There is no way to know from orbit. Scientists used to think it was easy - that if the planet's atmosphere was out of equilibrium, it has life, if it is in equilibrium not. James Lovelock suggested that we look for simultaneous presence of pairs of gases like oxygen and methane that react together.
An atmosphere can be out of equilibrium without life. For instance in early Mars it probably had a fair bit of oxygen, but through dissociation of water. Same also for Europa it's ocean may have oxygen formed in a similar way from the ionizing radiation from Jupiter.
But if you detected both methane and oxygen, his idea was, it might be a sign of life.
But - it's not so easy as that. First of all, soon after he suggested this - and deduced that Mars must be lifeless (which Viking seemed to confirm) - we started to find ways that life could be undetectable in this way.
First of all, you can get life that is isolated from the surface. At the time of James Lovelock's proposal, no life like that was known. But very soon after, then the first hydrothermal vent communities were found on the ocean floor. A planet with hydrothermal vent communities but no life on the sea surface or on land would have its atmosphere pretty much in equilibrium - no chance of detecting it from orbit.
Since then, we have found many other ways that life can survive in caves, or in the deep hydrosphere even - life that lives kilometers below the surface on the Earth, would be completely undetectable from the surface.
Then - was also found that Earth has been almost uninhabitable several times in the past, not recently but hundreds of millions to billions of years ago.
Most recently through snowball Earth - when - either nearly all the surface was frozen over ("slush ball") or even all the surface was frozen. Eventually came out of it probably through CO2 gas from the volcanoes warming up the atmosphere (cycling around CO2 earlier sequestered into the ocean floors as limestone - partly through life processes).
So anyway - it survived that and life continued. If you looked at Earth during its snowball phase, the atmosphere would be pretty much in equilibrium, and in the more extreme versions of snowball Earth, it might have no visible trace of life on the surface.
Earlier in its history it had huge impacts during the late heavy bombardment - sort of like the one that formed the Moon but smaller - and its oceans boiled and even the land went molten, quite probably, in some of the very early impacts a little over 3 billion years ago. Again would seem like a lifeless planet, but there is evidence is that life survived that, somehow, also.
Also - we now have detection of methane on Mars, in small quantities, at least most think so (still some doubts). Which if you went by James Lovelock's idea would suggest there is life there.
But that doesn't prove that there is life there, it turns out. Because it could be caused by serpentization.
As well as that, we have now found life in places like the Atacama desert, and the McMurdo dry valleys - where the life is there, but in tiny quanities and also, living very much in the slow lane. Microbes there can have lifetimes of thousands of years for a single microbe that is, before division. Slowly metabolizing. So what chance of detecting that from the effect on the atmosphere?
And - as well as that - many "cryptoendolith" type lifeforms. Microbes that live just below the surface of rocks. From orbit then the rocks would just have the spectrographic signature of rock. But perhaps mms below the surface, very sparse population of microbes.
And there are many ideas now for potential habitats on Mars, like these Atacama desert and McMurdo dry valleys. Just a few mms of water that may form briefly or below the surface. Or a bit of moisture in salt pillars. Or solid state greenhouse effect below layers of translucent ice near the poles.
So - there could be many lifeforms on Mars, and many habitats there, and we might not be able to see any evidence of them at all from orbit.
These methane plume observations might give us evidence of life. If so then the life is surprisingly abundant, maybe living in deep reservoirs hydrothermally heated.
But if not, there might be these sparse thinly populated slowly metabolizing populations of microbes, even higher lifeforms also like lichens. And we wouldn't know about them yet, no chance. You struggle to spot life in the Atacama desert or the McMurdo dry valleys on the surface, indeed for a fair while scientists thought there was no life there. And still discover new habitats there from time to time. You'd very likely see no evidence of any life there at all from orbit.
As an example, I did a calculation here, supposing photosynthetic life on Mars, and supposing that all of it was as habitable as the most habitable of the Mars like regions of the McMurdo dry valleys (very unlikely)
How Life May Exist On Mars With Atmosphere Close To Equilibrium
I used the measurements for oxygen production from microbial mats in Lake Hoare in the dry valleys:
Lake Hoare (Antarctic explorers)
I couldn't find an estimate for the residence time of oxygen in the Mars atmosphere, but assuming 4500 years like Earth, I made it that even if all of the Mars surface consisted of these warm seasonal flows etc, all as habitable as the most habitable areas of the dry valleys, and all that oxygen ended up in the atmosphere - then that would contribute 0.4 tons per square kilometer of oxygen to the atmosphere.
Which would contribute 0.0002% by weight of oxygen to the Mars atmosphere.
Curiosity measured 0.145% of oxygen in the Mars atmosphere - see Page on sciencemag.org
So obviously there is no chance at all of Curiosity detecting a 0.0002% contribution of oxygen from photosynthetic life on Mars. And - that's with wildly optimistic figures.
It's not plausible at all that entire surface of Mars is as habitable as Lake Hoare.
More likely, at most a few square kilometers total of surface as habitable as this over the entire surface of Mars, and so at most a few tons total of the oxygen would come from these habitats. That is a few tons total for the whole surface of Mars from those habitats. And what oxygen is produced, may not mix with the atmosphere anyway.
So, I think pretty clear we aren't going to detect life from orbit unless it is very abundant and also has direct connection to the surface by which it vents occasionally, as with the idea for the methane plumes.
Which it might have underground. We might detect methanogens that are abundant underground by these methods.
But that's by no means the only or even the most likely place to find life on Mars, a priori. It's just the one that perhaps we are most likely to be able to detect first, if it is there.
I don't think we are likely to detect cryptoendolithic life living on the surface of Mars and the top few mms and cms by these methods, with slow metabolisms and lifetimes of millennia per microbe.
The only way to find these harder to detect lifeforms, if they are there, is to send robots there - or just possibly might detect something from orbit spectrographic signatures, but that would be hard to do with such low abundances.
At some point we have to send landers and rovers, to look closely at the warm seasonal flows, and the Martian geyser's flow like features (the geysers are dry ice but the FLFs probably water based) and the salt deposits and drill into the rocks and drill into the shallow and also the deeper subsurface and examine the sand dunes, and the rock surfaces to see if there is any life that uses the night time humidity - and search for life in many different proposed habitats on Mars. I don't see how that is possible from orbit.
We can see the geology, plain to view from orbit. So different from Earth where the geology is hard to see from orbit. With a few ground measurements to confirm what we are looking at, we can pretty much work out a lot of the geology just from orbit. I think some planetary geologists may think that the search for life would be similar, that if we can't see it from orbit maybe it's not there or not on the surface. But life has its own rules, not tied down to particular geology. Similar geological features on the Earth may some of them have life abundantly, and some may be rare or even almost absent based on such things as small variations in the levels of humidity of the atmosphere etc.
There was nothing wrong with James Lovelock's idea, brilliant and widely acclaimed thinking. And lead him to his Gaia work. But science caught up with it and turned some of the data and other conclusions he relied on on their heads. Rather quickly indeed. His paper was in 1967. The discovery of hydrothermal vents was in 1977, ten years later.
For my calculations of the oxygen levels above, and more on the subject and links to find out more, see my Science20 article: How Life May Exist On Mars With Atmosphere Close To Equilibrium
An atmosphere can be out of equilibrium without life. For instance in early Mars it probably had a fair bit of oxygen, but through dissociation of water. Same also for Europa it's ocean may have oxygen formed in a similar way from the ionizing radiation from Jupiter.
But if you detected both methane and oxygen, his idea was, it might be a sign of life.
But - it's not so easy as that. First of all, soon after he suggested this - and deduced that Mars must be lifeless (which Viking seemed to confirm) - we started to find ways that life could be undetectable in this way.
First of all, you can get life that is isolated from the surface. At the time of James Lovelock's proposal, no life like that was known. But very soon after, then the first hydrothermal vent communities were found on the ocean floor. A planet with hydrothermal vent communities but no life on the sea surface or on land would have its atmosphere pretty much in equilibrium - no chance of detecting it from orbit.
Since then, we have found many other ways that life can survive in caves, or in the deep hydrosphere even - life that lives kilometers below the surface on the Earth, would be completely undetectable from the surface.
Then - was also found that Earth has been almost uninhabitable several times in the past, not recently but hundreds of millions to billions of years ago.
Most recently through snowball Earth - when - either nearly all the surface was frozen over ("slush ball") or even all the surface was frozen. Eventually came out of it probably through CO2 gas from the volcanoes warming up the atmosphere (cycling around CO2 earlier sequestered into the ocean floors as limestone - partly through life processes).
Perhaps Earth had patches of open ocean in the tropical regions, or perhaps it was completely covered in ice
Slushball Earth
Possible Snowball Earth models
So anyway - it survived that and life continued. If you looked at Earth during its snowball phase, the atmosphere would be pretty much in equilibrium, and in the more extreme versions of snowball Earth, it might have no visible trace of life on the surface.
Earlier in its history it had huge impacts during the late heavy bombardment - sort of like the one that formed the Moon but smaller - and its oceans boiled and even the land went molten, quite probably, in some of the very early impacts a little over 3 billion years ago. Again would seem like a lifeless planet, but there is evidence is that life survived that, somehow, also.
Also - we now have detection of methane on Mars, in small quantities, at least most think so (still some doubts). Which if you went by James Lovelock's idea would suggest there is life there.
But that doesn't prove that there is life there, it turns out. Because it could be caused by serpentization.
As well as that, we have now found life in places like the Atacama desert, and the McMurdo dry valleys - where the life is there, but in tiny quanities and also, living very much in the slow lane. Microbes there can have lifetimes of thousands of years for a single microbe that is, before division. Slowly metabolizing. So what chance of detecting that from the effect on the atmosphere?
And - as well as that - many "cryptoendolith" type lifeforms. Microbes that live just below the surface of rocks. From orbit then the rocks would just have the spectrographic signature of rock. But perhaps mms below the surface, very sparse population of microbes.
And there are many ideas now for potential habitats on Mars, like these Atacama desert and McMurdo dry valleys. Just a few mms of water that may form briefly or below the surface. Or a bit of moisture in salt pillars. Or solid state greenhouse effect below layers of translucent ice near the poles.
So - there could be many lifeforms on Mars, and many habitats there, and we might not be able to see any evidence of them at all from orbit.
These methane plume observations might give us evidence of life. If so then the life is surprisingly abundant, maybe living in deep reservoirs hydrothermally heated.
But if not, there might be these sparse thinly populated slowly metabolizing populations of microbes, even higher lifeforms also like lichens. And we wouldn't know about them yet, no chance. You struggle to spot life in the Atacama desert or the McMurdo dry valleys on the surface, indeed for a fair while scientists thought there was no life there. And still discover new habitats there from time to time. You'd very likely see no evidence of any life there at all from orbit.
As an example, I did a calculation here, supposing photosynthetic life on Mars, and supposing that all of it was as habitable as the most habitable of the Mars like regions of the McMurdo dry valleys (very unlikely)
How Life May Exist On Mars With Atmosphere Close To Equilibrium
I used the measurements for oxygen production from microbial mats in Lake Hoare in the dry valleys:
Lake Hoare (Antarctic explorers)
I couldn't find an estimate for the residence time of oxygen in the Mars atmosphere, but assuming 4500 years like Earth, I made it that even if all of the Mars surface consisted of these warm seasonal flows etc, all as habitable as the most habitable areas of the dry valleys, and all that oxygen ended up in the atmosphere - then that would contribute 0.4 tons per square kilometer of oxygen to the atmosphere.
Which would contribute 0.0002% by weight of oxygen to the Mars atmosphere.
Curiosity measured 0.145% of oxygen in the Mars atmosphere - see Page on sciencemag.org
So obviously there is no chance at all of Curiosity detecting a 0.0002% contribution of oxygen from photosynthetic life on Mars. And - that's with wildly optimistic figures.
It's not plausible at all that entire surface of Mars is as habitable as Lake Hoare.
More likely, at most a few square kilometers total of surface as habitable as this over the entire surface of Mars, and so at most a few tons total of the oxygen would come from these habitats. That is a few tons total for the whole surface of Mars from those habitats. And what oxygen is produced, may not mix with the atmosphere anyway.
So, I think pretty clear we aren't going to detect life from orbit unless it is very abundant and also has direct connection to the surface by which it vents occasionally, as with the idea for the methane plumes.
Which it might have underground. We might detect methanogens that are abundant underground by these methods.
But that's by no means the only or even the most likely place to find life on Mars, a priori. It's just the one that perhaps we are most likely to be able to detect first, if it is there.
I don't think we are likely to detect cryptoendolithic life living on the surface of Mars and the top few mms and cms by these methods, with slow metabolisms and lifetimes of millennia per microbe.
The only way to find these harder to detect lifeforms, if they are there, is to send robots there - or just possibly might detect something from orbit spectrographic signatures, but that would be hard to do with such low abundances.
At some point we have to send landers and rovers, to look closely at the warm seasonal flows, and the Martian geyser's flow like features (the geysers are dry ice but the FLFs probably water based) and the salt deposits and drill into the rocks and drill into the shallow and also the deeper subsurface and examine the sand dunes, and the rock surfaces to see if there is any life that uses the night time humidity - and search for life in many different proposed habitats on Mars. I don't see how that is possible from orbit.
We can see the geology, plain to view from orbit. So different from Earth where the geology is hard to see from orbit. With a few ground measurements to confirm what we are looking at, we can pretty much work out a lot of the geology just from orbit. I think some planetary geologists may think that the search for life would be similar, that if we can't see it from orbit maybe it's not there or not on the surface. But life has its own rules, not tied down to particular geology. Similar geological features on the Earth may some of them have life abundantly, and some may be rare or even almost absent based on such things as small variations in the levels of humidity of the atmosphere etc.
There was nothing wrong with James Lovelock's idea, brilliant and widely acclaimed thinking. And lead him to his Gaia work. But science caught up with it and turned some of the data and other conclusions he relied on on their heads. Rather quickly indeed. His paper was in 1967. The discovery of hydrothermal vents was in 1977, ten years later.
For my calculations of the oxygen levels above, and more on the subject and links to find out more, see my Science20 article: How Life May Exist On Mars With Atmosphere Close To Equilibrium
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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