Atmosphere of Venus

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Atmosphere of Venus
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Cloud structure in Venus' atmosphere, revealed by ultraviolet observations.
General information
Height	250 km
Average surface pressure	92 bar
Composition
Carbon dioxide	96.5 %
Nitrogen	3.5 %
Sulfur dioxide	0.015% (150 ppm)
Argon	0.007% (70 ppm)
Water vapor	0.002% (20 ppm)
Carbon monoxide	0.0017% (17 ppm)
Helium	0.0012% (12 ppm)
Neon	0.0007% (7 ppm)

Venus, the second planet from the Sun, has an atmosphere very different from that of Earth. In comparison to Earth it is much denser, heavier, and extends to a much higher altitude. Optical density of the atmosphere makes the surface impossible to see from outside without observing through radar mapping or other means, and due to the high albedo the surface was not imaged until the Magellan probe arrived in 1989.

Despite the harsh conditions on the surface, at about a 50 km to 65 km level above the surface of the planet the atmospheric pressure and temperature is nearly the same as that of the Earth, making its upper atmosphere the most Earth-like area in the Solar System, even more so than the surface of Mars. Due to the similarity in pressure, temperature and the fact that breathable air (21% oxygen, 78% nitrogen) is a lifting gas on Venus in the same way that helium is a lifting gas on Earth, the upper atmosphere has been proposed as a location for both exploration and colonization. ^[1]

[edit] Extent and composition

Venera 7, the first probe to survive the high atmospheric pressure of Venus and transmit data from the surface, though only for 23 minutes.

The atmosphere is divided into a number of sections depending on altitude. The troposphere begins at the surface and extends up to 65 kilometres (an altitude at which the mesosphere has already been reached on Earth). The bottom of this layer is the furnacelike surface with stifling air pressure and slow winds, and the top is where the temperature and pressure reaches Earth-like levels and clouds pick up speed to 70 m/s or more.

The stratosphere and mesosphere extend from 65 km to 95 km in height, and the thermosphere and exosphere begin at around 95 kilometres^[2], eventually reaching the limit of the atmosphere at about 220 to 250 km. Note that as the exosphere is the area in which the atmosphere of a planet blends into space, it is not possible nor useful to define it in exact terms. Even on the Earth itself the boundary of the exosphere is not clearly defined, extending to somewhere around 500 to 1000 km above the surface, but with an upper boundary reaching somewhere around 10,000 km.

The probe Venus Express arrived at Venus in April 2006 has recently shown through stellar occultation that the atmospheric haze extends much further up on the night side than the day side. On the day side the cloud deck has a thickness of 20 kilometres and extends up to about 65 kilometres, whereas on the night side the cloud deck reaches 90 kilometres in altitude, continuing even further to 105 kilometres as a more transparent haze. ^[3]

The atmosphere is composed mainly of carbon dioxide, along with a small amount of nitrogen and other trace elements. The amount of nitrogen is relatively small compared to the amount of carbon dioxide in the atmosphere, but as the atmosphere is so much thicker than that on Earth, the total amount of nitrogen on both planets is about the same, even though on Earth nitrogen makes up about 78% of the atmosphere. ^[4] The air pressure compared to Earth at the surface is immense: about 92 times that of the Earth, similar to the pressure found 910 metres below the surface of the ocean. In fact, the temperatures and pressures found on Venus's surface are so high that the carbon dioxide is technically no longer a gas, but a Supercritical fluid.

The enormous amount of CO₂ in the atmosphere creates a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 500 °C, hotter than any other planet in the solar system, even that of Mercury in spite of being located further out from the Sun and receiving only 25% of the solar energy that Mercury does. The average temperature on the surface is above the melting points of the metals lead (327 °C), tin (232 °C), and zinc (420 °C). The thick atmosphere also causes there to be little difference in temperature between the day and night side, in spite of the fact that the slow retrograde rotation of the planet causes a single day to be the length of 243 days on Earth, and thus 121 days of darkness before the sun rises again behind the clouds.

90% of the atmosphere of Venus is within 28 km of the surface, but as the mass of the atmosphere is so much greater, even above this area the atmosphere is of a higher atmospheric pressure than the surface of Earth until one reaches an altitude of over 50 kilometres. In comparison with Earth, the atmosphere of Venus has a total mass around 90 times greater, and while the atmosphere of Venus stays relatively thick past 50 kilometres and above, 90% of the atmosphere of Earth is within 10 kilometres of the surface. ^[5] On Earth, those who have reached 80 kilometres above the surface are designated by the United States as being astronauts, and the Kármán line, commonly used as the boundary between Earth and space, is located 100 kilometres above the surface. In comparison, on the night side of Venus clouds can still be found at 80 kilometres above the surface.

The area of the atmosphere most similar to Earth is at an altitude slightly above 50 km. According to measurements by the Magellan probe, the area from 52.5 to 54 kilometres has a temperature between 20°C and 37°C, and the area at 49.5 kilometres above the surface is where the pressure becomes the same as Earth at sea level ^[6]. Note that the atmospheric pressure on Earth also varies depending on altitude, and many inhabited locations have much less air pressure than locations at sea level. La Paz, the capital of Bolivia at 3600 metres has a pressure approximately 61% of that of the surface, and many locations are at higher levels, the highest being the town of Wenzhuan in Tibet, China where the atmospheric pressure is only about half that at sea level. As manned ships sent to Venus would be able to compensate for differences in temperature to a certain extent, anywhere from about 50 to 54 kilometres or so above the surface would be the easiest area in which to base an exploration or colony, where the temperature would be in the crucial "liquid water" range of 0°C to 50°C and the air pressure the same as habitable regions of Earth.

Though the planet's atmosphere is primarily composed of carbon dioxide, its minor constituents contain a range of interesting compounds, including some based on hydrogen, nitrogen, and sulfur as well as the large amounts of carbon and oxygen. Large amounts of the planet's Hydrogen are theorised to have been lost to space ^[7], with the remainder being mostly bound up in sulfuric acid (H₂SO₄) and hydrogen sulfide. Subsequently, hydrogen is in relatively short supply in the venusian atmosphere.

[edit] History

Through studies of the present cloud structure and geology of the surface combined with the fact that the temperature of the Sun has increased by 25% since around 3.8 billion years ago^[8], it is thought that the atmosphere of Venus up to around 4 billion years ago was more like that of Planet Earth with liquid water on the surface. The runaway greenhouse effect may have been caused by the evaporation of the surface water and the rise of the levels of greenhouse gases that followed. Venus' atmosphere has held a great deal of attention due to this by those studying climate change on Earth. ^[9]

There are no geologic forms on the planet to suggest the presence of water over the past billion years. However, though there is no direct evidence to support the existence of oceans on the planet, there is no reason to suppose that Venus was an exception to the processes that formed Earth and gave it its water during its early history, possibly from the original rocks that formed the planet or later on from comets. The common view among research scientists is that water would have existed for about 600 million years on the surface before boiling off, though some such as David Grinspoon believe that up to 2 billion years could also be plausible. ^[10]

[edit] Observations and measurement from Earth

Venus transits the face of the Sun on June 8 2004, providing valuable information on the upper atmosphere through spectroscopic measurements from Earth.

The upper atmosphere of Venus can be measured from Earth when the planet crosses the sun in a rare event known as a solar transit. The last solar transit of Venus occurred in 2004. Using quantitative astronomical spectroscopy, scientists were able to analyze sunlight that passed through the planet's atmosphere to reveal chemicals within it. As the technique to analyze light to discover information about a planet's atmosphere only first showed results in 2001^[11], this was the first opportunity to gain conclusive results in this way on the atmosphere of Venus since observation of solar transits began. This solar transit was a rare opportunity considering the lack of information on the atmosphere between 65 and 85 kilometres.^[12] The solar transit in 2004 enabled astronomers to gather a large amount of data useful not only in determining the composition of the upper atmosphere of Venus, but also in refining techniques used in searching for extrasolar planets. The atmosphere of mostly CO₂, absorbs near-infrared radiation, making it easy to observe. During the 2004 transit, the absorption in the atmosphere as a function of wavelength revealed the properties of the gases at that altitude. The Doppler shift of the gases also enabled wind patterns to be measured. ^[13]

A solar transit of Venus is an extremely rare event, and the last solar transit of the planet before 2004 was in 1882. The next solar transit is in 2012. After the solar transit in 2012 however, another one will not occur for 105 years.

[edit] Wind

The wind on Venus varies by altitude, to a much greater extent than that of winds on Earth. The wind near the surface is much slower than that on Earth. The winds on the surface move at only a few kilometres per hour (generally less than 2 m/s and with an average of 0.3 to 1.0 m/s ^[14]), but due to the high density of the atmosphere at the surface this is still enough to transport dust and small stones across the surface, much in the same way that a slow-moving current of water is able to move objects.^[15] In the cloudtops however, the wind speed suddenly picks up, reaching up to 95 m/s at the cloud top level. These high-speed winds circle the planet approximately every four days in a phenomenon known as "super-rotation". ^[16] As regular convection driven by differential solar heating would only create winds of a few metres per second it is not yet clear what causes the extreme and sudden change in wind speeds at the higher altitudes. It is hoped that the European Space Agency's probe Venus Express will shed light on this.

The wind speed at the equator is faster than near the poles, which is the reason for the typical lopsided V-type shape in the clouds that can be seen in many images of the planet.

A discovery of a large 'double-eyed' vortex at the south pole of Venus was made in the summer of 2006 by Venus Express soon after arriving at the planet from observations obtained from a distance further out than its current orbit, allowing it to observe the planet as a whole. Vortices exist at the poles on Earth and other planets, and with the super-rotation of the winds of Venus at high altitudes combined with recycling of hot air in the atmosphere confirmation of the existence of the vortex at the south pole came as no surprise, but the two eyes in the vortex have yet to be explained. ^[17] The first vortex on Venus was discovered at the north pole by the Pioneer Venus mission in 1978. ^[18]

[edit] Clouds

Photograph taken by the Galileo spacecraft en-route to Jupiter in 1990 during a Venus flyby. Smaller scale cloud features have been emphasized and a bluish hue has been applied to show that it was taken through a violet filter.

Clouds above the CO₂ layer are thick and are composed of sulfur dioxide and droplets of sulfuric acid. ^[19] These clouds reflect about 60% of the sunlight that falls on them, which is what obscures the surface of Venus from regular imaging. The reflectivity of the clouds causes the amount of light reflected upward to be nearly the same as that coming in from above, and a probe exploring the cloud tops could harness solar energy almost as well from below as above, enabling solar cells to be fitted just about anywhere. In addition to this, the extremely slow rotation of the planet would mean that a flying craft could maneuver itself so as to never experience night.

The cloud cover is such that very little sunlight can penetrate down to the surface, and the light level is only around 5000 lux with a visibility of three kilometres. At this level little to no solar energy could conceivably be collected by a probe. Humidity at this level is 0.1 percent. ^[20] In fact, due to the thick cloud cover the total solar energy received by the planet is less than that of the Earth.

Sulfuric acid is produced in the upper atmosphere by the sun's photochemical action on carbon dioxide, sulfur dioxide, and water vapor. Ultraviolet photons of wavelengths less than 169 nm can photodissociate carbon dioxide into carbon monoxide and atomic oxygen. Atomic oxygen is highly reactive; when it reacts with sulfur dioxide, a trace component of the Venusian atmosphere, the result is sulfur trioxide, which can combine with water vapor, another trace component of Venus' atmosphere, to yield sulfuric acid.

CO₂ → CO + O

SO₂ + O → SO₃

SO₃ + H₂O → H₂SO₄

Sulfuric acid rain that falls on Venus never reaches the ground, instead evaporating due to the heat before it reaches the whole way down in a phenomenon known as virga. ^[21]

[edit] Magnetic field

The magnetosphere shields the Earth from charged particles in the solar wind. The Venusian magnetic field is not nearly so strong, a mere 0.000015 times that of Earth

Due to its slow rotation and its estimated lack of internal thermal convection, Venus has a much weaker magnetic field than that of Earth. As a magnetic field keeps solar wind from knocking particles out of the atmosphere and into space, Earth has kept a relatively large supply of lighter elements in its air. On Venus however, it has been theorized that the much weaker weak magnetic field (only 0.000015 times that of Earth ^[22]) has caused lighter gases to be knocked out of the atmosphere, leaving only carbon dioxide and small traces of other elements and molecules.

[edit] Possibility of life

Due to the harsh conditions on the surface, little of the planet has been explored; in addition to the fact that life as currently understood may not necessarily be the same in other parts of the universe, the extent of the tenacity of life on Earth itself has not yet been shown. Creatures known as extremophiles exist on Earth, preferring extreme habitats. Thermophiles and hyperthermophiles thrive at temperatures reaching above the boiling point of water, acidophiles thrive at a pH level of 3 or below, polyextremophiles can survive a varied number of extreme conditions, and many other types of extremophiles exist on Earth.

However, life could also exist outside of the extremophile range in the cloudtops, and in the same way that bacteria have been found living and reproducing in clouds on Earth, it has been proposed that life could exist in the same area on Venus^[23]. Microbes in the thick, cloudy atmosphere could be protected from solar radiation by the sulphur compounds in the air.

The venusian atmosphere has been found to be sufficiently out of equilibrium as to require further investigation. Analysis of data from the Venera, Pioneer and Magellan missions has found the chemicals hydrogen sulfide (H₂S) and sulfur dioxide (SO₂) together in the upper atmosphere, as well as carbonyl sulfide (OCS). The first two are gases that react with each other, implying that something to produce them must be present. In addition, carbonyl sulfide is noteworthy for being exceptionally difficult to produce through inorganic means. Were it on Earth, this compound would be considered an "unambiguous indicator of life". Further, it's an often overlooked fact that one of the early Venera probes detected large amounts of chlorine just below the venusian cloud deck. ^[24]

It has been proposed that microbes at this level could be soaking up ultraviolet light from the sun as a source of energy, which could be a possible explanation for dark patches seen on UV images of the planet. ^[25] Large, non-spherical cloud particles have also been detected in the cloud decks. Their composition is still unknown.

[edit] Future exploration

Venus in Situ Explorer proposed by NASA's New Frontiers program.

In addition to the probe Venus Express now in orbit around the planet, the Japanese probe PLANET-C to be launched in 2010 will study the planet for a period of two years, including the structure and activity of the atmosphere. One of its five cameras known as the "IR2" will be able to measure the atmosphere of the planet underneath its thick clouds, in addition to its movement and distribution of trace components. With a varied orbit from 300 to 60,000 km, it will be able to take close-up photographs of the planet, and should also confirm the presence of both active volcanoes as well as lightning. ^[26]

The Venus In Situ Explorer, proposed by NASA's New Frontiers program is a proposed probe to would aid in understanding the processes on the planet that led to climate change, as well as paving the way towards a later sample return mission.^[27]

Another craft from the same program called the Venus Surface Explorer has been proposed to study the composition and isotopic measurements of the surface and the atmosphere, for about 90 days. A launch date has not yet been set. ^[28]

[edit] Proposed missions

Artist's conception of a balloon probe floating above the surface of Venus. Newly-proposed balloon missions would be able to stay afloat for hundreds of days.

After missions discovered the reality of the harsh nature of the planet's surface attention shifted towards other targets such as Mars. There have been a number of proposed missions recently however, and many of these involve the little-known upper atmosphere. The Soviet Vega program in 1985 dropped two balloons into the atmosphere, but these were battery-powered and lasted for only about two Earth days each before running out of power and since then there has been no exploration of the upper atmosphere. The NASA contractor Global Aerospace has proposed a balloon in 2002 that would be capable of staying in the upper atmosphere for hundreds of Earth days as opposed to two. ^[29]

A solar flyer has also been proposed by Geoffrey A. Landis in place of a balloon, and the idea has been featured from time to time since the early 2000s. Venus has a high albedo, and reflects most of the sunlight that shines on it making the surface quite dark, the upper atmosphere at 60km has an upward solar intensity of 90%, meaning that solar panels on both the top and the bottom of a craft could be used with nearly equal efficiency ^[30]. In addition to this, the slightly lower gravity, high air pressure and slow rotation allowing for perpetual solar power make this part of the planet ideal for exploration. The proposed flyer would operate best at an altitude where sunlight, air pressure and wind speed would enable it to remain in the air perpetually, with slight dips down to lower altitudes for a few hours at a time before returning to higher altitudes. As sulphuric acid in the clouds at this height is not a threat for a properly-shielded craft, this so-called "solar flyer" would be able to measure the area in between 45 km and 60 km indefinitely, as long as mechanical error or unforeseen problems do not cause it to fail. Landis also proposed that rovers similar to Spirit and Opportunity could possibly explore the surface, with the difference being that Venus surface rovers would be "dumb" rovers controlled by radio signals from computers located in the flyer above^[31], only requiring parts such as motors and transistors to withstand the surface pressure, but not weaker parts involved in microelectronics that could not be made resistant to the heat, pressure and acidic conditions. ^[32]

[edit] See also

• Terraforming of Venus
• Colonization of Venus
• Atmosphere of Mars
• Earth's atmosphere

[edit] References

1. ^ Colonization of Venus (pdf), Geoffrey A. Landis, Conference on Human Space Exploration, Space Technology & Applications International Forum,Albuquerque NM, Feb. 2-6 2003.
2. ^ Venus: Atmosphere, Encyclopedia of the Solar System, Donald M. Hunten, University of Arizona.
3. ^ Flying over the cloudy world – science updates from Venus Express, Venus Today, Wednesday, July 12, 2006.
4. ^ Clouds and atmosphere of Venus - Institut de mecanique celeste et de calcul des ephemerides
5. ^ The Environment of Venus, Carl R. (Rod) Nave, Department of Physics and Astronomy, Georgia State University.
6. ^ Venus Atmosphere Temperature and Pressure Profiles - Shade Tree Physics
7. ^ Gaia: A New Look at Life on Earth, James Lovelock, ISBN-13: 978-0192862181
8. ^ Newman, M. J. & Rood, R. T. Implications of solar evolution for the Earth’s early atmosphere. Science198, 1035–1037 (1977)
9. ^ Kasting J.F. (1988), Runaway and moist greenhouse atmospheres and the evolution of earth and Venus, Icarus, v. 74, p. 472-494
10. ^ Was Venus Alive? 'The Signs are Probably There' - By Henry Bortman, Astrobiology Magazine Managing Editor, 26 August, 2004
11. ^ First Detection Made of an Extrasolar Planet's Atmosphere, Robert Roy Britt, Space.com, 27 November 2001
12. ^ Venus' Atmosphere to be Probed During Rare Solar Transit, Space.com, 07 June 2004
13. ^ NCAR Scientist to View Venus's Atmosphere during Transit, Search for Water Vapor on Distant Planet, National Center for Atmospheric Research and UCAR Office of Programs, June 3, 2004
14. ^ Venus, atmosphere - The Encyclopedia of Astrobiology, Astronomy and Spaceflight
15. ^ Moshkin B.E., Ekonomov A.P., Golovin Iu.M. (1979), Dust on the surface of Venus, Kosmicheskie Issledovaniia (Cosmic Research), v. 17, p. 280-285
16. ^ Atmospheric Flight on Venus (pdf) - Geoffrey A. Landis, Anthony Colozza, and Christopher M. LaMarre. paper IAC-02-Q.4.2.03, AIAA-2002-0819, AIAA0, No. 5
17. ^ Double vortex at Venus South Pole unveiled!, European Space Agency, 27 June 2006
18. ^ First Venus Express VIRTIS Images Peel Away the Planet's Clouds, Emily Lakdawalla, 14 April 2006
19. ^ Krasnopolsky V.A., Parshev V.A. (1981), Chemical composition of the atmosphere of Venus, Nature, v. 292, p. 610-613
20. ^ Sterne und Weltraum, vol. 21, July-Aug. 1982, p. 282 Koehler, H. W.
21. ^ Planet Venus: Earth's 'evil twin' - BBC News, Monday, 7 November 2005
22. ^ 2004 Venus Transit information page, Venus Earth and Mars, NASA
23. ^ Astrobiology: the Case for Venus, Geoffrey A. Landis, Journal of The British Interplanetary Society, vol. 56, no. 7/8, July-August 2003, pp. 250-254
24. ^ Venus Revealed: A New Look Below the Clouds of Our Mysterious Twin Planet, David Grinspoon, ISBN-13: 978-0201328394
25. ^ Venus could be a haven for life, ABC News, Sep 28 2002
26. ^ Venus Exploration Mission PLANET-C, Japan Aerospace Exploration Agency; 2006/5/17 Update.
27. ^ New Frontiers Program - Program Description
28. ^ Venus Surface Explorer - Future Venus missions, solarsystem.nasa.gov
29. ^ Robotic Balloon Probe Could Pierce Venus's Deadly Clouds - Robert Myers, SPACE.com, 13 November 2002
30. ^ Exploring Venus by Solar Airplane (pdf), Geoffrey A. Landis, STAIF Conference on Space Exploration Technology, Feb. 11-15 2001
31. ^ Robotic Exploration of the Surface and Atmosphere of Venus, Geoffrey A. Landis, paper IAC-04-Q.2.A.08, 2004 International Astronautical Federation Congress, Vancouver WA, Oct. 4-8 2004
32. ^ To conquer Venus, try a plane with a brain, Paul Marks, NewScientist.com, 10:00 08 May 2005