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.