Other
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.u51a..03h&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #U51A-03
Other
5410 Composition, 5420 Impact Phenomena (Includes Cratering), 5455 Origin And Evolution, 5470 Surface Materials And Properties
Scientific paper
Since pre-Apollo years of the 1960s, it has been recognized that cratering on the moon must have been much more intense, averaged over the first few hundred My, than the average after 3500 My ago. This phenomenon is known as the "early intense bombardment." Initial interpretation of Apollo data raised the possibility that much of this cratering occurred in a single episode, or "spike" on the flux vs time curve, at about 3950 My ago, with a width of about 150 My. In some interpretations this was the primary source of all early cratering, known as a "catastrophic terminal bombardment." In one model Ryder has suggested that there was very little cratering before this. In other models, this is a spike superimposed on a declining flux, and there may have been various spikes. A host of problems remain. (1) Do we really have adequate dates for the lunar basins? The predominant opinion seems to be that virtually all visible basin were created in a burst within about 300 My. Confirming these dates would resolve the existence of the proposed catastrophe, which would then be constrained to involve numerous 50 and 100-km scale bodies hitting the moon. (2) How does intense cratering work to remove earlier samples of igneous crustal rocks and impact melts? The original suggestion of the catastrophe was in order to explain the paucity of pre 4000-My rocks in the lunar sample. Cumulative impacts tend to destroy early rocks whether or not they are concentrated in a catastrophe. In some models, the extended declining impact, due to megaregolith production, tends to destroy impact melts because they concentrate at the surface, while dredging up (and yet also pulverizing) crustal igneous samples from deep-seated reservoirs. (3) How severe is the absence of pre-4000 My impact melts? Their absence has been used as an argument for the existence of a cataclysm at 3950 My ago. But the details of item (2) need to be combined with actual distributions of impact melt ages and igneous rock ages to refine these discussions. (4) Do lunar meteorites show the same age distributions and properties as the front side Apollo samples? This may be a test of the hypothesis that Imbrium debris have contaminated the front side. (5) Do asteroids show the same age distributions as the lunar samples? Available models of a cataclysm at 3950 My ago suggest the impactors came from the outer solar system and therefore they should have affected the asteroid belt as well. (6) What is the significance of the "Genesis Rock," ALHA 84001, among the first 20 specimens from Mars given that astronauts were trained to look for lunar "Genesis Rocks" and couldn't find them? Mars should have been affected by the cataclysm, according to available models. Hartmann (2001) suggested it tells us that the Martian crust was not destroyed by plate tectonics as on Earth, and parts of the primordial Mars crust were exposed by erosion to provide the meteorite source. Hence Mars may be the only planet where we can access a primordial crust. The erosion must have happened after a putative cataclysm at 3950 My ago. (7) In short, did a cataclysmic spike at 3950 My ago, how big was it in terms of forming most lunar basins and other features, and how much of the total lunar cratering was concentrated in it?
No associations
LandOfFree
Review of Early Intense Bombardment and Associated Problems does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Review of Early Intense Bombardment and Associated Problems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Review of Early Intense Bombardment and Associated Problems will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1240103