venus


Venus is the second planet from the sun and the sixth largest.And until 1960s was ofiten considered a " twin sisiter " to the Earth cause venus is the nearest planet to us .And cause superficially the two planets seem to share many characteristics such as ,
  • Venus is only slightly smaller than Earth (95% of Earth's diameter,80% of Earth's mass).
  • Both have few craters indicating relatively young surfaces.
  • Thier densities and chemical compositions are similiar.

    But more detailed study of Venus reveals that in many important ways it is radically different from Earth.

  • Facts:-
    Sign:Venus Symble
    Orbit:108.200.000 km (0.72 AU)from sun
    Diameter:12.103.6 km
    Mass:4.869e24 kg
    Density:5.243 kg/m^3
    Minimum Distant from Sun:108 Million km(67 million miles)
    Maximum Distant from Sun:109 Million km(68 million miles)
    Minimum Distant from Earth:40 Million km(25 million miles)
    Rotation period about axis:243 days(retrograde)
    Revolution period about the sun:0.62 years
    Tilt of axis:177.178 degrees
    Surface Gravity:8.87 m/s^2(0.90 x Earth's)
    Average Temperature:457 C degrees(855 F degrees)
    Average Surface Temperature:730 K degrees
    Name in Roman/Greek mythology:Venus/Aphrodite
    Satellities:0(None)

    The first spacecraft to visit Venus was Mariner2 in 1962.It was subsequently visited by many others(more than 20 in all so far), including Pioneer Venus and the Soviet Venera7 the first spacecraft to land on another planet,and venera9 which returned the first photographs of the surface(left). Most recently,the orbiting US spacecraft Magellan produced detailed maps of Venus' surface using radar(above).

  • Venus' Orbit:-
     Venus' orbit is the most nearly circular of that of any palnet,with an eccentricity of less than 1%.
    Venus is peculiar in that its rotation is retrograde (in the opposite sense of the Earth and all other planets except Uranus) and because it is very slow a day on venus corresponds to 243 Earth's days.At praesent we have no solid explanation for why this is so.

    In addition,the periods of Venus' rotation and of its orbit are synchronized such that it always presents the same face toward Earth when the two planets are at thier closest approch.Whether this a resonance effect or merely a coincidence is not known.

  • Discovering venus:-
    In Discovering Venus we will talk about Earlier and Modern Views of Venus.
  • Earlier Views of Venus:
     In earlier times,there was considerable speculation concerning the possibility of life on Venus,sometimes with rather elaborate charcacteristics.Let's see one from earlier time a french man called,BERNARD DE FONTENELLE,in 1686 wrote that:

               I can tell from here...What the inhabitants of Venus are like;they resembl the Moors of Granada;a small black people,burned by the Sun,full of wit and fire,always in love,writting Verse,fond of Music,arranging festivals,dances,and tournaments every day.

    (Quoted in National Geographic,june,1975)

    Now we will know that all that description is rather unremarkable because it sounds like everyday human and turns out that monsieur de Fontenelle was quite incorrect about Venus and its inhabitants.
  • Modern Views of Venus:

    • In the last 30 years we have learned a great deal about our "sister" planet,and we now know that almost nothing on Venus is like that on the Earth.

    Much of the previous misconception can be traced to the difficulty of observing Venus beacuse it always coverd with a thick cloud layer. In the last three decades astronomers have learned how to peer through that cloud layer and un lock many secrets of this nearby but we this planet is unknown planet.

  • The Atmosphere of Venus:-
    As we know that Venus is covered by a thick layer clouds that make it impossible to see the surface for light in the visible part of spectrum.Missions to the surface of Venus (Russian Venera Spacecraft) indicate that the cloud deck bgains about 50 km above the surface.Spectral analysis of the thick cloud layer surrounding Venus suggests that the clouds are largly composed of sulphuric acid droplets.

    And as the cloud layer is rather featurless invisible light,but shows structure in other wave lenghths.
    A comarsion of the motion of the surface with that of the upper clouds indicates that while the surface takes about 8 months to rtotate,the clouds rotate all the way around the planet in about 4 days.This indicates that there are very high velocity winds (thats which we will talk about it after that) in the upper part of venusian atmosphere.

    The pressure of the atmosphere is about 90 times that of the Earth at the surface (about the same as the pressure at a depth of 1 KM. in Earth's oceans).

  • High Velocity winds:-
     As we know that a comparison of surface and upper cloud velocities indicates that there are winds in the upper part of venusian atmosphere,so we will talk about it now.

    There are high-velocity winds in the upper atmosphere,but the atmosphere below the cloud deck appears to be stagnant,with only very weak winds blowing at the surface. This winds with velocities as large as 300 (kph),these winds are comparable in speed to jetstreams in the Earth's atmosphere but extend over much larger regions in the case of Venus.

    And untill now it is not fully understood why these winds have such high velocities,and the contrasting stagnation of the lower atmosphere.

  • Different Wave Lenghths:-
    Different wave lenghths of light penetrate the atmosphere to varying degrees and therefore may be used to study different layers of the clouds and atmosphere.for example the following images show the clouds of venus imaged in Visible light (with a violet filter) (Left) ,ultraviolet light (Middle) ,and infrared light respectively (Right) click to any image to view the larger size of it.
                          

  • A Runaway Greenhouse Effect:-
     The Earth and Venus are near each other in the solar system,and are similar in size,density,and composition.and from our understanding of the origin of the Solar System,we would expect that their initial atmospheres would have been rather similar.

    untill now atmospheres of the two planets could hardly be much more different than they are.And there is a question How did this come to be?the reason is thought to lie in what is termed the "Runaway Greenhouse Effect".

  • Venus' Surface:-
     Our knowledge about the venus' surface comes from a limited amount of information by the series of Russian Venera landers,and primarily from extensive radar imaging of the planet.

    The radar imaging of the planet has been performed both from Earth-based facilities and from space probes.And the most extensive radar was Magellan orbiter in a 4-year period in the early 1990s.

    The Surface of Venus is smooth in many places,thought not as smooth as orginally expected. however,we find nearly the same gelogical features found on Erath like Canyons,Volcanos,Lava flows,Rift valleys,Mountains,Craters and Plains.there is difference that the surface appears to be a single crustal plat,with large-scale horizontal motion of crustal plates as found on Earth.

    The difference between this aspect of their geology is not understood.And the usual explanation is that Venus is a little behind the Earth in gelogical time scale,and its tectonic activity is just getting started.

    The global data set from radar imaging reveals a number of craters consistent with an average Venus surface age of 300 million to 500 million years.

    The large regions in Venus surface are 2 regions which extend to several Kilometers.These are called Istar Terra  in the northern hemisphere (about the size of Australia)   and Aphrodite Terra   along the equator (about the size of South America).

  • Hemispheric Views:
    The following images show 5 hemispheric views of the surface(Magellan Mission)


    The center image (a) is centered at the north pole.The other four images are centered around the equator of Venus at (b) 0 degrees logitude,(c) 90 degrees east longitude,(d) 180 degrees and (e) 270 degrees east longitude.

  • Example of Surface Features:
    we are now going to know more about surface features that have been discovered on Venus.

    Mountains


     Venus has many high mountains which appear to be in volcanic in origin.The bright region near the center in the image (a) above is Maxwell Montes,the highest mountain range on Venus;it reaches an elevation of 11 Km above average elevation (2 Km more than the elevation of Mount Everst above sea level on Earth).

    Volcanos and Lava Flows


     There is a strong evidence that volcanoes have erupted on Venus in the geologically recent past,and strong indirect evidence from observation like changing chemical compositoin of the atmosphere and the detection of lightning in certain regions that volcanoes are presently active on Venus,thought we don't yet have proof.

    One piece of evidence for recent volcanic activity is the presence in many regions of features that look like relatively new lava flows.

    The two images shown below illustrates a volcano about 3 miles in diameter near Paragon chasma (left) and an image of apparent recent lava flows in the Sif Mons region (right).

                      
    Note:
     You have to know that bright spots is differ from the dark one.
    The bright spots corresponds to rgions that reflect more radar waves than other regions.Thus,if you could actually see these regions with your eyes the patterns of brightness and darkness would probably not be the same as in these images.
    However,the basic features would still be the same.

    Rift valleys


     There are Rift Valleys as large as the East African Rift (the largest on Earth).The image shown below illustrates a rift valley in the West Eistla Region,near Gula Mons and Sif Mons.



    Meteor Craters


     We said before that Venus' surface is smooth but we don't know from where it got that,the Surface of Venus has been smothed by recent lava flows and by interaction with the corrosive atmosphere.The following images show a field of craters (right) and the largest crater found (left).

        

    The View from Venera


     Finally,we show two of the images taken by Russian Venera 14 lander.We can see in black and white the bottom of the spaccraft,rocks on the surface,and a portion of the horizon.


  • A comparison between Earth and Venus:-
    As we know that Venus is considered as the sister of Earth but we find alot of difference between them and we list it.

    1-The slow rotation period.
    2-The single-plate surface.
    3-The lack of a satellite.
    4-The exteremly weak magnetic field.
    5-The lack of water.
    6-The high surface temperature,and dense atmosphere.

    After that we find a great difference between Earth and Venus and in the next unit we will study Earth planet to know the difference.

  • Difficulties:-
  • There is some evidence of spreading and folding on Venus' surface and of recent volcanic flows.But there is no evidence of Plate tectonics as senn on Earth.Is this a result of the higher surface temperature?
  • The greenhouse effect is much stronger on Venus than Earth because of Venus' dense carbon dioxide atmosphere.But why did Venus evolve so differently from Earth?




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    Last Modified : 5 Sep. 2001
    Created By#C0115361 Team