[X] My Assistant

[alan]

At least three massive asteroids smashed into the Earth more than 3.2 billion years ago, and caused such destruction, they dramatically changed the structure and composition of the Earth's surface. This is according to new research from scientists at the Australian National University. The team uncovered evidence of major earthquakes, faulting, and volcanic eruptions that were so violent they dramatically changed the way the Earth's surface was forming. This happened during a period that the Moon also suffered heavy bombardment.

“Our findings are further evidence that the seismic aftershocks of these massive impacts resulted in the abrupt termination of an over 300 million years-long evolutionary stage dominated by basaltic volcanic activity and protracted accretion of granitic plutons,” Dr Glikson said.

“The precise coincidence of the faulting and igneous activity with the impact deposits, coupled with the sharp break between basaltic crust and continental formations, throws a new light on the role of asteroid impacts in terrestrial evolution,” Dr Glikson said.

http://www.universetoday.com/am/publish/ea...ids.html?582005

Could one of the geologist interpret this please.

[Guest_Richard Trigaux_*]

Thanks all for your precisions.




First the idea as what "something special" happened 3.2 billions years ago is the occurence of large (in the 1000kms) bassins on the Moon, which formed in a relatively short time. This simultaneousity hints at a special event, a group of large bodies with the same origin. This must not be confused with the initial bombing of all the planets at the origin of the solar system, which culminated 4.5 B years ago and decreased steadily since.

If the 3.2 B years event is not local to Earth, we may expect that Mare Caloris (Mercury) Hellas (Mars) and eventually others formed simultaneously. But we do not know the dates of these events.

But the fact that there is a group of large objects crashing on the Moon nearby in the same time makes that the idea of "A second Terrestrial satellite " not so "special pleading" as you say Bob Shaw. Of course other origins are possible, fo instance a large collision in the asteriod belt, and it is even more likely. (At the origin there was only some spherical asteroids, but numerous collisions occured which created the thousands angulous bodies we know today).

The original theory of the "second satellite of Earth" was in fact more complicated, it was rather a belt of debris: either an unique body never formed, or it broke appart. (Remember that at that time the Moon was much closer from Earth, so there was little place for the accretion of a third body, and matter rather formed a great number of small bodies, like today with the numerous little satellites at the rim of Saturn's ring). So when these bodies may have hit the Moon, due to the loss of mass, angular momentum rearanged the axis of rotation of the Moon, untill the next shock, so Bob Shaw, I think it would be rather difficult to infer orbital data of the impactors from the distribution of today marks.







Thank you tty for your precisions. The date you give (3.8 billion years) is what I was speaking about, but I did not remembered the figure. Perhaps this date is the date when the first continents began to form. Life seems much older, but this is known only from the evolution of DNA which has its own "clock" and we are not sure that this clock moves at a constant rate.






There was a discution about the effect of shockwaves on the opposite side of Mare Caloris, but these effects were purely tectonic, without melting of rock (But mountains were jumping and turned upside down like a pancake in its pan, if was rather better not to be here).




At last very large impact bassins like those we are discussing about (about 1000kms) could have formed in Earth past, but if they were into oceans, all the traces disappeared in 100 or 200 millions years, with the plate tectonics on the ocean floor. And 3.2 billions years ago, there was no large continents like today. So such impacts can be identified only by their ejecta (which may have covered the whole Earth). Are these ejecta basaltic/mantellic, or continental?





A last element is the evolution of life itself. It is now generally accepted that recent (less than Cambian time) impacts modeled the evolution of superior living beings. But 3.2 or 3.8 billons years ago, the skate of the evolution was rather setting the DNA code structure tself. Specialists have an hypothesis as what the genetic code is the result of an evolution in very ancient times. For instance there is a theory as what our three codons code evolved from a two codons code. So very ancien, but very large impacts may have driven the evolution of our DNA well before the appearance of our most ancient bacteria

Oooof record of the longest post to date!!!