Venus's surface features

Enormous strides have been made in understanding the nature of the surfaces of the solid bodies in the inner solar system and the processes that have shaped them. Venus, however, has been a particularly difficult planet to study. The picture that is emerging suggests that Venus may be the only member of the four terrestrial planets, besides Earth, that remains geologically active.

Overview

Of all the terrestrial planets, Venus is the most similar to Earth in size and geologic composition. At 12,258 kilometers in diameter, it is only slightly smaller (by 511 kilometers) than Earth, and its density is within 2 percent of being identical. It lacks the polar flattening, equatorial bulge, and planetary that Earth exhibits.

Geologic study of Venus is exceedingly difficult because the planet is perpetually shrouded from view by thick clouds. Its surface has never been photographed by Earth-based telescopes or from in orbit above the planet. However, in 1990 the Magellan spacecraft began generating high-resolution radar images of Venus’s surface and thereby began producing detailed maps with a of approximately 100 meters. A few panoramic photographs taken by several Soviet spacecraft have revealed the barren, rocky character of Venus’s surface in the proximity of their landing sites. However, a very high surface temperature averaging 750 kelvins has limited the operating life spans of spacecraft that have landed on Venus to a maximum of about two hours. Still, Venus has been the subject of very persistent research by space scientists and has yielded enough data about its topography and composition to permit informed speculation about the processes responsible for creating its surface. Scientific knowledge of the planet’s geologic features rests primarily on techniques involving radar imaging, while preliminary impressions of the chemical and structural nature of the surface have been provided through experiments conducted by Soviet spacecraft at several different landing sites.

The Venusian surface is generally smoother than that of any other terrestrial planet. Sixty percent of it lies within 500 meters of Venus’s mean radius of 6,051 kilometers. Because Venus has no equivalent to sea level, the mean radius is used as the baseline elevation for topographic measurements. Despite this prevailing uniformity, Venus does have some high mountains and deep valleys. The total range between the highest and lowest points on the planet is nearly 14 kilometers, a value that is similar to Earth’s.

Planetologists divide Venusian topography into several distinct types of terrain. Rolling plains dominate the globe and form an irregular, planet-girdling area covering more than 70 percent of the surface. About 16 percent of the surface lies below the level of the rolling plains. The remainder is divided among upland plains (0.5-2.0 kilometers above mean radius) and several types of true highlands. Among the latter are the regios, also called domed uplands. These are large, roughly circular areas that rise gently toward their centers, where they achieve heights of between 3 and 5 kilometers above the mean radius. They are thought to be situated over interior “hot spots,” which have caused the surface to bubble outward on a gigantic scale. Huge shield volcanoes sit atop many of the domed uplands, a fact that adds credibility to the theory that these landforms are similar to volcanic domes on Earth. Alpha Regio and Beta Regio, the first two surface features identified by Earth-based radar studies, are examples. Surfaces of the domed uplands are generally smooth, like those of the rolling plains, and appear to be the same age. Unlike plains, however, they seem to be crisscrossed by fault lines indicating crustal stresses. Two continent-sized highland areas lie within the mapped region of the surface, but together they account for only 8 percent of the planet’s surface. Ishtar Terra and Aphrodite Terra, about the size of Australia and Africa respectively, exhibit a rich variety of landscapes, including two types of mountainous terrain as well as areas of flat and complex plains.

The most common highland topography is one of ridges and valleys that intersect in chevron-shaped or chaotic patterns. This terrain is called tessera terrain, from the Greek word for “mosaic tile,” and resembles the deformation patterns that occur on the top of a moving glacier; there are no glaciers on Venus, however. In contrast, the tesserae are more dramatic but less common mountain systems that thrust their peaks 4 to 12 kilometers above the mean radius. The Maxwell Montes region of Ishtar Terra, which includes the highest known point, 11,800 meters above mean radius, is an example of the latter. It consists of a series of parallel ridges and valleys 15 to 20 kilometers apart. The Maxwell system appears much like the ridge and valley province of the Appalachian Mountains, although it is substantially higher. Its features, and those of at least three other mountain chains on Ishtar Terra, closely resemble those produced when plates of the Earth’s crust are thrust together by tectonic forces.

Contrasting with the mountain ranges are great rifts that cleave the surface to depths of up to 2 kilometers. One huge rift system stretches from east to west for more than 20,000 kilometers and can be traced as a series of chasms along the entire southern edge of Aphrodite Terra. From there, it continues across the rolling plains to link with Beta Regio. Another rift splits Beta Regio and continues to Phoebe Regio. This complex system consists of many related but distinct chasms, the largest of which is 3,500 kilometers long and 100 kilometers wide, with its deepest point lying 2.1 kilometers below the mean radius.

Among the most interesting surface features thus far discovered are Venus’s large volcanoes. Two excellent examples are Thea and Rhea Mons, which, along with several other volcanoes, rise from the Beta Regio dome. They appear to be situated on a fault that forms one edge of the great rift. The mass of material that has issued from them is greater than the total output of the volcanic mountains that have formed the Hawaiian Island chain. Both are shield volcanoes like Olympus Mons on Mars, formed by chronic, nonexplosive eruptions of lava that flow long distances before solidifying. Radar images show what may be geologically recent lava flows from both Thea and Rhea, and there is intriguing but very controversial evidence that these volcanoes may have been in eruption in the 1950s and again in the 1970s. Since that time no direct evidence of has been found by even the long-lived Magellan orbiter.

Impact craters, so characteristic of the surfaces of the Moon and Mercury and even fairly common on Mars, are apparently not nearly so plentiful on Venus. More than one hundred have been observed in radar images, ranging in diameter up to a maximum of 144 kilometers. However, the number of craters discovered is considered low, and the largest crater is only modest in size. These facts are considered to be important evidence that the present Venusian surface is not an ancient one. An old surface should bear numerous scars of past encounters with meteors, comets, and asteroids, as is the case with the Moon and Mercury.

Another class of circular geologic features bearing superficial similarity to impact craters is apparently an unrelated phenomenon. This group comprises the so-called coronae, of which at least eighty have been found. They average 500-800 kilometers in diameter, but their depth is only 200-700 meters, a fact that is inconsistent with an impact origin. Many researchers interpret the coronae to be collapsed bubbles in the crust, caused by localized heating from hot spots in the mantle beneath.

Many Soviet spacecraft failed, but four successfully took panoramic photographs of their landing sites. These pictures are remarkably similar in showing barren landscapes dominated by flat-topped rocks with relatively little loose, fine-grained material that might be described as soil. Closer inspection of images of the rocks shows that they seem to be partially exposed outcroppings of a horizontally layered rock mass that exhibits a marked tendency to break into platelike slabs. Additional measurements indicate that the rocks are of low density (1.5 grams per cubic centimeter) and high porosity. They have a bearing strength of only a few kilograms per square centimeter, meaning that they can be broken rather easily. These findings are regarded as surprising, for they are characteristic of sedimentary rocks on Earth. Under close inspection, the panoramic photographs seem to support this conclusion, showing what appear to be striations, indicating ripple marks and crossbedding, two common features of sedimentary deposits. Even the electrically nonconductive properties of the rocks, as revealed by their radar reflectivity, agree well with the behavior of sedimentary rock. If the surface rocks are indeed of sedimentary origin, they presumably formed from deposits of windblown sand.

The velocity of surface winds is low by terrestrial standards, not exceeding 1.3 meters per second. However, under Venus’s dense atmosphere, which is ninety times heavier than Earth’s, this velocity is more than sufficient to move fine-grained materials and raise dust. At a number of sites on the rolling plains, researchers have detected depressions that seem to be filled with volcanic ash, which may become lithified over time by yet unknown processes.

The mass of the planet, the density of its surface materials, and the relative abundance of certain elements present in its rocks all point to the likelihood that Venus, like Earth, experienced a planet-wide “meltdown” early in its history. The result was differentiation, a process in which the lighter elements migrated to the surface and the heavier elements settled toward the center. Escape of the residual heat from that meltdown is presumed to have been the major architect of the surface features observed on Venus, just as it has been on Earth. Whether the interior remains molten has not been determined. Venus unquestionably lacks a planetary magnetic field; because such a field is thought to be generated by planetary rotation around a molten iron core, this lack suggests that the interior of Venus has cooled and solidified. However, the evidence that volcanism has occurred on the surface in recent geologic times contradicts this view. It may be that Venus, which rotates 243 times more slowly than does Earth, simply does not spin fast enough to create the dynamo effect that gives rise to a magnetic field.

Methods of Study

The study of Venus by optical telescope has not been productive in elucidating the nature of the planet’s surface. The first successful attempts to penetrate Venusian clouds were made in 1961. Teams of American, British, and Soviet scientists were able to use large antennas to beam radar waves at the planet and receive faint return echoes. Earth-based radar studies of Venus have continued but are seriously limited by the fact that good results can be achieved only when the planet passes near the Earth, at which time Venus always presents the same “face.”

To gain a global picture of the Venusian terrain, the National and Space Administration (NASA)’s Pioneer Venus orbiter and the Soviet Venera 15 and 16 spacecraft carried radar-imaging instruments into orbit around Venus. In principle, the American and Soviet spacecraft operated similarly, combining synthetic aperture radar (SAR) imaging with radar altimeter measurements. The SAR images from Pioneer Venus cover 70 percent of the surface but are only about as detailed as those shown by a desktop physical relief globe. Veneras 15 and 16 reached Venus in late 1983. Equipped with larger radar antennas, they were capable of resolving surface details as small as one to two kilometers in size and could measure elevations to within 50 meters. Venera radar images look remarkably like high-altitude black-and-white photographs. Unfortunately, this imaging was obtained only for the northern quarter of the planet (from 90° to 30° north latitude).

Beginning in late 1970, a series of soft landings on Venus were made by Soviet Venera craft equipped to conduct experiments to detect the presence of certain rock-forming minerals. One approach used a gamma-ray to detect emitted by radioactive uranium, thorium, and potassium. Venera landers of the 1980s employed an automated drill that bored into the surface to obtain samples not contaminated by chemicals from the atmosphere or from the lander itself. The sample material was transferred to an automated laboratory inside the craft, where it was subjected to X-ray fluorescence. Results showed that the rocks at most of the landing sites appear to resemble basalt, an enriched with iron and magnesium. The exact composition, however, varied from site to site and, while not identical to that of Earth basalts, was chemically closer to Earth rocks than to Moon rocks.

At two locations, the experiments detected minerals more characteristic of granite, another common igneous rock. These findings are not seen to be in conflict with evidence that Venusian surface rocks may be sedimentary in nature, for the experiments detected the presence and ratio of identifying minerals but not the type of that contained them.

No evidence of water on the surface of Venus has been observed, but two discoveries suggest that such has not always been the case. If water was ever plentiful, it must have boiled away, so that the water molecules were dissociated into oxygen and hydrogen. Investigators have sought evidence for the “missing” oxygen and hydrogen, and some believe that they may have found both. Deuterium, a hydrogen isotope, has been detected to be one hundred times more abundant in the Venusian atmosphere than it is on Earth. Meanwhile, an experiment has shown that oxidized terrestrial basalts, when heated to the Venusian surface temperature, appear identical in visible and micrometer imagery to the surface rocks of Venus. The likeliest source for the deuterium and the oxygen to oxidize the is dissociated water molecules.

An intriguing possibility exists that Venus may harbor active volcanoes—perhaps the largest in the solar system as the large Martian volcanoes are extinct. Spacecraft and Earth-based observations have detected large amounts of sulfur dioxide, a common volcanic effluent, in the Venusian atmosphere. Moreover, sulfur dioxide content increased dramatically in the 1950s and again in the 1970s.

The first color pictures taken on the surface by the Venera 13 lander seemed to show that the landscape had an orange or amber tint, which proved to be an effect of sunlight filtered through the heavy overcast. Computer processing of the photographs has since shown that, in normal white light, the rocks are a uniform, colorless gray.

Previous spacecraft had performed preliminary radar investigations of Venus’s surface, identifying the major types of features on that surface and identifying prominent examples of each. However, high-resolution maps of the entire surface were lacking. The goal of NASA’s Magellan spacecraft was to use a synthetic aperture radar in prolonged orbit about Venus to produce a global map at a resolution even in excess of the best contemporary maps of Earth’s surface. Detailed geological interpretations and altimetry data were obtained in the process.

Magellan launched aboard the space shuttle Atlantis on May 4, 1989, and was the primary payload of the STS-30 mission. The spacecraft was deployed from the shuttle’s cargo bay, and was dispatched on a trajectory that concluded with orbital insertion about Venus on August 10, 1990. Magellan entered a highly elliptical orbit, often ranging from as little as 300 to as much as 8,500 kilometers above the surface. With Magellan in a polar orbit, the planet Venus rotated underneath it, thereby allowing the spacecraft to image a different ground track on each low pass. The spacecraft turned toward Earth as it climbed toward its highest orbital point and then transmitted the radar imagery it had collected during its low pass over Venus.

Science activities were slightly varied with each of Magellan’s six cycles, with the spacecraft’s orbit occasionally being altered for different research requirements. During the first cycle, Magellan concentrated on global radar mapping and imaged 84 percent of Venus. Later cycles filled in gaps and concentrated on specific features of interest. At a lower orbital late in its operational mission, Magellan was able to collect precise gravitational data as Venus slightly altered the spacecraft’s orbital parameters. Magellan was used to test aerobraking techniques by having the spacecraft fly through the upper portions of Venus’s atmosphere; its large solar panels experienced a retarding torque due to atmospheric drag. How the spacecraft responded to the atmosphere indirectly informed scientists about Venus’s atmospheric particle density as a function of altitude. With its primary and extended missions completed, flight controllers decided to send Magellan plunging into the upper atmosphere to remove it from orbit. Maneuvers were conducted to force the spacecraft’s orbit to decay due to orbital drag. On October 11, 1994, the final spacecraft maneuver was conducted. Controllers lost contact with Magellan the following day. Then, on October 14, Magellan was destroyed in the atmosphere. Although it could not be verified, many believed pieces of descending debris survived long enough to impact the surface.

Perhaps the most important result of Magellan’s intense investigation of Venus was leading researchers to determine a total lack of plate based on the two primary processes observed on Earth. Instead of continental drift and basin floor spreading, experts asserted that Venus’s global rift zones and coronae moved as a result of upwelling and subsidence of in the planet’s mantle. This suggested that Venus’s surface was quite young geologically speaking, perhaps less than 800 million years old. The enormous data set from Magellan was made available to interested researchers and individuals on compact disc. However, researchers published a study in Nature Astronomy in 2023 that suggested Venus may have experienced a period of tectonic activity.

Despite the extensive research with Magellan, many questions remained to be investigated. The European Space Agency (ESA) dispatched its Venus Express spacecraft to Venus in order to examine the planet’s atmosphere at infrared wavelengths. Venus Express launched on November 9, 2005, and was inserted successfully into Venus orbit on April 11, 2006. Its Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) was used to identify the amount of sulfur dioxide in the atmosphere between 35 and 40 kilometers and to monitor that constituent for changes in concentration over time that would indicate active volcanism. Venus Express’s for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) used stellar methods to determine the identity of atoms and molecules in the upper atmosphere at an altitude between 70 and 90 kilometers. SPICAV saw rapid drops in the amount of sulfur dioxide in the upper atmosphere, strongly indicating that Venus has active volcanoes. VIRTIS was then used to identify hot spots on the surface. Venus Express continued to collect data until it was decommissioned in 2014.

Context

In order to understand a system as complex as a terrestrial planet, it is necessary to have more than one example of how such planets function and evolve. Hence, the study of geologic processes on another world, far from being simply an esoteric and impractical inquiry, holds promise for improving human understanding of the forces that have acted on Earth and still continue to shape its surface. For this reason, the goal of Venusian geological studies is to discover the relationships and sequences of events that have resulted in the landforms that can be imaged. “Looking at Venus is like running the experiment that produced the Earth a second time,” according to Robert Kunzig, senior editor of Discover magazine. Indeed, scientists appreciate the opportunity to “run the experiment” again under slightly different conditions in order to see whether their set of explanatory theories can accommodate any observed deviations in the results. Venus presents a marvelous opportunity to test the plate tectonics theory on a planet that has many fundamental similarities to Earth but also exhibits numerous significant differences.

It is generally believed that Earth’s loss of internal heat is primarily a result of and occurs mainly along the 75,000-kilometer-long mid-ocean ridge. Seafloor spreading that results is responsible for producing a large expanse of young and renewable crust. Even the older continental masses are invigorated by the tectonic activity that is driven by this convective heat loss. Venus seems to have experienced similar horizontal crustal movements in the past and may still be experiencing them. The driving force, however, might be quite different. Most authorities interpret the present surface as having been formed through the release of heat at localized hot spots.

Another fundamental question is whether rocks making up Venus’s vast, rolling plains differ significantly in composition from those of the higher terrain. This issue is of interest because it bears on where and how crustal materials originated. It is not inconceivable that Venus could reveal hitherto unknown relationships that may have acted on Earth when the original continental rocks were first solidifying some 3.8 billion years ago.

The mystery of whether Venus once had oceans is of particular interest for two reasons. First, most scientists now accept as true evidence that Earth’s own atmosphere is beginning to warm as a result of increases in content. There is growing concern that atmospheric pollution may trigger a runaway “greenhouse effect,” similar to the process that appears to have happened on Venus. If Venus retains any “memory” of conditions before it became so hot, it will only be in the record of the rocks themselves. Second, scientists are still uncertain about how Earth got its abundant water in the first place. If Venus had oceans at a former time, that fact would have significant implications for theories of the origin of planetary water.

A large amount of the Venusian surface was mapped by Pioneer Venus’s radar, but its images were good enough only to show gross features of the surface and could not address cause-and-effect relationships. Better imaging has been obtained by the Arecibo radio telescope and the Soviet Venera 15 and 16 spacecraft, but the total area covered was too small to permit generalizations to be drawn. Together, all these data allowed planetary scientists the luxury of asking better questions, which might then be answered by the higher-resolution Magellan spacecraft’s synthetic aperture radar imaging system. Magellan produced a spectacular increase in knowledge of Venusian topography. Magellan imagery provided planetary scientists with the information needed to correlate the roles of volcanic activity, tectonic motion, and impact events in the formation and evolution of Venus’s surface features. Magellan established that some surface features resulting from tectonics and Venusian volcanoes have been active in recent geologic time, but some key questions are likely to remain unanswered until more complicated surface experiments can be conducted.

In June 2013, the ESA's Venus Express orbiter reported evidence that high speed winds on the surface of Venus increased in speed between 2006 and 2012. This was determined in part through an analysis of cloud features. According to researchers, the winds of Venus blow at super-hurricane-force.

By 2024, more than forty Venus missions had been launched by NASA, the European Space Agency, the former-Soviet Union, and the Japanese Aerospace Exploration Agency (JAXA). As of 2024, JAXA’s Akatsuki satellite remained the sole Venus orbiter since the decommissioning of the Venus Express in 2014. Nonetheless, Akatsuki has, since December 2015, been providing data on the Venusian atmosphere. Akatsuki's story is an interesting one. It was launched in May 2010 but initially failed to Venus orbit. Instead, it circled the sun for the next five years. In 2015, Japanese space engineers were able to maneuver the spacecraft to place it in Venus orbit, albeit a different one than originally planned. Akatsuki has contributed to the scientific body of knowledge through its imaging of Venus’ surface and cloud patterns. It is able to do this with a package of optics that include infrared, electro-optical, and ultra-violet cameras. Although Akatsuki has exceeded its planned service life, by early 2024 JAXA had not announced a mission termination date and it continued to collect data in Venus's orbit.

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