Comets: The Delivery System What'sNEW
Hoyle and Wickramasinghe also discuss another means of space travel which solves the radiation problem: comets. And even before that danger was known, the idea that comets could contribute to life on Earth was afoot. Among others, Isaac Newton endorsed it. "Newton considered the continual arrival of cometary material to be essential for life on Earth" (5).
Some Basic Facts about CometsComets are like cats. They have tails, and they do precisely what they want — David Levy
Comets, as astronomer Fred Whipple figured out, are made largely of ice. Much of the ice in comets is frozen water, but ices of other compounds such as carbon monoxide and carbon dioxide are also present. And comets contain, we have recently learned, a large amount of more complex organic compounds. These organic compounds may be limited to a mixture of molecules such as the original Miller - Urey experiment was able to produce, or they may be even more closely related to life; we can't be sure from here, yet. In the interior of a comet, under layers of opaque organic material, viable cells would be shielded from radiation. Of course, freezing slows or stops metabolism, so cells could exist there in suspended animation.
A few larger comets such as Halley's comet have orbits that bring them as close to the sun as Earth is. Even fewer comets, called "sungrazers," actually strike the sun, or pass so close that they are destroyed by it. Most comets reside at distances far beyond that of Pluto, in orbits not confined to the plane in which the planets' orbits lie. They are so numerous that the total mass of comets in solar orbit may be as great as the total mass of the planets. Slight gravitational disturbances caused by the outer planets or neighboring stars can change a comet's orbit completely, steering some closer to the sun, others completely away.
When a comet nears the sun, some of its surface material ablates away, making the comet's "tail." This process usually begins somewhere between the orbits of Jupiter and Mars. Some of the discharged material is gas, some of it is dust. Each makes a different kind of cometary tail. Dust and larger debris left by comets remain for a while in solar orbit. Earth often passes through the orbits of cometary debris, causing meteor showers such as the Perseid meteor shower around August 10 every year, when we cross the orbit of comet Swift-Tuttle.
Thousands of tons of cometary dust, debris and larger fragments fall to Earth every year. Starting in the late 1960's, U.S. military intelligence observers doing surveillance against enemy missile attacks began to observe and photograph comets and other objects as big as thirty to fifty meters in diameter exploding in the upper atmosphere. From 1975 to 1992, 136 such objects were observed — about eight per year. That information was kept classified until 1993-1994 (5.5). It's worth remembering that four billion years ago, when life on Earth first appeared, the number of comets nearing the sun was hundreds or thousands of times greater than it is now (6).
The study of comets today is rich with surprises. For example, comet Hyakutake, which was easily visible to the naked eye in March, 1996, was first discovered by a Japanese amateur astronomer using binoculars. Astronomers were surprised to learn "Hyakutake contains abundant ethane and methane, compounds never before confirmed in comets" (7-9). On March 27, 1997, NASA announced that a year-long study using Hubble and several Earth-based telescopes shows that the trace ices in the nucleus of comet Hale-Bopp are somehow segregated from water-ice (10). And on April 21, 1997, astronomers on the Canary Islands reported that Hale-Bopp has a third tail of a kind not seen before; it is composed of sodium gas (11). Following so many new findings, comet theorists are completely rethinking how comets are formed and what they contain. Perhaps in the process they should consider biological causes for some of the unexpected phenomena. For example, on Earth, ethane comes from methane, and methane is made from carbon dioxide by bacteria. This process could happen on comets as well.
Comets Reaching EarthMany objects that fall into Earth's atmosphere from space are destroyed by heat before they reach Earth's surface. Only the very largest objects have enough momentum to penetrate the atmosphere without slowing down much. The largest comets are in this category. Imagine the fate of living cells deeply embedded in the ice of a large comet. The high heat requirement to melt ice, and water's extremely high heat of vaporization could offer some protection to the cells during a fast trip through the atmosphere. And landing in the ocean would soften the impact. Still, the heat generated by such explosions can be enormous.
Christopher Chyba, Paul Thomas, Leigh Brookshaw and Carl Sagan wrote a study of this problem, published in Science in 1990, entitled "Cometary Delivery of Organic Molecules to the Early Earth" (12). They carefully calculate the heat generated by high speed impacts with Earth, and then conclude that life's building blocks (not whole cells) could arrive intact. It is reasonable to extend their conclusion to cells, by expanding the scope of their study. Chyba and his coauthors in 1990 admittedly do not examine the case of a comet exploding before impact. However most comets, indeed most large meteoroids of any type except iron ones, would explode before impact. In 1992 Chyba and Sagan (13) did address the explosion of comets in the atmosphere and found that for the delivery of intact organic compounds at least, this method of transfer was far more effective than comets that collide with the surface.
The best known atmospheric explosion of a meteoroid happened eight kilometers above Tunguska in central Siberia on June 30, 1908. The explosion flattened the forest for roughly 15 kilometers in every direction. The object was most likely an asteroid, perhaps 60 meters in diameter, because a comet would have exploded higher in the atmosphere. Our knowledge of this event is indirect because no one investigated the site until twenty years after the explosion (14). A similar atmospheric explosion, again over Siberia, occurred in 1947. We know that atmospheric explosions before impact by comets and asteroids are common. An explosion in the air would be much gentler than a collision with either Earth's hard surface or the ocean. Matter on the trailing side of a comet exploding in the atmosphere would be significantly slowed by the jolt. And matter located there would also be the best protected from the heat generated during atmospheric entry prior to the explosion.
In March 1965, an object estimated at 7 - 8 meters in diameter exploded 30 kilometers over Revelstoke, Canada. This time investigators arrived promptly and recovered many fragments a few millimeters in size. Most of these were not altered by heat, proving that a plausible delivery mechanism for cells exists (15).
In a new development, on May 28, 1997, NASA announced observations that comets as large as houses — "thousands per day" — actually break up and are destroyed at 600 to 15,000 miles above Earth. Dr. Lewis A. Frank, the principal investigator for NASA's Polar spacecraft instruments, described their descent as a "relatively gentle 'cosmic rain'." (16-16.2)
There are, as we see, several plausible methods of delivering cells in comets from space to Earth. Of course, our case for the presence of bacteria in comets would be stronger if we could actually find on Earth fossilized bacteria in a meteorite that came from a comet. Is this possible? Not all meteorites were necessarily ever embedded in comets. Most meteorites appear to be fragments of more solid objects such as asteroids or even other planets. However, a minority of meteorites, about 4 percent, are different from the rest. They are called carbonaceous chondrites. They contain a much higher percentage of carbon, from .35 percent to 4.8 percent versus a mean of .1 percent for ordinary meteorites. Their chemical composition makes them more susceptible to destruction by heat; the proportion of them entering the atmosphere may be as high as 50 percent (18). We know from the recovered fragments that the object that exploded over Revelstoke, for example, was a carbonaceous chondrite.
Among astronomers there are several theories about how carbonaceous chondrites came to be different (19). Recently scientists have determined that "salt-rich fluids analogous to terrestrial brines" flowed through the veins found in them (20), and that they were formed very early in the development of the solar system (21). The most accepted theory today is that they are remnants of spent comets.
By 1975, however, Nagy was no longer sure that the objects he and Claus had seen were extraterrestrial microfossils. In 1982, now looking at traces of amino acids from the Murchison meteorite, Nagy was cautious and noncommittal as to where they came from.
Nearly ten years later, German geologist and paleontologist Hans Dietrich Pflug also examined fragments of the Murchison meteorite. Using a new and difficult technique with the fragments, he isolated and photographed some startling things. The photographed forms resemble fossilized cells and virus particles. Pflug considered the arguments for and against Earthly contamination as the source for the fossils and is convinced that the fossils came from space (27). But he is officially noncommittal as to whether they are actually what they look like — cells and virus particles. "There is no convincing evidence that the forms are more than 'organized elements,' possibly some kind of prebiotic structures," he now says. Today the consensus is that all such fossils are Earthly contaminants, but the case is not settled. More research is needed.
Recently, new and better methods to conduct this kind of research have been developed by NASA researchers examining the meteorite from Mars designated ALH 84001. Fortunately, similar methods are now being applied to the Murchison meteorite, and the preliminary results are startling. Pictures made available in July, 1997, show microscopic forms in Murchison that look very lifelike (27.5).
Dust and SporesMost of the debris that comets shed is a fine dust. In fact, dust is the form in which the greatest volume of organic compounds is delivered to Earth; far more arrives that way than in larger meteorites or the largest impactors such as whole comets (28, 29). The fate of tiny dustlike particles falling to Earth is very different from that of larger objects. The effect of the thin upper atmosphere on particles as fine as single spores or viruses as they enter it at high speed can be to slow them while heating them only mildly, for only a few seconds.
Hoyle and Wickramasinghe calculated that a particle the size of a typical bacterium might get briefly heated to 500 degrees C. They then cite studies in which E. coli were subjected to close to twenty seconds of flash heating of up to 700 degrees C and survived (30). E. coli do not normally live in a hot environment; they live in our intestines. There are archaebacteria, however, that thrive at well above the boiling point of water, and many bacteria form heat-resistant spores. It is reasonable to assume that the sporulated forms of some of these could survive flash heating even better than normal E. coli can.
An array of cellular equipment that could help bacteria survive sudden heating is their set of "heat-shock" proteins. According to the textbook The Molecular Biology of the Gene (31):
"In almost all kinds of cells subject to heat shock, certain proteins (about 17 in E. coli) begin to be made much faster than usual.... Remarkably, some heat-shock proteins of widely different species are closely related; in fact, there are even similarities between those of bacteria and those of eukaryotes.... [W]e have no idea what function they have in common that is essential to save cells from the rigors of a sudden temperature rise...."
The response time of heat-shock proteins is less than a minute in the eukaryotic cells that have been observed for this response. In prokaryotic cells the response would be much quicker, because transcription is simpler and translation begins immediately. The writers of the quoted text do not consider that these proteins could help cells survive the heat of atmospheric entry. But why couldn't they?
After being released as dust by a comet, before entering the atmosphere, bacteria would be subject to radiation damage for a short while at least. Interestingly, certain bacteria, such as Deinococcus radiodurans, can survive a dose of nuclear radiation 3,000 times stronger than the dosage that would kill a human (32). There has never been radiation that strong on Earth. If life originated here from nonliving chemicals, and if evolution works according to the darwinian paradigm, the evolution of that survival capability is hard to explain. In fact, Dr. Kenneth Minton, one of the scientists who studied these bacteria, said, "Deinococcus radiodurans would be a good vector for panspermia" (33).
Anomalous Behavior by Some CometsWhen a typical comet nears the sun, it vents gases and dust from various places. This has been thought to happen because the sun's heat on the comet's irregular surface causes cometary material to boil off asymmetrically. However, some comets like Chiron and Schwassmann-Wachmann 1 exhibit unusual behavior; they sputter and brighten too far from the sun for this mechanism to work (34-37). Hale-Bopp is another comet in this category (38). Even Halley's comet had an unexpected outburst in 1991; it brightened by a factor of 300 while it was outbound and well beyond the orbit of Saturn. The strictly chemical explanations of these anomalous events rely on processes that would run out of supplies within a few hundred years.
M. K. Wallis and Wickramasinghe (39) proposed another explanation; namely, that a cycle of surface freezing, compression, and cracking may cause compressed liquid or gas to squirt from a comet's interior. A third possibility, which Hoyle and Wickramasinghe have considered, is that cellular metabolism may be under way on some comets. This metabolism could generate gasses that could, directly or indirectly, account for the brightening. The recent discovery of abundant methane and ethane in the coma of comet Hyakutake (40) strengthens this possibility. In any case, the anomalous brightening of these comets needs explaining. The presence of liquid water or even life on the comets could help explain it. More research would help here as well.
References1. Fred Hoyle and Chandra Wickramasinghe, Lifecloud: The Origin of Life in the Universe. Harper and Row 1978.
2. J. Secker, P. S. Wesson and J. R. Lepock, "Damage Due to Ultraviolet and Ionizing Radiation during the Ejection of Shielded Microorganisms from the Vicinity of 1 Solar Mass Main Sequence and Red Giant Stars" p 1-28 v 329 Astrophysics and Space Science. 1994. (Jeff Secker's website.)
3. Jeff Secker, Paul S. Wesson and James R. Lepock, "Astrophysical and Biological Constraints on Radiopanspermia" p 184-192 v 90 n 4 Journal of the Royal Astronomical Society of Canada. August 1996.
4. Paul Parsons, "Dusting off panspermia" p 221-222 v 383 Nature. 19 September 1996.
5. Donald K. Yoemans, Comets. New York: Wiley Science Editions 1991. p 350.
5.5. William J. Broad, "Earth is Target for Space Rocks at Higher Rate Than Thought" p B9,B14, The New York Times. 7 January 1997.
6. Christopher F. Chyba, "Extraterrestrial amino acids and terrestrial life" p 113-114 v 348 Nature. 8 November 1990.
7. Michael Mumma et al., "Detection of Abundant Ethane and Methane, Along with Carbon Monoxide and Water, in Comet C/1996 B2 Hyakutake: Evidence for Interstellar Origin" p 1310-1314 v 272 Science. 31 May 1996.
8. Kim Peterson, "Hyakutake Produces Another Surprise" p 1263-1264 v 272 Science. 31 May 1996.
9. Ron Cowen, "Bright Comet Poses Puzzle: Hyakutake's tails of mystery" ScienceNewsOnline, June 1, 1996.
10. "Hubble and IUE Hale-Bopp Observations Surprise Astronomers, NASA News Release 97-55, March 27, 1997.
11. Malcolm W. Browne, "Hale-Bopp Has 3d Tail" p B11, The New York Times April 22, 1997.
12. Christopher F. Chyba, Paul J. Thomas, Leigh Brookshaw and Carl Sagan, "Cometary Delivery of Organic Molecules to the Early Earth" p 366-373 v 249 Science. 27 July 1990. Internet Reprint Available
13. Christopher F. Chyba and Carl Sagan, "Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules; an inventory for the origins of life" p 125-132 v 355 Nature. 1992.
14. The Sibirian Center for Universal Catastrophe is a new resource on the Tunguska event.
15. Christopher F. Chyba, Paul J. Thomas and Kevin J. Zahnle, "The 1908 Tunguska explosion: disruption of a stony asteroid" p 40-44 v 361 Nature, 7 January 1993.
16. NASA newsrelease 97-112, 28 May 1997.
16.1. William J. Broad, "Tiny Comets May Have Big Impact" The New York Times, May 29, 1997.
16.2. see also NASA Sees Comets Entering Atmosphere, this website.
17. David W. Deamer and Gail R. Fleischaker, Origins of Life: The Central Concepts. Jones and Bartlett Publishers 1994.
18. Edward Anders, "Pre-biotic organic matter from comets and asteroids" p 255-257 v 342 Nature. 1989.
19. John G. Burke, Cosmic Debris: Meteorites in History. University of California Press 1986. p 304-307.
20. Ian D. Hutcheon, "Signs of an early spring," doi:10.1038/379676a0, p 676-677 v 379 Nature, 22 Feb 1996.
21. Magnus Endress, Ernst Zinner and Adolf Bischoff, "Early aqueous activity on primitive meteorite parent bodies," doi:10.1038/379701a0, p 701-703 v 379 Nature, 22 Feb 1996.
22. George Claus and Bartholomew Nagy, "A Microbial Examination of Some Carbonaceous Chondrites" p 594-596 v 192 Nature. 18 November 1961.
23. Bartholomew Nagy, George Claus and Douglas Hennessy, "Organic Particles embedded in Minerals in the Orgueil and Ivuna Carbonaceous Chondrites" p 1129-1133 v 193 Nature. 24 March 1962.
24. Harold C. Urey, "Origin of Life-like Forms in Carbonaceous Chondrites" p 1119-1233 v 193 Nature. 24 March 1962.
25. Harold C. Urey, "Biological Material in Meteorites: A Review" p 247-166 v 151 Science. 14 January 1966.
26. Keith Kvenvolden, James Lawless, Katherine Pering, Etta Peterson, Jose Flores, Cyril Ponnamperuma, I. R. Kaplan and Carleton Moore, "Evidence for Extraterrestrial Amino-acids and Hydrocarbons in the Murchison Meteorite" p 923-926 v 228 Nature. 1970.
27. Hans D. Pflug, "Ultrafine Structure of the Organic Matter in Meteorites" p 24-37 Fundamental Studies and the Future of Science, Chandra Wickramasinghe, ed. University College Cardiff Press 1984.
27.5. See Fossilized Life Forms in the Murchison Meteorite, this website.
28. Christopher Chyba and Carl Sagan, "Endogenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life" p 125-132 v 355 Nature. 1992.
29. Edward Anders, "Pre-biotic organic matter from comets and asteroids" p 255-257 v 342 Nature. 1989.
30. Fred Hoyle and Chandra Wickramasinghe, Our Place in the Cosmos. J.M. Dent Ltd. 1993. p 79.
31. James D. Watson, Nancy H. Hopkins, Jeffrey W. Roberts, Joan Argetsinger Steitz and Alan M. Weiner, The Molecular Biology of the Gene, 4th edition. Menlo Park, California: The Benjamin/Cummings Publishing Company, Inc. 1987. p 485.
32. Michael J. Daly and Kenneth W. Minton, "Resistance to Radiation" p 1318 v 270 Science. 24 November 1995.
33. Malcolm W. Browne, "Odd Microbe Survives Vast Dose of Radiation" p C1,C5 The New York Times 28 November 1995.
34. J. L. Elliot et al. (26 others). "Jet-like features near the nucleus of Chiron" p 46-49 v 373 Nature. 5 January 1995.
35. Ron Miller, and William K. Hartmann, The Grand Tour: A Traveler's Guide to the Solar System. Workman Publishing 1993.
36. Matthew C. Senay and David Jewitt, "Coma formation driven by carbon monoxide release from comet Schwassmann-Wachmann 1" p 229-231 v 371 Nature. 15 September 1994.
37. Alan S. Stern, "Chiron illuminated" p 23-24 v 373 Nature. 5 January 1995.
38. Nicolas Biver et al., "Substantial outgassing of CO from comet Hale-Bopp at large heliocentric distance" p 137-139 v 380 Nature. 14 March 1996.
39. M. K. Wallis and N. C. Wickramasinghe, "Comet Halley's Remote Outburst" p 228-230 v 112 The Observatory. 1992.
40. Douglas Isbell and Jim Sahli, "Chemical Measurements of Comet Hyakutake Suggest a New Class of Comet" release 96-108 NASA HQ Public Affairs Office. 31 May 1996.
Related ReadingM. E. Bailey, S. V. M. Clube and W. M. Napier, The Origin of Comets. Oxford: Pergamon Press 1990.
William J. Broad, "The Comet's Gift: Hints of How Earth Came to Life" p B9,B12 The New York Times April 1, 1997.
William J. Broad, "Spotlight On Comets In Shaping of Earth" p B7,B11 The New York Times June 3, 1997.
Cristiano Batalli Cosmovici, Stuart Bowyer and Dan Werthimer, eds., Astronomical and Biochemical Origins and the Search for Life in the Universe: Proceedings of the 5th International Conference on Bioastronomy, IAU Colloquium No. 161, Capri, July 1-5, 1996, Editrice Compositori 1997. Note especially:
Sara Schechner Genuth, Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton University Press 1997. (Lots about Isaac Newton.)
Roland Meier et al., "Deuterium in Comet C/1995 01 (Hale-Bopp): Detection of DCN" p 1707-1710 v 279 Science. 13 March 1998. "The observed values... imply a kinetic temperature above 30 K in the ... interstellar cloud that formed the solar system."
Bartholomew Nagy, Carbonaceous Meteorites, Elsevier Scientific Publishing Company 1975.
William R. Newcott, "The Age of Comets" p 94-109 v 192 n 6 National Geographic, December, 1997.
Paul J. Thomas, Christopher F. Chyba and Christopher P. McKay, eds., Comets and the Origin and Evolution of Life. Springer-Verlag, New York, Inc. 1997.
Related WebsitesThe Leonid Sample Return Mission from Space Science News at Marshall Space Flight Center, 16 November 1998.
Chemical Measurements of Comet Hyakutake Suggest a New Class of Comets: News release 96-108 from NASA JPL.
Meteorites and Comets: Organic Matter and Exobiological Hypotheses, 1834-Present: a large, categorized, annotated bibliography in "The Net Advance of Physics" by N. Redington and K.R. Keck at MIT.
SpaceViews: Comet Hale Bopp. SpaceViews is a monthly publication of the Boston chapter of the National Space Society.
Stratospheric Dust: part of Planetary Materials Curation at NASA JSC.
NASA researchers: Comet shower triggered life on Earth by Don Knapp on CNN Interactive, April 17, 1997.
Comets, maintained by Ron Baalke, JPL, NASA.
Comets, one of Bill Arnett's Nine Planets, an excellent resource.
The Comet Hale-Bopp Home Page at NASA's Jet Propulsion Laboratory, Pasadena, CA, contains more than 3,200 images, including one obtained by a California astronomer the night after the comet was discovered in July 1995.
The Near-Live Comet Watching System at NASA Headquarters, Washington, DC, contains more than 1,300 images. Astronomers from Australia, Asia, Europe and the Americas have submitted images, which have been captured by everything from professional observatory equipment to the backyard gear of an amateur astronomer. The archives include photographs of the comet over San Francisco; Dublin, Ireland; and Genoa, Italy.
UMass Astronomers Report Comets May Have Introduced Interstellar Chemicals to Earth by Arthur Clifford, University of Massachusetts Amherst, 9 October 1996.
SKY Online's Comet Page by Sky Publishing Corporation, 1998.