navigation image map next page table of contents previous page

Magellan has confirmed that Venus, like the other planets that have not developed a surviving ocean nor have had crust destroyed by plate tectonics, is dominated by volcanism, has had some tectonic adjustments, and still has a significant number of large impacts suggesting limited modification of a possibly still original outer crust. This is well-documented by the variety of Magellan images shown on this page. Models for the development of Venus are mentioned.

The following sequence of Magellan images, each briefly described, is representative of other than strictly volcanic types of surface classes and distinctive structural features that typify the venusian landscape (refer back to the table on p. 19-7 to review the nomenclature).

The next vertical view of the venusian surface exposes two unusual structures that are given quaint names - coronae and arachnoids to set them apart - that seemingly develop from fracture patterns in the lava plains.

Magellan image of "arachnoids" and "coronae" on Venus.

Smaller elliptical to circular structures, known as Corona(s) (or coronae, Latin for crown) are rimmed depressions may be similar to ring dikes known on Earth. Other coronas may be collapsed domes. They occur in large numbers mainly in the venusian lowlands.

The Aine corona near Aphrodite Terra.

A pair of elongate coronae in the Bahet region.

A perspective made from radar altimeter data shows the structure of a typical corona.

Perspective view of a corona's topography.

Coronae are widespread on the venusian surface; here are several that at this scale appear more subtle than the above views.

Russian scientists dubbed the broad web-like arrangement of linear features "Arachnoids," because of its resemblance to a spider's web.

An arachnoid variant of the corona structure.

A typical 'arachnoid' corona with swarms of fractures, on Venus.

19-31: Explain the fracture pattern in the above coronae. ANSWER

that The first shows broad plateaus and valleys in the Galindo region, with a corona in the foreground and a volcano-tectonic structure in the background. Below it is a view of the Sedna Planitia region, with a corona in the landscape on the right. The colors are not altitudes but are color-coded expressions of the variations in emissivity that are derived from the thermal data.

Much of Venus consists of lowland plains, as seen in this next image. Beneath that is a more dissected plains at a higher average elevation.

Typical venusian plains surface.

Perspective image of broad plateaus and valleys in the Galindo region on Venus.

Near surface winds on Venus sometimes produce discernible (at the resolution of Magellan) wind streaks:

Wind streaks on the venusian plains.

Close-spaced parallel ridges may be volcanic wrinkles or squeeze-ups through a series of fractures:

Parallel (some curved) narrow features on a volcanic surface which some interpret as wrinkle ridges or squeeze-ups.

Prominent NW-SE bright surface features, possibly some kind of parallel fracturing, cut the ridges. A better look at systematic fractures in a plains surface appears below:

Examples of parallel fracturing on the surface of Venus.

These NW-SE gashes are criss-crossed by numerous small ladder-rung like fractures, which may be analogs to regularly spaced megajoints found in certain terrestrial settings.

Parallel and crossing fracture systems are commonplace on Venus. These next images are typical of terrain that shows signs of extensional cracks, along with volcanic features.

Fractures cutting the venusian surface; note the anemone volcano in the upper right.

A thin fracture system (joint-affected?) in the Hestia region.

Some fractures are broad and can be classed as Chasma (Chasmata also used). Here is the Gumby Chasmata:

The Gumby Chasmata.

In the radar imagery, chasmae often appear dark (low returns), as seen in this image of Delvana Chasma in Beta Regio

The broad dark area is Delvana Chasma; light linear features to its right are mostly fractures.

Dali Chasma at the edge of Aphrodite Terra show up as a broad, flat canyon in this perspective image.

Perspective view of Dali Chasma.

Here is an elevationmap showing Dali Chasma, and a tributary, with directions of downward slope indicated.

Elevation map of Dali Chasma.

Coronas and domes often develop pronouncedradial fractures. This feature is call a "nova". The next two images show the Yavine Corona - a classic example of the fracture pattern. The bottom image is a computer-generated perspective view that suggests that the fractures are actually graben.

The nova fracture pattern on the Yavine Corona.

Perspective view of the Yavine Corona with nova fractures.

Another example shows the structures around a volcanic crater and a superimposed impact crater.

Fracture patterns in a corona setting.

Some structures defy easy categorization, as suggested here:

A feature of uncertain nature.

Another important venusian landform is called Tesserae. This is characterized by being higher in elevation, plateaulike except for the folds it contains, and often a criss-crossing pattern of ridges or wrinkles, and/or fractures and grooves. Here are three examples:

Example 1 of a tessera surface

Example 2.

Example 3.

A varieties of landforms (Montes) that resemble linear foldbelt mountains on Earth occur mainly in the higher elevation terrains such as Ishtar Terra. Most striking are the Maxwell Mountains

Radar image of the Maxwell Mountains.

Perspective of the Maxwell Mountains made from radar altimetry data; with sky effect added

An individual mountain in the Maxwell chain.

More widely spaced ridgelike structures may be mountains or volcanic squeeze-ups or some type of upwarp.

Broad wrinkles which may be upwarps or folds.

Peculiar terrain at Ovda Regio in the Aphrodite Terra is characteristic of long low mountain ridges, running NE-SW and spaced about 20 km (12 miles) apart; a dark band is a low valley.

Magellan image of the terrain at Ovda Regio in the Aphrodite Terra on Venus.

Elsewhere groups of steep mountains are probably volcanic structures undergoing erosion:

Color-coded perspective image of the Sedna Planitia region on Venus, with individual mountains.

About 1000 larger impact structures have been found on Venus. All are larger than 25 km; those smaller would have burned up passing through the dense atmosphere. These are probably young - estimates make them younger than 500 million years in age. In contrast to other planetary surfaces containing impact craters, those on Venus are evenly and randomly distributed rather than showing variable concentrations (density frequency).

The next scene is Golubkna impact crater, 30 km (18.5 miles) wide, which is nearly identical to impact structures found on the Earth, Moon, Mercury and Mars. It is distinguishable from calderas by the prominent, strongly reflecting ejecta deposits running up to the rim.

Magellan image of the Golubkna crater on Venus.

Visible in some impact craters are central peaks, a nest of terraces, and a wide apron of ejecta which stands out as bright because of the rough scattering surfaces. These are emphasized in this computer-generated perspective view of the Golubkna crater:

Computer-generated perspective view of the Golubkna crater.

Crater Dickinson below, formed in a lava plains, is perhaps partially filled by lava (much like the Moon's Tycho) although a partly exposed central peak was not submerged:

The impact Crater Dickinson, which may be partially filled with younger lava.

An impact crater, provisionally named to honor Nurse Clara Barton (U.S. Civil War), is 50 km wide, and has a ring of central peaks.

Crater Barton, one of the largest impact structures on Venus.

Smaller (6 km) but still distinctive with its separate ejecta rays is this impact crater in the Eistla region of Venus.

Another smaller impact structure.

Most of the impact structures occur singly, essentially spread randomly over the venusian surface. However, here are three close-spaced impact structures, ranging from 35 to 50 km wide, in the Eistla region.

A trio of impact crater in the Eistla region of Venus.

19-32: Look at the image below which pictures the largest (280 km; 174 miles) crater, Mead, found on Venus. Would you identify it as volcanic or impact in origin, and give reasons why? ANSWER

The largest crater on Venus, probably volcanic in origin.

Impact crater counts have been the prime data set for estimating the age(s) of venusian surfaces. As said above, about 1000 craters have been classified as impact from Magellan studies. Most are larger than 20-30 km since smaller ones aren't produced because of burn up of the incoming bolide during atmospheric passage. Based on expected frequencies of sizes on Earth and other planets, using an asteroid/comet flux calculated for the last billion solar system years, and taking into account the random distribution of craters on Venus, most venusian planetologists think the age of the entire surface is somewhere between 300000 and 500000 million years. This says, in effect, that the present venusian surface is young geologically. This rather surprising outcome needs explanation.

That explanation is still being debated. Various models have been proposed. Most 1) reject any plate tectonics activity, and 2) hold that volcanism has been active during this time period and hence the surface has been largely repaved. Tied to this is a model for what goes on in Venus' interior. One line of evidence requires a reasonable estimate of crustal thickness. It is not yet known whether the crust is tied to Venus' upper mantle and so behaves somewhat akin to the Earth's lithosphere. Gravity data from Magellan gives some clues. (Originally, gravity measurements were not part of the experiments but after a full set of radar and other data were acquired the JPL controllers decided to lower and circularize the orbit of Magellan so that variations in its velocity would reveal gravitational variations. Here is the resulting maps of the gravity studies:

Gravity maps for Venus.

Most workers trying to explain Venus' structure think that the crust is mostly likely 20-30 km thick. A few argue for thicker crust with one upper limit being 300 km. Actual determination would require future landings of survivable seismometers to measure P and S wave velocities, etc. The strength of this crust is also conjectural but the ability of high Terrae to exist now on the surface indicates a) some significant rigidity, and b) the likelihood that there is material of similar density protruding into the mantle as a deeper root, following isostatic principles. Laboratory tests suggest that venusian crustal rocks are stronger than their terrestrial counterparts, mainly because their water contents are probably low. Thus, despite the high temperatures at the surface, which should soften rocks and favor plastic flow, Venus can maintain high terrains.

The majority opinion favors convection currents in the mantle that react and interact with the crust. Currents within the crust itself probably exist but at the moment volcanism appears miniscule. But in the last half billion years, volcanism was rampant on Venus such that older surface were covered or perhaps foundered into the crust like lava rock slabs into a lava lake. In fact, the entire surface may have been covered by melt as a lava ocean. The present surface is likely a nth generation, as this melt-extrusion process would seem to have operated several times in the more distant past.

Although no missions since Magellan have provided new imagery and data serving to better understand the planet, experts have been sifting through the Magellan output to form general hypotheses as to its history. The most significant factor is the nearly identical size of Venus and Earth. This would imply that at least some terrestrial processes might occur in a similar manner on Venus. So far, volcanism, impact, and tectonic rifting appear to dominate today's venusian surface. There is a firm belief that Venus has experienced one or more major cycles of volcanism in the last billion years. All but the higher terrains display youthful features. Speculation by some workers support Venus as having a partial- to nearly total- coverage by a water ocean in its distant past, but no evidence of sedimentary deposits has survived the replating of the surface. Water would be supplied from subsurface melting and released during surface extrusion. Some of that water would enter the atmosphere of that time and would probably be lost to space by dissociation in which the hydrogen, at least, leaves the planetary environment. The water may have partially re-entered the venusian crust or was trapped there by some process; presence of water then might have promoted some degree of plate tectonic movements. Traces of water persist in the present-day atmosphere, perhaps an indication that volcanism still continues. If an ancient ocean did indeed exist, the possibility of life (once) cannot be ruled out. But the greenhouse warming that presumably was involved in evolving the present atmosphere would likely limit that life to forms of the "extremophile" type (see page 20-12). Depending on the temperature history, and the degree of shielding from solar radiation, any life that developed almost certainly did not go beyond primitive types.

Because of the great diversity of surface features revealed by Magellan, Venus ought to remain a high priority target for space exploration well into the 21st Century. Future spacecraft will likely continue to use (higher resolution; more versatile) radar as their prime sensor.NASA is renewing the Mercury Exploration program with the August 3, 2004 launch of MESSENGER, which will make several passes near Venus and the Sun (for gravity boosts of orbital velocity) and then reach orbit around Mercury in 2008. This sophisticated satellite looks like this (artist's rendition)

The Messenger satellite; destined to orbit Mercury.

Among its instruments are 1) MDIS = Mercury Dual Imaging System; 2) GRNS = Gamma Ray and Neutron Spectrometer; 3)XRS = X-ray Spectrometer; 4) MAG = Magnetometer; 5) MLA = Mercury Laser Altimeter; 6)MASCS = Mercury Atmospheric and Surface Composition Spectrometer; 7) EPPS = Energetic Particle and Plasma Spectrometer. Among its scientific goals are learning more about Mercury's crust, core, and magntic field. You can follow this mission by tracking its home page

As of now the only active mission to our sister planet is ESA's Venus Express which will spend about 500 Earth days (two venusian days) mapping the armosphere with 7 instruments. This spacecraft, shown below, was launched on November 8, 2005 and should begin data collection by May of 2006.

Artist's view of ESA's Venus Express.

We now move onto the last inner rocky planet - Mars - which shows an amazing diversity of features rivaling Venus and even Earth.

navigation image map next page previous page

Primary Author: Nicholas M. Short, Sr. email: [email protected]