The Sagan criteria for life revisited

May 21, 1999

It is now clear that organic chemistry has run rampant through the solar system and beyond.

        Carl Sagan, Scientific American, 1997 May 21, 1999: When the Galileo spaceprobe flew by Jupiter's moon Callisto last week, the detection of life on that strange and distant world was not among the scientific objectives. After all, Callisto's heavily cratered surface is a frigid -220o F and is scarcely protected from the ravages of space by a extraordinarily thin CO2 atmosphere. Indeed most astrobiologists concur that Callisto is an unlikely abode for life.

But even if Callisto was wet and warm and teeming with life, would Galileo have noticed? The question brings to mind an earlier Galileo flyby of another curious planet -- Earth.

Above: This view of Earth's southern hemisphere centered on the South Pole was created using images from the Galileo spacecraft taken during the December 1990 flyby.

When the Galileo spaceprobe swooped by Earth in 1990, all its instruments were pointed towards us. As Galileo flew toward our planet, the Earth was centered in the windshield and then again in the rear-view mirror as Galileo continued on its journey to Jupiter.

Galileo's close encounter with Earth framed one of the most difficult questions in astrobiology:

Can a modern space instrument tell if the Earth, or any planet, is a good candidate for harboring life?

As Cornell Professor, J.R. Vallentyne, put the matter in his opinion in 1965: "Apparent inherent limitations on temperature, pressure or chemical environment for living matter are geocentric myths."

To put the 1990 flyby in perspective, the late Carl Sagan and his colleagues published a 1993 Nature article on this question. According to Sagan, the Galileo spacecraft found clear signs of life during its flight past the earth including:

1. strong absorption of light at the red end of the visible spectrum, particularly over the continents. The light-absorbing pigment that causes this is chlorophyl, a molecule essential to plant life and photosynthesis. (Plants appear green because chlorophyl reflects green light and absorbs red and blues.)

2. spectral absorption features caused by molecular oxygen in Earth's atmosphere. The amount of O2 in our atmosphere is many orders of magnitude greater than is found on any other planet in the Solar System. An oxygen-rich atmosphere is a curiosity because oxygen slowly combines with rocks on the earth's surface. Maintaining the oxygen content requires some replenishing mechanism, in this case photosynthesis by plants -- the action of life.

3. infrared spectral lines caused by methane in the atmosphere. Although the amount of methane Galileo saw was miniscule -- about 1 part per million -- it is still important. In a oxygen-rich atmosphere like Earth's, methane should rapidly oxidize into water and CO2. Not a single molecule of methane would remain in equilibrium. Biological action such as bacterial metabolism in bogs replenishes the supply.

4. modulated narrowband radio transmissions. These emissions look nothing like natural sources of radio waves like lightning and plasma instabilities in Earth's magnetosphere. They are clear signs of a technological civilization.

Galileo's flyby of Earth was just the beginning of the first-ever control experiment in astrobiological remote sensing. The second part happened two years later, in 1992, when Galileo returned for a flyby of the moon.

Right: The false-color image of the Moon was taken in 1992 by the Galileo spacecraft enroute to Jupiter. The Sea of Tranquillity (Mare Tranquillitatis) is the blue area at right, the Ocean of Storms (Oceanus Procellarum) is the extensive blue and orange area on the left, and white lines radiate from the crater Tycho at bottom center. Three filters were used to make three separate exposures, combined in an exaggerated color scheme to emphasize composition differences - blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron. More information.

While Earth is known to be teeming with life, the Moon is believed to be the exact opposite -- cold, barren, and lifeless throughout its long geological history. What did Galileo see when it passed by the moon?

"Nothing," says David Noever, a NASA astrobiologist. "There was no evidence for life. No chlorophyll, no oxygen-methane atmosphere, no artificial radio transmissions. It was just as we would have expected, and consistent with the Sagan criteria."

Caveat Lunar

The Galileo flybys showed that we know how to identify life at a distance, at least the kinds of life we're familiar with here on Earth. However, things may not be as simple as they seem. Organic compounds have been discovered in some unlikely -- and almost certainly lifeless -- places, including amino acids in meteorites, organic molecules in interstellar clouds, and organic compounds called porphyrins in lunar soil.

Left: Unloading of Apollo 12 lunar soil and rocks in November 1969

The example of porphyrins on the Moon is particulary intriguing in the context of the Galileo flybys and Sagan's subsequent criteria for life. Porphyrins are the building blocks of brightly pigmented biomolecules such as hemoglobin and chlorophyll which reflect only certain wavelengths of visible light. Chlorophylls, for example, are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll captures or harvests the energy of sunlight--a kind of powerstation molecule underlying all life seen on earth.

The first 3 of Sagan's 4 criteria for life, as gleaned from Galileo's Earth flyby, are all related to porphyrins through the action of chlorophyll. Chlorophyll and photosynthesis are responsible for the spectral colors of plant-covered continents, for the oxygen content of the atmosphere and for its methane balance. Galileo didn't detect porphyrins during its flyby of the Moon, but they were there in quantities too small to see.

Right: Porphyrin molecules seem fully capable of biological wizardry on Earth. Put an iron atom in a porphyrin and the closely related oxygen-carrying blood molecule, hemoglobin, results. Put a magnesium atom in a porphyrin and the closely-related light-harvesting molecule, chlorophyll, is made. Put lunar soil specimen, 12023, into the lab for chemical analysis, and porphyrin shows up on the moon.

Here on Earth porphyrin organic compounds are useful biomarkers. For example, petroleum hunters look for porphyrins as markers of oil deposits and thermal maturity. They can be detected remotely without extracting organic matter to reveal oil shales and source rock that came from the decay of green plants.

Does the presence of porphyrins mean that there is or has been life on the Moon?

Not at all. The 1969 discovery of lunar porphyrins probably says less about the chances for biochemistry there, than about how common their generation may be elsewhere in the universe. In 1978 Simionescu et al. were able to produce porphyrins under laboratory conditions similar to those of primaeval Earth, before the genesis of life. They summarized the results in the journal Origins of Life:

"Experiments with gas mixtures intended to simulate the primaeval atmosphere of the Earth yielded many biologically important chemicals. Investigations into the synthesis of porphyrin-like compounds from methane, ammonia and water vapour were carried out by using high frequency discharges. Microanalyses of porphyrins showed that porphyrin-like pigments were formed in this way. The presence of divalent cations in the reaction system increased the yield of porphyrin-like pigments also involving the direct synthesis of their metal complexes. The ready formation of these compounds in abiotic conditions is significant, suggesting the possibility of their appearance during the early stage of chemical evolution."

Left: A close-up view of Apollo 12 lunar sample no. 12025, called Core Sample 1, and collected on the lunar surface, about 225 meters below the point where the Apollo 12 Lunar Module touched down. Soil sample 12025 is closely spaced in collection catalogs with the porphyrin-like pigments in Apollo 12 lunar soil sample 12023. Far Left: A brightly orange pigmented pebble-like lunar sample, Apollo 17 collection catalog.

The idea that the "stuff of life" is common even in lifeless places like the Moon is gaining momentum. On February 19th of this year an article in Science magazine reported one group's attempt to mimic an organic chemistry lab in outer space. The research team included a new breed of astrochemists--including Scott Sandford at the NASA Ames Research Center and the SETI Institute, both in Mountain View, CA, and lead author of the Science paper, Max Bernstein of Stanford University. Their experiments involved a class of complex carbon and hydrogen molecules, called polyaromatic hydrocarbons, or "PAHs." Like the porphyrins, these molecules are also part of the so-called CHNOPS elements--carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur.

To reproduce the chemistry of an interstellar molecular cloud, Bernstein's group followed a simple recipe:

1. mix carbon and hydrogen molecules, the PAHs, with water ice at minus 440 degrees Fahrenheit, the temperature inside an interstellar cloud; 
2. place these ice grains in a vacuum;
  3. shine ultraviolet light on them, the same type of radiation put out by nearby stars and re-radiated by glowing hydrogen gases.

Because of the extreme conditions, the likelihood of more complex, biologically useful molecules being formed seemed as remote as space itself. Instead, about 10 percent of the PAHs were converted to more biologically useful molecules such as alcohols, ketones and esters.

"These experiments take molecules that only an astrophysicist could love and transform them into something that ought to fascinate astrobiologists," comments Thomas Wdowiak, an astrophysicist at University of Alabama at Birmingham. "This shows there is a process that takes a rather abundant substance that exists in the universe and converts it to the kinds of things that are susceptible to the origin-of-life scenario."

Earth as we, the aliens, might see it....

On Christmas Eve 1968, Apollo 8 completed 10 orbits around the Moon and returned live television pictures back to our planet. Over half a billion people watched as Earth rose on the Moon's horizon. For many observers it was a transforming perspective.

Poet Archibald MacLeish wrote: ". . . to see the earth as it truly is, small and blue and beautiful in that eternal silence where it floats, is to see ourselves as riders on the earth together...."

For astrobiologists, the Galileo flyby invoked a similar transformation -- a first-time view of the Earth as an alien world. It affirmed that standards of proof may be the most interesting -- and vexing -- piece of the puzzle in the search for life among the stars. Meanwhile, scientists continue to push the limits of their understanding of both the biological and pre-biotic envelope for life, as we might know it, and how we might see it remotely from space--even when looking back directly at ourselves.  

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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