Comets, life delivery cosmic systems

Comets are like cats. They have tails, and they do precisely what they want — David Levyç+

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. 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 Earth

Many 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"  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 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. 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 .

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'." 

Reproduced from
http://www.panspermia.org/comets.htm