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4 Atmospheres: Planets and Comets INTRODUCTION The most powerful method that has been employed in ground-based observa- tions of planetary atmospheres is spectroscopy. Polarimetry, photometry of both reflected and emitted energy, and photography are also useful techniques, and both radiometric and radar observations can probe otherwise inaccessible regions in optically opaque atmospheres. With each of these techniques, the basic procedures are the same: to observe the given object, to interpret the observations with the help of laboratory calibrations and physical theory, and to develop comprehensive atmospheric models. Substantial progress in all three areas is required if a thorough understanding of planetary atmospheres is to be achieved. Our own atmosphere constitutes one of the greatest obstacles to ground- based observations by restricting them to severely limited regions of the electromagnetic spectrum. Techniques for overcoming this obstacle without resorting to space probes are rapidly becoming available and have permitted significant new observations. Among these methods are ultraviolet spectros- copy from rockets, infrared spectroscopy from high-flying aircraft, and the new interferometric spectrometer technique developed by the Connes. Since the latter instrument is presently ground-based, observations are confined to atmospheric windows, but the gain in resolution over conventional methods has produced an amount of new data comparable with that expected from 26
ATMOSPHERES: PLANETS AND COMETS 27 observations made above the atmosphere. Data of even higher quality may be obtained by using this instrument and its analogs at a drier site or on high- flying aircraft. Eventually it may be used in Earth orbit. ORIGINS OF ATMOSPHERIC GASES The commonly accepted division of the planets into two main groups is in accord with a difference in the composition of their atmospheres. The inner or terrestrial planets appear to have secondary atmospheres derived from out- gassing. The outer or Jovian planets have reducing atmospheres containing large amounts of hydrogen, which strongly suggest an origin similar to that of the solar system itself. Comets probably represent a distinct third category, originating from evaporated icy conglomerate material. There are composi- tional differences within these broad categories, and detailed studies of these differences should throw light on the formation and evolution of the planetary system. In the outer solar system, better values are needed for the relative abun- dances of both light and heavy elements and their isotopes. Present evidence suggests that relative abundances in the atmospheres of Jupiter and Saturn are similar to the solar values, but Uranus and Neptune have less light gases. These conclusions are consistent with observed planetary densities but cannot be accepted as definitive until more precise measurements are made. In particular, there is as yet no direct evidence for the presence of helium in the atmospheres of the outer planets, although it is generally assumed that this gas must be a major constituent. Knowledge of isotopic ratios would ma- terially aid in determining the kind and extent of fractionation (and possible isotope formation) that occurred in the early history of the solar system. From such data, it may be possible to deduce the intensity of solar activity during the Sun's earliest evolutionary phases. Among the inner planets, Earth appears to be the great anomaly. Assuming that our atmosphere is the result of crustal outgassing, its composition in the absence of chemical reactions with the crust and biosphere would be very similar to that presently observed on Venus and Mars, with the exception of the large amount of water found on Earth. It thus seems likely that the atmospheres of these other terrestrial planets are also secondary, but this assumption must be tested more rigorously. The reason for the abundance of water on Earth compared with Mars and Venus requires an explanation. Several possibilities have been suggested, but none is yet fully acceptable. The existence of life on our planet can explain the presence of methane and oxygen in the atmosphere, and life and water are both responsible for the relative lack
28 PLANETARY ASTRONOMY of free CO2. The relation between the atmosphere and life invites further study, particularly since it is generally assumed that the origin of life on Earth required a very different (reducing) atmospheric composition. The validity of this assumption can be tested in part by examining the atmospheres of the outer planets for signs of prebiological organic molecules, since these planets presently exhibit one type of atmosphere postulated for the primordial Earth. In this sense, the outer planets allow us to go back in time to examine condi- tions that may have existed in the early history of the solar system. Comets also provide such an opportunity. This is true, in particular, of the "new," long-period comets that are entering the inner solar system perhaps for the first time. The relative abundances of the elements found in comets can be expected to reflect the composition of the region of the solar system in which they were formed. Using these data and those on the atmospheres of Jupiter and Saturn, it should be possible to determine original abundances in the icy and gaseous materials that formed the solar system, in somewhat the same way that meteorites and the Earth's crust provide clues to the less volatile materials. Such an analysis is handicapped by the fact that the mole- cules observed in cometary spectra are only daughter products, resulting from evaporation and dissociation of the parent material in the nucleus. In fact, identification of the parent material is perhaps the most significant problem m cometary physics at the present time. Progress toward this goal can be expected with observations in the ultraviolet and infrared, particularly using high spatial resolution that permits study of gases very close to the cometary nucleus. ATMOSPHERIC COMPOSITION The gases known to be present in the atmospheres of planets and their satellites are given in Table 1; in each case the most abundant identified gas is listed first. The large number of minor constituents that have been identified in the Earth's atmosphere give an indication of the incompleteness of present knowledge of other planetary atmospheres. Supplementary Comments MERCURY: Observations suggesting a CO2 atmosphere now seem incorrect. The most recent studies indicate that the planet has no detectable atmosphere. VENUS: 13C16O2,12C16O18O, and 13C16O have been detected in addition to the abundant isotopic forms. H35C1 and H37C1 are both present. Isotopic ratios appear to be identical to telluric values. H2O may be present in detectable
ATMOSPHERES: PLANETS AND COMETS 29 TABLE 1 Gases Identified in Planetary and Satellite Atmospheres Gas Mercury No definite identifications Venus CO* CO, HC1, HF, H*O Earth N,, O,, HA A, CO,, Ne, He, CH., K, N,O, H,, O, O,, Xe Mars COS, CO, H,O Jupiter H,, CH,, NH, Saturn H2, CH.; NH»( ?) Uranus H,, CH. Neptune H* CH. Pluto No identifications Jovian satellites No definite identifications Titan CH. Triton No identifications amounts. MARS: 18C and 18O are found to be present in telluric relative amounts. JUPITER: Recent ultraviolet spectra from rockets revealed uniden- tified absorption at 2600 A and below 2100 A that may be caused by large organic molecules. A preliminary spectrum of the 8.5-13.5-/t region contains several puzzling features that require verification and explanation. SATURN: The question of varying amounts of ammonia in the planet's atmosphere has not been adequately tested. URANUS AND NEPTUNE: Previously unidentified absorptions near 7500 A have been shown to be caused by methane. The possibility that this gas may be responsible for other, presently unidentified absorptions in the red region of the spectrum must be explored. PLUTO: Photometry of the planet gives no indication of the brightening toward the ultraviolet that would be suggestive of Rayleigh scattering. No atmospheric constituents have been identified. JOVIAN SATELLITES: lo shows an anomalous brightening after eclipse that could be attributed to surface deposition of an atmospheric constituent. Europa also exhibits this effect, but with a smaller amplitude. However, there is no definitive evidence for the presence of atmo- spheres on any of the Galilean satellites. Cooling during eclipses suggests lunar-type thermal properties. Comets are not listed in Table 1, since cometary gases are excited differently âby resonance fluorescenceâat much lower densities, and consequently differ- ent types of chemical species are found. In the coma, identified species include Na, O, Fe, CN, CH, OH, NH, C2, C3, and NH2. Tail gases are predominantly ions, including such species as CO, N2+, and CO8+. A large number of ob-
30 PLANETARY ASTRONOMY served lines are unidentified. The ratio of 12C to 18C in comets should receive additional study; the better of the two measurements made to date suggests it is equal to the terrestrial value. The observed gases result either from photodissociation, or ionization of the stable parent molecules, or from chemical reactions occurring near the surface of the nucleus. These processes are poorly understood, in part because the nature of the parent material is unknown. Abundances have been determined only for those molecules whose resonance transitions happen to lie in the observable spectral region. It is highly likely that some of the most abundant species, which almost certainly include H, H2, and He, are among those not directly observed. Observation of the intensity of OH and O I is important in establishing total gas relative to "visible" molecules. Both absolute and rela- tive abundances of "visible" molecules are known to differ from comet to comet, but quantitative data are very weak and sketchy. The potential of ground-based observations of comets is far from having been fully exploited. The first spectrographic observation of a comet at coude dispersion and with fair spatial resolution was not obtained until 1957. Observations in the infrared and at radio wavelengths are in the pioneering stage. There are no spectra that include wavelength regions shorter than 3000 A. Very few accurate photometric or polarization measurements, par- ticularly at wavelengths corresponding to specific emission bands or to the continuum, have been made which concentrate on structures in the head and tail. Acquisition of the needed observational data is hampered by the fact that only bright comets, which appear infrequently and with little warning, lend themselves well to detailed astrophysical study. Further, comets differ consider- ably in their physical characteristics. A comet comparable with the great Sungrazer of 1965, in which a number of chemical elements were observed for the first time, may be expected only at average intervals of several decades. Observational planning should include steps to recognize suitable opportunities as early as possible in order to exploit them fully. The most pressing need for additional information on major constituents, next to accurate abundances of the gases listed in Table 1, concerns the abundances of N2 and He. Since neither of these gases has ground-state absorp- tion lines in the readily accessible spectral region, their presence and abun- dances must either be deduced indirectly or determined from Earth-orbital observations. With regard to minor constituents, isotopic forms of gases already identified, such as 18CH4 and 1H2H, should be sought and abundances estimated, and the possible presence of organic molecules and water vapor in the atmosphere of Jupiter further investigated. The search for these gases requires high-resolution ultraviolet and infrared
ATMOSPHERES: PLANETS AND COMETS 31 spectroscopy. The possibility of detecting some of them by means of their microwave resonance absorptions should not be overlooked. Finally, the interpretation of such observations requires a much better understanding than now exists of the formation of absorption lines in atmospheres containing absorbing and scattering particles. ATMOSPHERIC STRUCTURE The structure of a planetary atmosphere is determined in the most general case by the parameters of density, temperature, energy transport, velocity, and composition (both neutral and ionized species), as functions of height and time. In addition to their scientific importance, these quantities are of great interest to engineers designing spacecraft for atmospheric probing or landing on other planets. In principle, a temperature-density profile can be obtained from observations of absorption lines in planetary spectra. In practice, obser- vational and interpretive difficulties have seriously hindered progress. Experi- ence with satellite observations of the Earth can be expected to be very helpful for future planetary work. One of the most dramatic of recent advances in this area has been the de- termination of the total pressure of the atmosphere of Mars with a far greater precision than was previously possible. The determination was made from observations of strong and weak bands of carbon dioxide in the planet's spectrum. Application of this spectroscopic method to other planets has been much more difficult, because the presence of suspended matter in their atmo- spheres introduces serious complexities in the analysis of the absorption lines. The theory of radiative transfer in such atmospheres is presently being de- veloped and applied to the observations, with the result that a marked increase in knowledge of pressures and temperatures at different altitudes may be anticipated. Nevertheless, to provide rigorous tests of the theory, additional high-resolution studies of planetary spectra, for center-to-limb effects as a function of phase angle, are required. Another approach to understanding atmospheric structure is the direct measurement of temperature at wavelengths ranging from the 8-14-/* window to the radio region of the spectrum. The resulting temperatures may correspond to different levels in a planetary atmosphere or may be indicative of surface or subsurface conditions, depending on the strength of the atmospheric opacity and its wavelength dependence. Radio observations allow investigation of the lower atmospheres of cloud-covered planets and should contribute to studies of the atmospheres of Venus and the major planets. Models for the structure of an atmosphere must be able to explain the temperatures observed at all
32 PLANETARY ASTRONOMY wavelengths and the variation of temperatures with phase and position on the planet's disk (limb brightening or darkening). In the case of Venus, a large amount of such temperature data is on hand but has not yet been successfully incorporated into a consistent model atmo- sphere. No model which does not take winds into consideration is likely to be adequate. Measurements in the 10-14-/* atmospheric window indicate a temperature of 210-235°K. Radio observations made between inferior con- junction and quadrature show a decrease of blackbody disk temperature from about 650 °K at 6-cm wavelength to about 300-3 50 °K at millimeter wave- lengths. This variation is generally interpreted as absorption and re-emission by the atmosphere and clouds at higher, cooler levels of the radiation from hot, lower levels. The available data show a phase variation in the average disk brightness of about 10 percent at 0.8 and 3 cm, and of only a few percent at 10 cm, with minimum brightness after inferior conjunction, consistent with retrograde rotation. Limb darkening derived from Mariner II observations at 1.8 cm and from ground-based studies at 3 cm also indicates absorption in the atmosphere. Radar observations at 3.8 cm indicate at least 2.5 and possibly as much as 5-dB zenithal atmospheric absorption, which appears to vary slightly with time. The problem is thus to assimilate these data into a consistent atmospheric model. Such models are generally tied to some form of greenhouse effect in which solar radiation penetrates to the surface of the planet at short wave- lengths, while the thermal radiation re-emitted by the surface at longer wave- lengths is trapped by the atmosphere. If the observed spectrum is produced by absorption in a CO2-N2 atmosphere, very high pressures must exist; if by ab- sorption by water vapor, a great deal more water vapor must be present than seems consistent with observations; if by absorption by liquid water in the clouds, again too much water vapor is implied to be consistent; if by absorption by dust, large quantities of suspended material are required; and if by scatter- ing, large particle sizes are needed. In the case of Jupiter, thermal radiation dominates the radio spectrum for wavelengths shorter than about 3 cm. At longer wavelengths, the non- thermal synchrotron radiation of the magnetosphere increases in strength until, at 10 cm and beyond, the thermal component is very difficult to evaluate. The disk brightness temperature due to thermal radiation increases from 110- 140°K in the millimeter wavelength range to at least 250°K at 10 cm. Mea- surements at 20 /j, and from 8 to 14 p indicate that the planet is radiating more energy than it receives, i.e., it is intrinsically warm, a result that is important for theories of the formation of the planets. The temperatures at these wave- lengths are 150°K and 128°K, respectively. This spectrum suggests that the longer wavelength radiation is generated at lower levels in the atmosphere
ATMOSPHERES: PLANETS AND COMETS 33 where the temperature is higher, a result that is consistent with present ideas on the planet's atmospheric opacity. Some observations of the intensity of the radiation at short centimeter and millimeter wavelengths differ significantly from the usually observed values and could indicate atmospheric activity. This also is indicated by the variable visual band structure and the hot shadows observed at 10 ^. It is important to observe Jupiter over long time periods to determine whether these variations accompany other evidences of atmospheric disturbance. If construction of proposed new planetary radar facilities proceeds, it will soon be possible to obtain radar echoes from the Galilean satellites. The upper atmosphere of Jupiter can then be probed by studying the echoes received at the frequent times when one of the satellites is being occulted by the planet. Group velocity delay, extinction, and the Doppler phase shift of these echoes can all be used to gain information on the atmospheric scale height and dielectric properties. The observed disk brightness temperature of Saturn increases from about 100°K at short millimeter wavelengths to about 300°K at 20-on wavelengths, a spectrum very similar to that of Jupiter's thermal radiation. There is no evidence for a nonthermal component of radiation from Saturn. Unpublished high-resolution interferometer observations have been reported to show that the centimeter-range radiation is confined to the planetary disk, again suggest- ing atmospheric radiation. These studies require verification and extension. The intensity of radiation from Uranus and Neptune is extremely weak, and observations of their radio emission have been accomplished only recently. Three measurements of the disk temperature of Uranus give 220 ± 35°K at 1.9 cm, 159± 16°K at 3.75 cm, and 130±40°K at 11.3 cm. There is only one ob- servation of Neptune, 180±40°K at 1.9 cm. These fragmentary results are only a beginning in the study of the radio emission of these planets; the largest radio telescopes and most sensitive radiometers are required for additional research. In the case of the comets, the structure of the coma, head, and tail is easily visible. However, the distribution of the atoms, molecules, and associated dust is very specific; the details vary from comet to comet and within the same comet as a function of time. The excitation mechanisms are still not completely understood, and the interaction of the released gases and dust in comet tails with the solar wind requires much additional study. Monochromatic photog- raphy at high spatial resolution is one important method to obtain this information; spectra of the coma and tail using objective prisms or fast slit spectrographs provide another. Temperatures of cometary nuclei can be in- ferred from brightness measurements with infrared detectors. This can be done only on relatively large comets when they are near the Earth and with
34 PLANETARY ASTRONOMY the most modern detectors. Only one such measurement has been reported to date. Temperatures derived from analyses of molecular bands are difficult to interpret because of the nonequilibrium excitation mechanisms. GAS DYNAMICS Gas motions in comet tails are relatively easy to observe, but, as mentioned above, the relationship of these motions to the interplanetary medium through which the comets pass is not clear and constitutes an important area of research. One would like to understand the coupling mechanism between the ions in comet tails and the solar wind. It is not known, for example, whether comets have intrinsic magnetic fields, what is the origin of the system of tail rays, or what are the reasons for the sudden outbursts (observed as large increases in brightness) that are particularly puzzling in comets far from the Sun, even beyond Jupiter. Additional studies are required to define motions near the nucleus from which the gases are liberated. The characteristics of comet tails far from the Sun are also of interest. Theories of general circulation in planetary atmospheres are still in their infancy. Motions of clouds in the atmosphere of Mars have been mapped in a preliminary way to try to define the general pattern of the planetary winds, but the observations have been too fragmentary to permit a definitive repre- sentation. Motions in the atmosphere of Venus are even more difficult to identify, since the cloud cover of the planet is virtually featureless. However, thermal energy maps at different phases imply large-scale wind systems and "storms" which are very important energy-transport mechanisms. Recent evi- dence for a four-day retrograde cycle in the ultraviolet cloud pattern (as op- posed to the retrograde 243-day sidereal period for the planet itself) is ex- tremely interesting and should be further verified. For both of these planets, theoretical studies of general circulation would appear to offer considerable hope for improved understanding as knowledge of the relevant parameters increases. Detailed knowledge of temperature, pressure, and composition, as well as of the nature of the particulate matter suspended in the atmospheres, is important for this work. The theoretical study of angular momentum transfer on Jupiter, the nature of the pronounced equatorial currents on Jupiter and Saturn, and the nature and the motion of Jupiter's Great Red Spot have all received recent attention. Further work will require simultaneous determinations of the latitude, the longitude, and the shape of markings of all sizes. When such data are avail- able, a test of the hypothesis that the extremely rapid rotation of the major planets may give rise to novel hydrodynamic effects will be possible.
ATMOSPHERES: PLANETS AND COMETS 35 Observations made some years ago indicated that gaseous ammonia and the clouds in Jupiter's atmosphere rotate at different speeds. The effect has been observed twice, independently and at widely different times but in the interval between the two positive observations; negative results were obtained by other observers. It seems likely that new observations and, in particular, a systematic search for a possible latitudinal dependence, would be worthwhile. The determination of temperature variations, both spatial and temporal, in the atmospheres of these planets will be of great importance to theories of planetary interiors and atmospheric dynamics. That Jupiter's surface appearance varies has long been known. The variability manifests itself in the reflectivity of the planet; available measurements are inadequate to define a time scale for this variation, but the amplitude appears to be on the order of 0.5 magnitude. If a true period exists, which is doubtful, only a long series of new data will establish its duration. Variations in the magnetic field near the surface of Jupiter also bear on the dynamics of the atmosphere (see Chapter 5). ATMOSPHERIC AEROSOLS Every known atmosphere contains relatively large amounts of suspended matter, or aerosols. Aerosols are either condensates of a gaseous constituent of the atmosphere or particulate matter from meteorites or from the solid body with which the atmosphere is associated. Ignoring their presence can lead to serious misinterpretations of the observations. The opacity caused by such particles may contribute significantly to the planetary heat balance. Finally, planetary meteorology may be strongly affected by the presence of condensates able to absorb and liberate substantial amounts of latent heat during phase changes. Despite their importance, relatively little is known about aerosols in the atmospheres of other planets. Recent, very low values for the abundance of water in Venus' atmosphere have virtually eliminated the possibility that the planet's cloud cover consists entirely of ice crystals or water droplets. Dust seems the most reasonable alternative, but its composition is not known. Polarization measurements are being made over the full 180° phase angle in several wavelengths from 3000 to 10 000 A and should lead to new insights when interpreted with the help of computer programs capable of handling Mie scattering theory. However, the number of free parameters involved in making a fit to observations of this type is so great that unambiguous identi- fication is unlikely. Additional data are needed and may be forthcoming from radar and radio observations at a number of wavelengths. In the case of Mars, the presence of aerosols is manifested by several
36 PLANETARY ASTRONOMY effects: white clouds, yellow clouds, and the blue haze. The white clouds appear to be condensation phenomena, probably ice crystals. The yellow clouds are airborne dust, often brought into the atmosphere in sufficient quantity to obscure a large fraction of the planet's visible hemisphere. The blue haze, whose nature is not well understood, causes the customary absence of surface detail in photographs of the planet made at wavelengths below 4500 A. A baffling property of the blue haze is its variable transparency. It is commonly thought to be an atmospheric phenomenon, but suggestions ranging from ice-crystal clouds to a simple lack of contrast in surface features have been offered. High-resolution photography at selected intervals in the ultraviolet wavelengths is required to define the cutoff in surface detail and its temporal variation. Polarimetry at short wavelengths with high spatial resolution should also provide useful information. Jupiter's atmosphere has a rich assortment of cloud phenomena which are generally thought to result from condensation of atmospheric ammonia. However, the colors in the clouds, as exemplified by the famous Great Red Spot, have not been satisfactorily explained. The most likely source of the colors would seem to be complex organic substances in the atmosphere. This hypothesis can best be tested by spectroscopic analysis of atmospheric gases, as suggested in the section on Atmospheric Composition (page 28). The deeper layers of the atmosphere should contain water clouds; direct investigation may be possible with radar at short wavelengths or by infrared measures from airplanes. Atmospheric models have been proposed that take into account the effect of condensation on the temperature gradient, but they require elaboration and observational verification. On Saturn, the visible cloud surface may consist of ammonia crystals at very low temperatures; on the other hand, these clouds may be condensed methane or a mixture of the ammonia crystals and the methane. Clouds of condensed methane could exist in the low-temperature atmospheres of Uranus and Neptune; these clouds would be highest in the atmosphere. If the tempera- ture rises with decreasing altitude, the lower layers may contain ammonia and ultimately water clouds. Again it appears that these hypotheses must be ana- lyzed indirectly, although a few data may be provided by observations at radio frequencies (see the section on Atmospheric Structure, page 31). The particulate matter associated with comets manifests itself most strik- ingly in their tails. The tails of most new comets contain a large amount of dust that produces a continuum of scattered sunlight in contrast with the discrete emission lines of the ions. Under certain circumstances, the gas and dust components may be separated by the action of radiation pressure and the solar wind, leading to the phenomenon of comets with multiple tails. Photo- metric and polarimetric observations suggest particle sizes on the order of
ATMOSPHERES: PLANETS AND COMETS 37 0.5 /i; such measures should be extended to include a greater range of wavelengths. Like the gaseous components, the dust comes from the cometary nucleus, and spectral evidence of its composition is obtainable under proper conditions of excitation. Comets that approach the Sun rather closely exhibit the sodium D lines in spectra of the coma, while lines of iron, calcium, potassium, nickel, copper, chromium, and manganese were identified in the spectrum of the Sun- grazing comet of 1965. The opportunity for such observations is exceedingly rare, so that the possibility of even more direct study is attractive. This possibility stems from the well-established association between comets and meteor showers. Most of the cometary material entering the Earth's atmosphere when the planet passes through the comet's orbital plane disinte- grates before it reaches the ground. However, a growing body of evidence suggests that some of the carbonaceous chondrites may come from comets (see Chapter 3). The relationship between comets, asteroids, and meteorites de- serves much additional study, since meteorites are the only samples of extra- terrestrial "dust" that can be analyzed in the laboratory. INTERACTIONS WITH THE SURFACE The effects on the surfaces of solar system bodies of evolved or captured gases can be examined only with respect to Mars and Venus. The other objects have either no detectable atmospheres (Mercury) or no detectable surfaces (Jupiter). Comets have both surfaces and gaseous envelopes, but whether the gases have any effect on the surface once they have been liberated is not clear. It has been suggested that the process of liberation of the gases from the icy conglomerate nucleus may gradually transform the nucleus into a rather porous and spongy semisolid mass. This low-density, low-strength mass may be the parent body of the bright, large meteors which do not result in recoverable meteorites, despite well-observed trajectories. In the case of Mars, analysis of the Mariner IV pictures of the surface has shown significant deviations of the smallest craters from the frequency distri- bution curve that characterizes lunar craters. This disparity may be caused by erosion produced by wind-borne dust on Mars, a process that would obviously be lacking on the Moon. Such a mechanism is known to operate on Earth in arid regions, and it appears reasonable that winds capable of initiating the enormous Martian dust storms would have sufficient force to cause erosion. However, the details of the soil-moving process at very low pressures remain uncertain owing to the large extrapolation that is required to go from theories based on observations in terrestrial deserts at atmospheric pressure to expected
38 PLANETARY ASTRONOMY Martian conditions. Hence, experiments carried out at low pressure with a Martian mixture of gases are required to determine the wind speed and soil compaction at which dust begins to be picked up and, once airborne, the effect of this dust on solid surfaces of different strengths and compositions. Study of the surface of Venus, made possible by radar, is just beginning. As noted in the preceding section, the clouds that obscure the surface may be largely composed of dust. There is no direct evidence on the wind forces that would be required to dislodge dust and carry it into the atmosphere nor on the effects of such dust on surface terrain under conditions assumed to exist in the lower atmosphere. Some insight into these questions can again be gained by laboratory experiments on dust-raising and -transport mechanisms, in this case for high pressures and temperatures.
