA into proteins, or some other difference in the mechanism by which it uses DNA
- Life that is basically like Earth life - but with some novel solutions - for instance - using novel chemicals for photosynthesis - or with far more robust mechanisms for fixing damage caused by DNA or cosmic radiation.
- Step by step progression all the way from pre-biotic organics rich oceans, through the smallest living ancestral cells, perhaps 40 nm across, then to the smallest present day cells, 200 mm and all the way to modern life - this could fill in the huge gap in our knowledge about early evolution of life
- Evidence about origin of life - did it originate on Mars, on Earth, Venus, embryo planet - or was the early nebula or were the planets later on seeded by life on another planet that happened to pass through our solar system e.g. during the condensation of the nebula or the "late" heavy bombardment.
On Europa, and Encladus, might find any of those.
On Titan and Io, and a few other places, then we also have the possibility of finding life that is based on totally different principles from Earth life. E.g. not using water as a solvent.
And, together with study of comets, all this may give insight into what kind of chemicals were delivered to early Earth, Mars and Venus.
For Mars past life, see
Where Should we Send our Rovers to Mars to Unravel Mystery of Origin of First Living Cells?
For the ExoMars mission objectives
Call for ExoMars 2018 Landing Site Selection
For Curiosity biological objectives:
Objectives - Mars Science Laboratory
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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