Why is this should this be so for a planet that is about the same size and density as Earth?
One researcher thinks he knows why.
In a paper published in the journal Earth and Planetary Science Letters (available online from 8th of February), J. Huw Davies, argues that in the far past, The planet we now know as Venus, was formed by the near head on collisions of two almost identical sized bodies.
Unfortunately, the paper is restricted to subscription only, but here is a brief description of his argument, which explains what we observe in Venus today. The following is taken from the above paper, and is quoted directly (with some non-relevant parts edited out) to avoid confusion by myself. Please note this work is recognized to be that of J. Huw Davies, and is copyright of the author, and of the above mentioned Journal.
Did a mega-collision dry Venus’s interior?
(a) Two hot, approximately equal-sized planetary embryos colliding close to head-on, possibly in a manner leading to the final body having a retrograde rotation.
(b) Shock waves from the collision propagating into and fluidising both bodies. The hot turbulent fluids encourage reactions, including reactions between iron and water producing hydrogen that can escape Venus.
(c) Collision continues, the two cores coalesce, and hydrodynamic escape gathers pace.
(d) Some hours to days later, the body recaptures most of its shocked outflow. It has a very hot molten core, a molten mantle with droplets of iron, a primitive crust, overlain by an iron vapour, silicate vapour and atmophilic atmospheres. Note there is no liquid ocean. There will be turbulent mixing and entrainment between the various atmospheric layers.
(e) Some thousands of years later, as the body cools further the mantle would start to solidify from its base, and remaining liquid iron would penetrate as diapirs through to the core.
(f) Millions of years later the final planet has a primarily solid mantle, a liquid core, and thick carbon dioxide atmosphere. There is no reason why Venus should not collect a ‘late-veneer’ if one was collected by Earth
Figure 1a shows the start of the near head-on collision. As the collision proceeds increasing shocked volumes of the two bodies would change phase, also the water would be released from the hydrated minerals and could react with the free iron (figure 1b). There are at least two possible reactions between water and iron,
Fe + H2O = FeO + H2; and non-stoichiometrically 6Fe + H2O = 5FeHx + FeO (where x ~ 0.4) (Badding, et al., 1991, Ohtani, et al., 2005, Yagi and Hishinuma, 1995). The resulting hydrogen would escape the body, while the iron oxide (and iron hydride) would form part of the solid body.
(figure 1c) Modelling of the Giant Moon impact suggests the production of iron vapour and that much of this vapour would rise to the surface providing plenty of opportunity for reaction with water (Canup, 2004). The opportunity for interaction would be increased in this proposed larger Venusian collision and by the rotation of the two cores through their mantles before they coalesce forming the core of the final body. The carbonates would also devolatilise, but in contrast to hydrogen Venus would retain much of the denser carbon dioxide. The high temperatures would lead to a lot of vaporization producing a very thick atmosphere, and the lighter vapour would preferentially extend in the perpendicular plane. Any water in this plane that can interact with the solar UV would dissociate and release hydrogen. One is likely to produce also an iron and silicate vapour layers at the surface beneath an atmophilic layer (figure 1d). These layers would all be very hot and turbulent allowing a lot of mixing and reacting (Benz and Cameron, 1990). The event would lead to a probably completely molten body (figure 1d), and therefore any remaining water in the interior could be efficiently outgassed. In the interior one would have a liquid silicate (magma ocean) layer, through which iron would descend as growing drops, collecting in the liquid core (figure 1 d-f), penetrating through solidified mantle as large iron diapirs (Stevenson, 1990).
(figure 1e). The opaque atmosphere would lead to a blanketing effect and would keep the magma ocean liquid for much longer than if it was covered by a thin or no atmosphere
(figure 1f). Like current proposals for Earth, Venus is also likely to have received a ‘late veneer’ (Wood, et al., 2006). The process would leave a carbon dioxide greenhouse atmosphere. Therefore the magma ocean would re-equilibrate with the atmosphere and the predicted internal water content of Venus would be related to the water content of this early atmosphere. The atmosphere would then undergo differentiation by hydrodynamic escape. This process has left its mark, especially in the isotopic ratios of gases, including noble gases (Donahue, et al., 1982, Kasting and Pollack, 1983, Pepin, 1991), and would start from a reduced water budget. This though would have only limited influence on the water content of the interior.
Here is another Graphic, this time from NewScientist that shows what may have happened.
copyright NewScientist 23rd February 2008.
To sumarise, the two colliding bodies had different spin directions and speeds, therefore, the resultant Venus ended up with a slow, retrograde spin. Chemical reactions during the collision caused water to escape from the planet, or prevented recombination of Hydrogen and Oxygen. Also water reacted with the iron to produce iron oxide, and hydrogen (which is volatile enogh to escape the atmosphere) Carbonate rocks would vaporize releasing large volumes of CO2 producing a thick greenhouse gas atmosphere. The lack of water in the mantle of the planet would now not be conducive to the plate tectonics.
The test of this theory??? Look at the rocks on Venus and measure their water content.
Badding, J.V., Hemley, R.J., Mao, H.K., 1991. High-pressure chemistry of hydrogen in metals - Insitu study of iron hydride. Science 253, 421-424.
Benz, W., Cameron, A.G.W., 1990. Terrestrial effects of the Giant Impact, in: H.E. Newsom,
Canup, R.M., 2004. Simulations of a late lunar-forming impact. Icarus 168, 433-456.
Donahue, T.M., Hoffman, J.H., Hodges, R.R., Watson, A.J., 1982. Venus was wet – a measurement of the ratio of deuterium to hydrogen. Science 216, 630-633.
Kasting, J.F., Pollack, J.B., 1983. Loss of water from Venus .1. Hydrodynamic escape of hydrogen. Icarus 53, 479-508.
Ohtani, E., Hirao, N., Kondo, T., Ito, M., Kikegawa, T., 2005. Iron-water reaction at high pressure and temperature, and hydrogen transport into the core. Physics and Chemistry of Minerals 32, 77-82.
Pepin, R.O., 1991. On the origin and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Icarus 92, 2-79.
Stevenson, D.J., 1990. Fluid dynamics of core formation, in: H.E. Newsom, J.H. Jones, (Eds),
Wood, B.J., Walter, M.J., Wade, J., 2006. Accretion of the Earth and segregation of its core. Nature 441, 825-833.
Yagi, T., Hishinuma, T., 1995. Iron hydride formed by the reaction of iron, silicate, and water - Implications for the light-element of the Earths core. Geophys. Res. Lett. 22, 1933-1936.
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