Cometary lunar impacts?
Posted 05 November 2012 - 11:04 AM
Posted 05 November 2012 - 02:35 PM
I do know that Shackleton near the south pole is believed to be a trap for water deposited by cometary impacts. That's because sunlight never reaches the bottom of the crater.
Posted 05 November 2012 - 07:26 PM
Other than examining the material around a crater, I am not sure how discussing the source object's composition could be anything but speculation. I don't know and am speculating myself.
There are equations which relate the speed, density, and impact angle to the size of the resulting crater. But since you have several degrees of freedom here, I would guess there isn't a one to one correlation between object composition and resulting crater characteristics.
Posted 08 November 2012 - 05:26 PM
THE IMPACT RECORD OF DISRUPTED COMETS ON THE MOON.
INTERSTELLAR MATTER ON THE MOON.
Structure and Density of Cometary Nuclei
CRATER CHAINS ON THE MOON: RECORDS OF COMETS SPLIT BY THE
FREQUENCY OF ASTEROID’S TIDAL BREAK-UP.
Posted 09 November 2012 - 07:49 AM
With the current state of knowledge, can you say anything definitive? Not that I have seen.
Posted 09 November 2012 - 01:00 PM
It does raise a negative question that is useful to think about - Why aren't there more obvious cometary impacts on the Moon?
There doesn't seem to be any shortage of small cometary bodies in the inner solar system. For example, in 2008, SOHO announced its discovery of the 1,500 sungrazing comet.
In 1994, the US DOD released satellite observation data indicating that there were 136 airbursts of greater than 1-kiloton in force between 1975 and 1992 from ice-rock meteoriods, based on observing about 10 percent of the Earth's surface. On average, I read about 1 of these airbursts a year in the newspaper over the last four or five years.
This makes me wonder about why there is not more obvious evidence of small comet impacts on the Moon.
Clear Skies - Kurt
Posted 09 November 2012 - 09:00 PM
You make a good point about the frequency of events. I am surprised that someone hasn't caught an impact as it is occurring. You would think that by chance someone would have gotten lucky. Maybe it is just an issue with resolution.
Posted 10 November 2012 - 09:11 AM
Posted 10 November 2012 - 02:26 PM
Pete, there are many known real time lunar meteor impacts, and NASA's Meteoroid Environment Office (MEO) maintains a master list, including private observations like Ricks', that is up to 265 detections.
These are hardrock impacts. I was thinking in terms of a small comet impacter (less than 10 meters in dia.) with some-kind-of-a detectable tail on it, and such objects would transport hydrocarbons to the Moon's surface. Those are the size of objects seen in the SOHO sun grazer comet images of Kreutz group comets that "typically are tens of meters in size." http://sungrazer.nrl...php?p=FAQs#size .
But maybe the category of a "comet" misdefines the question. Most meteoriods are hard-ice-rubble mixtures, and they would also be transporting hydrocarbons to the Moon's surface.
If such a small cometary object (i.e. less than 50 meters in dia.) did hit the Moon, how would you distinguish the impact from a hard-rock impact crater?
The types of comet impacts that Greg was considering at the start of the thread are what we would recognize as true comets - larger comets that are broken up by tidal forces like Shoemaker Levy 9 - might be distinguished from "regular" impacts because they would produce a line of closely spaced craters - like the Davy Crater chain (Catena Davy):
It is the chain of craterlets that puts Catena Davy in the category of candidate or "suspected" comet impact on the Moon.
Clear Skies, Kurt
Posted 10 November 2012 - 10:30 PM
Clear skies, Greg