The desire to find life on another planet is as strong today among scientists and lay people as it has ever been. While this search had been largely confined to a relative handful of worlds in our own solar system, in recent years it has extended far beyond to extrasolar planets which are now being discovered at an ever faster pace. During this past week, yet another story about how scientists plan to search for life on distant extrasolar planets appeared on a popular space news web site as they frequently do. Given that all extrasolar planets are too distant to be reached by our current technology for an in situ examination, the article claimed that new telescopes could look for the spectral signatures of compounds like oxygen and chlorophyll to find plant life on these distant worlds.
Ignoring for the moment the improbability that a form of autotrophism like oxygen-producing photosynthesis will develop independently on a distant planet using a complex compound such as chlorophyll, the presence of oxygen has been shown not to be necessarily indicative of biological activity while the unique identification of complex organic compounds like chlorophyll based solely on remote sensing techniques is virtually impossible. Extraordinary claims require extraordinary evidence. Concluding that plant life exists on an extrasolar planet because of the presence of certain suggestive absorption bands and a reddening of an extrasolar planet’s spectrum is a huge and scientifically dangerous leap.
One thing that the past century has shown us is the extraordinary difficulty involved in the scientific search for life on other planets. While most people alive today may not realize it, we have already been down this road of assuming chlorophyll-laden plant life was responsible for a range of observed phenomena on an unreachable world only to be proven wrong. Instead, as new data became available, scientists of the time eventually discovered that their interpretation of the data was incorrect. That planet was our neighbor, Mars, and that time was the half century or so leading up to the beginning of the Space Age.
The Early-20th Century View of Mars
Because of the pioneering work of the famous (or maybe infamous, depending on your point of view) American astronomer, Percival Lowell (1855-1916), it was almost taken as an established fact by many during the first half of the 20th century that Mars possessed some form of life. Aside from Lowell’s fanciful interpretation of the origin of the purported canals of Mars and his belief in the presence of advanced forms of life there, there were observations that supported the more moderate view shared by many in the scientific community of the time that Mars harbored simpler forms of life. A firmly established wave of darkening was observed spreading over the spring hemisphere of Mars each Martian year which was widely interpreted as being the result of plants springing back to life much as happens on Earth each spring. This interpretation was bolstered by visual observations that the dark areas of Mars referred to as “seas” (although, by this time, they were already known not to be large bodies of water) appeared to have a distinct green hue just as one would expect from widespread plant life.
Percival Lowell as he appeared c1904. He was a famous popularizer of the study of Mars and is best known for his mistaken belief that the canals of Mars were built by a technologically advance civilization. (Library of Congress)
Other observations of Mars during this period lent further support to the view that the Red Planet could sustain simple life forms. The general consensus of the astronomical community at this time based on simple analyses of decades of photometric and polarimetric measurements of Mars indicated that the Martian surface pressure 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. No large bodies of water were detected on the surface and the climate was certainly colder than on Earth as a whole 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.
But not all observations of this era necessarily supported the view that the surface of Mars was covered with plant life. During the 1937 opposition of Mars, Canadian astronomer Peter Millman (1906-1990) used the 1.9-meter telescope at the David Dunlap Observatory outside of Toronto to obtain spectra of the dark “seas” and light “desert” areas of Mars to perform a detailed comparison of their colors. Even though he did find that the dark areas appeared to be relatively greener than the light areas, the color did not match the reflection spectrum of chlorophyll he obtained by observing fresh green leaves using the same equipment. While Millman could not discount the possibility of there being plant life on Mars using some variant of chlorophyll, it was unlikely to be using chlorophyll itself.
Canadian astronomer Peter Millman as he appeared in 1959. In addition to his work in 1939 to detect chlorophyll on Mars, he is better known for his work observing meteors. (Royal Astronomical Society of Canada)
Likewise, observations of the near-infrared spectrum of Mars during this time failed to find spectral features indicative of Earth-like foliage. But the famous Dutch-American astronomer, Gerard P. Kuiper (1905-1973) who is credited by many for reviving the field of planetary astronomy during the years leading up to the Space Age, noted that lichen also lacked these distinctive spectral features as well. Since lichen is frequently found in cold environments on the Earth, the general view during the 1950s was that Mars was covered with lichen-like plant life.
Sinton Bands
So as the Space Age was about to start, the general view of the astronomical community was that Mars was a colder version of the Earth with a thinner atmosphere. While Lowell’s fanciful view of Mars harboring a technologically advanced civilization certainly fueled the public’s appetite for science fiction stories, respected scientists rejected this view but generally believed that Mars seemed to be supporting some sort of simple, lichen-like plant life. 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 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 to 96 K 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 near 3.46 μm which resembled his IR spectrum of lichen. This finding and his conclusions were then published in The Astrophysical Journal – a peer-reviewed, professional astronomical publication that was as well-respected then as it is today.
Early cutaway drawing the the 200-inch (5-meter) Hale Telescope used by William Sinton to observe Mars in 1958. Click on image to enlarge. (Palomar/Caltech/Caltech Archives)
Encouraged by these initial results, Dr. Sinton repeated his measurements using an improved IR detector on the larger 5-meter Hale Telescope at the Mt. Palomar Observatory 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 (and greener) areas of Mars. While the features he observed were not a perfect match for any known plant life on Earth, he concluded that the features were definitely due to organic compounds such as carbohydrates produced by photosynthesis in 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 included IR instrumentation to measure these spectral features from close range on a pair of their 1M probes they launched towards Mars in October 1960 which, unfortunately, succumb to launch vehicle failures during ascent (see “The First Mars Mission Attempts“). Soviet engineers attempted it again with a pair of much more capable 2MV-4 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 and contact was lost three months before its encounter with Mars on June 19, 1963 (see “You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962“). As a result, there were no close up IR observations of the Sinton bands at this time.
The Soviet Mars 1 probe, launched on November 1, 1962, carried IR instrumentation to study the Sinton bands at close range during its planned encounter with Mars. (RKK Energia)
The Case for Martian Plant Life Unravels
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 Years 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 et al. 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 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 the ν1 bands of HDO (from the stretching of the O-D bonds causing vibration) 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.
Astronomer William M. Sinton as he appeared in his later years before losing a decade-long battle with ALS in 2004. (American Astronomical Society)
The situation for the rest of the evidence for the existence of plant life on Mars also quickly unraveled during this time. New IR spectral measurements made during the 1963 opposition of Mars by Soviet astronomer Vassili Moroz indicated that the surface pressure of the Martian atmosphere was less than a quarter of what had been previously believed – possibly a lot less. These measurements were confirmed by work by other astronomers published during 1964 including Gerard Kuiper and was finally firmly established by experiments performed by NASA’s Mariner 4 spacecraft during its encounter with Mars on July 15, 1965 (see “Mariner 4 to Mars“). The Martian atmosphere was eventually found to have just 0.6% the surface pressure of Earth and was too thin to support liquid water on the surface (for a detailed discussion of this subject, see “Zond 2: Old Mysteries Resolved & New Questions Raised”).
Further investigation showed that the amount of light scattered by fine dust now known to fill the Martian atmosphere had been underestimated in the simple Rayleigh scattering models used to analyze the earlier photometric and polarimetric data of Mars. This severely skewed the derived atmospheric pressure values far above what they actually are. Years later still, analysis of observations from NASA’s Mariner 9, which returned data from Martian orbit from November 1971 to October 1972, showed that the seasonal movement of dust across the surface of Mars was responsible for the wave a darkening that had been earlier interpreted as plant life returning during the Martian spring.
Conclusion
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. It is a perfect example of how the self-correcting scientific process is suppose 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 was covered in 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 (see “NASA’s Viking Mission & the Search for Life on Mars: The Experiments“). These lessons need to be remembered as future instruments start to scan distant extrasolar planets for signs of extraterrestrial life and claims are made that life has been detected because of the alleged presence of one compound or another. Our current expectations about the environments of extrasolar planets and any life they might harbor will almost surely be proven wrong just as our expectations about life on Mars a century ago.
Related Reading
“Zond 2: Old Mysteries Resolved & New Questions Raised”, Drew Ex Machina, July 17, 2014 [Post]
“NASA’s Viking Mission & the Search for Life on Mars: The Experiments”, Drew Ex Machina, July 28, 2022 [Post]
“The New Search for Life on Mars”, Drew Ex Machina, May 25, 2014 [Post]
“The Famous Mars Image That Never Was”, Drew Ex Machina, April 24, 2014 [Post]
“Mariner 4 to Mars”, Drew Ex Machina, July 14, 2015 [Post]
General References
Samuel Glasstone, The Book of Mars, SP-179, NASA, 1968
Gerard P. Kuiper, The Atmospheres of the Earth and Planets, University of Chicago Press, 1949
Peter M. Millman, “Is There Vegetation on Mars?”, The Sky, Vol. 3, No. 10, p. 11, August 1939
D.G. Rea, T. Belsky and M. Calvin, “Interpretation of the 3- to 4-Micron Infrared Spectrum of Mars”, Science, Vol. 141, No. 3584, pp. 923-927, September 6, 1963
D.G. Rea, B.T. O’Leary and W.M. Sinton, “Mars: The Origin of the 3.58- and 3.69-Micron Minima in the Infrared Spectrum”, Science, Vol. 147, No. 3663, pp. 1286-1288, March 12, 1965
James S. Shirk, William A. Haseltine and George C. Pimentel, “Sinton Bands: Evidence for Deuterated Water on Mars”, Science, Vol. 147, No. 3653, pp. 48-49, January 1, 1965
William M. Sinton, “Spectroscopic Evidence for Vegetation on Mars”, The Astrophysical Journal, Vol. 126., No. 2, pp. 231-239, September 1957
William M. Sinton, “Spectroscopic Evidence for Vegetation on Mars”, Publication of the Astronomical Society of the Pacific, Vol. 70, no. 412, pp. 50-56, February 1958
William M. Sinton, “Further Evidence for Vegetation on Mars”, Lowell Observatory Bulletin, Vol. 4, No. 15, pp. 252-258, 1959
William M. Sinton, “Further Evidence for Vegetation on Mars”, Science, Vol. 130, No. 3384, pp. 1234-1237, November 6, 1959
What are the conditions under which a thick oxygen atmosphere could form through abiotic means? Seems like you’d require a planet with a lot of water that gets separated into hydrogen and oxygen, and then some way to keep the oxygen stable in the atmosphere. If the conditions for that deviate from what we consider typical for rocky worlds below the 1.6 Earth radius level, then it could still be a good filter.
This topic of oxygen in the atmospheres of extrasolar planets is certainly worthy of a post on Drew Ex Machina in its own right. But until then, the topic of abiotic oxygen in the atmospheres of terrestrial planets has been covered in the following recent papers:
Robin Wordsworth & Raymond Pierrehumbert, “Abiotic Oxygen-dominated Atmospheres on Terrestrial Habitable Zone Planets”, The Astrophysical Journal Letters, Volume 785, Issue 2, article id. L20, April 2014 (preprint at http://arxiv.org/abs/1403.2713)
A. Segura et al., “Abiotic formation of O2 and O3 in high-CO2 terrestrial atmospheres”, Astronomy and Astrophysics, Volume 472, Issue 2, pp.665-679, September III 2007 (preprint at http://arxiv.org/abs/0707.1557)
Basically, oxygen can be produced by the photolysis of water and subsequent escape of the hydrogen and that oxygen will dominate on terrestrial planets with low concentrations of noncondensable gases like nitrogen and argon or planets with high concentrations of carbon dioxide and little liquid water, for example. There are also circumstances where abiotic oxygen can be present on prebiotic planets with the proper chemistry as described in this paper:
Domogal-Goldman et al., “Abiotic Ozone and Oxygen in Atmospheres Similar to Prebiotic Earth”, The Astrophysical Journal, Volume 792, Issue 2, article id. 90, September 2014 (preprint at http://arxiv.org/abs/1407.2622)
Ignoring for the moment the improbability of oxygen-producing photosynthesis evolving independently on another planet (there is nothing that we currently known about the evolution of oxygen-producing photosynthesis here on Earth to suggest it is inevitable) and the existence of many other forms of autotrophism that by chance or because of evolutionary favorable conditions could exist on other planets, the presence of oxygen can not be taken as proof of extraterrestrial life. The only thing it suggests is that some interesting chemistry might be going on.
Those seem like they’d be easy to screen for as well. If you don’t get a strong signal of nitrogen or some other non-condensing gas, or the planet has a very high concentration of CO2 and lack of the aforementioned gases, then you put the planet in the “possibly life-bearing but less certain” category. Same for if the planet is very young.
Nothing inevitable, although it is a very effective way to metabolize food and produce energy, and arose early in the planet’s history.
As for the other point, that falls into “false negative” category. The search for oxygen was just a way to try and find some proof of life – that there are other forms of life that don’t require oxygen or produce doesn’t invalidate that in of itself.
I can see photosynthesis being a likely thing to evolve on watery terrestrial planets in the surface water habitable zone, but there are several forms of it kicking around on Earth, e.g. the rhodopsin-based system used by various types of archaea, which is anoxygenic. Even with chlorophyll-based photosynthesis there are anoxygenic variants. So even on a planet whose biosphere evolves photosynthesis, there’s no reason that it would have to use chlorophylls, nor that it would necessarily produce oxygen (it may turn out to be a common trick, it may not be).
I didn’t know that, thanks.
The premise of some who advocate looking for oxygen IS that oxygen producing photosynthesis is inevitable or at very least common on planets with life. There is nothing to suggest that it is. There are plenty of lifeforms on this planet that use forms of autotrophism other than photosynthesis that have arisen before and probably afterwards. There are also forms of photosynthesis on this planet that do not produce oxygen. And, as has been pointed out, the presence of oxygen is not proof of life since there are abiotc means of producing oxygen in an atmosphere. The detection of oxygen is proof of only one thing: There is oxygen in the atmosphere. While it might be an interesting finding, one can not conclude anything one way or the other about the presence or absence of life based on experience of a single example of a planet with life and oxygen – the Earth.
Very useful example to bear in mind as regards this topic: we can identify that a feature in the Earth’s spectrum is due to chlorophyll, but we already know that there’s a biosphere here that uses chlorophylls. Without that knowledge, could we even go from the spectrum to inferring that the molecule causing the feature is actually chlorophyll? (I am sure comparative “xenobiochemistry” would be an utterly fascinating subject!) Inferring organic chemistry, let alone biochemistry from just a spectrum taken from remote distances seems to me a fairly hopeless task: last I heard we still don’t know for certain what the red pigment in Jupiter’s Great Red Spot is, and we have far more detailed information for Jupiter than we can hope to get from an exoplanet.
In fact, I’m not even entirely certain what the state of knowledge for exoplanet atmospheres is: sure there have been a lot of claimed detections of various molecules, but there have also been a fair few papers claiming that the supposed detections are due to noise and systematic errors in the datasets.
You hit the nail precisely on the head! While the spectral identification of relatively simple compounds are readily straightforward in the spectra of planets (e.g. oxygen, water, methane, etc.), more complex compounds (especially complex organic compounds with their myriad C-H bonds whose spectral signatures are subtly altered by adjacent bonds with other elements like N, O, P , etc.) are next to impossible to definitively identify remotely. When, for example, we see maps of chlorophyll concentrations in the Earth oceans generated using remote sensing data, scientists already know some particular signature is from chlorophyll-laden plankton because of the huge amounts of ground truth data gathered in situ to verify that interpretation. If we were performing the same scans of an alien planet, we wouldn’t know if that same spectral signature was from chlorophyll or any one of dozens if not hundreds of very different organic compounds or even mixtures of compounds without some prior experience with this particular planet or a MUCH larger sample of life-bearing planet than just one example (i.e. the Earth). Like I have tried to emphasize, the remote detection of life on extrasolar planets will not be as easy as some might lead us to believe. In fact, without a much broader knowledge of potential exobiologies than we currently have, it is simply impossible.
Thanks for the comment!
Any type of photosynthesis would, by definition, involve a pigment which absorbs light at certain wavelengths. Several years ago I read an article in Scientific American that explained why green was a good color for a planet with a yellow-white sun, even though that reflects near the peak wavelength of sunlight (forget the details). It also speculated on photosynthesis with suns of other colors (all I remember is that red suns should have black pigment in their plants). Maybe we could use this to help support (not confirm) biogenic production of oxygen, or just photosynthesis of some kind.
This speculation about the potential color variation of hypothetical chlorophyll-like pigments (whether they are involved in autotrophic activity that produces oxygen or not) is part of the problem. How is a hypothetical chlorophyll-like pigment whose identity is unknown and has most likely never had its spectral properties characterized under a range of physical and viewing conditions in laboratory studies going to be differentiated from any one of millions of other organic or inorganic compounds or mixtures of these compounds that have the same spectral fingerprint (to within measurement uncertainties)? The spectral identification of simple compounds like water vapor, carbon dioxide, ozone, methane, etc. is fairly straightforward. But in the case of very complex organic compounds which have lots of overlapping absorption bands related to its many C-H bonds whose properties (and absorption characteristics) are influenced by bonds with adjacent elements or groups as well as the physical conditions (e.g. pressure, temperature), the unique identification of a specific compound by means of remote sensing is difficult or impossible especially when one considers spectral measurement uncertainties. Even if oxygen is detected in the atmosphere of an extrasolar planet, that is not proof that life is present since there are abiotic means of generating oxygen in an atmosphere. And even if some spectral features are identified as being from terrestrial chlorophyll or some hypothetical variation, there would be no way to definitively prove it was not some other compound or mixture of compounds that have nothing to do with life without an in situ analysis.
Hi Andrew
I don’t usually add a comment so long after, but I’ve been reading old Planetary Astronomy papers by Kiess et al on the Nitrogen Oxides of Mars. Sinton was one of the astronomers who killed the Nitrogen Oxides means A Dead Mars scenario. Of course Larry Niven’s first few Known Space stories were set on the Nitrogen Oxide Mars, with the corpses of the subterranean Martians exploding into flame on contact with water.
While reading anything about Planetary Atmospheres in the 1950’s-early 1960’s, Kuiper’s “The atmospheres of the earth and planets” is always referred to. In one edition he nailed the modern day value for the surface pressure of Mars in c.1950, though throughout the 1950’s that drifted upwards to the 80 mb assumed by early Space Age Mars landing plans. Exactly what drove that rise in the estimate value?