However, would photosynthetic life on Mars be an easy to spot colour such as green? H.G. Wells in War of the Worlds speculates that it could be a vivid blood-red.
"Apparently the vegetable kingdom in Mars, instead of having green for a dominant colour, is of a vivid blood-red tint. At any rate, the seeds which the Martians (intentionally or accidentally) brought with them gave rise in all cases to red-coloured growths."
Of course we know now that Mars doesn't have vegetation like Earth, but it could have photosynthetic lichens and microbes. So, first, why does Earth life use a green pigmented chemical for photosynthesis and what might one expect to evolve on Mars? After all the green colour of Chlorophyll means that it absorbs light most strongly in the red and blue part of the spectrum, It's actually a bit of a puzzle why Earth photosynthetic life is green. It means that it rejects light in the strongest part of the solar spectrum. Why aren't leaves dark red, or dark purple, or indeed black?
Perhaps it evolved as a historical accident, and leaves are green because they get plenty of light, need to reflect some light away to resist dehydration, and there is no advantage in changing to black or purple. Or perhaps early life was indeed purple, absorbing green, like the modern haloarchaea, and photosynthetic green microbes originally evolved to take advantage of the light that the purple microbes rejected? That's the "Purple Earth Hypothesis" (abstract here). Or perhaps green is reasonably optimal anyway, once you take account of other effects?
At any rate, whatever the reason why we have green vegetation on Earth, yes, green photosynthetic life does work in Mars simulation experiments. So it could potentially have green photosynthetic life. But it's hardly the most efficient way to make use of the spectrum on Earth, and it would be even less effiicient on Mars with half the light levels of Earth and dust filtering out much of the light towards the blue end of the spectrum.
Solar radiation for direct light at the top of the Earth's atmosphere, and at sea level. Shows the linear visible spectrum superimposed.
As you can see, we get more green light than any other frequency, yet for some reason, most of our vegetation is green.
What about photosynthetic life which evolved on Mars, with half the light levels of Earth and with a fair bit of the light towards the blue end of the spectrum attenuated by the dust in the atmosphere?
Well actually, here on Earth we have many photosynthetic lifeforms that are are red, or pink, particularly, the haloarchaea. These are common in very salty water, and many of the suggested habitats on Mars are very salty. Of course that doesn't mean that Mars will have haloarchaea, but these are excellent candidates to survive on Mars, and they are red. So, it seems that photosynthetic life on Mars could be red or purple amongst other possibilitiees.
Lake Hillier in Western Australia, a saline lake noted for its pink colour. It's pink partly because of the purple haloarchaea, and partly because of red carotene accumulating in a green algae dunaliella salina.
The purple haloarchaea are amongst the best candidates for microbes to survive on Mars. They use Bacteriorhodopsin and Halorhodopsin for photosynthesis which resemble the pigment rhodopsin that we use for vision. Bacteriorhodopsin is a purple and absorbs green light most efficiently.
Carotenes are what makes a carrot red. They are involved in normal photosynthesis. They absorb UV, violet and blue light and scatter red and orange light. They dissipate some of it as heat so protecting other organics such as proteins and membranes from the damaging effects of UV light, which would be useful of course on Mars.
However, they also transmit some of the energy they receive to chlorophyll. Pigments that can transfer energy to Chlorophyll like this are known as "antenna pigments" and they do it by dipole to dipole coupling (the process is called Förster resonance energy transfer). In this photograph, ordinary green algae have been turned red by carotene which they produce as a protection and antenna pigment. There are many other antenna pigments such as Cholrophyll b and xanthophylls (which colour egg yolks and autumn leaves yellow) which have similar roles. Another exapmple is Lycopene which makes tomatoes red. Cyanobacteria and red algae also have phycocyanin and allophycocyanin which absorb orange light and a red pigment phycoerythrin which absorbs green light.
Deep dark red of algae in the crater lake of Mount Simba volcano at a height of 5,900 meters in the Altiplano, Chile. The microbes have developed special pigments to cope with extreme levels of UV. A few years ago the researchers measure what remains to this day the highest levels of UV measured in the world. Image credit SETI Institute/ NAI High Lakes Project
This next diagram shows in detail how these different pigments absorb light in different parts of the spectrum. The main thing to notice is how life uses a lot of different pigments to capture light in many areas of the spectrum. Chlorophyll a can only absorb a narrow band of light in the red part of the spectrum (688) and Chlorophyll b in the blue part of the spectrum. But as you can see, there are other antenna pigments that help it take advantage of other parts of the spectrum. Perhaps if there is chlorophyll based life on Mars, it will have these pigments, which are purple, orange, pink etc in colour.
Part of Figure 1 from this study of the colour of life on Earth and exoplanets.
Also, in low light conditions, photosynthetic life might be much more efficient at absorbing light than it is on Earth. Jack O’Malley-James of the university of St Andrews, Scotland, has suggested that life which evolved around red dwarf stars, especially binary star systems, could be dark in colour, or black, because it would receive far less visible light than Earth life does, so would need to make use of as much of it as possible.
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Examples of Earth vegetation with black flowers or leaves. When levels of light are low, then plants may become dark in colour or black to use as much of the spectrum as possible.
Possibilities for plant life under low light levels around other stars - perhaps in low light conditions the vegetation would be black.
So as well as the black plants, some red algae and brown algae are nearly black, and grow in depths around 270 meters where the light is less than 1% of surface light. Life on Mars might experience not dissimilar conditions. Especially since any photosynthetic life is likely to be hidden in cracks or beneath thin layers of dust or underneath the crust of rocks in order to shelter from UV light. The red iron oxides of the Mars surface are especially good at filtering out UV light so photosynthetic life might well use it as protection from sunlight. This cuts down the available light even more. So Mars:
This shows photographs taken by Opportunity during a dust storm from sols 1205 to 1236 (one month). Each horizon view has been compressed horizontally (but not vertically). By the end of this period it reached a visual optical depth tau 4.7 which means that 99% of the sunlight was blocked. However that is for direct light. Of course the dust will also scatter a lot of light, and if you include ambient as well as direct light then the figures are not quite so extreme.
This shows theoretical prediction of the combined direct and ambient light for different optical depths on Mars. Here the optical depth is a number which is larger, the more the light is blocked out. Tau 4.7 corresponds to around 99% of direct light blocked, for instance during a dust storm on Mars. However, much of that light is scattered so adding to the amount of ambient light.
The curves in this graph show the amount of the combined direct and ambient light. The numbers 10, 20, 30 etc show the angle of the sun from the zenith - so with the sun at 90 degrees from the Zenith it's horizon skimming with very little light, and if there is an dust it goes through thick layers of dust, with almost no illumination.
So for instance, to find out how much light you get on the surface at the height of a typical dust storm go to around 4.7 in this diagram. Of the original 600 watts you still get around 200 watts when the sun is vertically overhead, at its zenith, even during a dust storm. But you get it greatly reduced when the sun is closer to the horizon, as you'd expect, almost to nothing.
This shows the irradiance for direct sunlight. Very much less. During a Mars dust storm there is almost no direct sunlight with the sun at any angle from the zenith, even if it is directly overhead. Graphs from this 1999 paper. Our ideas of Mars have changed a bit since then but not enough to make these significantly out of date I think, at least for our purposes here. If you know of more recent graphs do say.
So, during a dust storm there is almost no direct sunlight. The amount of indirect sunlight is cut to a third even when the sun is directly overhead, at the tropics, and much more at other times of day even in equatorial regions (and much more so at higher latitudes). So, this is just my own suggestion here. Might some Martian life have very dark photosynthetic life in order to take advantage of as much light as possible during dust storms?
On the other hand, unless it is well sheltered by dust, or within rocks, it would also need to reflect away UV light for UV protection and may need to prevent desiccation. Perhaps another likely colour could be purple?
"Plants would appear darker under much dimmer, redder stars that emit more infrared than visual wavelengths of light but the color could vary widely"
Colour adjusted photograph by Tim Pyle of Caltech to illustrate possibilities for vegetation around other stars. Red dwarf stars particularly would have much more light in the red and infrared and photosynthetic life could have evolved to take advantage of it.
Or indeed, if it used iron oxides somehow for protection from UV light, it might be rust red in colour. So from those examples, photosynthetic life on Mars might well not be green. It could easily be various shades of red, yellow, pink, purple, or indeed almost black, amongst other possibilities. Kelp for instance and other forms of seaweed that are adapted to the lower light levelsi n the sea are often brown in colour, to absorb as much sunlight as possible.
Giant kelp is brown to absorb as much sunlight as possible. It's coloured brown because of the accesory "antenna" pigment Fucoxanthin which absorgs light in the blue-green to yellow-green part of the spectrum. Some photosynthetic microbes are dark in colour too. In the conditions on Mars with dimmer light and the dust storms, photosynthetic life on Mars might be dark like this to use as much of the sunlight as possible for photosynthesis.
Also, we don't know if Mars does have photosynthesis. If it doesn't, then we will be looking for life depending on chemosynthesis, in salty brines perhaps, just below the surface. If so, even if it doesn't photosynthesize, it can be damaged by UV. So, again, it's likely to use the red coloured surface rusty iron oxides for UV protection, or if it uses carotene for UV protection, it's again likely to be reddish in colour - for instance, one of our most ionizing radiation resistant microbes, Radiodurans ranges in colour from red to pink. It is that colour because of the presence of carotene for UV resistance, and is thought to have got its ionizing radiation resistance incidentally as a desert species from desiccation resistance (which has similar DNA damaging effects). Radiodurans requires oxygen (it's an obligate anaerobe) and can't photosynthesize, so it's not a candidate microbe for present day Mars. But it's an example to show that UV resistant non photosynthetic life could easily be reddish in colour on Mars.
So, even non photosynthetic UV resistant life might well be purple on Mars. This is one thing the National Geographic sequence got right. It seems quite possible that Mars could have purple lifeforms, or red, or pink, though it could also be many other colours such as black, orange, yellow, etc as well as green.
Rust coloured or red life would be hard to spot amongst the iron oxides even with filtered vision. And dark or black life, hidden beneath a layer of dust, or in the shadow of a crack, might be almost impossible to see by eye. If you crack open a rock on Mars, or srape away some dust, and the life is rust red, or dark brown, how would you recognize it?
So, present day life is going to be hard to spot by eye on Mars. Unless, that is, it is obviously novel, say a purple lichen that you can be pretty much certain never got to Mars on our spacecraft.
Lichen P. chlorophanum on a Mars analog substrate for the DLR Mars simulation experiments. - colour adjusted to a dark purple. If we saw a lichen or other multicellular lifeform on Mars, especially if coloured in some unusual non Earthly colour, it would be convincing evidence of native Mars life which we could spot visibly. At least, it would be, in early stages of human exploration (if we send humans to the Mars surface). In a Mars that's been settled by humans for some time, perhaps an unusually coloured lichen could be an adaptation of introduced Earth life, maybe partly through gene exchange with native Mars life.
Generally, multicellular life on Mars, large enough to see visually would be easier to spot, and it would be easier to show that it is from Mars, if it is. Also we'd be less likely to have accidentally introduced multicellular life than accidentally introduced microbes, in early stages of Mars exploration, if we do send humans to the surface.
However most of the suggestions for searches for life on Mars focus on microbes. If these microbes are mixed in with the dust or in the rocks, thinly distributed, and perhaps are reddish in colour, similar to the Mars surface, or a darker black in colour - how would we spot them visually?
Also, whatever life there is on Mars may be so sparse that if we go to the most habitable areas, perhaps we may find a total area of a few square meters of sparse, slowly metabolizing life, in an exploration region of several square kilometers. Or we might find a few spores in the dust, but perhaps have to examine a fair bit of dust to find them. So you are talking here about searching for something that humans may be unable to distinguish visibly, probably also in such sparse populations and hidden just beneath the surface, so that even with filtered vision it's impossible to detect it visually.