Venus's atmosphere

The atmosphere of Venus has a surface temperature of approximately 743 kelvins and a surface pressure of about ninety Earth atmospheres. Its clouds consist largely of carbon dioxide, and droplets of sulfuric acid rain down to the surface.

Overview

The second planet from the Sun, and the Earth’s immediate inner neighbor, is Venus. The planet is often called the Earth’s twin, because the masses and radii of the two planets are very similar. Venus’s mass is equivalent to 82 percent of the mass of the Earth. Its radius is only 5 percent less than the Earth’s. Under ordinary circumstances, no objects in the sky other than the Sun and the Moon surpass Venus in brightness. Viewing Venus with one or more of the other planets also visible in the sky shortly before sunrise, or briefly after sunset, can be an awe-inspiring sight. In ancient times, since Venus could be seen in the morning skies and in the evening skies, the planet was actually thought to be two different objects; it was given the names Phosphoros and Hesperus for the morning and evening stars, respectively.

Venus revolves once around the Sun every 224.7 Earth-days. It rotates on its axis once every 243.01 days. The inclination of its orbit is about 3.5° with respect to the ecliptic plane. Its orbit, like those of all the other planets, is an ellipse, but it is very close to being a perfect circle. The tilt of Venus’s axis of rotation is about 17.8° as compared with the Earth’s incline of 23.5°. As a result, any seasonal changes of weather on Venus would be less extreme than on the Earth. Remarkably, its rotation is retrograde, or in a backward direction, as compared with the direction of revolution, or clockwise, as seen from above the ecliptic plane. It is not known with certainty why Venus has a retrograde rotation. Virtually all other objects in the Solar systemrotate and revolve prograde, or counterclockwise as seen from above the ecliptic.

At each inferior conjunction (each time Venus and the Sun are aligned in the sky with Venus closer to the Earth than to the Sun), the same face of Venus points toward the Earth. This phenomenon may mean that Venus’s rotation is influenced by the Earth’s gravitational pull. However, some information indicates that the alignment at the inferior conjunction is not exactly perfect and therefore may be coincidental.

As seen from the Earth, Venus’s maximum disk size, or angular diameter on the sky, is about 0.02° and its minimum angular diameter is about 0.003° . The maximum or minimum size on the sky corresponds to the closest and farthest distances from the Earth. In contrast, the Sun and the Moon have an apparent angular diameter of about 0.5° . Given these observational circumstances, little can be learned about the planet’s atmosphere from telescopic observations. The planet’s average density is 5.25 grams per cubic centimeter, and the surface acceleration caused by gravity is 0.903 times that of the Earth. The escape velocity from Venus’s surface is 10.3 kilometers per second. That means Venus can retain its atmosphere virtually indefinitely, since most of the molecules at the top of the atmosphere will travel at speeds well under 10.3 kilometers per second.

Through a telescope, an image of Venus is somewhat disappointing as only the planet’s phases are obvious. Exceptional atmospheric and viewing conditions at good astronomical sites have allowed scientists to see and photograph subtle variations in shading on bright cloud tops of the planet’s lit side. However, no part of the surface of Venus is visible through telescopes because of its thick cloud cover.

Sulfur is probably released into the atmosphere of Venus by outgassing volcanic processes. Sulfur rises and bonds with water and oxygen to produce sulfuric acid. It appears as a fairly thick haze and is more strongly concentrated than the acid found in the battery of an automobile. These clouds are less murky than fog, with visibilities within them of perhaps several hundred meters. The top layers of these clouds are about 80 kilometers above the planet’s surface. The tops of the sulfuric acid-laden clouds have winds that move faster than 300 kilometers per hour, a speed comparable to that of the Earth’s jet streams. These high winds swirl around the planet in about four days. Circulating motions cause gas to rise near the equator and descend near the poles, probably a direct consequence of excess solar heating in the equatorial region.

The atmosphere of Venus is mostly carbon dioxide. Venus has undergone a spectacular greenhouse effect, caused by particles in the clouds trapping or absorbing infrared radiation. Most sunlight is reflected by clouds back into space. The Albedo of Venus is about 0.8; that is, 80 percent of incident radiation striking the cloud tops is reflected back into space. The 20 percent that penetrates the clouds warms the surface sufficiently to heat the surface rocks and terrain. These surface structures radiate, essentially at infrared wavelengths, and that radiation cannot penetrate the clouds and gets trapped. Heat builds, and the high temperature releases gases (mainly carbon dioxide) from rocky minerals into the atmosphere, which in turn drives the Greenhouse effect further. A cycle develops: the carbon dioxide traps the radiation, which heats the surface and atmosphere, which then triggers the release of more carbon dioxide. This continues until the entire atmosphere reaches a sufficiently high temperature (740 kelvins) to radiate as much energy back into space as it receives.

High above the clouds, the atmospheric layers called the exosphere consist mostly of hydrogen and helium. These strata are affected by intense incoming ultraviolet solar radiation. The radiation ionizes atoms, making them electrically charged. The uppermost layers form the ionosphere. Venus’s ionosphere is not as intensely ionized as the Earth’s.

Venus’s surface is quite flat compared with that of the Earth; however, there are at least three major elevated regions, and they exhibit features that influence the atmosphere. The largest, Ishtar Terra, is similar in size to Australia. The others are about the size of the largest islands in Indonesia. Many less elevated regions are also present. On Ishtar Terra, a mountain called Maxwell Montes is apparently higher than Mount Everest with respect to the surrounding flat terrain. Large volcanoes are present on each of the three plateaus. Volcanoes imply gas release from the planetary interior; therefore, Venus’s atmosphere is attributable in part to volcanic out-gassing. Although the atmosphere is composed mostly of carbon dioxide, there is some nitrogen and sulfur dioxide, very little water vapor, and trace amounts of various other gases. Sulfur dioxide is out-gassed by volcanoes on the Earth but is quickly diluted by rain and moisture. In contrast, the sulfur dioxide out-gassed in the dry atmosphere of Venus is very stable, accounting for the efficient production of sulfuric acid in the clouds and elsewhere.

Continent-building processes caused by plate Tectonics on the Earth may have occurred on Venus, though to a far lesser extent. This idea is based mostly on the fact that only a few elevated regions exist. Therefore, the outgassing brought on by a variety of volcanic actions related to Plate tectonics is probably slower and less effective on Venus, compared with the heat-induced gases, such as carbon dioxide, from rocks.

One other feature of the Earth’s atmosphere that probably does not exist to any extent on Venus is the production of high-level auroras or, as they are known on the Earth, the northern and southern lights. This is because Venus has little or no magnetic field. Since its rotation rate is very slow, one would expect a weak but nevertheless measurable magnetic field. Several theories have been put forth to explain this lack. One theory is that, like the Earth, Venus undergoes polarity changes in its overall dipolar magnetic field. The Earth’s magnetic field is explained by the dynamo hypothesis. The rotating core produces loop currents in the heated molten regions, which in turn produce a magnetic field. Geological and paleontological evidence suggests that the Earth’s magnetic field undergoes reversals of polarity at irregular intervals. During the reversal periods, little or no field is present. It is possible that Venus could be undergoing such a magnetic reversal phase. In fact, either it is in such a phase or the dynamo hypothesis is incorrect.

Methods of Study

The first attempt to send an interplanetary probe to Venus did not fare well. Mariner 1 launched on July 22, 1962. A software error caused its booster to veer dangerously off course while low in the atmosphere, and it was deliberately destroyed by the range safety personnel at Cape Canaveral.

Mariner2, a sister spacecraft to the failed first American Venus probe, launched successfully on August 27, 1962. Fortunately, this probe was able to fly within 34,833 kilometers of the Venusian surface on December 14, 1962. Although it carried no photographic equipment, Mariner 2 provided a treasure trove of new information about the shrouded planet. The spacecraft was outfitted with Geiger tubes, an ion chamber, a cosmic dust detector, a microwave radiometer, and a magnetometer experiment. Mariner 2 determined that the Venusian surface temperature was more than 670 kelvins. It detected neither a planetary magnetic field nor any Van Allen-like radiation belts about the planet. It continued to collect data about particles and fields in interplanetary space until contact was lost on January 3, 1963, at a distance of 87 million kilometers from the Earth.

Mariner5 was launched on June 14, 1967, and was sent to fly by Venus. This spacecraft also did not include cameras or television equipment. It carried radio science and ultraviolet experiments, as well as particle and magnetic field detectors. Mariner 5 encountered Venus on October 19, 1967, coming within 4,000 kilometers. The spacecraft investigated Venus’s cloud tops and the Solar wind interacting with interplanetary magnetic fields.

Early images taken by the space probes Mariner 10 and Pioneer Venus in reflected solar ultraviolet light revealed considerable variation in shading on Venus. Variation is caused by radiation coming from different levels of the clouds. The study of the motion of these clouds has indicated that the top layers can rotate at very high speeds, approaching perhaps 100 kilometers per hour. These high-strata, rapid wind velocities are in part caused by the hot, sunlit clouds transferring heat to the colder dark side. The cloud structure shows three distinct strata: a high, thick layer; a medium-high haze layer; and a lower, medium-thick layer. From this lower layer, it is essentially clear all the way to the surface.

The Pioneer atmospheric probes were sent to Venus' surface via a combination of small retrorockets and parachutes. The Soviet Venera landers measured a decrease in the wind velocity at lower altitudes. On the surface of Venus, winds are essentially gentle breezes. Heat transfer and exchange in the atmosphere are very dependent on the density and pressure of various layers. Lower levels of the atmosphere are under super-high pressures. Soviet Venera landers and the Pioneer space probes measured pressures of about 90 atmospheres at the surface. Heat transfer is so efficient that there is no large-scale difference between daytime and nighttime temperatures at the surface. Mariner 10 and Pioneer Venus ultraviolet images indicated an overall circulation pattern: Atmospheric gas rises at the equator and descends at the poles. With slow rotation of the planet, this circulation pattern is highly stable.

Four Venera landers managed to sit down on the surface, perform experiments, and obtain electronic images of the surroundings using a fish-eye lens or wormlike view of the terrain to the horizon. At both landing sites, rocks and the horizon in the clear atmosphere are visible. Rocks are clearly of volcanic origin. In some cases, their sharp edges indicate little or no erosion, suggesting recent volcanic origin. One would expect that erosion, under the high pressure and intense heat, would quickly deform and erode the rocks’ edges. Pioneer Venus detected the existence of sulfuric acid droplets, which at the high temperatures and pressures present is very corrosive.

Pioneer Venus was equipped with a radar ranger. Scientists could send radar beams to the surface of Venus from the spacecraft and measure the time interval from the emission of the radar to the subsequent receiving of the reflected echo. This procedure allowed accurate measurement of distances between the spacecraft and the surface. After a compilation of such observations, the Pioneer Venus mission team was able to provide a detailed map of the surface terrain for the first time. Better maps would have to await a more sophisticated radar system placed in orbit about Venus.

Vega 1 and 2 were ambitious missions involving identical carrier spacecraft that each delivered both a lander based on the Venera design and an instrumented balloon to Venus before both carrier spacecraft were then redirected to join an international group of spacecraft intercepting and studying Halley’s Comet near its 1985-1986 Perihelion passage. The carrier craft were outfitted with an imaging system, an infrared spectrometer, and a Spectrometer capable of ultraviolet through infrared observations, detectors of dust and micro-meteoroids, a plasma energy analyzer, a magnetometer, wave and plasma analyzers, a neutral gas mass spectrometer, and an energetic particle analyzer. The Vega 1 and 2 carrier craft encountered Venus on June 11 and 15, 1985, respectively, having several days earlier ejected their lander and balloon payloads. Neither carrier craft provided deep new insights into the nature of the Venusian atmosphere, but they did set the stage for the unique balloon payload and their results.

Venus’s atmosphere apparently provided a particularly strong wind gust when the Vega 1 lander was still 20 kilometers above the planet’s surface. This even activated the surface experiments early, and no results were produced after touchdown. Vega 2 operated properly and on June 15, 1985, safely touched down in the eastern Aphrodite Terra region. This lander determined the local Atmospheric pressure to be 91 atmospheres, with a surface temperature of 736 kelvins. It endured the extreme environment for just under an hour, but, before it failed, the lander determined a rock sample to be a variety of anorthosite.

The Vega 1 and 2 balloons floated in the planet’s atmosphere, providing data for about forty-six hours at an Altitude of approximately 54 kilometers. The balloons were small in mass and size (25 kilograms, about 55 pounds, and 3.4 meters, or 11 feet, in diameter) but were able to dangle a gondola assembly filled with instruments to sample and measure the Venusian atmosphere. The balloons began their mission after being deposited on Venus’s dark side. They sank to a depth of 50 kilometers before rising again to an altitude of 54 kilometers where they determined the pressure and temperature to be similar to those conditions on the Earth. However, wind speed was nearly that of hurricane status, and at this altitude, the carbon dioxide-rich atmosphere had a strong concentration of sulfuric acid with far less hydrofluoric and hydrochloric acid. Before losing electrical power, the balloons registered a variable vertical component to the atmospheric winds upon which they floated. Also they survived long enough to move from the dark side to the planet’s illuminated side.

The National Aeronautics and Space Administration’s (NASA’s) next probe to Venus was named after the great Portuguese explorer Ferdinand Magellan. Its goal was no less daunting than to use an imaging radar to map at least 98 percent of Venus’s surface to a resolution of 100 meters or less. Magellan was deployed from the space shuttleAtlantis on the STS-30 mission on May 5, 1989, and sent on its way toward the inner solar system. The spacecraft arrived in Venus orbit on August 10, 1990. Its synthetic aperture radar system was able to peer through the thick atmosphere. After four years of mapping, radar altimetry, and gravitational field measurements, NASA intentionally drove Magellan through the planet’s atmosphere on October 12, 1994. In one final experiment, information was inferred about the atmosphere as the spacecraft heated up, and contact was eventually lost when Magellan was destroyed.

The European Space Agency (ESA) launched the Venus Express spacecraft on November 9, 2005. Largely a twin of ESA’s successful Mars Express spacecraft, but modified to study Venus, Venus Express entered a nine-day-period polar orbit around Venus on April 11, 2006. Then for science operations to commence, that orbit was altered to have a twenty-four-hour period. Equipped with a penetrating radar, Venus Express began generating a surface map of the planet at resolutions even better than Magellan had achieved. However, the science objectives of Venus Express also included detailed atmospheric studies. Aboard the spacecraft were infrared, visible-spectrum, and ultraviolet instruments to observe Venusian atmospheric characteristics and determine temperature profiles as a function of altitude.

Venus Express discovered a rather unexpected double vortex feature located around the south pole of the shrouded planet. This remarkable find occurred on the spacecraft’s very first highly elongated orbit about Venus. Thus, Venus Express was able to examine the planet’s atmospheric patterns in ultraviolet and infrared from a global perspective (when far from Venus) and at close range (as it approached its low point in orbit). A vortex feature had been seen previously over the planet’s north pole by earlier spacecraft, but a double vortex with a stable structure was quite unusual. Invoking the high wind speed of the upper atmosphere and the Convection of rising hot air was insufficient to explain this double vortex. Using infrared sensors, Venus Express was able to map out windows in the atmosphere through which thermal radiation could escape to space. That modeling assisted scientists in determining cloud structures as a function of altitude above the tremendously hot planetary surface.

Some earlier probes had provided circumstantial evidence that lightning was present in Venus’s atmosphere, but others produced data strongly suggesting there was a total lack of lightning. In 2006, the magnetometer aboard Venus Express provided definitive data that lightning does occur in Venus’s atmosphere.

Venus Express made another important discovery in 2008 in detecting hydroxyl molecules. This was the first time on a planet other than the Earth that hydroxyl molecules had been detected; hydroxyl is a molecular ion consisting of one oxygen and one hydrogen atom bonded together covalently. Hydroxyl was detected at an altitude of 100 kilometers above the Venusian surface, which Venus Express accomplished by means of the Visible and Infrared Thermal Imaging Spectrometer, picking up the faint infrared light emitted by these molecules in a very narrow band of Venus’s atmosphere. That band appears to be only 10 kilometers thick.

Hydroxyl has been found around comets. However, planetary atmospheres produce the molecule in a very different manner from that involved in comets. Hydroxyl on the Earth is associated with the abundance of ozone in the upper atmosphere. Thus, detection of hydroxyl in Venus’s atmosphere suggests that Venus still retains some of the Earth-like aspects. Absorption of ultraviolet light by hydroxyl molecules is important to the heating balance of any planetary atmosphere. The hydroxyl data from Venus Express would greatly assist planetary scientists in fine-tuning their models of the Venusian atmosphere.

In late 2008, Venus Express, for the first time, detected water being lost from Venus’s daylight side. The previous year, this spacecraft’s Analyzer of Space Plasma and Energetic Atoms detected the signature of hydrogen being stripped away from the planet’s night side. The orbiter’s magnetometer was used to find hydrogen dissociated from water coming off the daylight side to be lost into space. Solar wind particles penetrate Venus’s atmosphere, since the planet lacks a protective magnetic field. Scientists believe that solar wind particles break water molecules into two parts hydrogen and one part oxygen. Oxygen and hydrogen have been found escaping the night side in the right proportion, but oxygen escaping from the daylight side was not seen in the 1:2 ratio required if the hydrogen seen comes from water. In any event, the solar wind mechanism was believed by many to be the means whereby, over time, Venus lost much of its original water. Venus Express continued to orbit the planet until 2014.

Venus-bound spacecraft missions have been conducted since 1961. Since 2023 more than 40 such missions have been launched by the United States, the former Soviet Union, the European Space Agency (ESA), and the Venus-bound spacecraft missions have been conducted since 1961. Since 2023 more that 40 such missions have been launched by the United States, the former-Soviet Union, the European Space Agency (ESA), and the Japanese Aerospace Exploration Agency. Three more Venus missions are scheduled to be initiated by the early 2030s. These include two NASA missions and one by the ESA. Three more Venus missions are scheduled to be initiated by the early 2030s. These include two NASA missions and one by the ESA.

Context

The Earth’s atmosphere has the potential to become more like that of Venus. There are two ways that such a situation could develop. If the Earth moved closer to the Sun, increased solar heat would release more carbon dioxide into the atmosphere. Limestone rock (calcium carbonate) and dissolved carbon dioxide in the oceans provide a tremendous store of trapped carbon dioxide. Under higher temperatures, the rocks and seashells would chemically release carbon dioxide, and the greenhouse mechanism would raise the atmosphere’s temperature. As a result, new carbon dioxide would be released and would speed up the greenhouse mechanism. The second way that the Earth’s atmosphere could become more like that of Venus involves pollution of the atmosphere to the extent that enough carbon dioxide accelerates the existing greenhouse effect.

More generally, the study of Venus’s atmosphere helps scientists to better understand terrestrial weather and climate. The Earth has an atmosphere composed mostly of nitrogen. The weather, however, is influenced primarily by water molecules and carbon dioxide molecules. These substances are found in the Earth’s atmosphere only in trace amounts; nevertheless, they are responsible for most of the heat transfer around the globe. On Venus, the weather is controlled by the atmosphere’s main constituent, carbon dioxide. The contrasts between weather processes on Venus and those on Earth have led to a more complete understanding of the latter. In an even larger context, comparative planetology studies of Venus, the Earth, and Mars contribute to a better understanding of the Earth’s complex weather system and atmospheric physics. Moreover, such a study helps us learn why three planets, all within the Sun’s habitable zone, could evolve so differently. To understand the Earth’s evolution fully, it is necessary to know why the Venusian atmosphere became thick in carbon dioxide at great pressure (so that a planetary greenhouse effect led to runaway temperatures), and also to understand why the Martian atmosphere became thin in carbon dioxide at low pressure and low temperature. The Earth, on the other hand, developed a nitrogen-oxygen atmosphere with traces of carbon dioxide; this led to reasonable temperatures and pressures and the development of a complex biosphere and interactive oceanic-atmospheric processes to maintain a dynamic equilibrium.

In January 2013, scientists discovered that the upper atmosphere of Venus streams outward in a manner that is comparable to a comet. According to scientists, the planet's ionosphere behaves very differently as a result of changes in the Sun's weather.

Bibliography:

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. Cambridge, Mass.: Sky, 1999.

Cattermole, Peter John. Venus: The Geological Story. Baltimore: Johns Hopkins UP, 1996.

Elkins-Tanton, Linda T. The Sun, Mercury, and Venus. New York: Chelsea House, 2006.

Esposito, Larry W., Ellen R. Stofan, and Thomas E. Cravens, eds. Exploring Venus as a Terrestrial Planet. New York: American Geophysical Union, 2007.

Fimmel, Richard O., Lawrence Colin, and Eric Burgess. Pioneering Venus: A Planet Unveiled. Washington, D.C.: National Aeronautics and Space Administration, 1995.

Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2008.

Grinspoon, David Harry. Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet. New York: Basic Books, 1998.

Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2005.

Izakov, M. "Large-scale Quasi-Two-Dimensional Turbulence and a Inverse Spectral Flux of Energy in the Atmosphere of Venus." Solar System Research. 47.3 (2013): 170–181. Print.

Marov, Mikhail Ya, and David Grinspoon. The Planet Venus. New Haven, Conn.: Yale UP, 1998.

Morrison, David, and Tobias Owen. The Planetary System. 3d ed. San Francisco: Pearson/Addison-Wesley, 2003.

Oschlisniok, J. "Microwave Absorptivity by Sulfuric Acid in the Venus Atmosphere: First Results From the Venus Express Radio Science Experiment VeRa." ICARUS. 221.2 (2012): 940–948. Print.

Rosenblatt, P. "First Ever In Situ Observations of Venus’ Polar Upper Atmosphere Density Using the Tracking Data of the Venus Express Atmospheric Drag Experiment (VExADE)." ICARUS. 217.2 (2012): 831–838. Print.

Spanenburg, Ray, and Kit Moser. A Look at Venus. New York: Franklin Watts, 2002.

"Venus." NASA Solar System Exploration, 28 Sept. 2023, solarsystem.nasa.gov/planets/venus/exploration. Accessed 29 Sept. 2023.

"Venus Overview." NASA Solar System Exploration, 28 Sept. 2023, solarsystem.nasa.gov/planets/venus/overview/#: Accessed 29 Sept. 2023.