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.
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.
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.
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.
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 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.
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.
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 “The New Search for Life on Mars”). 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.
“Zond 2: Old Mysteries Resolved & New Questions Raised”, Drew Ex Machina, July 17, 2014 [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]
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