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
