Hanmer-Articles-Venus-vs-Earth

From OttawaRasc

"If Venus is Earth’s sister planet, why don’t they look alike geologically?"

by Simon Hanmer

Basically, it all comes down to heat and water.

Lets look at the Earth first. We usually think of our planet, with its oceans full of water and forest cover on its continents, but if you could remove these you would see an extensive system of mid-ocean ridges that are long skinny mountain chains in the ocean basins that mark where hot magma (molten rock) erupted at the surface to create new ocean floor as the continents drifted or spread apart, the mountain chains of the Cordillera and the Andes along the west coast of the American continents where the ocean floor dives down or subducts beneath the continents, and the Himalayas that formed where the Indian subcontinent collided with the rest of Asia.

This picture is the typical planetary signature of what’s called “Plate Tectonics”, the process of continental drift. The key thing to understand here is that plate tectonics is the dominant way the Earth as a planet cools its interior. This is very important for planetary geology Rocky planets contain radioactive elements such as U (that breaks down to Pb) and K (that breaks down to Argon). Radioactive breakdown releases energy that turns into heat. These radioactive breakdown reactions generate new heat inside planets like the Earth. If such a planet doesn’t have an efficient cooling mechanism and accumulates that heat inside, the planetary interior can get so hot that it can melt with catastrophic consequences!

What about Venus? As Earth’s twin it’s about the same size, and because it has a very similar density, most planetologists think it is made of similar rocks to those that make up the Earth. But if we look at Venus with Magellan radar images, we find no ocean basins, no continents, no spreading ridges, no subduction zones, no collisional mountain belts and very few impact craters! If Venus is made of the same stuff as the Earth, it probably contains radioactive elements that produce heat, so why doesn’t it look geologically like the Earth?

80% of the Venusian's surface is made of gently rolling volcanic plains, dotted with large isolated volcanoes. The statistical study of impact craters suggests that the Venusian’s surface is no older than 500 million years in other words, it was completely resurfaced by massive volcanic eruptions that completely covered all the older craters at about the time that the first fossils were forming here on Earth

So why is Venus so geologically different to its sister planet Earth? There are a number of hypotheses, but the one that makes the most sense to me says that Venus has no water, either in its interior or on the surface and this is the critical difference between it and the Earth. Rocks are made of silicate minerals like quartz, feldspar, olivine and pyroxene. Silicate lavas are very sticky and stiff when they are dry, but flow like treacle when they are wet. Solid rock can also flow if it is hot enough and given enough time. But if the rock is very dry, it’s very strong and stiff and can hardly flow at all. Planet Earth has water at the surface that gets carried into its warm interior by the ocean floor material that slides down the subduction zones at the edges of the continents. This makes the Earth’s interior soft enough to flow, thereby allowing the plates to move and letting hot mantle material rise up under the mid-ocean ridges. The consequence of this is that the Earth’s interior is able to cool albeit very slowly. But dry Venus, with its stiff interior cannot flow, so plates can’t move and heat cannot escape. The heat builds up on a time scale of 100’s of millions of years until the upper part of the interior melts.

Molten rock (lava), even dry, is still softer than solid dry rock, so a catastrophic upward movement of lava can take place and quickly resurface the entire planet in a geologically short time. This catastrophic turnover of the upper part of the planetary interior allows heat to escape, things cool off and stabilize and the planetary interior returns to its normal strong, stiff state that cannot flow fast enough to allow tectonic plates to move around. Meanwhile, radioactive heat continues to be produced until the next catastrophic event occurs 100’s of millions of years in the future

So the next time you are peering at the cloud tops of Venus through a telescope, remember that the planetary geology hidden from view is very different from what we see on Earth and mostly because there’s not a drop of water to be found anywhere on the planet inside or out!

For more information on planetary geology, you might like to consult "The New Solar System", edited by Beatty, Petersen and Chaikin. Cambridge University Press, 4th edition, published in 1999, available from Sky Publishing. This is a beautifully illustrated book, though it gets a bit technical in places.