Astrobiology: A Cautionary Tale

by Paul Gilster on February 27, 2015

We’re discovering planets around other stars at such a clip that moving to the next step — studying their atmospheres for markers of life — has become a priority. But what techniques will we use and, more to the point, how certain can we be of their results? Centauri Dreams columnist Andrew LePage has been mulling these matters over in the context of how we’ve approached life on a much closer world. Before the Viking landers ever touched down on Mars, a case was being made for life there that seemed compelling. LePage’s account of that period offers a cautionary tale about astrobiology, and a ringing endorsement of the scientific method. A senior project scientist at Visidyne, Inc., Drew is also the voice behind Drew ex Machina.

by Andrew LePage


Every time I read an article in the popular astronomy press about how some new proposed instrument will allow signs of life to be detected on a distant extrasolar planet, I cannot help but be just a little skeptical. For those of us with long memories, we have already been down this road of using remote sensing techniques to “prove” life existed on some distant, unreachable world, only to be disappointed when new observations became available. But instead of a distant extrasolar planet, over half a century ago that planet was our next door neighbor, Mars.

Back when I was in high school in the late 1970s, I enjoyed spending time during study hall going through science books and magazines, old as well as new, in the school library. Among the interesting tidbits I read about were spectral features known as “Sinton bands” and how in the early 1960s these were considered the latest evidence of life on Mars. Of course by the time I was reading this, I knew from the then-recent results from the Viking missions that the explanation for these and other observations was simply incorrect. So what ever happened to these Sinton bands and the interpretation they were evidence of life on Mars?

In the years leading up to the beginning of the Space Age, the general consensus of the scientific community was that Mars was a smaller and colder version of the Earth that supported primitive plant life akin to lichen. This view was based on a large body of observational evidence gathered over the first half of the 20th century. A firmly established wave of darkening was observed spreading over the spring hemisphere of Mars each Martian year which was widely seen as being the result of plants coming out of their winter slumber much as happens on Earth each spring. This interpretation was bolstered by visual observations that the dark regions of Mars appeared to have a distinct green hue just as one would expect from widespread plant life.

Other observations of Mars during this period lent further support to the view that the Red Planet could support simple life forms. The general consensus of the astronomical community at this time based on analyses of decades of photometric and polarimetric measurements of Mars indicated that the surface pressure of the Martian atmosphere was about 85 millibars or about 8.4% of Earth’s surface pressure. Carbon dioxide and water vapor were detected and nitrogen was widely expected to be the major atmospheric constituent just as it was on Earth. No large bodies of water were visible on the surface and the climate was certainly colder than on Earth as a whole owing to Mars’ greater distance from the Sun, but the surface temperatures at the equator easily exceeded the freezing point of water during the summer so that liquid water was expected to be available. While not an ideal environment by terrestrial standards, it seemed that Mars had conditions that would be expected to support life much like the high arctic here on Earth.


Image: This was the best photograph of Mars available before the Space Age taken at the Mt. Wilson observatory in 1956 – the same year Sinton bands were discovered. Credit: Mt. Wilson Observatory.

To further test this view, American astronomer William Sinton (1925-2004) decided to use the latest technological advancements in infrared (IR) spectroscopy to obtain observations of Mars during its especially favorable 1956 opposition. On seven nights during the fall of 1956, Dr. Sinton used the 1.55-meter Wyeth Reflector at the Harvard College Observatory to make IR spectral measurements using a lead sulfide detector cooled using liquid nitrogen to vastly improve its sensitivity. He made repeated measurements between the wavelengths of 3.3 to 3.6 μm in order to sample the spectral region where resonances from the C-H bonds of various organic molecules would create distinctive absorption features. His analysis found a dip in the IR spectrum of Mars near 3.46 μm which resembled his IR spectrum of lichen. This finding and his conclusions were published in highly respected, peer-reviewed astronomical publication The Astrophysical Journal.

Encouraged by these initial results, Dr. Sinton repeated his measurements using an improved IR detector on the 5-meter Hale Telescope at the Mt. Palomar Observatory (then, the largest telescope in the world) during the following opposition of Mars in October 1958. His new observations had ten times the sensitivity of his original measurements and now covered wavelengths from as short as 2.7 μm out to 3.8 μm. In addition to absorption features attributable to methane and water vapor in Earth’s atmosphere, Dr. Sinton identified absorption features centered at 3.43, 3.56 and 3.67 μm that appeared to be weaker or absent in the brighter areas of Mars. Dr. Sinton concluded that inorganic compounds like carbonates could not produce the observed features. Instead they must be produced by organic compounds selectively concentrated in the dark areas of Mars that were already known to be greener. While the features he observed were not a perfect match for any known plant life on Earth, he concluded that they were due to organic compounds such as carbohydrates produced by plants on the surface of Mars. These findings and conclusions were again published in a well-regarded, peer-reviewed scientific journal, Science.

While there was naturally some healthy skepticism about the findings, they were seen by many as supporting the generally held view that Mars was the home of simple, lichen-like plant life. In order to better observe what became known as “Sinton bands”, the Soviet Union even planned to include IR instrumentation to measure these spectral features from close range on the first pair of spacecraft they launched towards Mars in October 1960. Unfortunately, both Mars probes succumbed to launch vehicle failures during ascent and never even made it into Earth orbit. Soviet engineers attempted it again with a pair of much more capable flyby probes of which only Mars 1 survived launch on November 1, 1962. Unfortunately, Mars 1 suffered a major failure in its attitude control system during its cruise and contact was lost three months before its encounter with Mars on June 21, 1963. As a result, there were no close-up IR observations of the Sinton bands at this time.


Image: The earliest Soviet Mars probes carried IR instrumentation to observe Sinton bands at close range including Mars 1 launched in November 1962. Credit: RKK Energia.

But even as the Soviet Union was struggling to reach Mars with their first interplanetary probes, the case for there being plant life on Mars and the Sinton bands being evidence for it was already beginning to unravel. Donald Rea, leading a team of scientists at the University of California – Berkeley, published the results of their work on Sinton bands in September 1963. They examined the IR spectra of a large number of inorganic and organic samples in the laboratory and could not find a match for the observed Sinton bands. While they could not find a satisfactory explanation for the bands, they found that the presence of carbohydrates as proposed by Dr. Sinton was not a required conclusion.

Another major blow was landed in a paper by another University of California – Berkeley team headed by chemist James Shirk which was published on New Year’s Day 1965. Their laboratory work suggested that the Sinton bands could be caused by deuterated water vapor – water where one or both of the normal hydrogen atoms, H, in H2O are replaced with the heavy isotope of hydrogen known as deuterium, D, to form HDO or D2O. Shirk and his team speculated that the deuterated water vapor was present in the Martian atmosphere with the implication that the D:H ratio of Mars greatly exceeded that of the Earth.

The final explanation for the Sinton bands came in a paper coauthored by Donald Rea and B.T. O’Leary of the University of California – Berkeley as well as William Sinton himself published in March of 1965. Based on a new analysis of Dr. Sinton’s data from 1958, observations of the solar IR spectrum from Earth’s surface and the latest laboratory results, it was found that the absorption features in the Martian spectrum now identified as being at 3.58 and 3.69 μm were the result of HDO in Earth’s atmosphere. The feature at 3.43 μm was, in retrospect, a marginal detection in noisy data and was probably spurious. The mystery of the Sinton bands was solved and, unfortunately, it had nothing to do with life on Mars.

Sinton bands were not the only causality of advances in technology and remote sensing techniques at this time. As more detailed ground-based observations of Mars were made during the 1960s and the first spacecraft reached this world, it was eventually found that all of the earlier observations that had been taken as evidence of life on Mars were either inaccurate or had non-biological explanations. After a half century of observations from space and on the surface, we now know that the Martian environment is simply too hostile to support even hardy lichen-like plants as had been widely believed before the Space Age.

This story about the rise and fall of the view that Mars harbors plant-like life forms should not be taken as an example of the failure of science. Instead, it is a perfect example of how the self-correcting scientific process is supposed to work. Observations are made, hypotheses are formulated to explain the observations and those hypotheses are then tested by new observations. In this case, the pre-Space Age view that Mars supported lichen-like plants was disproved when new data no longer supported that view. And our subsequent experience with the in situ search for life on Mars by the Viking landers in 1976 is further evidence not that Mars is necessarily lifeless, but that detecting extraterrestrial life is much more difficult than had been previously believed. These lessons need to be remembered as future instruments start to scan distant extrasolar planets and claims are made that life has been found because of the alleged presence of one compound or another. Past experience has shown that such interpretations can easily be incorrect especially when dealing with new observing techniques of distant worlds with unfamiliar environments.


Abelard Lindsey February 27, 2015 at 11:47

This is the rise and fall of the Lowellian Mars concept. Not that Mars had intelligent life on it (this was discredited by 1910 or so), but the idea that Mars had an atmospheric pressure comparable to Mt. Everest and was semi “Earth-like”, able to host simple plant life. the best SF description of this Mars is Clarke’s “Sands of Mars” novel. It was this concept of Mars that was rudely shattered by the Mariner 4 fly-by in July of 1965.

Gregory Benford February 27, 2015 at 13:29

An interesting parallel with different outcome is the disputes over methane on Mars. Prevailing opinion held that the Mars-orbital and Earthbound detections in the 2000s were wrong. Now Curiousity has shown the methane is real, with implications for subsurface life that we can pursue.

Alex Tolley February 27, 2015 at 13:57

Good article. As I recall. Lovelock dismissed life on Mars based on his Gaia hypothesis. Had Mars been coated with some form of organic material, like the tholins on comets, that would have been even more problematic.

I much prefer the approach that looks at non-equilibrium molecules, e.g. methane in an oxygen atmosphere. It isn’t perfect, but it seems a better approach to me. The issue with this technique is that the data so far is very low resolution, which makes characterization of molecules from these spectra rather crude. I don’t know whether this means we need better techniques, or just much larger telescopes to resolve planetary images.

william February 27, 2015 at 14:09

Very interesting article – I recall vaguely the Sinton bands that he talks about.

We best be careful though, how we handle our visiting spacecraft to other planets (or we may find life there, but it will be earthly life):

Joy February 27, 2015 at 15:40

I recall those expectations as well. My primary school teachers used materials which portrayed Mars as not much worse than Mt Everest. Particularly, I have a sad memory of attending with my brother a symposium at Stanford in July 1965 immediately prior to the Mariner 4 flyby. The presenter from NASA Ames anticipated a somewhat Earth-like terrain. The results, showing a heavily cratered surface (only a portion of Mars was imaged at high resolution and the flyby track was cratered) and confirming an atmospheric pressure an order of magnitude lower than expected were gutting. Bradbury’s Mars died that summer.

Luis February 27, 2015 at 17:27

How about we prove there is life on earth from Mars?

Abelard Lindsey February 27, 2015 at 17:58

The Methane on Mars may be the result of life on or in Mars. This is the most common explanation. However, it may instead suggest that Thomas Gold’s theory of abiotic Methane is correct. I bet more on Thomas Gold being correct than the existence of micro-organisms on Mars (although both could be true).

I think the Mariner 4 flyby had a far stronger effect on the public’s interest in space than most people realize. Prior to the flyby, Mars represented a new frontier in the public’s imagination, particularly the American public. That dream was dashed in July of 1965. I think this was the turning point when people began to loose interest in space. I think the result of this flyby affected the overall morale of Western, in particular American, civilization as a whole far more than most people realize today.

Ashley Baldwin February 27, 2015 at 18:01

Interesting to see if the parallel holds up with exoplanets . No helpful visiting probes here to clear things up. It’s going to be claim and counterclaim centred around biosignature SNR, signal to noise ratio and spectral resolution all the way up to a 16m space ELT TPF. CO2,O2,O3, dear old methane again and maybe even chlorophyll itself eventually . Ages before direct imaging allows more than point source resolution, so spectroscopy is going to be it , aided by precision astrometry for HBZ confirmation and hopefully massively improved asteroseismology to help constrain the important nature of the parent star itself which will be as key as the planet itself in supporting claims of life. All of this could easily begin within the next twenty years , indeed I expect it to.

ljk February 27, 2015 at 18:14

Drew, they just confirmed methane on Mars! Could it be… :^)

Andrew LePage February 27, 2015 at 18:26

@Abelard Lindsey February 27, 2015 at 11:47 & Joy February 27, 2015 at 15:40

Actually this idea that Mariner 4 single handedly destroyed the view of an Earth-like Mars is a bit of a modern myth. During the year leading up the launch of Mariner 4, analysis of new ground-based IR spectral results by Soviet and American astronomers acquired during the 1963 opposition of Mars was showing that the atmospheric surface pressure was as low as 17 millibars and probably lower. This realization that Mars has a thinner atmosphere than originally thought was beginning to be reflected in NASA Mars mission planning by the summer of 1964. Details of the changes in the understanding of the Martian atmosphere in 1964 are detailed here:

Then of course the “mystery” of Sinton bands was resolved in March 1965 and new analysis results from ground-based observations that started coming out during the first half of 1965 supported the newer view of a much thinner Martian atmosphere. By the time Mariner 4 Reached Mars, the old view of Mars was already in tatters. While there were certainly many who continued to cling to an Earth-like view of Mars, the pictures returned by Mariner 4 after its July 1965 flyby showing that Mars had a Moon-like cratered surface and lacked a magnetic field (which suggested Mars was “dead”) were the last nails in the coffin for an Earth-like Mars. The radio occultation experiment results by Mariner 4 that showed Mars had an atmospheric surface pressure of only 4 to 7 millibars essentially confirmed the “worse fears” from the latest analyses of new ground-based data.

E. F. Milone February 27, 2015 at 19:31

An interesting article on the Sinton bands. It may be relevant to mention that the darkening wave which had been thought to be green was in fact grey. If memory serves, it was O’Leary who reported the convincing photometry (at an AAS meeting).

G. Cecil February 27, 2015 at 23:29

35 years ago I had the privilege of taking grad obs. astronomical techniques from Prof. Sinton. He went through the history of his Mars NIR spectral observations, and the perils of systematic errors, showing us his annotated original strip charts. Now we have the luxury of quantifying errors by simulations, but results are only as reliable as our Bayesian priors.

Craig Watkins February 28, 2015 at 0:43

Of course there is a whole range of tantalizing results that could tease us, but isn’t it also possible that there would be a “smoking gun”? Couldn’t we also look at something that was almost the exact replica of earth’s atmosphere (at least as it would be measured in these new instruments) and find life to be easily the most compelling explanation? My excitement comes from the shear number of planets we’ll be able to measure. Eventually, I hope that we can find something very compelling even if our measurements are crude to begin with.

Joy February 28, 2015 at 1:19

@Andrew – years later I became aware of all of the depressing pre-Mariner 4 scientific results, but in July 1965, they were definitely not being brought to the attention of the general public, even by NASA’s own PR people except occasionally oblique and ambiguous references that Mariner 4 would help to settle ongoing scientific controversies. I recall Walter Cronkite’s dejected tone as he reported that the surface of Mars looked moon-like and dead. Agree with Abelard Lindsey, Mariner 4 was when the general public really got the message that “Mars ain’t the kind of place to raise your kids · In fact it’s cold as hell”. Mariner 4 had a huge psychological impact on the public, even if a the rather small community of planetary scientists had published advance warnings in journals with specialist readership. At the time I was at the youthful preteen “golden age” for reading SF and had a head full of Heinlein and Bradbury, which was much more the public zeitgeist than the cutting edge science.
PS: Abelard Lindsey’s science teacher must have used the same materials mine did, we both independently recall the analogy of Mars to Everest.

Ashley Baldwin February 28, 2015 at 4:28

For all the disappointment of Mars, it may well be it is still the place that life , or evidence is found beyond all reasonable argument to the contrary. Europan or Enceledan seas may bring hope but spectroscopic analysis ( again) of their “plumes” may be the best we have for a long time. I can’t see landers equipped with high grade drilling apparatus and very deep dive submersibles happening for decades at the least. Maybe we get lucky with a deeper drill on Mars or at a more promising landing site than currently accessible as that technology increases more quickly. I can’t admit to being optimistic but the hope remains. But bio signatures from parsecs and years of ensuing scientific toing and frowing is what I expect first.

DCM February 28, 2015 at 5:02

No life on nearby worlds is a positive matter. It means we can make use of those planets without harming any life. Of course future generations will claim there was life on Mars and that it was deliberately killed off so we could use it — my prediction for the future.
The more worlds we can occupy the better off we’ll be both in terms of our future around the sun and in case we find other life somewhere.

Andrew LePage February 28, 2015 at 9:53

@Joy February 28, 2015 at 1:19

I agree that the popular view of Mars as an Earth-like world was destroyed by the crater-filled images returned by Mariner 4 (as well as settle the “controversy” in the scientific community at the time you mentioned). It just goes to show the psychological impact of pictures over dry, uninspiring graphs and technical text (even for scientists!).

Abelard Lindsey February 28, 2015 at 13:30

“Abelard Lindsey’s science teacher must have used the same materials mine did, we both independently recall the analogy of Mars to Everest.”

I had a 4th grade teacher (she was old school) who showed us that Disney animation film that was made sometime in the late 50’s, the one with the ion propulsion systems and the funky life forms on Mars. This was in ’72-73, but definitely featured the “Lowellien Mars” of the 1950’s. Yes, I think it was the Mariner 4 flyby that first exposed much of the public to the realities of Mars.

Joy February 28, 2015 at 16:25

Ashley “But bio signatures from parsecs and years of ensuing scientific toing and frowing is what I expect first.”

Agree 100%. Even if Mars has a deep hot archaean biosphere, building huge telescopes is a lot easier and cheaper than putting a Halliburton drill rig on Mars. And drilling Europa is an order of magnitude more problematic.

Dmitri March 1, 2015 at 8:46

Regarding Curiosity methane finding on Mars. Right now the most stress is going on the fact “there is methane on Mars”. What everyone else is mission is the fact that the registered methane got consumed by some processes & not decaying over 300 years as the simple mathematic would predict. That leads to notion Mars environment & geology is used to consuming methane quickly. Have just watched SETI Institude weekly seminar on early Earth microbial mats I can’t stop myself thinking that the microbial mats might be the ones who consumed the methane instantly as it turned out microbial mats are the most efficient recycling machines in the whole planetary eco-system being far far more efficient than our tropical forests.

This involuntarily leads to thinking that this is sign of microbial activity although its unknown could static electric sparks in Mars sand storms or sand interaction by blowing wind be sufficient to erode all methane from local atmosphere in such short time span as being suggested currently as working hypothesis for explaining methane disappearance.

This neatly intertwines with fresh paper on possible methane based oxygen free life forms on Titan made by molecular dynamics expert & chemical engineer. They call their invention azotosome.

Alex Tolley March 1, 2015 at 13:03

Even if Mars has a deep hot archaean biosphere, building huge telescopes is a lot easier and cheaper than putting a Halliburton drill rig on Mars.

True enough, but does the telescopic approach tell us about life in the universe? Consider. Suppose most life is in the lithosphere as bacteria (and viruses) and has no detectable impact on the surface or atmosphere. Telescopes will therefore be just looking for the rarer macro-life, only. It is a bit like looking for the proverbial keys under the streetlight, because that is where the light is. This may well be the only way to look for life on exo-planets until we have interstellar probes, but it might still give us a very biased sampling of life.

I’m also not so sure that you need a rig on icy moons. Doesn’t a slug heated by radioactive decay make it much simpler to bore through ice? Mars is a more difficult nut to crack as it probably does need a rig to bore through rock. However, if life lives in the lithosphere, it probably does migrate to all niches where liquid water is sustainable for at least short periods, so shallow bores may be sufficient.

NS March 1, 2015 at 14:27

Joy: “…building huge telescopes is a lot easier and cheaper than putting a Halliburton drill rig on Mars. And drilling Europa is an order of magnitude more problematic.”

So should telescopes be our near-term emphasis, rather than probes? Ideally we would have both of course, but with (very) limited science budgets do telescopes make more sense for now?

NS March 1, 2015 at 14:35

Just ran into this. Apologies if it was linked before:

Abelard Lindsey March 1, 2015 at 20:43

Well, Methane is at least as common as water in the universe, and this attempt to construct a biomembrane based on stuff found in a liquid Methane sea is quite convincing. The question is if the polarity and surface tension of liquid Methane offers the same chemical capabilities to allow for the emergence of the complex organic molecules that these people simulated on computers.

The obvious next step is to actually construct their designs in a liquid Methane environment in a lab and see how well they work.

ljk March 2, 2015 at 13:31

Here is the incredible Disney animated speculation about life on Mars first aired on television in December of 1957:

Note the narrator even says these life forms might be possible only if the planet were a bit warmer and drier, so even then they were pretty certain there wasn’t much more than lichen on Mars if that. Of course a multicolor smudge just sitting on some rocks would hardly make for interesting viewing.
And who knows, with hundreds of billions of worlds in our galaxy, some of these creatures may indeed exist.

Mark Zambelli March 4, 2015 at 13:46

Thanks for the video link… the thing that strikes me most is the number of completely different ideas those brilliant animators explored… almost makes Mars look like a zoo for the galactic-federation. If only eh? Wonderful to see though.

@ Andrew LePage
Allow me to freely admit that even after all these years I’m still guilty of doing what you warn not to do. I’m naturally sceptical but I still still tend to leave my ‘salt’ at the door when I could really do with a pinch or two when keeping up with curent thoughts and ideas on a subject. It’s easy to get excited hearing about the capabilities of, say, the GPI (Gemini Planetary Imager) and it’s resolving power so when the talk turns to matters such as exo-planet atmosphere detections (for example) and what that means, it is hard for the lay-person to guage what is possible, what is probable and what is a flight-of-fancy. Reading your article above reminds me to try harder, ha ha.

I was born in 1971 so I’m an Apollo-baby. I’ve grown up with a moon shaped by impact craters rather than volcanic calderas and Mars has always been the planet it is today… I never experienced the disappointments referred to above. But this can’t be relevant, surely. Maybe it’s a psychology thing. I’m not gullible but I would subscribe to an ‘expert’ view over the next man’s, even (especially?) when it’s an optimistic projection about something or other.

Maybe it’s the lay-media who are at fault with regards to diseminating skewed info into the general public? Which makes for an even stronger case to pay even more heed to your warning in the main article. I’ll strive harder, thankyou for the reminder.

ljk March 5, 2015 at 12:09

India’s Mars orbiter is also monitoring the planet’s methane:

Abelard Lindsey March 5, 2015 at 13:13

Yep. That’s the Disney film I saw in 4th grade. As you can see it presented some rather funky life forms.

Abelard Lindsey March 5, 2015 at 13:16

I’m naturally sceptical but I still still tend to leave my ‘salt’ at the door when I could really do with a pinch or two when keeping up with current thoughts and ideas on a subject.

Not me, especially with regards to exolife. Remember that I’m the guy who thinks Nick Lane’s explanation on the origin of the Eukaryote is correct and that, consequently, the Earth is the only abode of complex life in the galaxy.

Michael March 5, 2015 at 16:07

@Abelard Lindsey March 5, 2015 at 13:16

‘Not me, especially with regards to exolife. Remember that I’m the guy who thinks Nick Lane’s explanation on the origin of the Eukaryote is correct and that, consequently, the Earth is the only abode of complex life in the galaxy.’

I too do believe that ‘technosapiens’, intelligent creatures such as ourselves that have developed technology to aid their expansionism are quite rare with around one or two per galaxy. There is no stipulation that technology will aid your survival absolutely and could in the end destroy you and your civilisation through say war or disease. If you look at the length of time that it took for us to evolve from the first life on Earth and the potential filter events it is a wonder we survived at all. But there is no stopping a similar evolutionary multi-cellular event occurring on another world, if and they are, just like chemical processes chance will play its hand out eventually given enough time. The more singled celled organisms out there the more likely multi-cellular ones will pop up sooner or later, they just need a cosy spot and plenty of time.

ljk March 5, 2015 at 17:09

Hyping up the possibility for life on Mars is nothing new:

Ironically, if you read the pre-Nineteenth Century speculations regarding life on the Red Planet, such famous pro-extraterrestrial life authors as Fontenelle did not put much stock in any beings there while at the same time giving worlds like Mercury and Venus much better chances for inhabitants:

Part of the reason given is that it is hard to see much on Mars in smaller telescopes due to its relatively small appearance. But you can hardly see anything on the other terrestrial type worlds in a regular telescope then or now, so go figure.

Marko Amnell August 12, 2015 at 12:15

Sorry about the late reply, but I found this interesting article when Googling for more information about the Sinton bands. They are mentioned by the British amateur astronomer Valdemar Axel Firsoff in his 1963 book _Life Beyond the Earth: A Study in Exobiology_. You note that the Sinton bands were initially “seen by many as supporting the generally held view that Mars was the home of simple, lichen-like plant life.” Firsoff considered the Sinton bands to be conclusive evidence of plant life on Mars. On page 196 he writes:

“Various explanations of the nature of the [Martian] maria have been proposed but have been made obsolete by W.M. Sinton’s discovery in 1957 and 1958 (lastly at the coudé focus of the 200 in. Palomar reflector) of three absorptions characteristic of the carbon-hydrogen bond in organic molecules in the infra-red spectra of the maria. It may be added that these absorptions were present in a weak form even in some bright regions, such as Arabia Amazonis.
“The terrestrial plants produce two characteristic infra-red absorptions at 3.41 and 3.51 microns (1 micron = 0.001 mm). The Martian absorptions occur at 3.43, 3.56 and 3.67 microns. The first two may be regarded as coincident with the terrestrial, but the third indicates the presence of organic compounds that do not normally occur in our vegetation although still within the carbohydrate chemical scheme.
“In these circumstances it is rather futile to dwell on the arguments for and against the _possibility_ of plant life on Mars. It is manifest and flourishing, or else it could not cause detectable absorptions.”

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