Venus

I. A brief history of Venus exploration. There have been 28 unmanned missions to Venus; some of the more significant are:

A. Mariner 4 (1962) - First interplanetary mission.

B. Mariner 10 (1973) - Photos of Venus at several wavelengths; revealed strong upper-level west-blowing winds.

C. Pioneer Venus orbiter (1978) - First topographic map (50 km resolution); first gravity map (resolution a few hundred km from -40° to 50° latitude.

D. Pioneer Venus probes (1978) - Atmospheric composition, temperature, and density with altitude.

E. Soviet Venera and Vega landers (1972 - 1984) - Crude elemental composition of soil; images of the surface.

F. Earth-based radar imagery, Goldstone and Arecibo observatories (1970's and 1980's) - Radar imagery at 1 km resolution of 1/4 of planet centered at 0° latitude and longitude.

G. Soviet Venera 15 and 16 orbiters (1983) - Radar imagery at ~1 km resolution of northern 1/4 of planet.

H. Magellan orbiter (1989) - Global radar imagery at 100 m resolution; global topography at 20 km resolution; near-global gravity at a few hundred km resolution.

II. Bulk properties of Venus:

A. Similarities to Earth:

82% of Earth's mass.

95% of Earth's radius.

Orbital radius is 0.72 AU.

Surface rocks are probably basaltic in composition.

B. Differences from Earth:

Retrograde rotation: 1 Venus day = 243 Earth days; 1 Venus year = 225 Earth days.

Dense CO₂ atmosphere, surface pressure of 90 bars.

Surface temperature of 450 C.

No significant magnetic field.

III. The major geologic terrain types on Venus.

A. Tesserae: Areally extensive (up to 1000's of km across) regions of highly deformed terrain. By definition, contains folds and fractures in two or more dimensions. This terrain type is unique to Venus, although many continental regions on Earth might look similar if erosion did not exist here. Although the deformation patterns sometimes appear complex, in many cases these patterns can be produced by only one or two episodes of deformation. Generally, tesserae appear to be stratigraphically older than surrounding terrain, rather than appearing to form from surrounding terrain. Tesserae comprise about 8% of the planet.

B. Volcanic plains: About 70% of the planet is relatively featureless volcanic plains that lie within 500 m of the mean elevation of the planet. These plains are remarkably flat, often having only tens of meters relief over 1000 km; however, unlike the great plains regions on Earth the Venusian plains have no sedimentary deposits and are more like the lunar maria. While they are relatively featureless, they are not totally featureless and do have assorted small volcanic features, fractures, wrinkle ridges, flow margins, lava channels, etc. While there are many small volcanic features on the plains, generally volcanic sources are not evident for most of the plains. Often broadly spaced fractures that form a coherent pattern for hundreds of km exist in the plains, and these fracture patterns parallel topographic uplifting or downwarping associated with formation of coronae, large volcanoes, and rifts.

C. Rifts: About 20% of the planet is covered by rift-volcano-coronae zones. Typically these zones appear to be stratigraphically the youngest features on the planet. There are several large rift systems crisscrossing primarily the midlatitudes of the planet. However, these rifts do not appear to be plate-tectonic spreading centers, but instead appear to represent extension of only a few tens of kilometers. The rifts can be up to 200 km across and 7 km deep. The rifts are typically anastamosing and have coronae embedded in them. At the Venusian equivalent of triple-junctions in the rift systems it is common for large shield volcanoes to occur.

D. Volcanoes: There are a few dozen very large shield volcanoes on Venus, often located at intersections of rift zones. They typically rise a few km above the surrounding areas, and have flows extending a few hundred km from the summit. There are hundreds of smaller volcanic features, ranging in size from small cones a few km across to features 100-200 km across.

E. Coronae: These are volcano-tectonic structures that are unique to Venus. Typically a few hundred km across, they are characterized by raised concentric ridges surrounded by a moat. Within the concentric ridge are often volcanic structures and radial fractures. These features are often within the rift zones, and the rifts divert around the coronae.

F. Ishtar Terra: This is a unique region on the planet about the size of Australia. It contains of the only elevated plateau on the planet, Lakshmi Planum, which has two large calderas within it. Lakshmi Planum is surrounded by the only true mountain belts on the planet. The tallest of these, Maxwell Montes, rises 10 km above the surrounding terrain. These mountain belts grade into surrounding tesserae.

G. Other miscellaneous features:

1. Ridge belts - a few sections of plains are crisscrossed by subparallel belts of ridges separated by tens of kilometers.

2. Large flow fields - There are a few effusive lava flows with dimensions and morphology similar to flood basalt features on Earth.

IV. Impact craters and implications for the resurfacing history:

A. The mean age of the surface.

1. There are ~900 impact craters on the surface.

2. The smallest impact crater is ~ 2 km in diameter, and atmospheric shielding prevents or hinders all but the largest meteoroids from making it to the surface to form a crater. Consequently, reliably age-dating the surface using impact craters can be done only with the population of craters larger than about 32 km in diameter.

3. Based on the number of craters per unit area larger than 32 km in diameter the "production age" of the surface is estimated to be about 500 My, with a factor of two error bars. The production age is how long it would take for the planet to accumulate the observed crater population if the planet started off with zero craters. If the planet is continually undergoing resurfacing at a constant rate then the production age is roughly the average age of the planet's surface.

B. The distribution of impact craters.

1. The spatial distribution of impact craters by itself appears spatially random; however, when overlaid on global topography or a geologic terrain maps a distinct trend appears: the rift-volcano-coronae zones are low in impact craters, particularly near large shield volcanoes.

2. There are two end-member models that one can think of to explain the global distribution of craters:

1. The planet was highly geologically active, and then suddenly “shut down” over a short period of time. Since that time only limited geologic activity has occurred. Craters have been emplaced randomly over time, and very few have been eliminated.

2. Volcanic and tectonic resurfacing occurs in small areas in a spatially random fashion, so craters are both being emplaced and being eliminated in a spatially random fashion, and the result is a spatially random crater distribution. Under this model one would expect to see a lot of craters in the process of getting eliminated, or partially filled with lava.

3. At first glance only a few percent seem to have been volcanically filled or tectonically deformed, suggesting that most of the craters have remained unmodified since emplacement. However, subsequent in-depth examination using stereo-derived topography revealed that craters with dark floors are shallower than bright-floored craters. Because most craters have radar-dark floors, it appears that most craters have been partially volcanically embayed.

4. Obviously embayed craters are spatially associated with the edge of crater-deficient areas, suggesting that those craters are on the edge of a recently resurfaced area.

V. The global gravity field.

A. There is a strong correlation between gravity highs of up to a few hundred mgal and large shield volcanoes. This suggests that most of the large shield volcanoes are currently active and supported by mantle plumes.

B. Most tesserae regions are significant topographic highs with very low gravity signals. This suggests that many tesserae regions are isostatically supported regions of thicker-than-average crust.

VI. The resurfacing history of Venus:

1. To account for the dark-floored craters, the distribution of craters, and observations of the different terrain types on the planet, the current versions of the end-member models used to account for the nearly spatially random distribution of craters can be modified as follows:

1. The planet has pretty much shut down geologically, and current activity is mostly confined to the volcano-rift-coronae zones. The shut down was slow enough that a lot of craters got partially filled with lava, but few were eliminated.

2. The planet is still very geologically active everywhere, but the way that craters get eliminated varies with geologic terrain type.

2. For the first end member, the big-picture scenario is the following:

1. The large amount of strain, their location at the bottom of the stratigraphic column, and their isostatic compensation all suggest that the tesserae are ancient remnants of a global volcanic-tectonic regime different from that which exists today. Perhaps plate tectonics existed on Venus in the past but ceased about a billion years ago as the mantle cooled.

2. The uniformity of elevation and crater density, and the coherent nature of deformation over large areas suggest that most of the plains were emplaced in a relatively short period, perhaps over 100-200 m.y.. Perhaps a global volcanic flooding of low-lying areas occurred after the cessation of plate tectonics.

3. The large gravity signals associated with shield volcanoes, and the stratigraphically young appearance of the rift-volcano-coronae zones suggest that these are currently active geologic features. The current volcano-tectonic regime has been dubbed "hot-spot" tectonics: Heat is transferred from the mantle by conduction at the base of the lithosphere with active volcanism over large mantle upwellings. Only limited horizonal movement of the surface occurs due to some dragging back and forth by underlying mantle convection.