Geology of the Solar System 5: Venus

Science!

There have been several satellite missions to Venus, so we know quite a bit about it.  It’s the second planet from the Sun, and is relatively close to Earth in terms of its distance from the Sun.  The Earth and Venus would have therefore formed in a very similar part of the solar system, making them very similar in composition according to the Solar Nebula Hypothesis.

Its rotation and orbit are even stranger than Mercury’s.  It rotates very slowly, once every 243 Earth days, and its rotation is reversed to what is seen on other planets.  It’s orbit around the sun takes 225 days, meaning that there is less than one Venus day per Venus year.

drunkenvenus

This oddity might have been caused by a glancing impact late in it’s accretionary history.

It’s size is very similar to Earth’s, and its density is 2.24 g/cm3, compared to Earth’s 2.52 g/cm3.  Because of its thick, greenhouse gas filled atmosphere, the surface temperatures can get hot enough to melt lead. The clouds obscuring its surface are composed of a tenuous mist of H2SO4 droplets (sulfuric acid). There is no appreciable magnetic field.

Internal Structure

Because it has a similar size, density, and distance from the Sun, we would expect it to have a similar composition as well.  The lack of a magnetic field could result from the core already being solid (as convection within a liquid iron core is needed to produce a magnetic field), or it could be because the planet rotates too slowly to move the materials around.

The composition of the crust is comparable to the basalts from ocean hot spots on Earth, though the high relief of volcanoes on Venus suggest highly viscous lavas, which would normally be a different composition on Earth.  Because of Venus’ high surface temperatures, its rocks are quite weak and tend to be more ductile rather than rigid or brittle.

Atmosphere

Carbon dioxide makes up about 96% of its atmosphere, with some nitrogen and other elements, and almost no water. Even though the sulfuric acid clouds reflect all but 20% of the sunlight, the carbon dioxide atmosphere has such a strong greenhouse effect that the surface is still extremely hot. Earth would have a similar atmosphere if not for its biosphere, which has locked up almost all of its carbon in rocks. The atmospheric pressure is 92 times that of Earth.

There is no evidence of liquid water having ever existed on Venus.  The H2 enriched atmosphere (there is more H2 than H1) suggests that any H2O has been boiled off, and any suface evidence of liquid water has been subsequently covered by volcanic activity.

In spite of the sulfuric acid clouds, there is no weathering on Venus’ surface from acid rain because the surface is so hot, liquid materials evaporate before they can hit the ground.

Major Geological Provinces

The surface lacks any heavily cratered terrains, suggesting that on average it is no older than about half a billion years. There are also no very small impact craters, suggesting that the Venusian atmosphere absorbs smaller bolides before they can hit the ground.

Lowlands (Yada Planitia) – 60% of Venus, with elevations below the mean planet radius (like sea level without the sea).  They are smooth and appear to be largely covered by lavas (solidified), except for a few areas with mountain belts caused by compression.

Uplands (Yada Regio) – 30% of the planet, dominated by extension and rifting, with volcanism over mantle plumes.

Highlands (Yada Terra) – 10% of the surface, and concentrated in two areas, called the Aphrodite Terra and Ishtar Terra. These are much higher above the mean planet radius than the previous two provinces, and show a lot of compressional features, suggesting they are created by downwelling and crustal thickening, rather than subduction (see above picture).

Volcanism

There are many more types of volcanoes on Venus than there are on Earth, and they are randomly and globally distributed.

Shield volcanoes tend to occur on the plains.  Some constitute huge laval fields, and they may have concentrations of several vents. Intermediate volcanoes are higher in altitude and cover smaller areas than many other volcanoes, suggesting very viscous or stiff lavas. They tend form high, steep-sided domes that look like pancakes.  Lava pancakes. Om nom nom. Large volcanoes are very similar to shield volcanoes, except that they’re… large… er. Calderas tend to be on the small side, and are formed when the crust collapses, forming a lava-spewing depression. Coronae are unique to Venus, and are formed by the extrusion of lava and sagging of crust, resulting in a circular feature with a raised, wrinkled rim and a central sag. And finally, there are also lava channels, which are pretty self-explanatory.

We haven’t been able to catch a volcano in the act of eruption, but fluctuations in Venus’ atmosphere suggest that some volcanoes are still active.

Tectonics

Tectonics on Venus (not plate tectonics like on Earth) are mostly caused by compression and extension of the crust over mantle plumes or hot spots.  There is some horizontal movement as well, but it is fairly limited to small areas. See the above picture in the geological provinces section.

This activity is caused by the planet releasing heat from its interior by convection, and results in volcanoes and upwellings over the plumes, and ridges and faults form mountains, rilles, and grabens form elsewhere by compression to compensate for the extensional movements.

Weathering and Erosion

At higher elevations, atmospheric SO2 chemically weathers Fe in the basalts, producing pyrite (shiny!). At lower elevations where temperatures and pressures are higher, oxidation occurs producing magnetite.

There is also some wind erosion and deposition as well.  Although wind speeds are fairly low on Venus, the dense atmosphere can get things moving at 1/10 the speed required to move things on Earth.

And that’s it for Venus!

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