Venus's surface experiments

An understanding of the geology of the other planets in the solar system is important for understanding the geologic past and future of Earth. Venus holds many clues to this understanding, including its surface geology, which appears to be mostly igneous and basaltic in nature.

Overview

The planet Venus is considered one of the terrestrial, or Earth-like, planets because of its position in the solar system, its planetary diameter, its geology, and other characteristics. Despite being called Earth’s twin because of those similarities, Venus is actually very different. The study of the planet Venus has been, at best, difficult because of its heavy cloud cover.

The best information on the Venusian surface and its soils came early from the Soviets, who focused on exploring the planet, successfully landing six spacecraft on the surface. Even though these craft operated for only limited amounts of time because of the planet’s extreme temperatures and the pressure of its atmosphere, the data provided by them have given astronomers and geologists important clues about the soils on Venus. Much of the information obtained by the Soviets can be compared with that known about Earth and to the data obtained from firsthand examination of the lunar rocks and soils. For example, photographs can be useful in examining the appearance of the soil, rocks, and their distribution. Images taken by Venusian landers can be compared with photographs of similar materials found on Earth and the Moon.

The first of the Soviet landers to provide clues to the Venusian soils was Venera 8, which made the first soft landing on Venus, on July 22, 1972, in a region generally thought to be like the rolling plains of Earth. The probe analyzed the surface and soils directly underneath it with a gamma-ray spectrometer designed to determine the chemical composition of surface material. Results showed that the soils under the Venera 8 were igneous, or volcanic, in origin. The layer was found to be approximately 4 percent potassium, approximately 200 parts per million uranium, and approximately 650 parts per million thorium. The layer was also determined to have a density of approximately 1.5 grams per cubic centimeter (in comparison, water has a density of one gram per cubic centimeter at a temperature of 277 kelvins). From these data, astronomers and geologists were able to ascertain not only that the soils under Venera 8 were igneous but also that they were probably similar to the granites or basalts found on Earth.

In 1975, Venera 9 and 10 provided an even better look at the Venusian soils. Each lander transmitted an image that showed the soil and rocks surrounding it. These photographs revealed rocks that were, on average, twenty centimeters wide, about fifty to sixty centimeters long, and slablike in appearance. A few of the rocks showed evidence of volcanic origin. Many of the rocks had jagged edges, which demonstrates little erosion, although some did show signs of weathering. This relative lack of erosion surprised many astronomers and geologists. They had believed that, because of the planet’s extremes in temperature, atmospheric pressure, wind velocity, and chemical composition, the photographs would show well-eroded landscapes. Astronomer Carl Sagan, among others, hypothesized that low wind velocities at the surface levels of Venus produce little effect on the rock. Apparently, the Venusian surface temperature stays fairly constant and thus does not create much wind. Chemical analysis of the rocks again showed the elements potassium, uranium, and thorium. Nevertheless, the sites differed in the type of rock material. At one Venera lander site, the rocks were basaltic in appearance, similar to those lining Earth’s oceans. At the other site, the rocks were more like granite, similar to that found in Earth’s mountains. The rocks appear to be relatively young in age. This would indicate that the planet has been geologically active in the geologically recent past. The Venusian soil in the areas observed photographically appeared to be loose, coarse-grained dirt. It was also evident from the photographs that Venus (or at least parts of it) is a dry and dusty planet. Radar images from other Venera missions, as well as Pioneer Venus and the Magellan spacecraft, verified this for the rest of the planet.

The Soviets continued their studies of the Venusian surface with two additional spacecraft, Venera 13 and 14. These two spacecraft performed similar examinations but in a much more complex manner. Rather than single images, near-panoramic views of the landing sites were produced. Photographs showed rocks somewhat similar to those found at the Venera 9 and 10 landing sites. Rocks also showed evidence, however, of what appears to be thin layering, ripple marks, and fracturing, especially around Venera 14. Some rocks showed evidence of erosion. On Earth, rocks that show layering—such as sandstone and limestone—are usually sedimentary. Based on the photographs and measurements made by the spacecraft, several Soviet scientists suggested that the Venusian rocks might be sedimentary, but that has not been confirmed.

The possible cause or causes of the erosion remain unknown. In the absence of water, several possibilities have been suggested. These include chemical weathering or erosion caused by nearby volcanism and its resulting ash, dust, and lava. Chemical weathering seems the most likely explanation. Venus’s thick atmosphere has cloud layers laced with sulfuric acid, which rains down as a caustic, corrosive agent on the surface.

Both spacecraft collected a cubic centimeter of Venusian soil for analysis. The probes utilized an X-ray source to stimulate emissions from the collected soil samples. This chemical analysis revealed that the samples were similar to basalt in composition, although the basalts differed at the two sites. Near the Venera 13 landing site, the type of basalt found is referred to as leucitic high-potassium basalt, while near the Venera 14 landing site, a tholeiitic basalt, similar to that found on the ocean floors on Earth, was found. The soil itself appeared fine-grained, and the photographs revealed many small rocks. It has been speculated that this also indicates that weathering processes of some type are at work, breaking down larger rocks into smaller ones, eventually reducing them to soil.

Another pair of Soviet probes, the Vega 1 and 2 spacecraft, landed on Venus in June 1985. Vega 2 results revealed a Venusian soil and surface that are again similar to basalt. Nevertheless, the new data also revealed a surface rich in the element sulfur, which is usually associated with volcanism. This presence has provided another clue to the surface and geology of Venus.

Venera 8 landed about 5,000 kilometers east of an area referred to as the Phoebe region. Venera 9, 10, 13, and 14 all landed between 900 and 3,000 kilometers east of the raised areas known as Beta Regio and Phoebe Regio. Even though the craft landed on and took samples from an area that could be of the same or similar geologic makeup, they have given astronomers and geologists a good idea of the planet’s surface composition.

The probes produced mostly photographic data, although some chemical analysis was conducted on-site. Thus, any discussion of soil samples is based on the evidence reported by these spacecraft since no samples have ever been returned to Earth for detailed study. Spacecraft data have enabled astronomers and geologists to begin to understand not only the surface of the planet Venus and its chemical makeup but also the planet’s evolutionary path. Scientists continued to collect data about Venus in the twenty-first century. The Akatsuki spacecraft entered Venus’ orbit after being launched in 2010, and the Parker Solar Probe and the Solar Orbiter have both made multiple fly-bys of the planet.

Knowledge Gained

It appears that Venus may still be an active planet geologically, which scientists inferred from the discovery of high concentrations of sulfur in Venus’s atmosphere. Thus, its soils, for the most part, must be considered with that fact in mind.

Analysis of Venusian rocks around the landers provided scientists with interesting but sometimes confusing data. For example, the fact that most of the rocks appear to lack signs of erosion at first seemed puzzling. An understanding of the weather patterns on Venus and the planet’s atmospheric chemistry, however, has led to the development of theories relating the small-scale erosion to a low wind velocity at the surface because of its virtually uniform temperature. The rocks themselves appeared to be mostly igneous in nature. Most igneous samples appeared to be similar to basalt, much like those rocks and materials that line Earth’s ocean floors. Some of the rocks resembled granite, like those that form Earth’s mountains. However, despite apparent volcanic origins for Venus’s crust, some specimens appeared to be sedimentary. This led to further questions which remain unanswered. Although the sedimentation process on Earth is usually accomplished by water, present-day Venus has no water, nor is there any evidence of water in its near past. The origins of this phenomenon remain unknown.

Analyzed samples varied slightly from site to site, as was expected by geologists, since samples on Earth also differ. In fact, the variation of Earth samples is greater than that of the limited Venusian ones. Nevertheless, potassium—a key element in igneous and especially basaltic materials—was detected, as were uranium and thorium. Geologically, the rocks are relatively young, presenting additional evidence that Venus is a planet that may be experiencing continuous changes. Fine-grained soils were found at some sites, while coarser soils appeared at others. At one site, at least, smaller rocks led scientists to theorize that erosion does occur on the planet, thus producing soil.

Context

When the planets of the solar system are categorized, one usually finds two major groupings: the terrestrial or Earth-like planets and the Jovian or Jupiter-like planets (sometimes also called the gas giants). Venus, because of its relative size, atmosphere, position within the solar system, and surface, is naturally among the set of terrestrial planets: Mercury, Venus, Earth, and Mars.

An understanding of the nature of the terrestrial planets, their atmospheres, planetary geologies, and soils can give astronomers and geologists clues to the pasts not only of these worlds but also of our own—revealing how these planets were formed, what geological changes they have undergone, and how they might be related. Venus holds many clues to the formation of the solar system. Unfortunately, observations of the Venusian surface are nearly impossible because of the dense atmosphere that surrounds the planet. Orbiting spacecraft provide information regarding the general geologic contours on the surface—the planet’s mountains, valleys, craters, and other surface features—but hard evidence of the nature of the surface, particularly its soil, can come only from the surface of the planet. Prior to the landing of Soviet probes, no information about the Venusian surface existed.

Materials sampled and photographed in the vicinities of the Soviet landers proved to be mostly igneous in nature. Additional on-site chemical analysis showed these materials—both rocks and soils—to be similar to granite or basalt. Basalt-type materials are not unique to the second planet from the Sun. These materials have been found on Mars in the vicinities of the American Viking landers, in samples brought back from the Moon by the American Apollo crews, and by uncrewed Soviet spacecraft. As Soviet spacecraft became more sophisticated and knowledge of the harsh Venusian environment grew, landers were able to provide data on the surface of Venus, among other things. Additional information may provide scientists with clues to the past of the terrestrial planets, part of which is hidden in the Venusian surface and soil. Perhaps more important, Venusian soil information may provide clues to Earth’s future, particularly regarding our planet’s fragile environment.

Comparative planetology is essential for achieving a more complete understanding of our own planet. As Venus, Earth, and Mars started out relatively similar in the early solar system, and all three are in the habitable zone, why is it then that Venus is devoid of water and hot with a thick atmosphere of carbon dioxide, Earth is capable of supporting life, and Mars has no liquid surface water and is cold, with a thin atmosphere of carbon dioxide? Only when that question is answered will scientists have a clear idea of Earth’s complex planetary environment.

Work performed along the way to that understanding has included spacecraft dispatched to the veiled planet Venus by both the Soviet Union and the United States. American spacecraft have only flown by Venus or studied it from orbit. The Japanese Aerospace Exploration Agency and the European Space Agency have also produced orbiters that have either orbited the planet or made fly-bys. The National Aeronautics and Space Administration (NASA) has had more interest in exploring Mars than Venus. However, because the Soviets have had only bad luck when it comes to Martian exploration, they have emphasized the study of Venus with flyby craft, landers, orbiters, and even balloons temporarily floating within its hellish atmosphere. NASA announced plans to return to the study of Venus in the 2020s through the launch of two robotic probes, the DAVINCI+, a probe that will fall into the planet’s atmosphere, and the VERITAS, which will enter orbit around Venus. In the meantime, the European Space Agency’s (ESA) Venus Express began orbiting Venus on April 11, 2006, conducting mapping operations and other scientific investigations of Venus’s surface and atmosphere. In October 2012, the ESA agreed to continue funding the orbiter through December 2014. The Japanese spacecraft Akatsuki has been orbiting Venus since 2010.

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