IAL 13: Venus

Don't Panic


  1. Introduction
  2. Venus Facts
  3. The Venusian Atmosphere
  4. The Venusian Runaway Greenhouse Effect
  5. Venusian Weather
  6. Venus and Plate Tectonics
  7. The Formation and Continuing Geological Evolution of Venus
  8. The Fate of Venus

  1. Introduction

  2. Venus has had a long history.

    At least when at its brightest, it is the 3rd brightest astro-body coming in after the Sun and the Moon.

    Because it is an inner planet it is always close to the Sun in the sky: maximum elongation is about 46 degrees (Ze2002-15).

    Thus, it is a morning and evening star: in fact THE Morning Star and Evening Star of history. See the Venus as the Morning Star of Evening Star in the figure below (local link / general link: venus_sunset.html).

    Mercury is also a morning and evening star, but it is NOT nearly as noticeable.

    Venus is also the unearthly planet that comes closest to us.

    But from Earth-based telescopes in the visible, Venus is almost a featureless yellowish-white (HI-170).

    What we are seeing is the highly reflective cloud cover that totally covers Venus.

    Only with using other frequency bands to study Venus and sending spacecraft to orbit and land on Venus has Venus' nature elucidated somewhat.

    The most important spacecraft till 2022 are:

    1. the NASA Magellan spacecraft (1989--1994) that did radar mapping of the whole Venus surface.

    2. the Soviet Venera spacecraft (1961--1985) which included landers. The successful landers penetrated the Venusian atmosphere and landed on the Venusian surface. They survived only a few hours at most because of the extreme heat and pressure they were subjected to.

  3. Venus Facts

  4. Just some basic Venus facts---which, of course, beg for an explanation---and there is some explanation, but also some "Just So."

    1. Tabulated Facts:

      See below Table: Venus Facts

      Table:  Venus Facts
      Quantity                      Value
      Venus symbol                   : Also the female gender symbol
      Mean distance from the Sun    0.7233 astronomical units
      Eccentricity of orbit         0.0068 :   which is the smallest planetary
                                               eccentricity:  i.e., the orbit
                                               is the most circular.
      Axis tilt to orbital pole     2.7°: which is very small.  Venus probably has almost NO
                                      seasons because of this and the small eccentricity.
      Mean inclination to Ecliptic  3.39471°
      Orbital period                224.701 days 
      Rotational Period             243.0226 days :  and it is a 
                                                     retrograde rotation 
                                                     (i.e., clockwise looking
                                                     down from the north ecliptic pole).
      Venusian day                  116.750 days and it is a
                                                     retrograde rotation
      Equatorial radius             6052 km ≅ 0.95 R_Earth_equatorial
      Mass                          4.870*10**24 kg ≅ 0.815 Earth masses
      Mean density                  5.24 g/cm**3 = a bit less than mean Earth density
      Uncompressed mean density     4.2 g/cm**3 ≅ uncompressed mean Earth density, a model-dependent result
      Surface gravity               0.903 Earth gravities
      Surface temperature           ∼ 470 C ≅ 740 K all the time :  it's hot

      References: Cox-294,295, Se-418,467, HI-169, Wikipedia: Venus.


        Question: Given the very small axis tilt of Venus and very small eccentricity of Venus' orbit, one would expect Venus to have:

        1. extreme seasons.
        2. almost no seasons.
        3. seasons with variations similar to those of the Earth.

        Answer 2 is right.

        Every day (and they are very long days too: 116.750 days) is much like any other day on Venus weatherwise.

        In fact, changes during a day are probably what one could call the "seasons". But except for lighting, those changes are NOT very extreme either.

        The total temperature variation may only be about 10 K from hot to cold regions (Ze2002-184). We discuss why below.

    2. Venusian Orbit, Axial Rotation, and Day:

      Venus is unusual for a planet in having an axial rotation period of 243.0187 days (relative to the observable universe, of course) longer than its year (224.695 days).

      Also unusually, its axial rotation is retrograde: i.e., it rotates westward or clockwise looking down from the north celestial pole (NCP).

      These unusual rotation characteristics violate the trends that we believe the protoplanetary disk set up.

        Question: What is the favored explanation of the unusual rotation characteristics?

        1. An unusual tidal locking effect of the Sun.
        2. A gravitational perturbation by Earth.
        3. A giant impactor in the early Solar System that gave Venus a random rotational period and rotation direction.

        Well answer 3 seems to be favored one, but answer 1 is also considered. Answer 2 seems ruled out. See Wikipedia: Venus: Orbit and rotation.

      The Venusian day is fully explicated in the figure below (local link / general link: venusian_day_explication.html).

    3. Transits of Venus:

      Venus transits (i.e., the passage of Venus across the face of Sun: i.e., the solar photosphere) and their history are explicated a bit in the figure below (local link / general link: venus_transit.html).

  5. The Venusian Atmosphere

  6. At the surface, pressure is about 90 atmospheres, the temperature is about 740 K (470 C), and the composition is 96 % CO_2.

    Venus has a runaway greenhouse effect because of that super-dense, CO_2-rich atmosphere.

    This causes Venus' surface temperature to be 740 K (or 470 C) which is hotter than Mercury's mid-day temperature of about 600 K.

    Also Venus' surface temperature is pretty constant with location and time of day for reasons will go into below.

    Table:  Venusian Atmospheric Composition by Number
    Gas                      Percentage 
    CO_2                     96.5 
    N_2                       3.5
    SO_2                      0.015
    water vapor
    sulfuric acid (H_2SO_4)
    hydrochloric acid (HCl)
    hydrofluoric acid (HF)   
    and other stuff          <0.03
    References: HI-171; CW-35; Se-467.

    The Venusian atmosphere is explicated in the figure below (local link / general link: venusian_atmosphere_layers.html).

    A bit of the surface of Venus is shown in the figure below.

  7. The Venusian Runaway Greenhouse Effect

  8. Venus, of course, tends to be hotter than the Earth because it is closer to the Sun.

    Science fiction writers once speculated that the high reflectivity of the Venusian clouds might keep the surface from being super-hot. They envisioned a perpetual rain on a swampy Venus inhabited by swamp creatures: e.g., Ray Bradbury in The Illustrated Man.

    Recall NOT a lot of sunlight of does get to the surface: only about 3 % (Ze2002-184) which is a lot less than the 40 % of the sunlight that gets to the Earth's surface (Ze2002-157).

    But the runaway greenhouse effect caused by the dense CO_2 atmosphere still makes the surface super-hot: about 740 K (470 C) which is hot enough to melt lead and tin (CAC-52).

    See IAL 11: The Earth: The Greenhouse Effect for the explanation of the greenhouse effect.

    The closeness to the Sun was the key factor that set Venus on the road to perdition.

    After Venus formed and cooled enough to hold any kind of an atmosphere, volcanic outgassing (see figure below: local link / general link: volcanic_outgassing.html) probably gave it a mainly CO_2 and water vapor atmosphere that may have been much like Earth's early atmosphere.

    Venus was just too hot for liquid water either ever or for retaining it if it ever existed.

    The water vapor in fact was largely lost. Convection probably carried it to high altitudes where UV sunlight broke it up and the hydrogen escaped.

    1. The water vapor on Venus today if converted to liquid would cover a perfectly round Venus to a depth of only 0.3 m.

    2. The original water content has been estimated to be equivalent to a covering of 25 m from isotopic analysis.

    3. Earth's water covering a perfectly round Earth would give a 3000 m depth ocean (Se-468).

    Without liquid water there was no major sink for CO_2.

    So the hydrogen escaped leaving Venus increasingly dry.

    The outgassing of CO_2 continued creating the super-CO_2 atmosphere of today and the runaway greenhouse effect.

    The extreme heat of the surface baked sulfur, chlorine, and fluorine out of the ground, and so gave rise to vapors of sulfuric acid (H_2SO_4), hydrochloric acid (HCl), and hydrofluoric acid (HF).

    Volcanoes are also probably injecting sulfur compounds into the atmosphere (FK-251).

  9. Venusian Weather

  10. Well it's NOT fully understood.

    The high altitude weather at the cloud layers is best known because we can see it and probes have gone down through it.

    But what we do know about the surface is that the temperature is extremely uniform from equator to poles and from day to night side.

    The total variation may only be about 10 K from hot to cold regions (Ze2002-184).

    Now the Moon and Mercury are slow rotators with days of 29.5 and 176 Earth days, respectively. They both show extreme daily variations in temperature: i.e., 300 K for the Moon and 500 K for Mercury.

    Venus has a very efficient and simple (as far as we've seen) global circulation pattern driven by heat. (Remember we are seeing the cloud layer mostly, NOT the surface very much.)

    The pattern is best described as two giant VORTICES that are centered on the poles with jet stream winds of up to of order 300 km/hr always moving west mainly at the cloud layer.

    The whole atmosphere seems to rotate west with a period of about 4 days (Se-467--468; FK-250).

    The vortices split at the equator and spiral to the poles. They make a distinctive Y pattern (Se-467--468; FK-250). See the figure below.

    The atmosphere is heated on the day side most intensely near the equator.

    The hot air convects to the poles and night side at relatively high altitudes.

    But somehow the pattern is two symmetric western spirals that are mirrored about the equator, and NOT an axially symmetric flow around the Sun-Venus line---which is the counterfactual case illustrated in the figure below (local link / general link: venusian_atmosphere_convection_false.html).

    The slow rotation does somehow impose a western pattern wind pattern and gives rise to two convection cells: a northern hemisphere one and a southern hemisphere one (FK-250).

    The air rises and the equator and swirls westward to the poles where it sinks flows back in a westward??? swirl to the equator. The westward flow distributes the heat to the night side of Venus.

    The circulation flow is in the main cloud layer where one has the high speed winds of 350 km/hr. The air flows around Venus in about 4 days (FK-250).

    Earth has 6 main convection cells due to greater rotation speed: 3 in each hemisphere that are mirror images of each other: a N/S tropical cell, a N/S temperate cell, and a N/S polar cell (FK-250).

    And it seems that the slow rotation causes Venus to NOT exhibit local cyclones and anticyclones that make Earth's weather so unpredictable and variable (Se-468).

    Turbulence with vortices occurs in fluids when one trys to change fluid speeds too quickly. For examples of vortices on Earth, see the figure below (local link / general link: earth_blue_marble.html).

    Venus cloud pattern pretty much looks the same all the time.

    As far as we can tell Venus does NOT have much weather variation. Recall it has little seasonal variation.

    On Venus probably every Venusian day of 116.75 days (retrograde is probably much like any other.

    We know less about the surface winds on Venus, but they are much slower than the cloud layer winds it seems on average: maybe about 1 m/s or 4 km/hr (CW-37).

    Remember the surface air is awfully dense (or of order 1/10 of water), and so has a lot of inertia (i.e., resistance to acceleration).

  11. Venus and Plate Tectonics

  12. Thanks to the radar mapping of the Magellan probe and earlier orbiters (see Wikipedia: Venus: Radar mapping), we know a lot about Venusian geology---but still far less than there is to know.

    Of course, we have NO SEISMOLOGY. The Soviet VENERA LANDERS only survived a few hours on the surface at most: no time to wait for earthquakes even if they had seismometers.

    We have no Venus rocks, but the VENERA LANDERS did take some pictures and did some crude analysis.

    Although the atmosphere of Venus is radically different than Earth's we might expect the geology to be somewhat similar with the vast exceptions of NO water erosion and weathering and NO sedimentation.

    The subject of PLATE TECTONICS will show up a significant difference.

    To investigate PLATE TECTONICS, we will first consider some Magellan mosaic radar images of Venus. See the figure below.

    Now the radar mosaics are cute, but how about a TOPOGRAPHICAL ELEVATION MAP. See the figure below.

    Does Venus have PLATE TECTONICS?

    1. There is the granitic rock which together with the terras suggests continents.

    2. Some trenches resembling oceanic trenches have been found in the lowlands (HI-175).

    3. There are some fold-mountains in the Ishtar terra (Se-471). This indicates some horizontal crustal motion, but that mountain range is NOT large.

      There are no large arcs of fold mountains that often indicate plate boundaries on Earth???.

    4. There seems to be a rift in the Aphrodite terra (CW-37) perhaps indicating a divergent boundary.

    5. The terras cover a distinctly smaller fraction of Venus than the continental crust does on Earth. The terras cover about 24 % of Venus and continental crust covers about 41 % of Earth (HI-175; Cox-253).

        Question: If the water surface of the Earth covers about 71 % of the surface, how come the continental crust covers 41 % of the surface?

        1. Bad math.
        2. Some of the water surface is under the continents.
        3. Some continental crust is submerged.

        Answer 3 is right.

        Remember the continental shelves extend well beyond the coasts.

    So the evidence is MIXED on whether there is Venusian PLATE TECTONICS.

    Perhaps there is bit of plate tectonic action on Venus, but it never fully into full plate tectonics with extensive continents and arcs of fold mountains as on EARTH.

    Perhaps much of Venus can be described as hotspot volcanism where rising MANTLE PLUMES of magma lead to volcanoes and perhaps much of the uplands.

    Giant mantle convection cells (leading to plate boundaries) may NOT be extensive.

    Why is PLATE TECTONICS at least under-developed on Venus and perhaps NOT really present at all?

    As of yet, there is no definitive answer, but it may be that the surface heat of Venus makes the surface layers too plastic for plate tectonics with rigid plates (Se-476).

    Perhaps abundant liquid water is needed as a lubricant (Gri-172,173). Hydrated rocks may be needed for allow plates to slide.

    On the other hand the bit by which Venus is SMALLER than Earth may be essential difference.

  13. The Formation and Continuing Geological Evolution of Venus

  14. Venus probably formed much like Earth.

    It built up from protoplanets and planetesimal.

    It was hot from formation and radioactivity.


    No GIANT IMPACTOR created a moon, but one may have given Venus it's unusual rotation characteristics as discussed above.

    Venus did suffer the HEAVY BOMBARDMENT, but continuing geological activity has erased the EARLY CRATERING just as on Earth.

    Venus is almost the same size as the Earth and very probably has the same composition of refractories and radioactive elements.

    Thus, its primordial-radiogenic heat geology (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat) should continue to the present.

    Venus like the Moon and Mercury and probably the Earth, had lava flooding of huge basins.

    And Venus still has lava plains---the gently rolly lowlands that cover 60 % of the surface (HI-174,175).

    But those lava plains are NOT the original lava plains.

    1. Impact Cratering on Venus:

      Venus like the EARTH has been re-surfaced probably several times.

      We know this for sure because of impact cratering.

      Venus has about 900 observed impact craters.

        Question: 900 impact craters is:

        1. far more than the Earth has.
        2. far fewer than the Earth has.
        3. about the same as the Earth has.

        Answer 1 is right.

      Venus has far more than the Earth's 170 craters, but far less than the Moon's and Mercury's quasi-infinity of craters (Se-473).

      For a model of a impact crater on Venus, see the figure below.


        Caption: A 3-d model of Venus impact crater Golubkina based Magellan radar mapping.

        The color is false, of course, but was chosen to emphasize structural detail and simulate the colors seen on the surface by the Soviet Venera landers.

        The vertical scale may be stretched to emphasize the features.

        This seems to be a typical impact crater with central peak, flatish floor, and slumping rim walls.

        Credit/Permission: NASA, before or circa 2003 / Public domain.
        Download site: Views of the Solar System by Calvin J. Hamilton. CJH states the image is Public domain.
        Image link: Itself.

      The relatively small number of craters on Venus tell us that the surface has been re-surfaced and it can be estimated from the crater counts that the current surface is on average somewhere in the range of 500--800 Myr old (HI-177). (The time scales vary with reference source.)

      The whole surface is probably younger than 1 Gyr.

      The implied renewal rate does seem to be about five times slower than for the Earth on average (CW-34).

      On the other hand the Earth has some very old patches that Venus may lack (HI-177). (But how do we know this????.)

      More cratering and slower renewal than Earth can at least partially be accounted for by the LACK of water erosion.

      The only things that can erase craters are volcanic lava flows and ash deposition and small amounts of fold-mountain construction and wind erosion.

        Cratering to erase craters is a slow process after the heavy bombardment and probably negligible on a planet where the surface is continually being renewed.

      An interesting point about Venusian craters are that there are none smaller than about 3 km in diameter (Se-473).

        Question: Why are there no small craters?

        1. The small impactors burn up or break up in the heavy atmosphere.
        2. The small craters are selectively erased by volcanic action. Volcanoes prefer forming near small craters.

        Answer 1 is right.

        Of course, there is NO micrometeoritic erosion on Venus pulverizing the surface to regolith as on the Moon and Mercury.

    2. Other Geologic Processes on Venus:

      What are the forces of continuing geologic change besides cratering?

      There is wind erosion as mentioned above, but this is slow compared to the absent water erosion.

      So we come to VOLCANISM driven by primordial-radiogenic heat.

      Volcanic action creates new lava flows and ash layers that cover old terrain.

      Some horizontal crustal movement initiated by lava flooding may created limited fold mountains (Se-471).

      Shield volcanoes such as occur on Earth and Mars are present. See the figure below (local link / general link: volcano_shield_cartoon.html) showing a cartoon of a shield volcano.

      An example of a
      shield volcano on Venus is Gula Mons. See the figure below.


        Caption: A model 3-d view of Gula Mons and Crater Cunitz on the western part of Eistla Regio.

        Lava flows and faults appear as bright in the image.

        Eistla Regio is about at the west end of Aphrodite Terra and south of the east end of southmost reaching part of Ishtar Terra.

          Eistla is a Norse giantess. A regio is a large area marked by reflectivity or color distinctions from adjacent areas, or a broad geographic region.

        The image is based on radar data from the Magellan orbiter The colors are meant to imitate those shown by the Soviet Venera 13 and 14 landers and are used to enhance surface structure.

        The elevations have been stretched by a factor 22.5 ??? to enhance visualization ( NASA NSSDC Photo Gallery Venus).

        You are looking from an elevation of 0.78 km to the north-east toward Gula Mons which is 1,310 kilometers away.

        Gula Mons is a 3 km high volcano: it is a normal shield volcano (Se-473), but the stretching has made it look a bit unnatural???. The height is relative to the Eistla Regio ( NASA NSSDC Photo Gallery Venus).

        Gula is a Babylonian earth mother: symbolically a creative force---I just quote the sources.

        Impact Crater Cunits is named for Polish mathematician and astronomer Maria Cunitz (1610--1664). It is in the foreground and 215 km from the observer. The crater diameter 48.5 km. It shows the rim and flatish inner floor with a central peak of a typical large impactor crater.

        JPL Multimission Image Processing Laboratory produced the image for release at a 1991mar05 JPL news conference.

        Credit/Permission: NASA, JPL 1991 / Public domain.
        Download site: Views of the Solar System by Calvin J. Hamilton.
        Image link: Itself.

      One volcanic feature unique or nearly unique to Venus are coronae.

      The singular is corona, but this is NOT to be confused with the solar corona.

      For an example of a corona, see the figure below.

      There are many other geological features on Venus (such as pancake domes), but we won't consider them here.

      The locations of volcanoes, coronae, terras, etc. are probably random and determined by the random initial conditions, plus gigayears of continuous geological evolution.

      One odd last thing to remark is that Venus has no measurable GLOBAL MAGNETIC FIELD.

      But Venus really should have a molten iron core like the Earth.

      Maybe the slow rotation of Venus is NOT sufficient to generate a dynamo effect (Se-476).

  15. The Fate of Venus

  16. There will be continued geological activity on Venus, but probably NOT a lot of qualitative change over the next few gigayears???.

    Then Venus will get vaporized in the post-main-sequence-phase Sun in ∼ 5 Gyr (see Wikipedia: Sun: After core hydrogen exhaustion).

    The two figures below (local link / general link: sun_evolution.html; local link / general link: sun_red_giant.html) show what happens to the Sun As Time Goes By (1931).