Our knowledge concerning the surface of Venus comes from a limited amount of
information obtained by the series of Russian Venera landers, and primarily
from extensive radar imaging of the planet. The radar imaging of the planet
has been performed both from Earth-based facilities and from space probes.
Photos showed mountains, craters, and plains and that most of the rock is basalt which is typical of volcanism.
The most extensive radar imaging was obtained from the Magellan orbiter in a 4-year period in the early 1990s. As a consequence, we now have a detailed radar picture of the surface of Venus. The adjacent animation shows the topography of the surface as determined using the Magellan synthetic aperture radar (black areas are regions not examined by Magellan). An MPEG movie (303 kB) of this animation is also available. Magnellan could detect detail as small as 100m.
One can also measure the smoothness of the surface. The smoother regions appear darker when radio waves are impinged on such areas because they can fly off in an arbitrary direction. For very rocky and rough surfaces, the radio waves bounce around, and have less directions that the waves can escape so they tend to come back in the direction they came. (Think about watering a flat surface with a powerful hose, the water sprays all over the place. The water would be like the radio waves.) Thus smooth surfaces tend to look darker than rough surfaces with the radar imaging technique.
65 percent of Venus is ROLLING PLAINS and the rest is highlands and few percent mountains.
The rolling plains are smooth lava flows but with about 1000 craters. The craters are spread uniformly over the surface (in contrast to our moon) and look new. None are older than 500 million years. This suggests that the older ones were washed out by lava flows in prior periods of geological activity.
Also the craters are all large. This is because the small meteorites are dissipated in the thick atmosphere and do not reach the surface.
Volcanos are common on Venus and show up bright in radar images but they are not found in chains as on the Earth.
Thus Volcanism is not the result of plate motion, sea spreading or SUBDUCTION ZONES as on Earth but up-welling from hot-spots. The peaks are shallow and called SHIELD VOLCANOS with very fluid lava.
Volcanism usually occurs on Venus in circular CORONAE (not to be confused with stellar coronae!!). These are 2000km bulges where molten lava/magma has up-welled the surface and then retracted, and allowed the surface to pull back. PANCAKE DOMES are evidence for lava outflow in these regions. One usually sees a network of concentric fractures in these regions.
There are lava channels on Venus, one is as long as the Nile river.
There are two "continents", which are large regions several kilometers above the average elevation. These are called Ishtar Terra and Aphrodite Terra. They can be seen in the preceding animation as the large green, yellow, and red regions indicating higher elevation near the equator (Aphrodite Terra) and near the top pole (Ishtar Terra).
That is, we see faults and wrinkled mountain ranges, but this seems to be motion within a single pliable plate rather than interaction between separate rigidly moving plates.
Why the two planets differ in this aspect of their geology even though we believe them to have similar interiors is not well understood. The usual explanation is that Venus is a little behind the Earth in geological time scale, and its tectonic activity is just getting started.
It is thought that the crust is less dense on Venus than that on the Earth and so this accounts for it being more pliable. It is also very hot so it is not very stiff or rigid and is more bendable.
Earth loses 70 percent of its heat through the rifts between plates, venus gets rid of its heat through volcanos that rise from within the crust and coronae regions of volcanic activity.
It is thought that rather than losing heat through motion between plates as on Earth, venus loses heat by the periodic sinking of parts of pliable crust, with lava overflowing such regions. This is the OVERTURN model.
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| Hemispheres of Venus (Ref) |
The center image (a) is centered at the North Pole. The other four images are centered around the equator of Venus at (b) 0 degrees longitude, (c) 90 degrees east longitude, (d) 180 degrees and (d) 270 degrees east longitude. The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 spacecraft. (Here is a more extensive discussion of these hemispheric views.)
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| A Volcano (Ref) | Apparent Lava Flows (Ref) |
In all of
these radar images you should bear in mind that bright spots correspond to
regions that reflect more radar waves than other regions. Thus, if you could
actually see these regions with your eyes the patterns of brightness and
darkness would probably not be the same as in these images. However, the basic
features would still be the same.
There are rift valleys as large as the East African Rift (the largest on Earth). The image shown below illustrates a rift valley in the West Eistla Region, near Gula Mons and Sif Mons. However this is not believed to be the separation between two rigid plates (as it would be on the Earth) but rather a weakening and spreading within a single plate, perhaps on its way to form two plates.
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| Rift valley on Venus |
The perspective in cases like this is synthesized from radar data taken from different positions in orbit.
The East
African Rift on Earth is a consequence of tectonic motion between the African
and Eurasian plates (the Dead Sea in Israel is also a consequence of this same
plate motion). Large rift valleys on Venus appear to be more a consequence of more
local tectonic activity, since the surface of Venus still appears to be a
single plate.
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A Field of Craters | The Largest Crater (Ref) |
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| The surface of Venus from Venera 14 (Ref) |
1. Venus formed only slightly closer to the sun than Earth. Its density and size tell us it should have dense interior but we are puzzled by absence of magnetic field. (25,000 times weaker than that of Earth).
This also means that Venus is NOT protected from the solar wind and the solar wind penetrates very close to the planet developing a BOW SHOCK much nearer to Venus than the similar effect produces on Earth and a denser ION TAIL (see fig 23-12 of text) for Venus.
2. Cratering shows large craters and young cratering impacts compared to our moon but old compared to Earth. This is based on frequency and structure of impacts. Also there are no small craters.
3. There was lava flooding but no water flooding because the water was dissociated by the hot surface temperature which evaporated liquid water and then UV radiation from the Sun dissociated the water.
4. Slow surface evolution: There is little surface erosion because of the absence of water, but there is volcanic activity and lava flows. The overturn model for crust motion may explain how Venus loses heat and could maintain a solid core, and avoid having a strong magnetic field. (Solid core means no dynamo to sustain the field, as discussed earlier.)
