Don,
despite the graphical beauty of your work, I see the following issue:
You used essentially /today's/ Mars texture that is full of craters and filled it with water in the lower areas.
However Mars' atmospheric pressure is only about 1% of that on Earth. One implication of this is that many asteroids have made it down to Mars' surface in the course of time without being destroyed by the heat due to friction with the atmosphere. This is dramatically confirmed in present day Mars 'surface: a dense crater landscape. So the low atmospheric pressure and the richness in craters matches in case of Mars.
On the other hand, your terraformed Mars appears to have a much denser atmosphere more or less like Earth, I'd suppose. Now because of that dense atmosphere the Earth surface is NOT a crater landscape as we know very well...
So how did so many asteroids at the (early) times of your terraformed Mars make it through the dense atmosphere to hit the ground before burning away??
See my point?
Bye Fridger
Crater formation and erosion
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Cham wrote:Fridger,
the explication is simple : this terraformed Mars is from the future, not in the past.
OK, thanks, I obviously don't read enough SciFi stuff these days. I thought the idea was that a long time ago, Mars had a dense, earthlike atmosphere and plenty of water as recent findings seem to suggest.
Bye Fridger
I'm willing to guess that ersosional processes would need a few hundred thousand years to erase most of the smaller and mid-sized craters. The large ones would likely need a few million years. All of that is supposing that the terrafomration of Mars would create a self-sustaining geological cycle; even then, a lot of the features would probably last a long time. After all, even some realtively small craters on Earth are tens of million of years old.
I think that even a fully active Mars would retain a cratered face for a long, long time.
...John...
I think that even a fully active Mars would retain a cratered face for a long, long time.
...John...
"To make an apple pie from scratch, you must first create the universe..."
--Carl Sagan
--Carl Sagan
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Spaceman Spiff
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Off-topic, but I detect a slight misconception:
The density of atmosphere of the terrestrial planets suc as Venus, Earth and Mars is not an impediment to the formation of craters by asteroid bombardment.
In fact Venus has many well-preserved craters of sizes over 10's of km despite an atmosphere many times denser than Earth's, which is taken to indicate Venus once had plate tectonics (no huge basins remain) but weathering is now virtually non-existent.
The rarity of craters on Earth is due to water-based weathering and plate tectonics. Earth has some vestiges of very large craters left, but these typically lie in the rather immune 'cratons', the most obvious example is the one in the Hudson Bay of Canada, north of St. James' bay. There is also one in Germany some 30km across centred on Nordlingen.
Mars' craters are preserved largely because no plate tectonics seemed to have operated, the Northern plains it seems are dried ocean floors, which may have filled with sediment. Once terraformed, Mars' craters would take millions of years to erode from the 'normal map'!
Spiff.
t00fri wrote:One implication of this is that many asteroids have made it down to Mars' surface in the course of time without being destroyed by the heat due to friction with the atmosphere.
...Now because of that dense atmosphere the Earth surface is NOT a crater landscape as we know very well...
So how did so many asteroids at the (early) times of your terraformed Mars make it through the dense atmosphere to hit the ground before burning away??
The density of atmosphere of the terrestrial planets suc as Venus, Earth and Mars is not an impediment to the formation of craters by asteroid bombardment.
In fact Venus has many well-preserved craters of sizes over 10's of km despite an atmosphere many times denser than Earth's, which is taken to indicate Venus once had plate tectonics (no huge basins remain) but weathering is now virtually non-existent.
The rarity of craters on Earth is due to water-based weathering and plate tectonics. Earth has some vestiges of very large craters left, but these typically lie in the rather immune 'cratons', the most obvious example is the one in the Hudson Bay of Canada, north of St. James' bay. There is also one in Germany some 30km across centred on Nordlingen.
Mars' craters are preserved largely because no plate tectonics seemed to have operated, the Northern plains it seems are dried ocean floors, which may have filled with sediment. Once terraformed, Mars' craters would take millions of years to erode from the 'normal map'!
Spiff.
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Spaceman Spiff wrote:The density of atmosphere of the terrestrial planets such as Venus, Earth and Mars is not an impediment to the formation of craters by asteroid bombardment.
In fact Venus has many well-preserved craters of sizes over 10's of km despite an atmosphere many times denser than Earth's, which is taken to indicate Venus once had plate tectonics (no huge basins remain) but weathering is now virtually non-existent.
The rarity of craters on Earth is due to water-based weathering and plate tectonics.
Spiff.
Spiff,
are you really saying that Earth has about the same high rate of asteroid bombardment on its surface as the Moon, Mars etc? If I got you correctly, you argued that the difference is mainly that water-based weathering on Earth is very effective in washing all those many craters out.
In view of the HUGE number of visible craters on Moon, Mars and Venus, our home planet would then appear as a really dangerous place
?
Your argument concerning Venus high atmospheric pressure along with its manifest crater landscape sounds convincing.
As to the plate tectonics effecting a "washout" of craters: does one imagine that boundaries between tectonic plates that have shifted apart are filled with magma emerging from deeper regimes? If this process would happen vividly enough, a number of craters can vanish with time.
How about erosion effects on Mars associated with sand storms that are VERY frequent there? Why have the many craters not been entirely filled by sand over such a large time span? Why is water-based weathering so much more effective?
++++++++++++++
Why don't we see MANY craters in our extended deserts on Earth, where water-based weathering is virtually absent?
++++++++++++++
Questions...
Bye Fridger
Fridger,
Water weathering includes the effects due to water freezing in cracks. Apparently the expansion of the ice is much more effective in demolishing craters and mountains than is the friction due to airborne particulates.
Don't forget that these effects happen for hundreds of millions of years. Our current deserts were not deserts long ago.
Water weathering includes the effects due to water freezing in cracks. Apparently the expansion of the ice is much more effective in demolishing craters and mountains than is the friction due to airborne particulates.
Don't forget that these effects happen for hundreds of millions of years. Our current deserts were not deserts long ago.
Selden
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selden wrote:Fridger,
Water weathering includes the effects due to water freezing in cracks. Apparently the expansion of the ice is much more effective in demolishing craters and mountains than is the friction due to airborne particulates.
What does "apparently" mean in proven terms? Scientific References? You sure know, I am a "WEB-page"-click allergic
. What would be the rate of asteroid influx on Earth's /surface/ since the last ice age, if I count the numbers of visible craters on Mars and use some conservatve time estimate for its "stationary state" .
Don't forget that these effects happen for hundreds of millions of years. Our current deserts were not deserts long ago.
Certainly, I didn't forget about the effects of time. But despite the familiar changes of climate on Earth, "short" and "long" times on the scale of climatic changes should be quantitatively compared with the time scales for weathering wash-out effectiveness for craters.
Otherwise I have problems with real insight. Also I ALWAYS care to distinguish between what is KNOWN from real evidence and what is folklore. So what was the basis of your estimates?
Bye Fridger
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"Apparently" means that I couldn't find any primary references during a quick search, only what seemed to be instructional documents which did not include referencest00fri wrote:selden wrote:Fridger,
Water weathering includes the effects due to water freezing in cracks. Apparently the expansion of the ice is much more effective in demolishing craters and mountains than is the friction due to airborne particulates.
What does "apparently" mean in proven terms? Scientific References? You sure know, I am a "WEB-page"-click allergic
What would be the rate of asteroid influx on Earth's /surface/ since the last ice age, if I count the numbers of visible craters on Mars and use some conservatve time estimate for its "stationary state" .
Estimating the ages of meteor craters on Mars seems to be difficult for periods that short or that recent. All I've been able to find are descriptions which mention ages in the MegaYears.
e.g.
http://www.msss.com/http/ps/age2.html
http://www.psi.edu/projects/mgs/chron04a.html
Traces of Catastrophe by B.M.French of the Smithsonian Institution,
http://www.lpi.usra.edu/publications/bo ... intro.html
seems to have a reasonable summary of large terrestrial craters with references. From it I get the impression that the errors in determining the terrestrial crater formation rate for large meteor craters must be quite large. Chapter 1 has a histogram of known meteor crater ages: It shows fewer than 150 of any age, and only about 45 are younger than 80 MegaYears.
Estimating terrestrial influx rates from the ages of meteors found in Antartica is a topic of ongoing research.
e.g.
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2345.pdf
Don't forget that these effects happen for hundreds of millions of years. Our current deserts were not deserts long ago.
Certainly, I didn't forget about the effects of time. But despite the familiar changes of climate on Earth, "short" and "long" times on the scale of climatic changes should be quantitatively compared with the time scales for weathering wash-out effectiveness for craters.
Otherwise I have problems with real insight. Also I ALWAYS care to distinguish between what is KNOWN from real evidence and what is folklore. So what was the basis of your estimates?
Bye Fridger
I'm not sure what you mean by "estimates". Of what?
Destruction of surfaces by ice?
http://www.geo.ua.edu/intro03/Eros.html
mentions that formation of salt crystals is an important factor in erosion in dry climates, which I'd forgotten. But it includes no numeric values or references.
Selden
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Spiff, Selden,
many thanks for contributing/collecting further information on this interesting topic.
Obviously, these matters range outside the domain of my professional expertise, still my common sense as a scientist always wants to be satified. For deepening my intuitive understanding, further evidence would be much appreciated, as long as it concerns /serious/ sources that clearly separate fact and fiction.
@Selden: I really think this discussion now should be moved to Physics & Astronomy as suggested by Spiff.
Bye Fridger
many thanks for contributing/collecting further information on this interesting topic.
Spiff wrote:I am not out to make you 'admit you are wrong'.
Obviously, these matters range outside the domain of my professional expertise, still my common sense as a scientist always wants to be satified. For deepening my intuitive understanding, further evidence would be much appreciated, as long as it concerns /serious/ sources that clearly separate fact and fiction.
@Selden: I really think this discussion now should be moved to Physics & Astronomy as suggested by Spiff.
Bye Fridger
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t00fri wrote:
How about erosion effects on Mars associated with sand storms that are VERY frequent there? Why have the many craters not been entirely filled by sand over such a large time span? Why is water-based weathering so much more effective?
Bye Fridger
As luck would have it, there seems to be a large sand storm now on mars. The HiRise camera has been sending hi resolution images for almost a year now. After this storm subsides, I am sure that a few of the areas that were scanned with the camera will be rescanned and any differences or effects of the storm, if any, will be shown and can be measured.
cartrite
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Spaceman Spiff
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Grief! Leave the forum for a couple of hours, and the place gets reorganised beyond recognition! 
OK, my response once again, appended here:
In the topic "UPDATE!! - Ultimate Terraformed Mars Texture" (http://www.celestiaproject.net/forum/viewtopic.php?t=8877) on page 10, there started an off-topic query about crater erosion of terraformed planets. Questions were Asked, and at 07:30, Wed Aug 22, 2007, I posted this:
Then at 11:03, Wed Aug 22, 2007, Fridger posted:
... and so my pains for being a know it all, I try and answer:
Not quite: remove the word 'high': the Late Heavy Bombardment finished 600 million years after Solar System formation. Earth (obviously) has a similar rate of bombardment as Venus or Mars - definitely the moon - then and now. The rate is now vastly reduced: most asteroids are 'used up'. The impact basins of the moon, such as Imbrium, would have formed during the Late Heavy Bombardment. At that time, the Earth had virtually no continents, and plate tectonics would be subducting the whole crust in a few hundred million years. Yet Tycho is thought to be some 100 million years old, and there are similar sized craters on Earth that have ages of 100's of millions of years.
See above. Empirically not. Unless you count them as a source of mass extinctions and worry about that at night.
By the way, Venus has only a few hundred craters, and no basins, which has been used to date its surface to maybe 500 million years old. The (cratered) surfaces of the moon and Mars are supposed to age back to about 3.9 billion years ago.
That's one way. Sea floor craters end up being subducted and as virtually none of the sea floor is older than 200 million years, one sees that the lifetime of sea floor craters is very short compared to the Earth's.
I've already mentioned a vast crater in Canada. Most of Earth's craters are on land (or rather, continent). Land based craters can be destroyed if they are adjacent a continental margin undergoing orogenesis (e.g., the Andes), or a rift valley is forming and shield volcanoes - or worse, basalt flood plains (as in the Deccan Traps in India) - cover the crater. The only places on Earth spared this are the cratons, the old centres of the main continents. Key examples are eastern Canada, South Africa and western Australia. Only tiny areas of those pieces of crust date back to before 3.9 billion years ago - the first minicontinents that might have survived the Late Heavy Bombardment.
In the meantime, for younger craters, their basins are quicky filled in by sediments from water weathering and their mountain walls are quickly ground down - especially by glaciers if applicable. Take the Yucatan crater that is a candidate for causing the KT boundary extinction (65 million years ago). It is far from a tectonic boundary, but is completely filled in by sediment, and its walls would have been stark mountains, but these are eroded several kilometers lower. The crater is maybe 300km across yet now hidden.
Actually, you'll find large Martian craters have flat interiors: they have been filled in, but maybe by sediment not dust. I think you need water ablation (as Selden mentioned) to produce sand (and dust) in vast quantities. I think Earth has more sand and dust than Mars. It's just that most of it is wet.
By the way, the Tharsis bulge and Elysium volcanoes are shield volcanoes sitting over basalt flood plains that have covered vast areas, easily burying any basins there. Also, recently MARSIS on Mars Express found the northern plains to have many buried craters and basins underneath.
If you scroll around the Sahara on Google Earth, you'll find hundreds of dry river beds.
This geological/palaeontological stuff is widely available in any popular science books on Earth history, also textbooks since the 80's. The matter about which terrestrial planet has more craters and why is rather self-evident now. That's why I felt no need for references.
I hope that ends 'show and tell' for today!
Spiff.
OK, my response once again, appended here:
In the topic "UPDATE!! - Ultimate Terraformed Mars Texture" (http://www.celestiaproject.net/forum/viewtopic.php?t=8877) on page 10, there started an off-topic query about crater erosion of terraformed planets. Questions were Asked, and at 07:30, Wed Aug 22, 2007, I posted this:
Spaceman Spiff wrote:The density of atmosphere of the terrestrial planets suc as Venus, Earth and Mars is not an impediment to the formation of craters by asteroid bombardment.
In fact Venus has many well-preserved craters of sizes over 10's of km despite an atmosphere many times denser than Earth's, which is taken to indicate Venus once had plate tectonics (no huge basins remain) but weathering is now virtually non-existent.
The rarity of craters on Earth is due to water-based weathering and plate tectonics. Earth has some vestiges of very large craters left, but these typically lie in the rather immune 'cratons', the most obvious example is the one in the Hudson Bay of Canada, north of St. James' bay. There is also one in Germany some 30km across centred on Nordlingen.
Mars' craters are preserved largely because no plate tectonics seemed to have operated, the Northern plains it seems are dried ocean floors, which may have filled with sediment. Once terraformed, Mars' craters would take millions of years to erode from the 'normal map'!
Spiff.
Then at 11:03, Wed Aug 22, 2007, Fridger posted:
t00fri wrote:Spiff,
are you really saying that Earth has about the same high rate of asteroid bombardment on its surface as the Moon, Mars etc? If I got you correctly, you argued that the difference is mainly that water-based weathering on Earth is very effective in washing all those many craters out.
In view of the HUGE number of visible craters on Moon, Mars and Venus, our home planet would then appear as a really dangerous place
Your argument concerning Venus high atmospheric pressure along with its manifest crater landscape sounds convincing.
As to the plate tectonics effecting a "washout" of craters: does one imagine that boundaries between tectonic plates that have shifted apart are filled with magma emerging from deeper regimes? If this process would happen vividly enough, a number of craters can vanish with time.
How about erosion effects on Mars associated with sand storms that are VERY frequent there? Why have the many craters not been entirely filled by sand over such a large time span? Why is water-based weathering so much more effective?
++++++++++++++
Why don't we see MANY craters in our extended deserts on Earth, where water-based weathering is virtually absent?
++++++++++++++
Questions...
Bye Fridger
... and so my pains for being a know it all, I try and answer:
t00fri wrote:are you really saying that Earth has about the same high rate of asteroid bombardment on its surface as the Moon, Mars etc?
Not quite: remove the word 'high': the Late Heavy Bombardment finished 600 million years after Solar System formation. Earth (obviously) has a similar rate of bombardment as Venus or Mars - definitely the moon - then and now. The rate is now vastly reduced: most asteroids are 'used up'. The impact basins of the moon, such as Imbrium, would have formed during the Late Heavy Bombardment. At that time, the Earth had virtually no continents, and plate tectonics would be subducting the whole crust in a few hundred million years. Yet Tycho is thought to be some 100 million years old, and there are similar sized craters on Earth that have ages of 100's of millions of years.
t00fri wrote:In view of the HUGE number of visible craters on Moon, Mars and Venus, our home planet would then appear as a really dangerous place
See above. Empirically not. Unless you count them as a source of mass extinctions and worry about that at night.
By the way, Venus has only a few hundred craters, and no basins, which has been used to date its surface to maybe 500 million years old. The (cratered) surfaces of the moon and Mars are supposed to age back to about 3.9 billion years ago.
t00fri wrote:As to the plate tectonics effecting a "washout" of craters: does one imagine that boundaries between tectonic plates that have shifted apart are filled with magma emerging from deeper regimes?
That's one way. Sea floor craters end up being subducted and as virtually none of the sea floor is older than 200 million years, one sees that the lifetime of sea floor craters is very short compared to the Earth's.
I've already mentioned a vast crater in Canada. Most of Earth's craters are on land (or rather, continent). Land based craters can be destroyed if they are adjacent a continental margin undergoing orogenesis (e.g., the Andes), or a rift valley is forming and shield volcanoes - or worse, basalt flood plains (as in the Deccan Traps in India) - cover the crater. The only places on Earth spared this are the cratons, the old centres of the main continents. Key examples are eastern Canada, South Africa and western Australia. Only tiny areas of those pieces of crust date back to before 3.9 billion years ago - the first minicontinents that might have survived the Late Heavy Bombardment.
In the meantime, for younger craters, their basins are quicky filled in by sediments from water weathering and their mountain walls are quickly ground down - especially by glaciers if applicable. Take the Yucatan crater that is a candidate for causing the KT boundary extinction (65 million years ago). It is far from a tectonic boundary, but is completely filled in by sediment, and its walls would have been stark mountains, but these are eroded several kilometers lower. The crater is maybe 300km across yet now hidden.
t00fri wrote:How about erosion effects on Mars associated with sand storms that are VERY frequent there? Why have the many craters not been entirely filled by sand over such a large time span?
Actually, you'll find large Martian craters have flat interiors: they have been filled in, but maybe by sediment not dust. I think you need water ablation (as Selden mentioned) to produce sand (and dust) in vast quantities. I think Earth has more sand and dust than Mars. It's just that most of it is wet.
By the way, the Tharsis bulge and Elysium volcanoes are shield volcanoes sitting over basalt flood plains that have covered vast areas, easily burying any basins there. Also, recently MARSIS on Mars Express found the northern plains to have many buried craters and basins underneath.
t00fri wrote:++++++++++++++
Why don't we see MANY craters in our extended deserts on Earth, where water-based weathering is virtually absent?
++++++++++++++
If you scroll around the Sahara on Google Earth, you'll find hundreds of dry river beds.
This geological/palaeontological stuff is widely available in any popular science books on Earth history, also textbooks since the 80's. The matter about which terrestrial planet has more craters and why is rather self-evident now. That's why I felt no need for references.
I hope that ends 'show and tell' for today!
Spiff.