Recent press attention has been directed toward two new science papers involving NEOs. These are: "The Origin of Life in Comets" by Napier et al. (published in the current issue of The International Journal of Astrobiology ) and "Comet May Have Exploded over North America " by Kennett et al. (discussed on the National Science Foundation website). Each of these papers makes extraordinary claims about the role of NEOs in the origin and evolution of life. However, these extraordinary claims are not backed up by compelling evidence, as discussed below. (Note that these criticisms represent my own personal opinion only, although I have profited from discussions with several colleagues).
1. DID LIFE BEGIN IN COMETS?
"The origin of life in comets" (in The International Journal of Astrobiology )
W.M. Napier, J.T. Wickramasinghe & N.C. Wickramasinghe
Cardiff Centre for Astrobiology, Cardiff University , 2 North Road , Cardiff CF10 3DY
Abstract: Mechanisms of interstellar panspermia have recently been identified whereby life, wherever it has originated, will disperse throughout the habitable zone of the Galaxy within a few billion years. This re-opens the question of where life originated. The interiors of comets, during their aqueous phase, seem to provide environments no less favourable for the origin of life than that of the early Earth. Their combined mass throughout the Galaxy overwhelms that of suitable terrestrial environments by about 20 powers of ten, while the lifetimes of friendly prebiotic environments within them exceeds that of localised terrestrial regions by another four or five powers of ten. We propose that the totality of comets around G-dwarf sun-like stars offers an incomparably more probable setting for the origin of life than any that was available on the early Earth.
This paper concludes that it is a factor of a trillion-trillion (10 to the 24 th ) more likely that life arose in comets than on terrestrial planets like the Earth. However, this "incomparably more probable" number is arrived at only by a series of assumptions, some not very secure.
Although reported as a relative probability of origin of life, this number is derived primarily from an estimated ratio of the volume of all solar-system comets to the very limited volume of water-saturated clays ( Darwin ' small warm ponds) at the surface of the Earth. To associate this ratio of volumes with the probability of life, one must make several assumptions:
- Life originated in clay (This is just one of many possibilities)
- Comets contain lots of clay (But the scientists who publish data from the Deep Impact mission do not refer to clay, but to a variety of silicates.) This assumption that Earth has very little clay while comets are full of clay is the key to their argument.
- Comets maintain liquid water (and wet clay) throughout their interior volumes over millions of years, or more. (The evidence for past water activity in comets is usually ascribed to very short-lived radioactive heating at the dawn of the solar system, not a persistent situation.) Small objects such as comets cool much more rapidly than large ones, such as planets.
- Life needs only water and clay and organics to form (no mention is made energy sources.)
- Life once formed in comets and frozen there survives for hundreds of millions of years, perhaps even to the present (neglecting that it may be killed by lack of nutrients or of energy, or by cosmic ray irradiation.)
Added note: The three authors of this paper have questioned some of my points above. Principally, they say that their model does not require the presence of abundant clay in comets, only of abundant liquid water. On the subject of water, they say that while most comets probably cool quickly, comets more than 100 km in diameter can retain liquid water interiors for longer. Finally, they state that in their interstellar panspermia model (Napier 2007, Internat. J. Astrobiol. 6, 223-228), microbes need only survive in the comet for about 50,000 years.
Following is a press report with further criticisms:
SCIENTIST: CALCULATIONS PROVE LIFE BEGAN IN COMET
By Ker Than, Staff Writer
posted 16 August 2007 in LiveScience and Space.com
Life almost undoubtedly began in space, and specifically in the hearts of comets, rather than on Earth, a new study claims.
Chandra Wickramasinghe, an astrobiologist at Cardiff University in the United Kingdom , and his team say their calculations show that it is one trillion trillion times more likely that life started inside a slushy comet than on Earth.
"The comets and the warm watery clay pools in comets are settings in which the organic molecules are transformed into living structures in comets," Wickramasinghe said. "That transformation is more likely in some comet somewhere in the galaxy than in any small pond on the Earth."
The new findings will be detailed in an upcoming issue of the International Journal of Astrobiology.
But while most scientists are willing to concede that fallen comets might have delivered some of the water and organic materials necessary for life to Earth, critics say that Wickramasinghe's proposal that life originated in comets which subsequently crashed on our planet -- an idea called panspermia -- is speculative and not supported by evidence.
"It looks to me as if their conclusions are constructed from a series of speculations, none of which is based on much evidence. It is a theory built on air, not solidly grounded in scientific facts," said David Morrison, a senior scientist at the NASA Ames Research Center in Moffett Field , California , who was not involved in the study.
Wickramasinghe and his colleagues' idea rests on the assumption that comets are full of porous clay particles that can hold water in a liquid form for eons.
Cometary missions such as Deep Impact have found evidence for a variety of silicates existing inside comets, but not clay per se, Morrison said.
The "assumption that Earth has very little clay while comets are full of clay is the key to their argument, and it is at best speculation," Morrison said.
It is also an open question as to whether comets do indeed contain liquid water inside them and whether other star systems support comets at all, let alone clay-, water- or life-bearing comets. "No comets have been discovered yet around other stars," Morrison said in an email interview.
Paul Falkowski, a biochemist at Rutgers University in New Jersey , also does not think that the site of life's origins can be figured out using simple calculations. "These basic kinds of things are dependent on the beginning initial assumptions. I don't know that we know the odds," Falkowski said. "We know the odds for exactly one planet, and it happened once, so everything else is a game."
Recent work by Falkowski and his team suggests that life would have difficulty surviving unprotected in deep space where comets reside. In research detailed in the Aug. 6 issue of the journal Proceedings of the National Academy of Sciences, the team recovered highly degraded microbial DNA from 8 million-year-old Antarctic ice and estimated that DNA on Earth has a half-life of only about 1.1 million years. In other words, every 1.1 million years, half of the DNA disappears.
The researchers say cosmic rays are the culprits and think that DNA-or any other complex organic molecule-would have a difficult time surviving for long in space, where radiation levels are much higher than on Earth.
"The radiation flux on the surface of this planet is one-tenth to one-one-hundredth to that of space," Falkowski told SPACE.com. "So when you go into a situation where you don't have a magnetic field protecting you from cosmic background radiation, the amount of damage to DNA would be incredibly high."
Falkowski's team estimates that DNA would survive only a few hundred thousand years in space, essentially ruling out interstellar pollination of life by comets as well as the potential for life to survive in space for very long.
2. DID AN EXPLODING COMET KILL NORTH AMERICAN MEGAFAUNA AT END OF LAST ICE AGE?
"Comet May Have Exploded Over North America 13,000 Years Ago"
August 14, 2007 (from NSF website; also presented at AGU meeting May 23, 2007, with discussion available on YouTube at http://www.youtube.com/view_play_list?p=741568C2D58A9793)
NSF web announcement: New scientific findings suggest that a large comet may have exploded over North America 12,900 years ago, explaining riddles that scientists have wrestled with for decades, including an abrupt cooling of much of the planet and the extinction of large mammals.
The discovery was made by scientists from the University of California at Santa Barbara and their colleagues. James Kennett, a paleoceanographer at the university, said that the discovery may explain some of the highly debated geologic controversies of recent decades.
The period in question is called the Younger Dryas, an interval of abrupt cooling that lasted for about 1,000 years and occurred at the beginning of an inter-glacial warm period. Evidence for the temperature change is recorded in marine sediments and ice cores.
According to the scientists, the comet before fragmentation must have been about four kilometers across, and either exploded in the atmosphere or had fragments hit the Laurentide ice sheet in northeastern North America .
Wildfires across the continent would have resulted from the fiery impact, killing off vegetation that was the food supply of many of larger mammals like the woolly mammoths, causing them to go extinct.
Since the Clovis people of North America hunted the mammoths as a major source of their food, they too would have been affected by the impact. Their culture eventually died out.
The scientific team visited more than a dozen archaeological sites in North America , where they found high concentrations of iridium, an element that is rare on Earth and is almost exclusively associated with extraterrestrial objects such as comets and meteorites.
They also found metallic microspherules in the comet fragments; these microspherules contained nano-diamonds. The comet also carried carbon molecules called fullerenes (buckyballs), with gases trapped inside that indicated an extraterrestrial origin.
The team concluded that the impact of the comet likely destabilized a large portion of the Laurentide ice sheet, causing a high volume of freshwater to flow into the north Atlantic and Arctic Oceans .
"This, in turn, would have caused a major disruption of the ocean's circulation, leading to a cooler atmosphere and the glaciation of the Younger Dryas period," said Kennett. "We found evidence of the impact as far west as the Santa Barbara Channel Islands."
The National Science Foundation's Paleoclimate Program funded the research. NSF contact is Cheryl Dybas <firstname.lastname@example.org>
This hypothesis of a recent comet impact is based on indications of extraterrestrial chemical signatures (such as iridium and metallic microspherules) found at more than a dozen archaeological sites in North America . But the proposed interpretation involving impact from a large comet is both internally inconsistent and is at odds with geological evidence.
The posted report suggests an impact on North America about 13,000 years ago by a 4-km-diameter comet. Something that large would not have been slowed significantly by its passage through the atmosphere. The explosion of roughly 10 million megatons would thus have made a crater at least 50 km across. Such a large crater formed on land this recently would be the most obvious impact feature on the planet. Yet it is not there, and one can't just wish it out of existence. The ejecta should also be obvious, yet that too is not found.
Impact by a 4-km comet or asteroid is extremely rare: for an asteroid the average interval is about once per 10 million years, for a comet as much as once per billion years. Yet this event is supposed to have happened 12,900 years ago. Normally in science, extraordinary claims require extraordinary levels of evidence.
Added note: Several colleagues have informed me that these authors seem to be describing a comet that broke apart before impact, either fragmenting in the atmosphere or perhaps suffering a fate similar to that of Comet Shoemaker-Levy-9, which was pulled apart by the tidal forces of Jupiter. However, these suggestions do not save the hypothesis; quite the contrary. Calculations show that neither a stony object nor an icy comet will fragment in the atmosphere if it is several kilometers in diameter. And even if it did break apart at an elevation of (say) 100 km, this is only a few seconds before impact, so the material cannot spread very far laterally - perhaps of order 100 km, not the several thousand km that would be needed to distribute the impact over much of North America. The alternative, that the comet was pulled apart by tidal forces, would have required that it pass extremely close to the Moon on its way to hitting the Earth. Any impact by a 4-km comet or asteroid is extremely unlikely to have happened so recently, as noted in the critique above. To require that it also skim the surface of the Moon before hitting Earth drops the probability by several more factors of ten. In fact, such a comet break-up scenario for Earth is not likely to have happened even once in the past 4 billion years, let alone just 13,000 years ago. Yet another suggestion is that the widespread archeological evidence might have been due to secondary craters. But one only gets secondary craters if there is a primary crater, and as noted above, there is none.