archive: SETI Sagan: NASA adopts Sagan Criteria for Life

SETI Sagan: NASA adopts Sagan Criteria for Life

Larry Klaes ( )
Fri, 21 May 1999 08:52:06 -0400

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>Subject: Sagan: NASA adopts Sagan Criteria for Life
> Space Science News home The Sagan Criteria for Life Revisited
>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
>earlier this month, 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
>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. More information.
>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?
>Above A candidate line-up: How would a spacecraft flyby pick out the lone
>unusual suspect that harbors life? As Cornell Professor, J.R. Vallentyne,
>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
>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.)
>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
>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.
>infrared spectral lines caused by methane in the atmosphere. Although the
>amount of methane Galileo saw was miniscule -- about 1 part per million --
>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
>replenishes the supply.
>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
>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
>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
>Sagan criteria."
>Caveat Lunar
>The Galileo flybys showed that we know how to identify life at a distance,
>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
>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
>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
>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
>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
>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
>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
>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
>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
>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:
>mix carbon and hydrogen molecules, the PAHs, with water ice at minus 440
>degrees Fahrenheit, the temperature inside an interstellar cloud;
>place these ice grains in a vacuum;
>shine ultraviolet light on them, the same type of radiation put out by
>stars and re-radiated by glowing hydrogen gases.
>Sign up for our EXPRESS SCIENCE NEWS delivery
>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
>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
>proof may be the most interesting -- and vexing -- piece of the puzzle in
>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.
>Web Links
>Additional Reading
>Lunar pigments - Porphyrin-like compounds from an Apollo 12
>sample--Bunnenberg, E. et al, 1971.
>Nature, Vol. 365, No. 6448, pages 715-721; October 21, 1993.
>Porphyrin from Apollo 11,12, and 14--Conference on the Organic Analysis and
>Carbon Chemistry of Lunar Samples: Their Significance for Exobiology,
>Park, Md., Oct. 26-28, 1971.) Space Life Sciences, vol. 3, Oct. 1972, p.
>Porphyrin-like compounds genesis under simulated abiotic
>conditions--Simionescu, C. I. et al., 1978
>Biological Role of Porphyrins and Related Structures. Adler, Alan D. Academy
>of Sciences; New York, 1975, p 267.
>Botnikov, G. G. "The detection of iron porphyrin proteins by the method of
>biochemiluminescence in the search for extra-terrestrial life." COSPAR,
>PRESS, INC., 1965, P. 1-12. 20 REFS. Vallentyne, J. R.
>Related Links
>The Search for Extraterrestrial Life--Scientific American article by Carl
>Sagan, Jan. 1997
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