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 structures. These are distinctive structural edifices that typify the venusian landscape (refer back to the table on page 19-7 to review the nomenclature). At least some of these features are related to (caused by) volcanic activity.
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.
Smaller elliptical to circular structures, known
as Corona(s) (or coronae, Latin for crown) are rimmed depressions that 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.
Coronae are widespread on the venusian surface; here are several that at this scale appear more subtle than the above views.
Russian scientists dubbed coronae with a broad web-like arrangement of linear features "Arachnoids," because of the resemblance to a spider's web.
19-31: Explain the fracture pattern in the above coronae. ANSWER
Much of Venus consists of lowland plains, as seen in this next image.
Near surface winds on Venus sometimes produce discernible (at the resolution of Magellan) wind streaks:
Close-spaced parallel ridges may be volcanic wrinkles or squeeze-ups through a series of fractures:
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:
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. This next image is typical of terrain that shows signs of extensional cracks, along with volcanic features.
Some fractures are broad and can be classed as Chasma (Chasmata also used). Here is the Gumby Chasmata:
In the radar imagery, chasmae often appear dark (low returns), as seen in this image of Delvana Chasma in Beta Regio
Here is an elevation map showing Dali Chasma, and a tributary, with directions of downward slope indicated by red arrows.
Coronas and domes often develop pronounced radial fractures. This feature is call a "nova". The next two images show the Yavine Corona - a classic example of this fracture pattern. The bottom image is a computer-generated perspective view that suggests that the fractures are actually graben.
Another example shows the structures around a volcanic crater and a superimposed impact crater.
Some structures defy easy categorization, as suggested here:
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 two examples:
A variety 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
More widely spaced ridgelike structures may be mountains or volcanic squeeze-ups or some type of upwarp.
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.
Elsewhere groups of steep mountains are probably volcanic structures undergoing erosion:
About 1000 larger impact craters have been found on Venus. All are wider than 25 km; smaller ones would have failed to form because the causative bolides burned up passing through the dense atmosphere. These structures 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 randomly distributed and have variable (uneven) concentrations.
The next scene is Golubkna impact crater,
30 km (18.5 miles) wide, which is nearly identical to large impact structures, with central peaks and terraces, 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. 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:
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.
Smaller (6 km) but still distinctive with its separate ejecta rays is this impact crater in the Eistla region of Venus.
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.
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
As with Earth, the Moon, Mars, and several Giant planet satellites, parts of Venus have been mapped in terms of units of relative ages, using crater density and superposition to determine this. Here is one map of a "quadrangle" on the venusian surface:
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 300 and 500 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:
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 active volcanism manifesting these currents 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. From observations so far, there is no clear evidence that the convections currents set up the conveyor belt tectonics that characterize Earth.
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 postulates 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 (evidence obliterated by volcanic overplating). 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.
The writer has not seen this idea put forth by planetologists: The high atmospheric and surface temperatures now on Venus would mean that surficial materials are only a few hundred degrees Celsius from attaining their melting points. With the resultant reduced thermal gradient, less subsurface heat would be needed to activate and sustain the volcanism that today dominates the venusian surface.
As of now the only active mission devoted exclusively to our sister planet is ESA's Venus Express which has spent about 500 Earth days (two venusian days) mapping the armosphere with 7 instruments. This spacecraft, shown below, was launched on November 8, 2005, arrived on April 11, 2006 and began full data collection by May of 2006. In the interim, the orbit is undergoing adjustment to be less elliptical and at lower altitudes.
The two principal objectives in this mission are to gather more detailed information about the venusian atmosphere and about possible active volcanism. The 7 instruments onboard are shown in this sketch and in the list below it:
ASPERA (Analyser of Space Plasma and Energetic Atoms)
MAG (Venus Express Magnetometer)
PFS (Planetary Fourier Spectrometer)
SPICAV/SOIR (Ultraviolet and Infrared Atmospheric Spectrometer)
VeRa (Venus Radio Science Experiment)
VIRTIS (Ultraviolet/visible/near-infrared Mapping Spectrometer)
VMC (Venus Monitoring Camera)
The first images, received on April 13, were of the south polar region of Venus, showing a vortex in the cloud pattern:
Cloud patterns vary over time during the venusian day to day: as depicted in this sequence:
Venus Express is providing better looks at cloud shapes and distribution:
This Venus Express image shows the twin vortex in the north polar region:
">There is a well-developed vortex pattern in the clouds at the south pole:
One objective of the Venus Express mission is to obtain superior temperature maps of the planet's surface, using mainly the VIRTIS sensor. This next illustration shows the first results (on the left) compared with a map of much the same area as sensed by Magellan's thermal sensor. The temperature variation involved ranges from 453° Celsius and 479° C.
Temperatures in the global atmosphere remain relatively stable and constant during the venusian day. Some variation has been noted, particularly in the cooler atmosphere, as evident in this sequence showing the south polar region as warmer (yellow/red) than the northern region (green/blue):
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.
We now move onto the last inner rocky planet - Mars - which shows an amazing diversity of features rivaling Venus and even Earth.
Primary Author: Nicholas M. Short, Sr.