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Posted by: Juramike Jul 14 2009, 02:33 PM

space.com article on the possible detection of granite by IR signature on some of the ancient-looking plateaus:
http://www.space.com/scienceastronomy/090714-st-venus-map.html


Granite is formed by chemical processing with water in the mantle.
The lower density granite bloops up and forms continental material.

The argument is that if there was processing of subsurface rock by water, there may have been oceans and plate tectonics.

Posted by: nprev Jul 15 2009, 12:32 AM

Exciting & interesting, but gotta play skeptic.

For one thing, there's certainly nothing like a definitive spectral signature of granite here, just a putative compositional difference based on thermal emission (with a strong correlation to altitude... ) Second, exposed granite on Earth weathers away relatively quickly; I've seen some pretty rotten granite in the Rocky Mountains. Admittedly, water's not a serious player on Venus' surface, but it seems reasonable to assume that H2SO4 is pretty common, perhaps even more so at higher elevations from atmospheric precipitates?

I dunno. Guess we have to build a lander or two & go find out for sure!

Posted by: dburt Jul 15 2009, 12:58 AM

Me too, but for additional reasons. Large bodies of granitic rocks on Earth commonly are believed to have used subducted water as a flux (dissolved substance that lowers the melting point), but such a process cannot yet be demonstrated for Venus, which may have lacked both a granitic continental crust and liquid water. In addition, many other dissolved substances besides H2O, including sulfates and carbonates, can act as fluxes in magma (molten rock). Furthermore, if higher elevations on Venus have a different spectral response, this presumably could be caused by temperature effects (including coatings of frost-like mineral condensates) alone.

Even in the absence of fluxes, granitic and intermediate rocks, generally in small amounts, can be formed by the classical, experimentally proven "Bowen mechanism" - extreme fractional crystallization of basaltic magma. This mechanism is how small bodies of granite are believed to have formed on Earth's Moon, for example, in the virtual absence of water. It was once believed to be how all granites formed.

Agree - granite problem probably needs a lander or two.

-- HDP Don (edited after reading original article)

Posted by: qraal Jul 16 2009, 09:20 AM

I agree with your conclusion but sulfuric acid decomposes thermally before it ever goes near the solid surface.

Need those landers - especially some nuke-powered rovers.