NASA Facts the Planet Venus

8/7/2019 NASA Facts the Planet Venus

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NASAFactsAn Educational Publicationof theNational Aeronautics andSpace AdministrationN F 86/12-77

The Planet VentisOne of a series of NASA Facts about theexploration of Venus.In 1978 NASA will send two spacecraft to Earth'snearest planetary neighbor, Venus. This is thePioneer-Venus mission. One spacecraft will orbitthe planet and send scientific information back toEarth for months. The other will carry four probes

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that will detach and plunge into the atmosphere ofVenus to tell us about its meteorology, itscomposition, and its complex chemistry, but theyare not designed to survive after impact.Venus is a fascinating planet because it is


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almost a twin of Earth but has developed quitedifferently so that our type of life cannot live on itssurface. Scientists want to find out why itdeveloped the way it did, and in so doing, gaingreater insight into the nature of the processesthat have molded our Earth and brought it to theconditions of today.Venus as a Member of the Solar System

After the Sun and the Moon, Venus is thebrightest object in the sky. If you know where tolook you can sometimes see Venus in broaddaylight. Like other planets Venus appears tomove through the constellations of the Zodiac,but it is never seen far from the Sun because it isan "inferior" planet. That is, it travels along anorbit within that of the Earth. Venus takes only 225days to go corT pletely around the Sun. Theposition of Venus east or west of the Sun isreferred to as an elongation. At eastern elongationyou can see it as a bright, star-like object in theevening sky. At western elongation you have tolook for it rising before the Sun in the earlymorning sky (Figure 1).Appar en t D a i lyPath o f Set t ing SunG r ea tes tEasternPath of VenusE l o n g a t i o n jVbit

V e n u s N e a r \\ 1 V e n u s N e a r S u p e r i o rIn fer io rCon junc t i onC on jun c t i onHorzon_.L+^`^^' `.E'r' S u n B e l o wHor izon

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Su nBe l owHor izonFigure 1. Venus moves from superior conjunction to the e a s t o ft h e S u n and thus appears a s an evening s t a r . A s it progressesaround its orbit, i t p a s s e s through inferior conjunctionbetween Earth and Sun and then becomes a morning star.rising before the Sun.

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When Venus passes between Earth and Sun,and thus comes closest to the Earth, it passesthrough what astronomers call inferiorconjunction. On the far side of the Sun it passesthrough superior conjunction (see Figure 2). Thepositions of greatest elongation correspond toquadratures on the orbit.

/Venus\Super iorCon junc t i on\\I^Su n^II1I k Q u a d r a l u r eI\\InferiorC on jun c t i on

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Figure 2. The orbits of Venus and Earth show the positionsthe conjunctions.

Venus as a PlanetThe mass, diameter, and density of Venus areall only slightly less than those of the Earth. Butits surface is hotter, its atmosphere much densand its rotations much slower than those of theEarth. Venus is one of the three planets of thesolar system that do not have satellites; the oth

are Mercury and Pluto. Also Venus does not hava significant magnetic field.The diameter of Venus is 12,100 km (7519 mi)and its average density is 5.25 times that of wateThis high density may imply that Venus has a coof nickel and iron like the Earth.The surface of Venus cannot be observedvisually from Earth because Venus is shrouded ia thick blanket of clouds. However, radar wavesfrom Earth stations penetrate to the surface andastronomers have used them to map the surfaceof Venus. In addition, the surface emits radiowavesmicrowaveswhich penetrate the clouand can be received on Earth. Astronomers usethese radio waves to estimate the temperature the planet's surface.


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In the early 1960's when these measurementswere first made, astronomers were surprised todiscover that the surface of Venus is extremelyhot. The microwaves carry the message that it isabout 430C which is hot enough to melt zinc.They also told us that the temperature is the sameboth day and night on the surface of Venus.The surface temperature of Venus is muchhigher than that of the Earth. This difference isattributed to the dense atmosphere of Venuswhich traps incoming solar radiation and prevents

Figure 3. In visible light Venus is a featureless, bright, yellowishglobe. In ultraviolet light the clouds of Venus have prominentpatterns as shown in this mosaic of pictures taken by NASA'sspacecraft, Mariner 10, as it flew by genus on the way toMercury.heat from being reradiated into space. This istermed the "greenhouse effect."A telescope reveals no details in the yellowishclouds of Venus. However, if these clouds arephotographed in ultraviolet light, dark shadingscan be seen that rotate in a period of about 4 days.These shading- have also been seen more clearlyin ultraviolet pictures of Venus taken from apassing spacecraft (Figure 3).But the planet itself rotates much more slowlythan its cloud tops. ,'Again the discovery was madeby radio waves. Transmitted from the Earth by abig antenna, these waves are reflected by Venus.Radar astronomers can tell from the form of the

radio echo that Venus rotates once in 243.1 dayin a direction opposite to the rotation of the EarThis is called a retrograde direction. A day (thetime from one sunrise to the next) on Venus lasabout 117 Earth days (Figure 4).Another peculiar fact about the rotation ofVenus is that it seems to be linked to the Earth.Each time g enus passes closest to the Earth atinferior conjunction, the same face of Venus isturned towards the Earth. Most astonomersbelieve this is just a coincidence, but it may nobe.Astronomers do not know why Venus rotatesslowly. The other planets (except Mercury) rotaquickly on their axes. Mercury has been slowedtidal friction because of its closeness to the Subut Venus is not only rotating slowly, it is alsorotating in the "wrong" direction. The Sun coulnot have caused this. One possibility, proposedsome years ago, is that Venus once had a bigsatellite that revolved around it in the retrograddirection. This satellite crashed into Venus andstopped the planet's rotation, or perhaps evenpushed it a little into rotating the opposite way.Astronomers looked at the spectrum of lightreflected from the clouds of Venus and were abto find that its atmosphere contains much (97 %

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Ve nus T i me s/j ; 7% 4 :00 amI Orbi tMid n ig^ ^ ^ Y Y YI/ 'I\0 8:00 pmRotat ionJ(.^ ,:pp pm

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Axia l Rota t ion: 243.1 Earth daysOrbi ta l per iod : 225 Earth days ( = Venus year)Noon t o Noon : 1 1 6 . 8 E ar t h d ays ( = Ve nus d ay)Venus year : 1 .93 Venus daysFigure 4. A combination of retrograde rotation about its axi243.1 days and orbital revolution about the Sun in 225 daysmake a day on Venus 116.8 Earth days long, as shown in thdiagram.

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carbon dior.ide. This again is a contrast to Earthwhich only has a small amount (0.03%) of carbondioxide in its atmosphere. The reason there is somuch carbon dioxide in the atmosphere of Venusis thought to be because Venus is a dry planet.The presence of oceans of water on Earth allowedEarth's early atmosphere of carbon dioxide toreact with the rocks and form carbonates. Today,most of Earth's carbon dioxide is bound up ascarbonates in rocks. Also living things on Earthused carbon dioxide from the atmosphere toprovide ca , `-on for their bodies. Oxygen wasreleased b^ plants to form Earth's oxygen-richatmosphere of today.Another big difference between Earth andVenus, as mentioned earlier, is that Venus doesnot have a magnetic field of any significance. As aconsequence its atmosphere has no protectionfrom the blizzard of high-energy particlesprotons and electronsknown as thesolar wind. These particles are held away from theEarth by our magnetic field. But they speeddirectly into the upper atmosphere of Venus.The Surface of Venus

Details on the surface of Venus were virtuallyunknown until two Soviet Venera spacecraftlanded on it in 1975 and returned pictures. Earlierspeculations had ranged from steaming swampsto dusty deserts, from carbonated seas to oceans

of bubbling petroleum. All were wrong, thoughdesert concept seems closest to what is nowknown. Photographs from the Veneta spacecra(Figure 5) revealed a dry rocky surface fractureand changed by unknown processes. There arerocks of many different kinds and a dark soil. Ttwo spacecraft landed about 2000 ! g y m (1200 mi)from each other. One landed on an ancient plator plains area. At this site there are rockyelevations interspersed with a relatively dark,fine-grained soil. This soil seems to have resulfrom a weathering of the rocks, possibly by achemical action. The rocky outcrops are genersmooth on a large scale, with their edges bluntand rounded. The dark soil fills some of thecavities in the rocks. By contrast the other Venlanded at a site where there are rocks which looyounger and less weathered, with not muchevidence of soil between them.Measurements made by the Russian spacecshowed that all these rocks on Venus have adensity and radioactive content similar toterrestrial basaltic rocks.In 1962, craters were discovered on radar maof the planet (Figure 6). Subsequently, radarastronomers found evidence of large lava flowsgreat circular basins like those of the Moon, aneven chasms and volcanoes like those on MarsGenerally, however, the surface of Venus is norugged as Earth and Mars. Radar measuremenof altitude variation on Venus amount to only

Figure 5. Ttie first pictures of the surface of Venus were returned to Earth by two Soviet Veneta spacecraft that landedon Venus in 1975. The top picture is from Veneta 9 and the bottom from Veneta 10. They show rocky terrains which aresurprisingly different at the two sites, the Veneta 10 site showing evidence of much greater age, inferred from theweathered nature of the rocks.

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chlorine and fluorine, and ice crystals. The cloudsare pale yellow in color and reflect about 85% ofthe sunlight failing upon them. Their true naturewas not discovered until a few years &go, whenastronomers used a variety of observations toshow that the clouds are composed of droplets ofsulfuric acid.

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0255075100Wi nd Speed , m / sFigure 7. Space probes and ground-based observations haveestablished these details of the structure of the Venusianatmosphere. The curve across the lower part of the sketchshows the wind speed at altitudes measured by one of theSoviet spacecraft.The general composition of the atmospherewas not known until 1932 when carbon dioxide

was discovered by the use of a spectroscope toanalyze the light from the planet. Space probeshave confirmed that the atmosphere of Venus isabout 97% carbon dioxide, and that it is almost100 times as dense as Earth's atmosphere.The atmospheres of Earth and Venus probablyoriginated from radioactive heating of bothplanets. As the rocks became hot, lightweight

about 5 km (3 mi) compared with Earth's range of20 km (12 mi) between the top of Mt. Everest andthe bottom of the Marianas Trench, and Mars'range of 30 km (20 mi) between the top of OlympusMons and the bottom of the Hellas basin.The existence of craters and other features onthe surface of Venus suggests that the planet hadbeen subjected to a bombardment from spaceearly in its history, just as the other inner planetsof the solar system were bombarded, and thatafterwards there were volcanic activities, lavaflows, and possibly some mountain building. Theevolution of Venus may have progressed to aneven more Earth-like state than that of Mars. Butthe absence of water on Venus prevented it frombecoming like the Earth is today.

F igure 6. Radar scanning of the surface of Venus using theDeep Space Antennas of NASA's network, revealed craters a sshown in this picture of an equatorial region. The dark bandacross the p icture covers an area for which radar data were notobtained.

The Atmosphere of VenusThe atmosphere of Venus (Figure 7) wasdiscovered by a Russian astronorner in the 16thcentury when the planet was seen to cross the

face of the Sun in transit. The black disc ofVenus silhouetted against the Sun was distortedby the planet's atmosphere when close to theedge of the Sun. Also, other astronomers saw theatmosphere extending the arms of the crescentVenus.The brilliant clouds of Venus were at one timethought to be clouds of water droplets. Otherspeculations included dust, exotic compounds ofO R I G IN A L P A G E I SO F PO O R Q U A L I T Y


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gases such as carbon dioxide and water "boiledoff" (outgassed) from the mantle and crust. IfVenus went through as much outgassing as theEarth, it would be exr ected to have had originallyan atmosphere consisting of carbon dioxide in anamount 100 times the amount of all Earth'spresent-day atmosphere, and of water equivalentto the amount in the Earth's oceans.The Earth has held water in its oceans becauseits surface is not hot enough to evaporate all thewater into its atmosphere, which has a region of lowtemperature (a cold trap) in the stratosphere thatprevents water vapor from rising to a height atwhich solar ultraviolet radiation could break watermolecules into oxygen and hydrogen.On Venus, which is closer to the Sun, the planetwas probably hot enough originally for all thewater to evaporate into the atmosphere. In such awater-saturated, hot atmosphere there could beno cold trap, and water would break down intooxygen and hydrogen. The hydrogen would haveleaked off into space. The oxygen might havereacted with surface rocks to oxidize them, butthis is only speculation.Another theory is that Venus was not formedfrom the same basic materials as the Earth anddid not have much water to begin with.Whichever way it happened, Venus is very shorto water today. Its atmosphere is extremely dry,especially above the clouds.Another effect of the dense atmosphere ofcarbon dioxide is to trap incoming solar radiationand prevent heat radiation from the surface andthe lower atmosphere from escaping back intospace. The surface temperature of 480C (900F)permits very active chemical reactions betweenthe atmosphere and the surface materials, andmany substances that on Earth would be bound inthe rocks are probably released into theatmosphere of Venus. These include chlorine andfluorine.

Like on Earth, sulfur is probably released intothe atmosphere of Venus. This sulfur rises highinto the Venusian atmosphere where it combineswith water and oxygen to produce sulfuric acid. Itis a haze of droplets of sulfuric acid, moreconcentrated than the acid in an automobile'sbattery, that forms the very deep clouds of Venus.These hazy clouds are not thickly opaque like rainclouds of Earth, but tl,ay have extremely deeplayers. Their tops are 80 km (50 mi) above thesurface of Venus, and their bottoms are about 50km (30 mi) above the hot surface of the planet.Above and below the sulfuric acid clouds areother thinner haze layers of unknowncomposition. But there seems to be a clear regionfor about 20 km (12 mi) above the surface. Thus,while Earth's first 20 km are cloud filled, on Venus

these first 20 km of atmosphere are clear. Venuclouds and weather begin at the level of Earth'supper stratosphere.Close to the surface of Venus there are onlyslight breezes, whereas at the top of the sulfuriacid clouds the winds reach 360 km/hr (220 mi/hcomparable to Earth's jet streams. The high winof Venus sweep around the planet and make itappear that the topmost regions of the clouds(visible in ultraviolet light as dark and lightshadings) whirl around the planet in about 4 dayThere are also circulating motions within theatmosphere. Regions of rising gas are locatedalong the equator where solar heat affects theatmosphere most. Other regions of descendinggas are located toward the poles. The atmospheis known to be in circulation because thetemperature is virtually the same on the day andnight sides and from equator to poles. Thereforethe atmosphere of Venus must be constantlyagitated like water coming to the boil in asaucepan.Down close to the surface, thermal (heat)chemical reactions occur in the atmosphericgases and between the atmosphere and thesurface rocks. Above the sulfuric acid cloudschemical reactions are controlled by theultraviolet solar radiation; they are photochemicreactions. Within the droplets of the clouds othchemical reactions take place in a liquid phase.The chemistry of the atmosphere of Venus is thextremely complex, and it is not yet thoroughlyunderstood.High above the clouds and the hazes is theupper atmosphere of Venus which changes intoregion of mainly helium and hydrogen, called thexosphere, and then into interplanetary space.Incoming solar radiation affects the atoms andmolecules of the upper atmospheric gases andionizes them, making them electrically chargedThis forms an ionosphere somewhat similar toEarth's ionosphere. Both ionospheres have layeWe use the layers in Earth's ionosphere to reflecertain frequencies of radio waves around theworld for long-distance radio communications.The layers of the ionosphere of Venus are notintensely ionized as those of Earth's ionosphereand are generally lower and thinner. Also thetemperature of Venus' ionosphere is much lessthan that of Earth's ionosphere. So while Venuslower atmosphere is hotter than that of the Earthits upper atmosphere is cooler.As already mentioned, because Venus does nhave a magnetic field of any consequence, itsupper atmosphere receives directly the solar wof protons and electrons streaming from the SuThis causes a sharp boundary for Venus'

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ionosphere, at an altitude of only 350 to 500 km(200 to 300 mi) on the day side of the planet. Onthe night side, the ionosphere trails out behindthe planet in a "tail" extending downwind intoin!.-rplanetary space (Figure 8).

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Pl aA n aT;SolarWind Shade..a'Cavity

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Figure 8. Because Venus has an insignificant magnetic field, itinteracts differently than Earth with the solar wind, whichpenetrates very close to the planet as compared with the Earth.This sketch shows the bow shock where the solar wind isslowed down by the atmosphere of Venus. Behind the planet isa cavity and a plasma tail.

There are many unanswered questions aboutVenus which scientists hope will be answered bynew information to be provided by NASA'sPioneer-Venus spacecraft. Such questionsinclude:What causes the weather on Venus?Is it really a greenhouse effect that makes theplanet so hot?What causes the dark markings in the Venusclouds?What are the various constituents of the Venusatmosphere?How does the ionosphere interact with the solarwind?

How these and other questions about Venusmight be answered by the Pioneer-Venus missionwill be discussed in another fact sheet of thisseries.

Further ReadingVenus, Young, A. and Young, L. , ScientificAmerican, Vol. 233, Sept. 1975, pp. 71-78The Atmosphere of Mars and Venus, Eshleman,Von R., Scientific American, March 1969,pp. 79-88The New Venus, Ksanfomalita, L.V. et al., NewScientist, Vol. 73, January 20, 1977, pp. 127-12Surface of Venus, Evidence of Diverse Landformby Radar Observations, Malin, M.C., andSanders, R.S., Science, Vol. 196, May 27, 1977pp. 987-994Mariner 10 Science Reports, various authors,Science, Vol. 183, March 29, 1974, pp. 1289-13Venus Exploration With the Veneta 9 and Vener10 Spacecraft, Icarus, Vol. 30, No. 4, 197?,pp. 605-625Student Projects1.Planetary O rbits and Ap paritions

You are given the following information:Mean distance of Venus from the Sun is 107.5million km. Mean distance of the Earth from theSun is 149.6 million km. Period of Venus in itsorbit around the Sun is 225 Earth days. Period oEarth in its orbit around the Sun is 365.25 Earthdays.If Venus was in inferior conjunction (see Figure on April 6, 1977, work out when the next inferiorconjunction will take place. Note that when Venhas made one circuit of the Sun, Earth has onlycompleted part of a complete circuit.A NASA Pioneer-Venus orbiter, launched towarVenus in May or June, will arrive in the vicinity othe planet in December. A probe spacecraftlaunched in August will also reach Venus inDecember. Is Venus a morning or an evening, , star" at the time both spacecraft arrive there?If Venus is brightest in the sky of Earth about 36days before or after inferior conjunction, whatdate in December should the spacecraft arrive aVenus if Venus were to be its brightest as seen Earth?2.The Atm osp here of Venus

You are provided with information obtainedfrom a probe that penetrates the atmosphere ofVenus. This probe carries instruments to measuthe speed of the wind and the temperature of theVenus atmosphere. While the wind speedinstrument gave good data points all the way to

ORIGINAL PAGE ISOF POOR QUALITY

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the surface, the temperature measuringinstrument failed at high temperature about 8 kmabove the surface. These are the data points:

Height km Wind Speed m/s Temperature C70 135 5065 135 3060 130 1956 120 4750 87 10045 68 13939 45 17734 9 9 22330 41 26726 40 29 819 34 32715 25 20 110 4 4006 1 1 0

faster noes Venus move along its orbit (in km/s)than the Earth? Why is this?If you have access to a telescope (a 3-inchaperture telescope is suitable), also makesketches of the phase of Venus at eachobservation and prepare a series of drawingsshowing how the planet changes in apparent sizas its phase changes. Why is this? Again use yoorbital diagram of Earth and Venus seen fromabove the Sun. Approximately how far from Eartwill Venus be when it passes between Earth andSun at inferior conjunction?

Make a graph showing wind speed andtemperature plotted against height above thesurface of Venus. Try to fit a s rnooth curvebetween your data point;. ;%,e there any wildpoints that do not fit' Do you think these are realor a fault of the instrime-it? If you reject thesepoints do you get smoctn curves? Can youextrapolate the temperature curve to estimate thetemperature at the surface? What is thistemperature?The base of the clouds is at an altitude of 50kilometers. Do you see anything unusual aboutyour curves at that altitude? Explain what may behappening in the Venus atmosphere to the bottomedge of the clouds.3. Observation

When the two Pioneer-Venus spacecraft arelaunched to Venus, the planet can be seen in theevening sky. Once each week observe whereVenus is, relative to the constellations, and makea continuing sketch of its movement relative tothe stars during the voyage of the spacecraft toVenus. Watch for the time when Venus changesits eastward movement relative to the stars to awestward movement. Why does this happen? Asketch of the orbits as viewed from above with thepositions of the planets every week will help youarrive at an explanation. You can derive themovement of the planets around theirapproximately circular orbits by using theinformation provided in Project 1. How muchU.S. GOVERNMENT PRINTING 0MCE : 1976 0-256-640

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NASA Facts the Planet Venus

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