Friday, May 04, 2007

Venus: the forgotten planet

I was going to bed last night, and it occurred to me that I never hear much about Venus. We read about Mars, Jupiter, Saturn, Pluto (not even a planet!), Mercury ... but not Venus. Venus, the planet the most like earth -- save for one minor detail -- no tectonics. Oh, and a bit of greenhouse gas issue ...

A conspiracy? Probably not, since Wikipedia has a great article (emphases mine):
Venus - Wikipedia, the free encyclopedia

... About 80% of Venus' surface consists of smooth volcanic plains. Two highland 'continents' make up the rest of its surface area, one lying in the planet's northern hemisphere and the other just south of the equator. The northern continent is called Ishtar Terra, after Ishtar, the Babylonian goddess of love, and is about the size of Australia. Maxwell Montes, [named after James Clerk Maxwell] the highest mountain on Venus, lies on Ishtar Terra. Its peak is 11 km above Venus' average surface elevation; in contrast, Earth's highest mountain, Mount Everest, rises to just under 9 km above sea level. The southern continent is called Aphrodite Terra, after the Greek goddess of love, and is the larger of the two highland regions at roughly the size of South America.

... Venus has a number of unique surface features. Among these are flat-topped volcanic features called farra, which look somewhat like pancakes and range in size from 20–50 km across, and 100–1000 m high; radial, star-like fracture systems called novae; features with both radial and concentric fractures resembling spiders' webs, known as arachnoids; and coronae, circular rings of fractures sometimes surrounded by a depression. All of these features are volcanic in origin....

...Venus has several times as many volcanoes as Earth, and it possesses some 167 giant volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. However, this is not because Venus is more volcanically active than Earth, but because its crust is older. Earth's crust is continually recycled by subduction at the boundaries of tectonic plates, and has an average age of about 100 million years, while Venus' surface is estimated to be about 500 million years old...

... on Venus, about 85% of craters are in pristine condition. The number of craters together with their well-preserved condition indicates that the planet underwent a total resurfacing event about 500 million years ago. ....Without plate tectonics to dissipate heat from its mantle, Venus instead undergoes a cyclical process in which mantle temperatures rise until they reach a critical level that weakens the crust. Then, over a period of about 100 million years, subduction occurs on an enormous scale, completely recycling the crust. [jf: I think this means the turnover starts ever 500 million years, but it takes 100 million years to exhaust the mantle heat and restabilize the crust.]

... Venus has an extremely thick atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The pressure at the planet's surface is about 90 times that at Earth's surface—a pressure equivalent to that at a depth of 1 kilometer under Earth's oceans. The enormously CO2-rich atmosphere generates a strong greenhouse effect that raises the surface temperature to over 400 °C (752°F). This makes Venus' surface hotter than Mercury's, even though Venus is nearly twice as distant from the Sun and receives only 25% of the solar irradiance.

Studies have suggested that several billion years ago Venus' atmosphere was much more like Earth's than it is now, and that there were probably substantial quantities of liquid water on the surface, but a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere. Venus is thus an extreme example of climate change, making it a useful tool in climate change studies.

... The permanent cloud cover means that although Venus is closer than Earth to the Sun, the Venusian surface is not as well heated or lit. In the absence of the greenhouse effect caused by the carbon dioxide in the atmosphere, the temperature at the surface of Venus would be quite similar to that on Earth. ...

... lack of an intrinsic magnetic field at Venus was surprising given that it is similar to Earth in size, and was expected to also contain a dynamo in its core. A dynamo requires three things: a conducting liquid, rotation, and convection. The core is thought to be electrically conductive, however. Also, while its rotation is often thought to be too slow, simulations show that it is quite adequate to produce a dynamo. This implies that the dynamo is missing because of a lack of convection in Venus' core. On Earth, convection occurs in the liquid outer layer of the core because the bottom of the liquid layer is much hotter than the top. Since Venus has no plate tectonics to let off heat, it is possible that it has no solid inner core, or that its core is not currently cooling, so that the entire liquid part of the core is at approximately the same temperature. Another possibility is that its core has already completely solidified.

... If viewed from above the Sun's north pole, all of the planets are orbiting in a counter-clockwise direction; but while most planets also rotate anticlockwise, Venus rotates clockwise in "retrograde" rotation. The question of how Venus came to have a slow, retrograde rotation was a major puzzle for scientists when the planet's rotation period was first measured. When it formed from the solar nebula, Venus would have had a much faster, prograde rotation, but calculations show that over billions of years, tidal effects on its dense atmosphere could have slowed down its initial rotation to the value seen today.

A curious aspect of Venus' orbit and rotation periods is that the 584-day average interval between successive close approaches to the Earth is almost exactly equal to five Venusian solar days. Whether this relationship arose by chance or is the result of some kind of tidal locking with the Earth, is unknown....
The major flaw with this article is it doesn't make clear why Venus lacks plate tectonics. There's a throwaway line about "dry crust", so maybe the theory is that Venus' solar proximity led to early water loss, the loss of water led to arrested plate tectonics (and to the CO2 accumulation and greenhouse gas effect?), then arrested plate tectonics and greenhouse gases led to a volcanism dominated globe with a hot ultra-dense atmosphere ...

I think The Onion recently suggested Dick Cheney is Venusian, which probably explains the GOP's attitude towards CO2 accumulation.

PS. If you know a planetary person, can you ask them to edit the article to clarify the tectonics/CO2/volcanism relationships?

PPS. I had a high school science teacher who was a fan of Velikovsky's "When World's Collide". Messed up my thinking for a few months. So I thought it was curious that we still don't have confident explanation for the retrograde orbit. (Alas, Velikovsky's explanation was inconsistent with basic physics.)

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