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:
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.
