Valles Marineris(Grand canyon of MARS) (APOD 30 Jul 2006)

[gerald-nz]

Looking at the APOD for 30th july (above) seems to me the explanation is a bit fanciful(crack in planet?)
Looking at the same thing under a microscope on a ball-bearing I would say it's been caused by a touching scraping near-miss by another round body,notice parallel striations above and below the main canyon.

[karlhein]

Has anyone calculated whether Valis Marinaris (sic) would be above sea level or under sea level if there was surface water on Mars. I wonder what our mid ocean ridges and/or trenches at tectonic plate boundries would look like if they were visible from the air.

[JonS]

I think gerald-nz has got it right. Valles Marineris has a lot of features that are hard to explain geologically, but are consistent with a grazing collision with another body. Consider: the valley is nearly straight, tapering at the ends and deepest in the middle; the center area looks as though an explosion blew out material to the north; crater trails run parallel to the valley; such an impact would have pulverized an enormous quantity of rock, creating a global dust layer (exists); the impact would have caused titanic floods if water bodies had been present (evidence exists); material would have been blown out into space (Martian meteorites have been found on Earth); the impact probably would have fractured the crust and triggered volcanoes (nearby Tharsis region). And on and on.

Looking at the APOD for 30th july (above) seems to me the explanation is a bit fanciful(crack in planet?)
Looking at the same thing under a microscope on a ball-bearing I would say it's been caused by a touching scraping near-miss by another round body,notice parallel striations above and below the main canyon.

[orin stepanek]

http://antwrp.gsfc.nasa.gov/apod/ap060730.html
I don't know; it looks kind of like a boiled egg that cracked open. I wonder if the crust cooled too fast causing the internal pressure to crack open the Martian crust.
Orin

[aichip]

It would come as no surprise if it started as a rift from the planet cooling, but then was eroded heavily by the seas retreating on the surface. We could then see how its features would make perfect sense.

First, the non-moving crust (having no plate tectonics) would cool and crack, creating a rift. Then the water entering the crack would strike lava (and this is known to happen on Earth even today, look at the Red Sea in the last month) and would boil and create a great deal of erosion.

Now, the cooling, sinking area would have the effect of drawing more water flow into the area as it shrank and you get the many tributaries that lead into the valley.

Just looking at the processes we see here, it is easy to explain this feature. After all, anyone who knows about gravitation and the Roche limit would also know that a passing planet or asteroid could not create this valley without creating far worse and more detectable side effects.