Discussion in 'Biology & Genetics' started by BenTheMan, Mar 23, 2007.
Why do you deny the fact that the entire scientific community rejects your claims?
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No, I'm not. I'm assuming they were as huge as the ones that cratered the surface of the moon, or Mercury, or Mars. While they would make a big splash on one side of the planet, the other side would be relatively unaffected. I'm not saying that it wouldn't have affected the evolution of life, but rather that it would not necessarily have precluded such evolution. If you have some articles on that with good facts [and not just speculations], I would appreciate reading them. A huge impact on one side of the planet every 100 years does not appear to me to show a preclusion of life's development.
From a popular work, but one whose author's have impeccable scholarly credentials:
Early in the history of the solar system the impact of very large objects was much higher, and objects struck Earth that were as big as Mars. During the first 600 million years of Earth's history, there were impacts of bodies 100 kilometres in size that individually delivered enough energy to heat and sterilize Earth's surface down to several kilometres.The larger impacts would have vaporised the oceans and parts of the crust.
From Ward, P.D. & Brownlee, D. Rare Earth page 46 Copernicus Books ISBN 0-387-95289-6
Slightly more technical, but with implicit revelation of the energy magnitude of these events is Glickson,A.Y. Asteroids and Archaean crustal evolution: Tests of possible genetic links between major mantle/crust melting events and clustered extraterrestrial bombardments In Lunar and Planetary Inst., International Conference on Large Meteorite Impacts and Planetary Evolution p 29-30
Portion of abstract
The identification of microtektite-bearing horizons containing spinals of chondritic chemistry and Ir anomalies in 3.5-3.4-Ga greenstone belts provides the first direct evidence for large-scale Archaean impacts. The Archaean crustal record contains evidence for several major greenstone-granite-forming episodes where deep upwelling and adiabatic fusion of the mantle was accompanied by contemporaneous crustal anatexis. Isotopic age studies suggest evidence for principal age clusters about 3.5, 3.0, and 2.7 (+/- 0.8) Ga, relics of a ca. 3.8-Ga event, and several less well defined episodes. These peak events were accompanied and followed by protracted thermal fluctuations in intracrustal high-grade metamorphic zones. Interpretations of these events in terms of internal dynamics of the Earth are difficult to reconcile with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by mantle rebound response to intermittent extraterrestrial asteroid impacts is supported by (1) identification of major Archaean impacts from microtektite and distal ejecta horizons marked by Ir anomalies; (2) geochemical and experimental evidence for mantle upwelling, possibly from levels as deep as the transition zone; and (3) catastrophic adiabatic melting required to generate peridotitic komatites.
More specifically, Grieve, R. A. F.; Pesonen, L. J. The Terrestrial Impact Cratering Record Tectonophysics, Vol. 216, No. 1/2, p. 1 - 30
Portion of abstract
By analogy with the lunar record and modelling of the effects of very large impacts, it has been proposed that biological and atmospheric evolution of the Earth could not stabilize before the end of the late heavy bombardment ≡3.8 Ga ago.
Walter, while most of my posts are lightweight, replete with ad hominems, whimsical nonsense and minor irrelevancies, if I am speculating I shall say so and when, on the rare occassion, I make a technical statement it will not be speculation. You can bank on it. But I shall always be delighted to provide citations. If you require further support in this instance please let me know. There is a cornucopia of work in this area.
It appears that the second article, with major events possibly dated at 3.8, 3.5, 3.0, 2.7 billion years ago conflicts with the assumption that cellular life arose circa 3.6 billion years ago and that it could not have arisen when major events were still transpiring. This actually implies that such major events could not destroy/prevent life's evolution commencing circa 3.6 billion years ago [if not earlier].
The first article suggests impacts with multiple Mars-sized objects. Those would have added significantly to the mass of earth, and it would not therefore be accurate to assert that earth's size and crust stabilized roughly 4.5 billion years ago, the presumed age of the earth, if such were the case.
Instead, that first article suggests that earth's crust would not have solidified until roughly 3.8 billion years ago.
Almost sounds as if he were there to watch it all happen.
I don't necessarily believe it, but I don't necessarily dispute it either. It does not, to me, appear that he has evidence for such Mars-sized impacting objects, and that rather that is conjecture on the author's part. The craters on the moon were caused by impacts of much much smaller pieces, more along the lines of the second article's impact events, which are not shown to be global, but rather local.
In any event, whether the time-window for life's evolution to the first cells was 200,000,000 years or 1,000,000,000 years is likely irresolvable with our current knowledge, and 200,000,000 years does appear adequate.
The prevailing view for the formation of the moon is that it is a consequence of the proto-Earth being struck by a Mars sized object. This is common knowledge and has been the view since the early 80s.
Your objections have all the appearance of argument from incredulity. I have no intention of encouraging that approach by further discussion.
Since we have only gross speculation to suggest how life might have arisen it is completely groundless to say 200 million years appears adequate. It may be. It may not be. At this point we cannot say.
Actually, I believe I stated the basis for the objections quite succinctly.
So you do not accept the current consensus view that the moon was formed through the impact of a Mars sized object with the proto-Earth? You do not accept that the impact of 100 kilometres diameter bolides would have the potential to evaporate the oceans? You appear to think the maximum size of impactors on the moon can be deduced from the size of the largest craters? I am especially interested in your response to the last point.
Further documentation of the current consensus that you wish to object to on no other apparent grounds than incredulity.
Sleep, Norman.H. Annihilation of ecosystems by large asteroid impacts on the early Earth
Nature, Volume 342, Issue 6246, pp. 139-142 (1989).
Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.
Abe.Y, A Proto-atmosphere and the Environment of the Earth During Accretion American Geophysical Union, Fall Meeting 2001, abstract #U51A-09
Abstract: The earliest surface environment of the Earth is reconstructed in accordance with a recent planetary formation theory. The recent planetary formation theory suggests 2 stages of planetary formation; the stage of runaway growth followed by the stage of giant impacts. A Mars-sized planet forms in 106 years at the stage of the runaway growth. Since solar nebula likely exists at this stage, the proto-Earth at this stage should have attracted the nebula gas and have a distended solar-composition (H2<ETH>He) atmosphere. Also, we expect impact degassing from Earth-forming planetesimals to form a degassed atmosphere, which sometimes called `a steam atmosphere.' Hence, a mixed proto-atmosphere of solar-type and degassed components would have formed. Though the structure of the mixed atmosphere embedded in the nebula gas is not well understood yet, it would be similar to that of degassed atmosphere with extended upper atmosphere while the planet is relatively small. Then, the surface temperature is controlled by that of the degassed atmosphere. Since the actual energy release occurs intermittently, the atmosphere may cool before the next planetesimal impact if the mean impact interval of planetesimals is longer than the cooling time of a hot atmosphere. However, judging from the frequency and thermal effects of planetesimals impacts, we consider that the proto-atmosphere has a strong thermal blanketing effect and a surface magma ocean is formed. Thus, dissolution of volatile components into the mantle is expected. Segregation of metallic iron from silicate also occurs at the super heated impact points. This will also lead to reaction of volatile components with metallic iron. Very large infrequent impacts are expected at the giant impacts stage. Though the solar nebula has likely been lost by this stage, the mixed proto-atmosphere would have survived the nebula dissipation, because the atmosphere is tightly bounded by the EarthOs gravity field. Each impact may drive off the existing atmosphere. However, it is not able to desiccate the interior of the Earth. Therefore, the atmosphere will soon recover through degassing or re-accretion of impact-generated circumterrestrial disk. It should also be noted that refractory Moon can be formed even from a volatile containing disk. Since a long impact interval is expected, the atmosphere may condense to form oceans between impacts at this stage. The interior of the Earth, at least at the upper mantle region, remains in a partially molten state during this stage and chemical differentiation is expected.
I have emboldened the key phrases in this abstract.
Ophiolite, you seem to miss the point I was making.
The articles you posted showed a series of major impact events at about 3.8, 3.5, 3.0 and 2.7 bya [presumptively each series lasting several million years, then reducing significantly in frequency of impacts until the next series].
However, the earlier supposition [posted by others] was that life arose circa 3.6 bya.
You also postulated that those major impact events would preclude life. Those are two contradictory statements.
I said that those series of major impacts, if life was around since circa 3.6 bya, shows that life can survive through such major impacts.
Or do you dispute the series of impact ages [3.5 bya, 3.0 bya and 2.7 bya] in the article which you yourself posted? Or do you dispute the presumed date of origin of life at 3.6 bya and believe it is closer to 2.7 bya [at the conclusion of the last of the series of major impact events]?
On a more general discussion, there is ZERO evidence that a Mars-sized object ever impacted earth. That is pure conjecture on the part of that author. While that is one of the theories of moon formation [Mars-sized impactor hitting earth and showering out a spray of ejecta], it is certainly only speculation or conjecture.
If a Mars-sized object impacted earth, it would indeed melt the entire planet [already mostly molten anyway], while adding to its mass. Since the supposition is that earth's crust formed/solidified circa 4.5 bya, and no significant change in mass occurred thereafter [age of the earth], such Mars-sized object would have been at the earliest history of earth. That is what is meant by the age of the earth - when it acquired its current approximate mass of heavy materials [iron, nickel, silicates, aluminates, etc.], and its crust solidified - circa 4.5 bya. While certainly we see evidence [as per the article you cited showing a series of major impact events] for a series of major impacts thereafter, such as pock-mark the moon, Mercury, and to a lesser extent Mars [which are now mostly eroded], those did not add significantly to the mass of those planets, but did heavily crater the surfaces.
You should note that there are also other viable theories for both the formation of the moon, and the formation of earth, other than a Mars-sized object striking proto-earth, and ejecting mantle material to form the proto-moon. The fact that the moon's orbit is in earth's plane of orbit about the sun would be a large coincidence [though not impossible] in the earth-impactor theory, but a direct consequence in cloud-condensation theory separating into two bodies [proto-earth and proto-moon] in orbit about each other.
Also, the article showing the series of impact events does not attempt to explain why there were long periods of few impacts [hundreds of millions of years] followed by short periods of intense impact events. This would imply that there were events in our solar system that were transpiring to give rise to large numbers of impactors at those ages [3.8, 3.5, 3.0 and 2.7 bya] which took wild elliptical orbits, only to be swept clean from the inner planetary orbits over the course of a few million years, as they hit the sun, Mercury, Venus, Earth and its moon, and Mars, removing them from that region of our solar system.
So, what could have given rise to such a series of events that generated millions of impactors?. Colliding planets in the asteroidal belt region is my best guess.
Walter it has become clear to me in reading your responses that you know precious little about planetary formation theory. You dismiss the Mars mass impactor theory as if it were the work of a single author and remains the opinion of that single author. That is not the case. It is widely accepted by the large majority of those working in this field. You appear to have been blissfully unaware of it until this exchange of ours. Of course I can find you a driller's handful of counter theories, one of which may be right, but they certainly have not attracted the almost universal support of the Mars impactor.
Next, you say there is no evidence for this event. Well bullcrap in a large articulated lorry. I take it from this you do not believe we went to the moon, for it is the composition of the moon rocks that provides some of the most compelling evidence for this theory.
I had grave doubts about using the particular paper which you have got so hung up on. I suspected you might improperly zero in on the post 3.6by ages, but the paper is such a fascinating one for broader reasons that I decided to stay with it.
Why are you opposed to the consensus view on the effects of the Heavy Impact Phase? You have not given a single valid reason apart from a misinterpretation of a document that I have provided. Your position continues to look like an argument from incredulity.
The impactors were multi-sourced. If I can find it I'll post you an interesting analysis of isotope ratios that demonstrates the ocean water had to be derived from multiple sources. There were still plenty of planetesimals floating around the inner solar system. Jupiter was still tossing cometary bolides in and out, and yes, protoplanets were colliding in the asteroid belt and elsewhere.
I believe I explained it quite well. The article shows several periods of major impact until as recently as 2.7 bya, whereas your contention was that such impacts preclude life directly contradicts the assertion that life arose circa 3.6 bya. Do you not believe that there were several phases of heavy-impact, over the course of some nearly two billion years [4.5 bya to 2.7 bya], as the article says. Anyway, thanks for posting it, I was not aware of that excellent evidence.
Also, I am well aware of the "consensus" of opinion that a Mars-sized impactor formed the moon circa 4.5 bya. The composition of the moon is more like the mantle of earth, than the average of the earth. Other theories, other than the "Mars-sized impactor theory" also explain that. That is circumstantial evidence only, thus far, and not very convincing [for myself, though the majority are sometimes easily swayed].
For an example where the "consensus" of view was wrong [including my own!] regarding another area of science [nuclear physics], check out the physics section where I posted regarding the nuclear-decay-rates being affected by the earth-sun distance. Everyone's been wrong on the "consensus" that such decay rates are not affected by outside environmental influences. It begs for an explanation. Go to it!
I am temporarily suspending my side of this discussion. My recollection of your posting character was that it was solid and scientific. I have not found that to be the case in this thread. You are using cheap debating tricks (e.g.though the majority are sometimes easily swayed. A trifle snide, don't you think?) and frankly I find your claims to have been well familiar with the Mars mass impactor theory difficult to reconcile with your initial posts in this thread.
I am going to conduct a careful review of a selection of your posts in order to decide if it is worth continuing this discussion with you. Please feel free to be angered, amused, or wholly indifferent to this seemingly patronising attitude.
If it's not too much trouble, would a moderator like to spin this discussion of into a thread in Biology titled, say, Constraints on abiogenesis from bolide impacts.
Separate names with a comma.