Space & Astronomy/Space and Astronomy (October 1993).iso/mac/TEXT/SPACEDIG/V16_3/V16NO376.TXT

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Date: Sat, 27 Mar 93 05:40:51
From: Space Digest maintainer <[e-mail]>
Reply-To: [e-mail]
Subject: Space Digest V16 #376
To: Space Digest Readers
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Space Digest                Sat, 27 Mar 93       Volume 16 : Issue 376

Today's Topics:
                 Chicago area cosmonaut lecture times
Gravity waves, was: Predicting gravity wave quantization & Cosmic Noise
                 In what craft did Glenn orbit the E
                 Magellan Update - 03/22/93 (2 msgs)
Speculation: the extension of TCP/IP and DNS into large light lag enviroments
   Stockman, Mark, and Keyworth (was Re: Flight time comparison...)
            Timid Terraformers (was Re: How to cool Venus)
                    Venus Atmosphere Paper (long)

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----------------------------------------------------------------------

Date: Fri, 26 Mar 1993 23:31:20 GMT
From: Dennis Newkirk <[e-mail]>
Subject: Chicago area cosmonaut lecture times
Newsgroups: sci.space

Cosmonaut Dr. Georgi Grechko will be presenting a lecture about
his years of involvment in the Soviet/Russian space program in
early April in the Chicago area.

  Tue, April 6 at 7:30 PM at Harper College, Building J, Room 143
  Wed, April 7 at 7:30 PM at Chicago Police Dept. 14th District
    Office Auditorium for Wright College.
  Thur, April 8 at 10:00 AM and 11:30 AM at Chicago Museum of Science and
    Industry
  Thur, April 8 at 6:00 PM at Museum of Science and Industry for
    Chicago Council on Foreign Relations (admission $12).

All appearance are free and open the the public except for the last one.

Dr. Grechko is currently head of an atmospheric physics lab of the
Russian Academy of Sciences. Grechko made 3 spaceflights, one
to the Salyut 4 space station for 29 days in 1975, one to the Salyut 6
space station in 1977 for 96 days, and one to the Salyut 7 space
station for 8 days in 1985. Before joining the cosmonaut corp, Grechko
was involoved in ballistics planning for Sputnik, Vostok 1 which
launched the first person into space, and Luna 9 which returned
the first pictures from the surface of the moon. He also trained
to fly missions to the moon in the late 1960's.

Dr. Grechko's visit to Chicago is sponsored by the Chicago Society
for Space Studies, one of four area chapters of the National Space Society.
Groups co-sponsoring lectures include the Planetary Studies Foundation in
Palatine and the Chicago Council on Foreign Relations and the Museum
of Science and Industry.

------------------------------

Date: Fri, 26 Mar 1993 21:41:11 GMT
From: Cameron Randale Bass <[e-mail]>
Subject: Gravity waves, was: Predicting gravity wave quantization & Cosmic Noise
Newsgroups: sci.space,sci.astro,sci.physics,alt.sci.planetary

In article <[e-mail]> [e-mail] (Nick Haines) writes:

>The curvature in which we're interested is thus a property of the
>surface (or space) itself, and does not require the concept of an
>`embedding space.' Since we can never observe such a space, why
>suggest it exists? It's not required by our theory, it's no part of
>our description of the universe, and is thoroughly bogus.
>
>Should this go in the FAQ?

     No.  There is no intrinsic reason we should restrict inquiry to the
     "ant's eye view".  If it is useful to embed the space in another,
     we should go right ahead.

     "Existence" is a rather tenuous concept in this context.  Do
     complex numbers "exist"?  How about tensors?  How about the
     "space" itself.  Why do you think physical space is some sort
     of local manifold describable by our mathematics?

                              dale bass

------------------------------

Date: Fri, 26 Mar 1993 20:46:13 GMT
From: Dave Michelson <[e-mail]>
Subject: In what craft did Glenn orbit the E
Newsgroups: sci.space

In article <[e-mail]> [e-mail] writes:
>
>it wasn't a ship it was a mercury CAPSULE.  i believe it was called
>freedom 7.

Nope.

Freedom 7 was flown by Alan Shepard on the first suborbital flight.

Friendship 7 was flown by John Glenn on the first orbital flight.

>also he wasn't the first man to orbit the earth in a u.s.
>spacecraft.

I guess the secret is out!  Eisenhower made a SECRET flight into
SPACE aboard SCORE in 1958.  For reasons of NATIONAL SECURITY, we
were TOLD that SCORE was an experimental comunnications SATELLITE.
In FACT, SCORE was AN acronym for SPACE CRAFT carrying ORBITING REPUBLICAN
chief EXECUTIVES.

>answer tomorrow.

I beat YOU to it :-)

---
Dave Michelson                             University of British Columbia
[e-mail]                                  Antenna Laboratory

------------------------------

Date: Fri, 26 Mar 1993 22:01:45 GMT
From: Eric H Seale <[e-mail]>
Subject: Magellan Update - 03/22/93
Newsgroups: sci.space,sci.astro,alt.sci.planetary

[e-mail] (Steve Derry) writes:
>Another alternative would be to map small selected areas of high interest
>and play the data back at the current 1200bps rate.  By the time that TEX
>and cycle 5 gravity mapping is complete, the target areas could be selected.
>If they were small enough, and spaced far enough apart, then the data could
>be stored onboard during mapping orbits (only mapping over a small latitude
>range), and played back at slow rate after the target area has been covered.
>Alternatively, portions of the data could be played back between mapping
>passes, but this would make operations a bit more complex.

Unfortunately, the hardware just wasn't designed to do this.  Working
from memory here (I was an AACS guy, not telecom), the Magellan telecom
scheme works like this.  You have a carrier signal at some frequency.
1200 bps engineering and high-rate science data are then superimposed on
this signal via two separate sub-carriers.  The composite signal gets
shipped off to the amplifiers and out the antenna.  I'll attempt a
drawing:

   ---------             ---               ---              ----------
  | carrier |  -------->| + |------------>| + |----------->|Amps, etc.|
   ---------             ---               ---              ----------
              ^                 ^
              |                 |
                     ----------         ---------
            | 1200 bps |       | hi-rate |
            | modulat'n|       |   mod.  |
             ----------         ---------
              ^                 ^
              |                 |
            -------------       ---------
                   | engineering |     | science |
           |    data     |     |   data  |
            -------------       ---------

Problem is, the Magellan tape recorders (Magellan only has one antenna,
so radar data has to get taped and then played back) can only talk to their
subcarrier box (the official name of this "box" escapes me, but it does
the data modulation, as I recall).  So, aside from the fact that the
recorders can't run slow enough to output data at "just" 1200bps, they
couldn't put that signal onto the engineering subcarrier anyway.

For a while, project folks were working on a scheme to read short pieces
of tape-recorded data into the on-board computer's memory, then play it
back slowly over the 1200bps engineering data stream.  The read-out,
though, can't "take over" the 1200bps stream -- it only gets a part of
it (I think about 100bps).  Now consider that most of the "missing" 1%
of Venus consists of missed orbits (so your radar data will be
occasional complete swaths, each of which nearly fills the surviving
tape recorder).  Now, the tape recorder holds a couple of gigabytes of
data, transmit it back at 100 bps when you have DSN coverage (last I
heard, about 6 hours every day after TEX)...

As for why high-rate playback can't occur in the first place, both
telecom "strings" are wounded.  In one, the signal "addition" circuitry
is dead (high-rate data subcarrier doesn't even show up); in the other,
part of the telecom circuitry is outputting a spurious tone (kind of a
microwave "whistle") that sits very nearly on top of the high-rate data
subcarrier signal.  The only way to get rid of this tone is to heat the
transmitter to just short of the point where the solder on the circuit
boards will start to melt...

Another "slight" wrinkle to this situation is the question of staffing.
In the interests of saving development money, Congress mandated that
Magellan need lots of care & feeding from the ground (autonomy = $).
So, you need to hang on to the people that generate mapping parameters
for the radar and attitude control, all the radar ops folks, lotsa
flight people to do the "tweaking" for the tape-recorder-to-computer
kludge, etc.  Now consider that most of these people have already been
"encouraged" to find other employment, and you'll get the picture.

My $0.02
Eric Seale

------------------------------

Date: 26 Mar 1993 23:47:29 GMT
From: "Peter G. Ford" <[e-mail]>
Subject: Magellan Update - 03/22/93
Newsgroups: sci.space,sci.astro,alt.sci.planetary

In article [e-mail], [e-mail] (Steve Derry) writes:
>Another alternative would be to map small selected areas of high interest
>and play the data back at the current 1200bps rate.  By the time that TEX
>and cycle 5 gravity mapping is complete, the target areas could be selected.
>If they were small enough, and spaced far enough apart, then the data could
>be stored onboard during mapping orbits (only mapping over a small latitude
>range), and played back at slow rate after the target area has been covered.
>Alternatively, portions of the data could be played back between mapping
>passes, but this would make operations a bit more complex.
>

It's a nice idea, but you cannot just isolate a small portion of a raw
radar signal and extract from it a high-resolution image of a small
patch of ground.  The surface echoes are re-distributed in time and in
frequency, and you'd need about a megabyte of raw Magellan data in
order to generate the first patch of high-resolution image. After that,
the relationship is quite linear.  Also, I don't think that the tape
recorders that store the radar data can be down-linked at 1200bps,
since the latter is intended for engineering data only, and the
high-rate telemetry takes a different path.

Peter Ford
MIT Center for Space Research

------------------------------

Date: Sat, 27 Mar 1993 01:10:42 GMT
From: Tom A Baker <[e-mail]>
Subject: Speculation: the extension of TCP/IP and DNS into large light lag enviroments
Newsgroups: alt.internet.services,sci.space

In article <[e-mail]> [e-mail] (M. Sean Bennett) writes:
>As man moves outward into space it will become essential to provide an information
>structure for communication of data.
>
>    The current set of protocols make no alowance for light 'lag' between
>targets of wide divergence. (Mars-Earth). The current DSN is expensive to

Now hold on there.  Even terrestrial protocols take notice of "lightspeed
delay".  And transmissions over satellite links in the Clarke orbit
require special parameters for their error correction protocols.  They
don't use XMODEM or Kermit; there is something like a 570 mS round trip
time, so the handshaking is arranged with that in mind.

>    We need some form of ISO standard (I know they are hard to set,
>but if NASA/GlavCosmos publish a protocol it will be the defacto standard)

I do think we will indeed have a standard, when the need arises.  Until
that time, we will have to wait to learn of the resources available then.

I for one would be tickled pick if we could set up optical fiber cables
between the two planets.  And don't say that is flatly absolutely
impossible.

tom

------------------------------

Date: 27 Mar 93 01:55:52 GMT
From: Pat <[e-mail]>
Subject: Stockman, Mark, and Keyworth (was Re: Flight time comparison...)
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (Dave Michelson) writes:
>sure, the best of intentions.  It's not the first time technical people
>have jumped on such a bandwagon (von Braun did so at Peenemunde) and not

And I thought It was because Von Braun wanted to further the cause
of aryan peoples everywhere:-)

No, No,  Dennis, put down the flame thrower.

Quick, Bill, Get me my Nomex Underwear, AAAIIIEEEEEEEE....

------------------------------

Date: Fri, 26 Mar 1993 20:36:14 GMT
From: Paul Dietz <[e-mail]>
Subject: Timid Terraformers (was Re: How to cool Venus)
Newsgroups: sci.space

In article <[e-mail]> [e-mail] (John F. Woods) writes:

> However, the surface of venus is probably oxygen-poor; most of the
> carbon dioxide in the atmosphere was baked out of the surface rocks,
> and if they ever cool below red-heat, they may be ready to react with
> whatever atmosphere remains.  It might be embarassing to blow off the
> entire current excess, cool Venus off a bit, and then suddenly wind up
> with a vacuum when the surface rocks suck all the remainingt carbon
> dioxide back in... :-)

What does CO2 have to do with the oxygen content of Venus's crust?
Most of the oxygen on the inner planets is in the form of silicates,
mostly Mg-Fe silicates in the mantles.  The very top of Venus is
likely somewhat reduced compared to Earth, as there has been no
biological carbon pump to maintain an oxidation gradient across the
lithosphere, but there is still plenty of oxygen in the rocks.

CO2 is not going to be sucked out of the atmosphere by reaction with
oxygen-poor materials.  Instead, it reacts with silicates to make
carbonates and silica (or, with oxides to make carbonates).

Talk of terraforming Venus should also keep in mind that the *crust*
has to be cooled off.  This could take longer than just cooling the
atmosphere, as rock is not very thermally conductive.

    Paul

------------------------------

Date: Fri, 26 Mar 1993 21:22:20 GMT
From: Chris Schiller <[e-mail]>
Subject: Venus Atmosphere Paper (long)
Newsgroups: sci.space

Below is a paper on the atmosphere of Venus that I wrote for a class
a few years ago.  I thought it might be helpful in the recent discussions
here.  Reading through it again, I feel that it is a good
examination of the atmosphere, but some of the "must have" statements
are probably on shakey ground.  Sorry the tables and figure are not
present, but news is still ascii.  References are at the end.

Chris Schiller
[e-mail]


--------------------------------------------------------------------



          The planet Venus was studied through the use of a spacecraft
      for the first time in 1962 by the Mariner II craft.  It had
      been studied by Earth based instruments from the time of the
      introduction of the telescope.  After Mariner II both Soviet
      and American probes were sent to Venus to collect data which
      could not be obtained from Earth.  Since the surface is
      obscured in the visible spectrum by opaque clouds, and
      the geography is relatively uninteresting, the atmosphere
               ^^^^^^^^^^^^^^^^^^^^^^^^ [what the hell is
                                                    Magellan for?  I think
                                                    I should probably
                                                    retract this
                                                    statement]
      has attracted the overwhelming majority of study.  A great deal
      of data were obtained through the seventies by mostly Soviet
      probes which included soft landers.  These missions climaxed
      in 1978 with the Soviet Venera 11 and 12, and the US
      Pioneer-Venus probes, of which the orbiter portion still
      operates and continues to provide data.
           A commonly accepted model of solar system evolution
          holds that Venus and Earth were formed of similar materials
      under similar circumstances.  This is one of the more important
      reasons for studying Venus: by understanding Venus we can
      better understand the Earth.  Many similarities exist between
      Earth and Venus.  Venus has .8 the mass of the Earth, .9 of the
      radius of the Earth, and is only 30 percent closer to the sun
      than the Earth.  There are also many significant differences.
      Venus has a much hotter, opaque atmosphere.  Surface temperature
      and pressure are 735 K and 90 atmospheres respectively.  The
      planet has an axial rotation period of 243 days and very little
      inclination.  Water is almost absent on the planet.  Venus
      has obviously evolved an atmosphere very different from Earth's
      atmosphere.
           The most important aspect of the atmosphere is probably
          its chemical composition.  The positively identified
      gases in the lower atmosphere include CO2, N2, H20, CO, HCL,
      HF, SO2, S3, He, Ne, Ar, and Kr. (Moroz 1981)  Some of
      the gases measured by the Pioneer-Venus gas chromatographs
      are summarized with their concentrations in table 1.
      (Oyama et al. 1979)




                              Table 1
          Carbon dioxide and nitrogen make up all but .1 % of the
      atmosphere with water having a mixing ratio in the range
      1 X 10^-5 to 1 X 10^-3.  The different probes gave widely
      varying measurements of the mixing ratio of water either
      through experimental errors or through real variations
      in the abundance of water.  The identification of O2 by
      Pioneer-Venus and of CO by other landers in the lower
      atmosphere suggests that the lower atmosphere is not
      in thermodynamic equilibrium since they cannot exist
      together in equilibrium.  (Moroz 1981)  Sulfur was expected
      to be found in the lower atmosphere in the form of COS,
      but was instead found in the form of SO2.  Only trace
      amounts of COS were found by the landers.  It was found that
      the ratios of the masses of the measured noble gases to total
      planetary mass (except for 40 weight Ar) is much greater than
      the same ratio for the Earth.  This suggests either a
      much greater endowment in the formation of the planet, an
      addition to the inventory by solar wind or a collection
      of solar wind irradiated matter.  The ratio for Ar 40 is
      1/4 that of the Earth.  This may be due to different
      planetary tectonics. On Venus the crust may not have been
      heated and overturned as it has on Earth, and the Ar 40
      is still trapped in rocks below the surface.
      (Pollack and Black 1982) The upper atmosphere consists mainly
      of CO2, CO, N2, O, and He with profiles shown in figure 1.


                             Figure 1
      Oxygen is the most abundant above 155 km, with CO2 overtaking
      at the 155 km height.  The source of the atomic oxygen and carbon
      monoxide is probably the dissociation of CO2 by radiation, and
      these gases are transported to the lower atmosphere by
      eddy diffusion.  Above 200 km, He becomes the largest constituent.
      As with water, there is very little H2 in the atmosphere
      of Venus.  Any original H2, along with any produced from
      the decomposition of water, has probably been outgassed to space.
           Measured temperature profiles are shown in figures 2, and
      with a blow-up of the lower levels, figure 3.  Below 40 km the
      lapse rate of the measured temperatures dT/dz is about 8 K/km.



                       Figure 2                    Figure 3
      This is very close to the adiabatic lapse rate, suggesting that
      there is a very good mixing of the atmosphere at these altitudes.
      (Seiff et al. 1979).  Above 40 km, the data deviates from the
      adiabatic rate to fit more closely the profile expected for
      a gas in radiative equilibrium.  The most opaque clouds occur
      in the 49-50 km range, are heated by incoming radiation,
      and give rise to convective motion just above this level.
      The data from the descending probes become erratic at this level.
      Figure 2 shows that at about 85 km the atmosphere becomes
      isothermal. This region extends up through the 110 km altitude.
      Multiple measurements have shown that for a given latitude,
      in the 65 to 100 km altitude, the day to night variations
      in temperature are less than 5 K.  This shows that there
      must be a very strong atmospheric circulation from the dayside
      to the nightside.  Large temperature variations in a latitudinal
      direction, however, have been observed.  Temperatures will
      drop from their equatorial highs some 25 K in the 60-80 deg
      latitude range, and then rise back to the equatorial values
      at the poles.  This characteristic is probably caused by
      a large, overturning cell with compression at the pole, and
      rising gases at the 60-80 deg range. (Taylor et al. 1979)
      The high surface temperature means that any water, CO2, or
      N2 in the surface rocks have outgassed to the atmosphere.
      Therefore, most of the original amounts of these gases
      must have been added to the atmosphere, with very little
      trapped in the mantle.  This is in contrast to Earth, where
      water is trapped by temperature in the oceans, and CO2 is
      trapped in sedimentary rocks as a result of the liquid water.
           The clouds of Venus, although by mass a small part of
          the total atmosphere, are very important.  They
      block the view of the planet from 70 km to a large portion
      of the electromagnetic spectrum including visible light.
      Their reflectivity of short wavelengths, and their absorption
      of long wavelengths is a driving force in the climatic
      equilibrium of Venus.  They provide a large part of the filtering
      required for greenhouse heating of the lower atmosphere.  Four
      distinct layers were observed by the Pioneer-Venus landers.
      An upper layer, approximately 10 km in depth starting at
      68 km, contains particles 3 um in diameter and less.
      A middle layer, separated from the upper by a distinct
      boundary, is 6 km thick, and consists of 8 um or less
      particles.  The lower layer, only 4 km thick, is the densest
      layer, and the most opaque.  It also contains particles of
      all sizes less than 8 um.  A sub-layer is also present,
      extending from the bottom of the lower layer at 48 km, to
      approximately 31 km altitude.  This layer is better
      characterized as a region of haze of less than 1 um particles.
      Measurement of the index of diffraction of the cloud layers
      match those of 80% sulfuric acid (H2SO4) solution in water with the
      corresponding particle size.  This match becomes less distinct
      in the lower layers, where absorption spectrum show that other
      admixtures must also be present, with FeCl2H2O, HBr, and
      elementary sulfur being likely candidates. (Moroz 1981)
      The clouds of Venus are much less optically dense than those
      of the Earth.  Absorption which would take place in tens
      of meters on Earth, takes kilometers on Venus.  Large variations
      in the density, particle size, and altitude of the clouds were
      found by the different probes in different locations on the
      planet.  These variations are probably real.  The circulation
      required for mixing of the atmosphere to obtain matching
      thermal profiles must be intense.  The Venera 11 and 12
      spacecraft also detected low frequency pulses during descent
      to Venus.  These are likely caused by thunderstorm activity
      with cloud to cloud discharge causing the pulses.  The presence
      of lightning opens the possibility of nitrogen compounds
      forming in the lower atmosphere.
           The atmosphere of Venus must be mixed in some manner
          to redistribute the energy of the incoming solar radiation.
      This energy must be moved away from the equatorial dayside
      region.  This is the region of greatest heating.  The slow
      rotation of Venus cannot provide much help in moving the energy.
      A cross-section of measured wind velocities by three
      Pioneer-Venus probes are shown in figure 4.



                              Figure 4
      The atmosphere of Venus rotates in the same direction as
      the planetary rotation, but with a much higher velocity than
      the surface at most altitudes. This is termed "super-rotation".
      The angular velocity of Venus' atmosphere is small because
      of its relatively slow rotation.  This causes the coriolis
      force to be small everywhere, and the Rossby number to be
      much larger than one.  No cyclones or anti-cyclones should
      be found on Venus, and none have yet been observed.  The
      meridional wind at 50-60 km as seen in figure 4 is toward
      the equator, while the wind at 65 km is toward the poles.
      This implies that a Hadley cell is operating to redistribute
      energy to the polar regions.  The atmosphere is heated at
      the equator.  The gases rise, and flow outward to the poles,
      where the gases cool.  They then flow back to the equatorial
      region to replace the gases which rise.  Since the planet has
      almost no spin inclination to the sun, there are no seasonal
      variations in redistrubution patterns.  The mechanism for
      super-rotation is presently still debated, but a simulation
      which provides reasonable matching of wind profiles relies
      on momentum transfer provided by the meridional Hadley cell
      and large scale eddies. (Moroz 1981).  This general hypothesis
      is supported somewhat by studies of data from 1982 observations
      of circulation. (Baker and Leovy, 1985) and (Limaye et al. 1987)
      The 1982 data also shows a solar locked divergence of clouds
      around the local noon, as one would expect from the heating
      in that region. (Limaye 1987)  The observations seem
      to point to a general solar tide with angular momentum
      derived from interactions with the meridional circulation.
           Above 100 km, the incoming ultraviolet radiation is
          intense enough to ionize any atoms or molecules present.
      Typical electron densities are shown in figure 5.




                                 Figure 5
      These look very similar to those found on Earth.  The profile
      is caused by an increasing atmospheric density irradiated by
      a more and more obscured incoming UV source.   The Venus
      ionosphere was found to be extremely variable on both
      the dayside and the nightside.  Orbiters would measure
      a substantial density of ions on one orbit, and on the
      subsequent one would find almost no ionized species for the
      same altitude.  The Venus ionosphere is different from Earth's
      ionosphere in the respect that it does not interact with
      a magnetic field produced by the planet.  The Earth has a
      relatively strong magnetic field which shapes and controls
      the free ions.  It also interacts with the solar wind, causing
      effects not seen at Venus.  The lack of a magnetic field
      at Venus permits a stronger interaction of the ionosphere
      with the solar wind.  The solar wind is also denser at Venus'
      orbit.  The solar wind exerts a pressure on the ionosphere
      of Venus, and compresses it.  This is shown simplified
      in Figure 6.



                               Figure 6
      The ionosphere is therefore sensitive to fluctuations in the
      solar wind.  A one-dimensional model (Stein and Wolff, 1982)
      shows that variations in the solar wind dynamic pressure over
      tens of minutes can produce tens of kilometers of height
      variation in the density of the ionosphere.  The ionosphere
      compresses, and then expands-a breathing.  This can have
      several effects.  The compression results in heating, the
      expansion in cooling.  The breathing could also contribute
      to ion transport to the nightside.
           The Venus probes discovered a substantial density of
          ions in the upper atmosphere of the nightside of the
      planet.  Since there is no source of energy on the nightside,
      these ions must have come from the illuminated side of the
      planet.  A two-dimensional model (Whitten et al. 1981) using
      ion density differentials, shows that the observed nightside
      ionosphere can be explained by diffusion of ions from the
      dayside.  The ion drift velocities across the terminator
      estimated by the model are able to approximate those observed.
      The nightside ionosphere has also been observed to have large
      fluctuations in density.  Correlations (Cravens et al. 1981)
      between the disappearance of the nightside ionosphere and
      increased solar wind activity have been made.  The solar
      wind has great effects upon the both the dayside and nightside
      ionospheric components.
           A large amount of discussion has focussed on the origin
          and evolution of the atmosphere of Venus because of its
      intimate relation to that of the Earth.  The matter that
      makes up the planet has been theorized to come from three
      sources.  The original proto-planetary matter from which the
      primary planet condensed is likely the largest source.
      Subsequent collection of inner-planetary matter, both from
      the inner and outer solar system is the next source.  The
      final source is matter from the sun transported on the
      solar wind.  Once the planet condensed, an outgassing of H
      and He occurred.  This outgassing was either catastrophic or
      gradual depending on conditions such as temperature.  Explanations
      have been theorized for the differences in the abundance of
      gases including H2O, CO2, and the noble gases.  Other
      differences include the deuterium to hydrogen ratio of Venus
          being 100 times that of the Earth.  Table 2 shows abundances
      for several gases on Venus, Earth, and Mars.  The first
      column is the mixing ratio of the present atmosphere.  Column
      x gives an enhancement factor for any near surface reservoirs,
      and column r gives the ratio of the sum of the first two columns
      to the total planetary mass.



                                 Table 2
           The simplest, and most trivial explanation is that Venus
          was originally endowed with the present differences, or close
      to them, and that collection of matter and outgassing played
      insignificant roles.  This is considered possible, though
      unlikely.  Another model assumes that the Earth and Venus
      formed with similar original compositions, and differing
      conditions caused them to diverge to their present state.
      This is the most widely accepted version, and is supported
      by much of the data.  A great deal of the total mass of
      the gas CO2 is locked in sedementary rocks on the Earth.
      There is no such sedementation on Venus and almost all of the
      available CO2 is in the atmosphere.  If one accounts for the
      CO2 in Earth's rocks, the ratios of the mass of CO2 to the
      planetary mass is approximately the same.  This suggests
      that both planets were endowed with equal proportions
      of CO2.  The abundance of free oxygen on Earth is
      explained by the continual replentishment by the biosphere.
      On Venus, without a biospheric source of O2, the gas
      would have combined long ago with other constituents.
      The abundance of the noble gases on Venus has been theorized
      to have resulted from collection of solar wind implanted
      matter (Bogard 1987).  In this scenario, the matter which
      became Venus was implanted with noble gases from the solar
      wind while blocking this accretion in the pre-Earth matter.
      This matter was collected by the proto-planet in its inner
      orbit, and the gases remained because they were to massive
      to outgas to space.  This is supported by the presence of
      noble gas implanted rocks from lunar sources.  Venus, with
      its lack of magnetic field is also better able to collect
      matter from the solar wind.  However, the present rate of
      noble gas flow in the solar wind, if extrapolated over 4 billion
      years, is still not sufficient to account for the noble gas
      abundance on Venus.  The rate may have been greater in the
      earlier stages, but this is considered not probable.
      The near absence of water on Venus must be explained in relation
      to the relative abundance of it on Earth.  The high temperatures
      on Venus mean that any water must be in the gaseous state.
      This permits a certain fraction to be exposed to decomposing
      UV radiation in the upper atmosphere.  Once freed, the hydrogen
      will outgas to space.  The decomposed water will be replaced
      with more from below, and the process will continue until
      all of the hydrogen part of the water has escaped the planet.
      The ratio of deuterium to hydrogen is explained in this
      case by the retention of the original deuterium due to
      its much slower outassing rate.  It has been proposed
      (Grinspoon and Lewis, 1987) that the amount of water left
      on Venus cannot be left from the original in the above process.

      With no outside source, the mixing ratio of water, over the
      5 billion years of possible outgassing, must be smaller than
      observed.  They suggest that cometary impacts have provided
      a continuous, if not punctuated, source of water on Venus.
      The deuterium to hydrogen ratio predicted by this model,
      however, is larger than the observed ratio.  Such
      disprepancies might be explained by very recent cometary
      impacts such as parts of the same proposed comet cloud
      that may have caused the cretacious extinctions on Earth.
           The process which led to the high temperatures on Venus,
          which in turn led to the loss of water and the accumulation
      of carbon dioxide, have also been studied.  A study of
      the runaway greenhouse effect (Kasting 1987) suggests that
      a solar flux 1.4 times that presently at the Earth would
      cause thermal runaway.  This was approximately the flux
      at Venus early in the solar system's history.  His model
      is independent of CO2 in the atmosphere.  A planet with
      the amount of water now on Earth, with a solar flux
      equal to 1.4So, would have its temperature rise, which
      would evaporate more water, which would in turn absorb
      more radiation.  This positive feedback would result in
      a surface temperature of 1500 K, giving a steam atmosphere,
      and a subsequent loss of hydrogen by photodissociation and
      outgassing.  Once the process is started it insures that
      all the water, including any trapped in surface rocks, is
      lost because of the loss of hydrogen.  Earth, possibly
      because of its lower solar flux, cloud cover, or extraction
      of CO2 from the atmosphere, has remained a cooler planet and
      retained the present water abundance.
           Planetary probes to Venus in the 1970's amassed a large
          amount of data on the atmosphere of the planet.  The
      atmosphere's composition, structure, and dynamics were
      measured.  From the data scientists have theorized reasonable
      models for the clouds, the ionosphere, and the chemical
      processes involved in the present atmosphere and in its
      evolution.  Continued study is required to fully
      understand the mechanism of super-rotation, the clouds in
      the polar regions, and formation of the cloud layers.
      The chemical composition the surface rocks would be of
      special interest.  The Magellan probe, scheduled for launch
      in early 1989, will provide high-resolution maps of the
      planet along with other data available through its high
      power radar.  Unfortunately, no surface lander or descent
      spacecraft is currently scheduled for a mission to
      Venus.



                        References

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      Bogard, D. D.:1987, On the Origin of Venus' Atmosphere, Icarus 74, 3

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      Cimino, J.:1982, The Composition and Vertical Structure of the Lower
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End of Space Digest Volume 16 : Issue 376
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