Venus plate tectonics

Plate tectonic activity, which is responsible on Earth for subduction zones, spreading centres and obducted ophiolites, as well as associated ore deposits of Cu, Cr and Ni described in 8.6, appears to have been less significant on other terrestrial planets. As a result, local enrichments of these and other transition elements (apart from Fe and Ti) are probably absent on the Moon, Mercury, Venus, Mars and the asteroids. Since Fe and Ti minerals are predominant on terrestrial planets, electronic spectra of Fe2+ and Fe3+ in silicates and oxides influenced by Ti4+ and Ti3+ are expected to dominate remote-sensed spectra of their surfaces. 

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

Venus is similar in size to the Earth and might be expected to have differentiated to a similar extent. However, while the early accretion history might have been similar (with the exception of the absence of a moon-forming event), silicate differentiation did not proceed according to the familiar plate tectonic mechanisms. There is, of course, no data on the interior of Venus, and so planetary degassing characteristics must be deduced from limited atmospheric data and observations of volcanic activity at the surface. 

Relatively recent (<1 Ga) volcanic resurfacing of Venus may have erased earlier evidence for plate tectonics, and there is some discussion on whether Mars may have experienced sea-floor spreading early in its history (e.g. Connemey et al. 1999 see also Stevenson 2001). 

Water, in liquid oceans, is what makes Earth s tectonic history different from Venus and Mars. On Earth oceanic crust and hence plate is cooled quickly by water, because the surface is close to 0 C, not 500°C as on Venus. This cooled plate thickens and becomes dense more quickly than it would on Venus. Plates fall into the astheno-sphere as a steady regular process. Andesite volcanism is fluxed by water given off by subducted oceanic crust. Mars is so cold that reintroduction of water to the interior does not occur. The only volcanism in the past billion years on Mars appears to have been from deep-rooted plumes. 

Neither Mars nor Venus has well-defined smoothly moving plates, thousands of kilometres long from ridge to subduction zone. Plate tectonics needs water to help cool the new plate. 

This mixing turns the Earth inside out and changes its chemistry. One theory says that we have so much nitrogen in our atmosphere because it was released from the mantle via the motion of plate tectonics. Mars and Venus have no plate tectonics and no nitrogen. Other gases like CO and hydrogen sulfide and subterranean minerals saturate the water at Earth s seams, providing a constant input of heat and chemicals for life to use. 

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