Date: Sat, 27 Mar 93 05:40:51 From: Space Digest maintainer Reply-To: Space-request@isu.isunet.edu Subject: Space Digest V16 #376 To: Space Digest Readers Precedence: bulk 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) Welcome to the Space Digest!! Please send your messages to "space@isu.isunet.edu", and (un)subscription requests of the form "Subscribe Space " to one of these addresses: listserv@uga (BITNET), rice::boyle (SPAN/NSInet), utadnx::utspan::rice::boyle (THENET), or space-REQUEST@isu.isunet.edu (Internet). ---------------------------------------------------------------------- Date: Fri, 26 Mar 1993 23:31:20 GMT From: Dennis Newkirk 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 Subject: Gravity waves, was: Predicting gravity wave quantization & Cosmic Noise Newsgroups: sci.space,sci.astro,sci.physics,alt.sci.planetary In article nickh@CS.CMU.EDU (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 Subject: In what craft did Glenn orbit the E Newsgroups: sci.space In article <1469100030@igc.apc.org> tom@igc.apc.org 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 davem@ee.ubc.ca Antenna Laboratory ------------------------------ Date: Fri, 26 Mar 1993 22:01:45 GMT From: Eric H Seale Subject: Magellan Update - 03/22/93 Newsgroups: sci.space,sci.astro,alt.sci.planetary sdd@larc.nasa.gov (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" Subject: Magellan Update - 03/22/93 Newsgroups: sci.space,sci.astro,alt.sci.planetary In article 1ovckaINN2kl@rave.larc.nasa.gov, sdd@larc.nasa.gov (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 Subject: Speculation: the extension of TCP/IP and DNS into large light lag enviroments Newsgroups: alt.internet.services,sci.space In article <1ovhnjINNpv7@gap.caltech.edu> sean@ugcs.caltech.edu (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 Subject: Stockman, Mark, and Keyworth (was Re: Flight time comparison...) Newsgroups: sci.space In article <1993Mar25.235008.22396@ee.ubc.ca> davem@ee.ubc.ca (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 Subject: Timid Terraformers (was Re: How to cool Venus) Newsgroups: sci.space In article <24318@ksr.com> jfw@ksr.com (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 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 chris@cdc.hp.com -------------------------------------------------------------------- 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 Primary: Moroz, V. I.:1981, The Atmosphere of Venus, Space Science Reviews 35, 1 Other: Baker, N. L., Leovy, C. B.:1985, Zonal Winds near Venus' Cloud Top Level, Icarus 69, 202 Bogard, D. D.:1987, On the Origin of Venus' Atmosphere, Icarus 74, 3 Bougher, S. 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