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Cometary lunar impacts?

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#1 Greg Moore

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Posted 05 November 2012 - 11:04 AM

Are there lunar features visible with amateur telescopes that researchers have reason to believe are the results of cometary impacts rather than meteoritic/asteroidal? I would assume there must be differing parameters for craters produced by faster moving less dense objects,and variation in associated debris generated.
Greg

#2 luigis

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Posted 05 November 2012 - 02:35 PM

I'm not sure.

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.

#3 daveCollins

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Posted 05 November 2012 - 07:26 PM

My books describe craters in terms of what can be seen, not the type of object that created them. In particular they are described in terms of their size, complexity, and special features such as double walls.

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.

#4 canopus56

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Posted 08 November 2012 - 05:26 PM

Hi Greg. This is a reading interest of mine, and here's my bibliographic list. The short answer is Melosh's 1994 paper ("Crater Chains on the Moon"), in which the authors suggest two _candidate_ possible lunar comet impacts: "Davy chain to between 200 and 250 km for a chain near the crater Abulfeda." Think in terms of a Shoemaker Levy 9 like comet (but much smaller) that is small enough to get ripped up by Earth-Lunar tidal forces. The 1997 article suggests Reiner Gamma as a _candidate_. Happy reading! Clear Skies, Kurt

THE IMPACT RECORD OF DISRUPTED COMETS ON THE MOON.
http://www.lpi.usra....97/pdf/1138.PDF

INTERSTELLAR MATTER ON THE MOON.
http://www.lpi.usra....12/pdf/1275.pdf

Structure and Density of Cometary Nuclei
http://www.lpi.usra....metsII/7016.pdf

CRATER CHAINS ON THE MOON: RECORDS OF COMETS SPLIT BY THE
EARTH'S TIDES?
http://www.lpi.usra....94/pdf/1447.pdf

FREQUENCY OF ASTEROID’S TIDAL BREAK-UP.
http://www.lpi.usra....12/pdf/6320.pdf

#5 daveCollins

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Posted 09 November 2012 - 07:49 AM

Thanks for the links. I did read them and my impression is that the authors have no idea as to whether or not there are lunar features due to comets. They are simply speculating about possibilities. In fact, it isn't even known what compositions they have.

With the current state of knowledge, can you say anything definitive? Not that I have seen.

#6 canopus56

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Posted 09 November 2012 - 01:00 PM

I agree, that's actually right. Yes, the state of knowledge is exploring a hypothesis. The word "candidate" indicates that.

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.

http://www.nasa.gov/...1500comets.html

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

#7 daveCollins

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Posted 09 November 2012 - 09:00 PM

Kurt, I appreciate your thoughtful posts. I was a little concerned that you might feel I was being negative. That isn't really were I am coming from. I attempt to apply critical thinking about what I read and then respond accordingly.

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.

#8 azure1961p

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Posted 10 November 2012 - 09:11 AM

I believe a CN member Rick evans had an impact video this small flash light looking like it was authentic anyway.

Pete

#9 canopus56

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Posted 10 November 2012 - 02:26 PM

Dave, we're good. I took your criticism as trying to keep the amateur community focused on what is known by professionals with certainty, and that is an important value for the community when talking to the public at star parties. We should try to clearly separate what the professionals have a consensus opinion on from the "fuzzier" exploration theories at the edges of astronomical science.

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.

http://www.nasa.gov/...all/news/lunar/

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):

http://www.lpi.usra....e/?AS12-51-7485
http://www.lpod.org/...rch&cat=0&pos=8

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

#10 Greg Moore

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Posted 10 November 2012 - 10:30 PM

Thank you for your responses, folks, and what great links, Kurt! I was away on my last trip up to Cherry Springs for the year (had several good hours of sky) and I apologize for my tardy expression of gratitude.
Clear skies, Greg






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