Venus
Remember that in terms of physical characteristics, Venus is as close to a twin to Earth as we have in the solar system. Venus has 81.5% of the mass of Earth and its diameter is only 650 km less that of Earth's. So, if we were expecting a planet to develop a similar surface pattern and a functioning system of plate tectonics, Venus would be that planet. Yet if you look at the topography you see none of the long linear features that are the subduction zones, mountain belts and spreading ridges on Earth. What then do we see? Well, here is a slightly different version of the topography that extends from -66.5 to 66.5 degrees in latitude (the polar regions are missing) with labelled features:
The most prominent feature on Venus is a large elevated area, referred to as a highland, Ishtar Terra. It is located near the pole and also hosts the largest mountain on Venus, the 11 km high Maxwell Montes. Near the equator are two more highland areas, Ovda Regio and Thetis Regio, which together make Aphrodite Terra. Sometimes you will see these two are referred to as "continental" areas (usually with quotes) but when you examine their boundaries, you will notice that these are not like the continents we have on Earth. Because of the significant differences of large scale surface features it is generally agreed that Venus does not have plate tectonics as we do on Earth. What then does the surface show?
Volcanoes
There are many different volcanoes on Venus. Let's take a look at some interesting ones, starting with a large one:Sapas Mons
There are intermediate size volcanoes, we'll start with the Tick (not this one):Sapas Mons is a large volcano approximately 400 kilometers (250 miles) in diameter and 1.5 kilometers (.9 miles) high located on a topographic rise in Atla Regio. The summit consists of two mesas with flat to slightly convex tops and smooth surfaces that appear radar-dark in the image. The sides of the volcano show numerous bright overlapping flows that provide the edifice with a roughly radial outline. Many of the flows appear to be flank eruptions. Radial fractures clearly transect the flows to the east and south. Darker flows in the southeast quadrant are probably smoother than the bright flows closer to the eruptive center.
This image is Copyright © by Calvin J. Hamilton.
The Tick
And then there are weird pancake-like volcanoes:Scientists nicknamed this type of volcano a tick. About 65.6 kilometers (41 miles) across at the base, this volcano has a flat, concave summit 34.8 kilometers (22 miles) in diameter. The sides of the volcano are characterized by radiating ridges and valleys. The rim of the volcano to the west appears to have been breached by dark lava flows that emanated from a shallow summit pit 5.4 kilometers (3.4 miles) in diameter and traveled west along a channel. A series of coalescing, collapsed pits 2 to 10 kilometers (1.2 to 6.2 miles) in diameter is 10 kilometers (6.2 miles) west of the summit rim. The black square represents missing data.
This image is Copyright © by Calvin J. Hamilton.
Pancake Volcanoes
This cluster of four overlapping domes is located on the eastern edge of Alpha Regio. The domes average about 25 kilometers (16 miles) in diameter with maximum heights of 750 meters (2,460 feet). These features can be interpreted as viscous or thick eruptions of lava coming from a vent on the relatively level ground allowing the lava to flow in an even lateral pattern.
This image is Copyright © 2002 Calvin J. Hamilton.
Lava Flows and Lava Channels
We find many examples of lava flows and channels that must be the result of lava flow (remember - no water):Festoon Flows
A volcano on the plains between Artemis Chasma and Imdr Regio displays a sheet of thick radar-bright flows and broad flow lobes. This type of flow has been name "festoon" and only three have been found on Venus [Head et al., 1992]. The lobes and flows show prominent transverse ridges that have an average spacing of about 750 meters (2,460 feet). The flow features are associated with a complex domical structure about 100 kilometers (62 miles) across and 1 kilometer (.6 miles) in relief. They are surrounded at a lower elevation by plains surfaces that expose radar-bright volcanic deposits [Moore et al., 1992]. These materials extend some 360 to 400 kilometers (220 to 250 miles) from the volcano. They appear to overlie the radar-dark, lowland plains that dominate this region of the surface.
This image is Copyright © by Calvin J. Hamilton.
Ammavaru Lava Flows
This is a mosaic of the Ammavaru volcanic complex and associated outflow channel in the Lada Terra region. The channel displays a broad U-shaped outline across the image that extends from a collapse source on the southwest flank of Ammavaru (upper left), through reaches that are anastomosing (lower left center) and distributary (lower center), to terminal flow deposits east of a breach in the north-trending ridge (upper right). The channel is more than 1,200 kilometers (750 miles) long.
This image is Copyright © by Calvin J. Hamilton.
Deformed crust called Tessera
So far the story is one of extensive volcanism. But there are regions on Venus which appear to be composed of highly deformed crust and which may be the oldest portions of the surface. This is one example, an area referred to as Alpha Regio.Alpha Regio, a topographic upland approximately 1300 kilometers across, is centered on 25 degrees south latitude, 4 degrees east longitude. In 1963, Alpha Regio was the first feature on Venus to be identified from Earth-based radar. The radar-bright area of Alpha Regio is characterized by multiple sets of intersecting trends of structural features such as ridges, troughs, and flat-floored fault valleys that, together, form a polygonal outline. Directly south of the complex ridged terrain is a large ovoid-shaped feature named Eve. The radar-bright spot located centrally within Eve marks the location of the prime meridian of Venus. Magellan synthetic aperture radar data is combined with radar altimetry to develop a three-dimensional map of the surface. Ray tracing is used to generate a perspective view from this map. The vertical scale is exaggerated approximately 23 times. Simulated color and a digital elevation map developed by the U. S. Geological Survey are used to enhance small scale structure.
Image Number: WEB12179-2011
Credit: Image by Magellan Spacecraft, NASA/JPL
If you examine this image at full resolution you will notice a complex set of lines that cross-cut and truncate each other. This is taken as evidence of some kind of tectonic forces.
What does it all mean - Summary
It is obvious from the above descriptions that Venus is very different from the Earth. Its surface is dominated by volcanic features though there is evidence of some deformed crust. You will have noticed some craters in the images above, but not many. The image at the top of the page is of a crater. Part of the reason for the relative lack of craters is that the atmosphere is so dense that small meteorites break up during entry. But the relative lack of large craters is an indication that the surface is relatively young. Indeed, it is estimated that the surface is only 300 - 600 million years old. To put this in context, if the earliest reptiles on Earth had not been too busy mucking about in the Carbonifereous swamps and had instead developed space travel, they might have seen an entirely different Venusian surface.
So Venus was able to resurface itself, but in a very different manner than the Earth. On Earth we have the system of plate tectonics, which continuously resurfaces the planet. It has been proposed that the resurfacing on Venus occurs during a global crustal overturning event that takes place over a relatively short time. The reason for this may be the lack of surface water on Venus. Water has a great influence on many tectonic processes such as the melting point of rocks and the strength of rocks. Adding water makes it easier for rocks to flow during deformation and causes rocks to melt at lower temperatures. One way to think of this is that water lubricates plate tectonics on Earth. Without water the machine of plate tectonics is stuck. It does not work, but the radioactive decay within the planet (recall module 2) keeps producing heat. That heat eventually builds up to a critical stage where volcanic eruptions dominate the surface (the planet boils over) and subduction is wide spread. For a relative short time, ~100 million years, the planet is incredibly active and a tremendous amount of heat is released from the mantle. After that resurfacing event the planet enters another lengthy period of relative inactivity.
One major take-away of this is: Venus may be Earth's sister planet, but the siblings diverged significantly during their early history. Atmospheric differences and the lack of water led to significant differences in the tectonic evolution of the planets. We are able to live with plate tectonics on Earth. A Venusian style of resurfacing would be difficult to survive.
