PDA

View Full Version : Moon craters and age of Earth



kamaz
2011-Dec-30, 02:27 PM
Over at Pharyngula (http://scienceblogs.com/pharyngula/2011/12/nice_argument_for_the_age_of_t.php) someone is describing how to use a photo of the lunar farside to argue with the YEC crowd:



I usually point out that almost all of the craters were formed by asteroids smashing into the planet, and that the Moon has over 250 craters with a diameter of 100 km or more. After explaining that Earth is just as likely to be struck by large asteroids as the Moon (is more likely to be struck, in-fact, due to its greater gravitational well), I then ask them to consider what their time-scale entails: that Earth should be struck every couple of decades by an asteroid capable of completely ejecting an area about the size of New Hampshire (not to pick on New Hampshire). Since such an event has never been observed and there are no well-preserved impact structures anywhere close to this size range, I then suggest to them that the only sensible conclusion is that Earth is much older than they had thought.


Hold on here. Something is wrong. If the Moon is cratered and Earth is not, then the simplest explanation is that the surface of the Earth is younger then the surface Moon. (We can see that rule at work on the Moon, where younger maria are less cratered than older highlands). Thus, if the Moon was 4.5 Billion years old, and the Earth was 6 thousand years old, then the former would be cratered, and the latter would not -- exactly what we observe! The guy's argument only works because the YECs believe that the Moon is also 6000 years old.

But, since we know from radiometric dating that both Earth and Moon are roughly 4.5 billion years old, what is the reason that Earth has so few impact craters? Is it because of having atmosphere, plate tectonics, erosion or something else? Thanks.

NEOWatcher
2011-Dec-30, 02:37 PM
But, since we know from radiometric dating that both Earth and Moon are roughly 4.5 billion years old, what is the reason that Earth has so few impact craters? Is it because of having atmosphere, plate tectonics, erosion or something else? Thanks.
I would say because the impact frequency is not evenly distrubuted over that 4.5 billion years. The moon existed during the heavy bombardment period.

kamaz
2011-Dec-30, 03:01 PM
The moon existed during the heavy bombardment period.

So did the Earth.

Jeff Root
2011-Dec-30, 03:36 PM
I have always found it difficult to believe that plate tectonics
was a very major direct factor, and assumed that erosion was
the primary eradicator of craters. Of course, plate tectonics
can lift up part of a continent, changing drainage patterns
which increases erosion from some places and increases
deposition in other places. There must have been few large
impacts after the late heavy bombardment era, since few
large craters are visible. Very small craters are prevented
by the atmosphere.

-- Jeff, in Minneapolis

Nowhere Man
2011-Dec-30, 03:40 PM
[url="http://scienceblogs.com/pharyngula/2011/12/nice_argument_for_the_age_of_t.php"]what is the reason that Earth has so few impact craters? Is it because of having atmosphere, plate tectonics, erosion or something else? Thanks.
All of the above. Earth got the same proportion of impactors that the moon did. But most of the craters have been eroded away over the past few billion years. And roughly 3/4 of them were in the ocean where they can't be seen.

Fred

NEOWatcher
2011-Dec-30, 05:40 PM
So did the Earth.
Yes; but the ones on the moon didn't erode.

kamaz
2011-Dec-30, 08:14 PM
Jeff Root and NEOWatcher have pointed me to the answer by mentioning Late Heavy Bombardment, so I went to read the wiki article on the subject: http://en.wikipedia.org/wiki/Late_Heavy_Bombardment

Turn out that most of these lunar craters have been created during LHB, ca. 3.8 billions years ago. Earth would get similarly cratered, but there's a twist: apparently on Earth, LHB resulted in destruction of most of the crust (melting most of the surface), which is why we don't have these craters any more. I'm inclined to buy this explanation, because Earth, being heavier, would attract more impactors than the Moon, suffering more damage. The earthly craters which were created after the crust resolidified (like those in the lunar maria) were much smaller and have eroded away since.

So as far as debunking YEC is concerned, I believe that this argument is wrong. The guy who came up with this apparently believes that the bombardment rate has been constant over the lifetime of the solar system, but this is not the case.

grapes
2011-Dec-30, 11:42 PM
I have always found it difficult to believe that plate tectonics
was a very major direct factor, and assumed that erosion was
the primary eradicator of craters. Of course, plate tectonics
can lift up part of a continent, changing drainage patterns
which increases erosion from some places and increases
deposition in other places. About three fourths of the earth surface is ocean, and ocean floor is recycled by plated tectonics quite quickly, relatively speaking. There's very little of it that is more than 200 million years old. And 3.8 billion years ago, there was even less continental crust. And, craters can be covered up by erosion, not just worn away. Plate tectonics allows that to happen, as parts are lifted.

But, to the OP, really? There are folks who accept an old moon but a young earth?

slang
2011-Dec-31, 12:27 AM
Hold on here. Something is wrong. If the Moon is cratered and Earth is not, then the simplest explanation is that the surface of the Earth is younger then the surface Moon.

It is. But that's not the point. YEC's deny tectonics and other geologic explanations anyway, so going that route is fruitless.


(We can see that rule at work on the Moon, where younger maria are less cratered than older highlands). Thus, if the Moon was 4.5 Billion years old, and the Earth was 6 thousand years old, then the former would be cratered, and the latter would not -- exactly what we observe! The guy's argument only works because the YECs believe that the Moon is also 6000 years old.

But the YEC crowd doesn't argue that the moon is 4.5 Bn years old, they believe it was "created" at the same time as the Earth. 6000 years ago!


But, since we know from radiometric dating that both Earth and Moon are roughly 4.5 billion years old, what is the reason that Earth has so few impact craters? Is it because of having atmosphere, plate tectonics, erosion or something else? Thanks.

Radiometric dating is vehemently denied to be accurate by the YEC crowd. So the "we" in "we know" excludes YEC's.


So as far as debunking YEC is concerned, I believe that this argument is wrong. The guy who came up with this apparently believes that the bombardment rate has been constant over the lifetime of the solar system, but this is not the case.

That doesn't matter at all w.r.t. to the YEC argument. If we have no such impact craters, nor historic recollection of such events, on Earth over the past 6000 years, how could they exist on the Moon, if both were 6000 years old and receive an equal ish amount of impacts?

You seem to have missed this part:


They will tell me that Earth is somewhere between 6,000 - 10,000 years old, and, when prompted, that the rest of the universe is the same age as well.

Rhaedas
2011-Dec-31, 12:34 AM
During the Archean period (2.5-3.8 billion years ago) the crust was still fluid and thin, and only protocontinents were being formed, so any major impacts wouldn't have left anything that would last until now.

kamaz
2011-Dec-31, 12:43 AM
I initially liked the argument presented in the OP, because it would be a very simple and elegant way to refute YEC claims without invoking radiometric dating. Then I realized that this argument has some very serious problems:

1. It requires explaining why the Earth is not cratered (thus my question in the OP)
2. It relies on misunderstanding of the cratering process, i.e. assuming that cratering happened at a constant rate, when in reality most of the craters were produced during LHB
3. Once we introduce the concept of bombardment period a YEC supporter will claim that LHB happened, say, 1 year after creation. (In fact, YECs have long ago developed a theory which tries to link LHB with the Flood: http://www.answersingenesis.org/articles/tj/v13/n1/crater ).
4. Refuting that requires reference to radiometric dating of Apollo samples. However, if we are to talk radiometric dating, then we could as well focus on terrestrial rocks.

Overall, the argument quoted in the OP has holes big enough to drive a truck through and cannot be substantiated without reference to radiometric dating. The fact that YEC proponents buy it anyway is amusing.

kamaz
2011-Dec-31, 12:49 AM
During the Archean period (2.5-3.8 billion years ago) the crust was still fluid and thin, and only protocontinents were being formed, so any major impacts wouldn't have left anything that would last until now.

Aha! So the Moon has solidified sooner than Earth did, and that's why the Moon has evidence of LHB while the Earth has none? That's logical: the Moon is smaller, so it should cool down faster. Thanks.

slang
2011-Dec-31, 01:06 AM
I initially liked the argument presented in the OP, because it would be a very simple and elegant way to refute YEC claims without invoking radiometric dating. Then I realized that this argument has some very serious problems:

1. It requires explaining why the Earth is not cratered (thus my question in the OP)

And science explains that quite nicely, with evidence 'n all. But that's a burden of proof issue, isn't it?


2. It relies on misunderstanding of the cratering process, i.e. assuming that cratering happened at a constant rate, when in reality most of the craters were produced during LHB

Not really. Only if you look at it from a science view, and a much older moon. It's not an argument to prove exactly how old the moon is, just that it appears to be much older than the Earth.


3. Once we introduce the concept of bombardment period a YEC supporter will claim that LHB happened, say, 1 year after creation. (In fact, YECs have long ago developed a theory which tries to link LHB with the Flood: http://www.answersingenesis.org/articles/tj/v13/n1/crater ).

This is true, of course. But then they will need to show some evidence for that. Not that they would bother. But I agree, for me the interesting thing about this argument is how they will explain it away. Because they will. In a for them convincing, and for us amusing, way.


Overall, the argument quoted in the OP has holes big enough to drive a truck through and cannot be substantiated without reference to radiometric dating. The fact that YEC proponents buy it anyway is amusing.

Not very surprising considering all the other stuff they swallow without questioning.

NEOWatcher
2012-Jan-03, 01:03 PM
It is. But that's not the point. YEC's deny tectonics and other geologic explanations anyway, so going that route is fruitless.
I guess they deny wind and rain too?

slang
2012-Jan-03, 10:23 PM
I guess they deny wind and rain too?

Probably claim it's introduced after the Fall.

(Only partly joking...)

grapes
2012-Jan-04, 09:36 AM
Keep the discussion to the topic at hand, not speculation about religion.