A MEETING WITH THE UNIVERSE

Appendix A-2

Planetary Exploration

Mercury

Ground-based radar measurements determined (1965) that the rotation period is 59 days, not 88 days as had long been believed.

Mariner 10 made the first spacecraft flyby of Mercury in 1974 (in fact, it flew past Mercury three times), and obtained several thousand photographs.

Among the results of the Mariner 10 investigations were the following:

The mass of Mercury was accurately determined.
Any residual atmosphere has less than a million -billionths the pressure of the Earth's atmosphere at sea level. However, a trace of helium, perhaps derived by outgassing from Mercury's interior, was found.
It was discovered that Mercury has an internal magnetic field, similar to but weaker than that of Earth.
Mercury's surface is heavily cratered and resembles that of the Moon.
A huge circular impact basin (Mare Caloris), about 1300 kilometers (810 miles) in diameter, was discovered.
A planetary feature unique to Mer cury was found, consisting of long scarps, or cliffs, that apparently were produced by compression in a major shrinkage of the planet.
Flat plains, perhaps lava found.
Mercury was found to be closer to a perfect sphere than is the Earth.

The surface of Mercury:
ancient craters and a strange trench.

Venus

Ground-based radar measurements in the early and mid-1960s showed that the rotation period of Venus is about 240 days and that the rotation is retrograde, so that it spins in the opposite sense with respect to the Earth.

The 1962 flyby of Venus by Mariner 2 led to an accurate determination of the planet's mass. This spacecraft, and ground-based observations, also measured high temperatures, around 300° C, in the atmosphere of Venus.

Ground-based observations detected various minor components of the Venus atmosphere, including the gases hydrochloric and hydrofluoric acid (1967), carbon monoxide (1968), water (1972), and sulfuric acid (1978). In 1973, it was found that the upper cloud layer is composed of sulfuric acid droplets.

The changing swirls of Venus' thick atmosphere, recorded day by day by the Pioneer Venus spacecraft.

From the USSR's Venera 7 in 1970, it was found that the atmosphere is largely carbon dioxide, with a pressure at the planetary surface about 100 times that of the Earth's atmosphere. Venera 7 also measured a sur face temperature of about 500° C.

Mariner 10 flew past Venus in 1974. Among the results were:

Venus has no significant magnetic field.
A notable disturbance in the solar wind is produced as it flows past Venus.
Venus is closer to a perfect sphere than the Earth.
Ultraviolet images of the atmosphere revealed streamline and circulation patterns, including Y- and C-shaped structures.
The upper atmosphere was found to rotate much more rapidly (once in about 4 Earth days) than Venus itself, a result also suggested by ground based measurements.
Hydrogen and helium were detected in the atmosphere.

In 19 75, the Venera 9 and 10 lander spacecraft found that the surface of the planet is firm and rocky. Their other results included:

The measured surface winds had velocities of about 3 to 13 kilometers per hour (2 to 8 miles per hour).
A significant amount of sunlight reaches the surface.
The surface rocks vary in radioac tivity and have compositions resembling those of granite and basalt.

The 1978 Pioneer Venus mission in cluded both atmospheric probes and an orbiting spacecraft. Among the results were:

The atmosphere was found to circulate in large planetwide systems, much simpler than the circulation patterns on the Earth.
A collar of polar clouds was dis covered, which may be part of a large atmospheric circulation vortex.
At least four distinct cloud and haze layers were found at different altitudes.
The haze layers contain small aero sol particles, perhaps sulfuric acid droplets.
Accurate measurements of the at mospheric composition showed that it was about 96 percent carbon dioxide, 4 percent nitrogen, with small amounts of water, oxygen, and sulfur compounds. Rare gases such as argon and neon detected in the atmosphere suggest that Venus is richer in volatile elements than the Earth or Mars.
The surface of Venus was found to be generally smooth, much less irregular than that of the Earth.
Radar measurements from the Pio neer Venus Orbiter and from ground based observatories revealed the presence of two broad plateaus as well as apparent volcanic structures, craters, and canyons.

Lightning and thunder are present on Venus, according to results from the Venera 11 and 12 landers in 1978.

Mars

Ground-based observations detected water vapor on Mars in 1964.

Mariner 4 flew past Mars in 1965 and photographed a heavily cratered, moonlike surface.

Mariner 4 found that the Martian atmosphere is thin, with less than 1 percent the pressure of the Earth's atmosphere, and is composed largely of carbon dioxide.

The Mariner 6 and 7 flybys took place in 1969. Instruments on these spacecraft found that nitrogen is virtually absent from the atmosphere and that solid carbon dioxide ("dry ice") occurs in the clouds and near the polar caps.

Mariner 6 and 7 found that the dust particles in the Martian atmosphere probably consist of silicate materials derived from the planetary surface.

The first soft landing on Mars was executed by the USSR Mars 3 spacecraft in 197 1; the spacecraft ceased operating 20 seconds after the landing.

Landfall on Mars:
the Viking 1 spacecraft approaches the Red Planet.

Mariner 9 became the first Mars orbiting spacecraft in 1971. It obtained over 7300 photographs. Among the results of this mission were:

Mars is actually a two-part world, with an ancient cratered surface in the Southern Hemisphere and a geologically younger surface, with volcanoes, canyons, and dry river channels, in the Northern Hemisphere.
The Martian volcanoes include a few huge ones, rising to heights of as much as 25 kilometers (16 miles), with fresh looking lava flows.
Mars has a huge valley (Vallis Marineris), about 5000 kilometers (over 3000 miles) long.
Sinuous channels, with braided and streamlined formations, appear to be former river beds.
Landforms resembling lava flows occur in flat regions.
There are layered deposits in the Martian polar regions, perhaps indi cating glacial periods in past times.
Solar ultraviolet light is not absorbed by the atmosphere and hence reaches the surface of Mars.
Periodic global dust storms were observed in detail.
The two small moons of Mars were photographed and found to be very dark and to have irregular shapes and cratered surfaces.

dark photo of the sun slipping below the Martian horizon

Martian sunset: the Viking I lander records the last fading light on the red surface of Mars.

The Viking 1 and 2 landers and orbiters reached Mars in 1976. Among their many findings were:

The highly oxidized soil produced unique chemical reactions in the life detection instruments (see Appendix section on Exobiology).
The reddish color of the soil is due to oxidized iron. The soil is fine-grained and cohesive, like firm sand or soil on Earth.
The surface rocks resemble basalt lava, and the soil chemistry is like that of weathered, altered basalt.
There are water and sulfur com pounds in the soil.
The sky is not blue but pink, its color caused by fine suspended particles of red dust.
The polar caps are largely made of water ice.
The winds at the surface of Mars are light, about 24 kilometers per hour (15 miles per hour).
The surface temperature ranges from about -84° C (-120° F) at night to -29° C (-20° F) in the afternoon.
Fog and clouds occur despite the fact that the water content of the atmosphere is less than 0.1 percent that of the air on Earth.
The surface pressure of the atmo sphere, only about 0.8 percent that of the Earth's atmosphere, varies seasonally in accord with the evaporation of the polar caps.
The isotopic ratios of carbon and oxygen in the Martian atmosphere resemble those of the Earth's atmosphere.
The atmosphere has been modified over time by the escape of nitrogen to space; this has produced nitrogen isotopic ratios that differ from those on Earth.
Mars may have had a much denser atmosphere in the past, and could have had liquid water on its surface. The abundances of rare gases such as argon and neon suggest that Mars has a lower volatile content than either the Earth or Venus.
The Martian moons are grooved, in dicating that fracturing may have occured; they may be asteroids that were captured by Mars.

Viking 2 photo of the rock strewn martian surface

The Viking 2 lander,
surrounded by a croud
of Martian rocks.

Jupiter

The Planet

According to a theory formulated in 1958, the interior of Jupiter in cludes a large core of metallic hydrogen.

Pioneer 10, in 1973, and Pioneer I 1~ in 1974, made the first flybys of Jupiter; both spacecraft survived pas sage through the asteroid belt and through the Jovian trapped radiation. Among the findings from these space craft were:

Aside from its polar flattening, Jupiter is very symmetrical and has no gravitational anomalies; it behaves like a liquid planet.
Jupiter is slightly more massive than had been indicated from ground-based observations.
As suggested from ground-based measurements, Jupiter emits twice as much energy as it receives from the Sun.
The temperature at the center of the planet may be 30,000° C.

The Atmosphere

From ground-based studies, it was deduced that the atmosphere of Jupiter must consist mostly of hydrogen and helium, in an approximately 2 to 1 ratio. The bright bands in the atmosphere ("zones") are cooler than the dark ones ("belts").

A theory proposed in 1958 interprets the great Red Spot as the top of a rotating column in the atmosphere.

Among the results of the Pioneer 10 and 11 missions were:

The general banded structure of the Jovian atmosphere is not present near the poles; there, oval circulation patterns develop.
At the poles, a thick, aerosol-free, or "blue sky" , atmosphere was found.
The bright zones consist of rising cloud masses at higher altitudes, while the belts are descending masses that allow a deeper view into the atmosphere.
Two cloud layers are present; the thick, low cloud deck is topped by a thinner, clearer region.
Detailed study showed rapid motions among the clouds and changes in the wind speeds.
Bright plumes of warm material were observed rising from deep in the atmosphere.
Changes in the flow patterns of the Red Spot were observed between 1973 and 1974; the Red Spot and other features were interpreted as hurricane-like storms.
Helium was detected, confirming the earlier deduction of the Jovian atmospheric composition.

photo of the swirling atmosphere of Jupiter

Swirls and storms in Jupiter's atmosphere, recorded by Voyager 1.

Voyager 1 and Voyager 2 flew past Jupiter and its moons in 1979; among the findings from this pair of space craft were:

Eruptions of warmer material from below are signalled by brightenings followed by cloud spreading.
Along the boundaries of the belts and zones, there occur atmospheric jet streams, plumes, and extensive turbulence.
There is actual mass movement, rather than simply wave motions, in the Jovian atmosphere.
An east-west wind structure is present in the polar regions.
The Red Spot rotates counterclock wise in about 6 days, as estimated from ground-based observations.
The fraction of helium by volume in the Jovian atmosphere is 0.11.
Aurorae were discovered in the polar regions and are related to the magnetic field lines and currents between Jupiter and its moons.
Lightning "superbolts" were de tected in the cloud tops.
There is intense ultraviolet emission from the atmosphere.

close up photo of Jupiters prominent red swirling covering

Stormy weather around Jupiter's
great Red Spot.

Minor components of the Jovian atmosphere that have been detected at various times include deuterium (heavy hydrogen) compounds; organic molecules such as ethane and acetylene; water vapor (found using the Kuiper Airborne Observatory in 1975); carbon monoxide; phosphine (PH₃); and germane (GeH₄).

The Magnetic Field and Trapped Radiation of Jupiter

The discovery of naturally generated radio bursts from Jupiter was accomplished with a ground-based radio telescope in 1955. Theory indicated that a strong magnetic field must be present in order to account for the bursts, and additional radio observations showed that the occurence of many bursts is related to the position of the Jovian moon Io.

Jupiter's moon lo, with a large active volcano.

Among the findings of Pioneer 10 and 11 were:

The magnetic field was detected and found to be huge. The bow shock of the Jovian magnetosphere had a measured width of 26 million kilometers (16 million miles). The Jovian magnetic tail extends to beyond the orbit of Saturn. If the magnetosphere were visible in the sky from Earth, it would appear larger than the Sun or Moon.
The Jovian magnetic field is 10 times stronger than Earth's and contains 20,000 times as much energy.
The axis of the magnetic field is tilted 11 degrees from the Jovian rotation axis and is offset from the center of Jupiter in a manner similar to the axis of the Earth's field.
The Jovian magnetic field has the opposite sense to Earth's field; the "north" magnetic pole is at the south pole of Jupiter.
The magnetic field fluctuates rapidly in size on the sunward side of Jupiter because of pressure variations in the solar wind, an effect studied in further detail by the two Voyager spacecraft.
Energetic protons were found and measured in the Jovian radiation belt.
It was discovered that streams of high-energy atomic particles are ejected from the Jovian magnetosphere and travel as far as the orbit of the Earth.
Electric currents were detected flowing between Jupiter and some of its moons, particularly Io.

Among the findings of Voyager I and 2 were:

Within the Jovian magnetosphere is a huge sheet of plasma (a gas of electrically charged atomic particles), 4.8 million kilometers (3 million miles) in diameter. The plasma sheet rotates along with Jupiter and its magnetic field.
Some of the magnetospheric plasma has a remarkably high temperature, in the millions of degrees. Among the high energy particles in the plasma are ions of hydrogen, sulfur, and oxygen.
A sulfur-rich plasma, which was not present in 1973, was detected near Io, with a temperature of about 100,000° C.
The electrical current between Jupiter and lo was measured at about 5 million amperes.
It was found that radio emissions may be generated in the plasma near Io.

The Moons and Ring of Jupiter

Groundbased studies of the jovian moons in the 1970's revealed the presence of two additional small moons and the existence of water ice on the surfaces of the three outermost Galilean satellites, Europa, Ganymede, and Callisto.

Findings about the jovian satellites from Pioneer 10 and 11 included:

The two outermost Galilean moons, Ganymede and Callisto, are less dense than the Earth's Moon.
Io was found to be 28 percent more massive than had been calculated from groundbased observations and to be 1.22 times more massive than our Moon.
A thin atmosphere, about 1/20,000 of the density of Earth's atmosphere and extending to about 115 kilometers (70 miles) above the surface was discovered on Io.
A cloud of sodium vapor around Io and its orbit, detected from the ground, was detected and found to be immersed in a cloud of hydrogen as well.

photo of Io's orange hot volcanic surface

Io: a volcanic landscape sculptured by flows and eraptions of molten sulfur lava.

Results of the Voyager I and 2 missions included:

Three additional moons were discovered.
Amalthea, the small inner moon of Jupiter, was found to have an elliptical shape. In effect it is a big rock, about 265 x 140 kilometers (165 x 87 miles) in size.
Volcanic eruptions were discovered on Io, the first evidence of active volcanism found outside the Earth. The volcanism is probably induced by tidal heating. The erupted plumes extend up to 320 kilometers (200 miles) above the surface, and the volcanic activity varies over times of a few months.
A hot spot on Io, 150° C warmer than the rest of the surface, and associated with volcanic features, was discovered.
Europa was found to be a world of essentially flat terrain, with virtually no topographic relief. It is marked by intersecting linear features, not cracks, and the surface is very young, as indicated by the lack of impact craters.
Both cratered and grooved terrains are present on Ganymede; the icy crust of this moon appears to have been globally deformed.
Ganymede was found to produce distinct disturbances in the jovian magnetic field and trapped radiation belts as far as 200,000 kilometers (124,000 miles) away.
Callisto's crust is ancient and heavily cratered, with large impact basins.
A ring around Jupiter was discovered inside the orbit of the innermost moon. It apparently extends down to the top of the jovian atmosphere.

Saturn

The Planet

Ringworld: giant Saturn and two of its moons.

Among important recent findings from Earth-based observations of the Saturn system have been the detection of a deuterium compound in the atmosphere of Saturn, indications that the small ring particles are composed of or covered by ice, the detection of ice on the moons Rhea, Iapetus, and Dione, and the discovery of methane in the atmosphere of Titan.

The first spacecraft flyby was accomplished by Pioneer Saturn in 1979. Among the findings were:

Confirming ground-based studies, Saturn has an internal heat source like Jupiter and radiates about twice as much energy as it receives from the Sun.
As suspected, Saturn must have in ternal shells of liquid and metallic hydrogen, small amounts of helium, ammonia, and water, and perhaps a small rocky core.
A magnetic field was discovered around Saturn, larger than the Earth's, but smaller than that of Jupiter. It is five times as weak as predicted by theory.
The axis of Saturn's magnetic field is aligned parallel to the planet's rotation axis, contrary to the circumstances in both the Earth and Jupiter.
The boundary of the magnetic field varies due to changes in the pressure of the solar wind on the sunward side, as was found in the case of Jupiter.
The atmosphere of Saturn has weak bands, rather than the conspicuous belts and zones seen on Jupiter.
There is a high haze, perhaps composed of crystals of ammonia ice, above the clouds.
Apparent high-speed jet streams were detected in the atmosphere.
Confirming ground-based measure ments, the cloud-top temperatures were measured at about -200° C (-330° F), only about 73° C (130° F) above absolute zero.
Radiation belts were discovered that are weaker than those of Jupiter. The radiation is absorbed (cut off) by the rings and moons of Saturn. Cutoffs in the radiation data were used to infer the presence of additional rings and moons beyond those already known from visual observations.

The Voyager 1 encounter (November, 1980) provided a closer look at Saturn and its surroundings. Some of the new discoveries were:

The details of Saturn's atmosphere appear similar to Jupiter's, with alternating light and dark bands and circulating storm systems.
Wind speeds up to 1500 kilometers per hour (930 miles per hour) were measured near Saturn's equator; these winds are four to five times as fast as those measured on Jupiter.
Unusual atmospheric features in clude a ribbonlike wave feature, large and small clouds, and a red oval similar to, but smaller than, Jupiter's Red Spot.
Aurorae were observed in the atmosphere above Saturn's poles.
Although lightning bolts were not observed on Saturn, radio signals typical of lightning flashes were recorded. The signals may be produced by electrical discharges in the rings rather than in Saturn's atmosphere.

The Rings

photo of light browm belts encircling the planet Saturn

Saturn's atmosphere: belts of quiet colors.

The Pioneer Saturn flyby in 1979 made several new discoveries about the rings:

The rings consist largely of particles several centimeters in diameter. They are extremely cold and possibly composed of frozen water and other ices.
An extensive cloud of hydrogen was discovered around the rings.
Two new rings (called F and G) were discovered, and a gap between rings was confirmed.

Voyager 1 provided much more detail on the beauty, complexity, and some times baffling nature of the rings. Among the discoveries were:

The six known rings are actually composed of hundreds of tiny, thin ringlets with intervening spaces, so that the whole ring system looks something like the grooves in a phonograph record. Even the Cassini division, once thought to be empty space between the A and B rings, contains several dozen ringlets. There are far too many rings to be explained by our present theories of how planetary rings form and remain stable.
Elongated radial features that last for hours to days were observed in the 13 ring. These "spokes" may be clouds of electrified dust rotating around Saturn above the plane of the rings.
The thin outer F-ring, discovered by Pioneer Saturn, was resolved into three distinct but intertwined ringlets. This braided ring structure is very difficult to explain; it seems likely that both electrical and gravitational forces are at work.
Two small moons, one on each side of the F-ring, may act as "shepherds", their gravitational attraction keeping the ring particles on track between the orbits of the two moons.

photo of Saturn illustrating the different layers of atmosphere

Saturn's atmosphere, color-enhanced to bring out details.

The Moons

Nine (possibly ten) moons had been detected from Earth. The encounter of Pioneer Saturn (September, 1979) provided several new discoveries:

At least two new moons were dis covered by Pioneer and ground-based observations.
Accurate masses were determined for the moons Rhea and Iapetus.
The cloud-top temperature of Titan was found to be very low, about -200° C (-330° F), and a hydrogen cloud was discovered around Titan.

Saturn's small white moon, Diane against the orange giant

Saturn's moon Dione against the red back ground of Saturn itself.

A much closer look at several of Saturn's moons was provided by Voyager 1. The new results included:

Six tiny, unnamed moons were photographed, some of them for the first time. Satellites 10 and 11 share the same orbit and must frequently undergo some orbital "evasive actions" to avoid colliding. Satellite 12 shares the orbit of the larger moon Dione. The shepherd Satellites 13 and 14, on either side of the thin F-ring, may exert gravitational forces to keep the ring in place, while Satellite 15, located just outside the large A-ring, likewise may help keep that ring in place.
The inner moons Mimas, Tethys, Dione, and Rhea all have heavily cratered surfaces like those of the Moon and Mercury, although Saturn's moons are composed largely of water ice. This shows that meteorite bombardment, even as far out as Saturn, has been a major process in shaping the solar system. Mimas is marked by a huge impact crater that is fully one fourth the diameter of Mimas itself. This crater makes Mimas look like a staring eyeball, and the impact that formed it was almost intense enough to blast Mimas into fragments.
The inner moons also show traces of internal geological activity. Tethys has a rift-like valley that stretches 800 kilometers (500 miles) across its surface. Dione shows several sinuous, branching valleys. Both Dione and Rhea have bright, wispy streaks on their surfaces.
Although Enceladus orbits between two heavily cratered moons, Mimas and Dione, it seems smooth and en tirely uncratered, as viewed from Voyager l.
Titan, Saturn's largest moon, has a diameter of 5120 kilometers (3180 miles), which makes it smaller than Jupiter's moon Ganymede. Titan's dense, hazy atmosphere is at least 400 kilometers (250 miles) thick and completely shrouds the surface from view Titan's atmosphere was found to be mostly nitrogen (like the Earth's), with minor methane and other hydrocarbons. At the surface, the pressure of Titan's atmosphere is at least twice that on the Earth. The surface temperature, about -175° C (-280° F), is low enough to permit lakes and streams of liquid nitrogen to form on its surface.
Long-range Voyager 1 photographs of Iapetus, an outer satellite, confirmed Earth-based observations by showing that the satellite has light and dark faces, but no explanation for this puzzling difference was found.

Uranus

A wholly unanticipated system of thin and narrow rings was discovered around the planet Uranus in 1977 by telescopic observations from the Kuiper Airborne Observatory. There may be as many as nine of the very dark rings.

Neptune

Cloud patterns were detected in the atmosphere of Neptune by ground based observations in 1978.

Pluto

All studies of Pluto thus far have been done from the ground. In recent years, a surface layer of frozen methane has been detected, and a moon (Charon) was discovered. It appears that Pluto is much smaller, less dense, and less massive than previously believed.

Asteroids and Meteorites

Although nearly all asteroid observa tions thus far have been made from the ground, the asteroid belt was safely traversed by Pioneer 10 in 1973 and subsequently by other spaceprobes.

Based on the study of reflected sun light, asteroids have been classified into six mai or CORIDositional families since 1970.

Since 1970, diameters have been estimated for an increasing number of asteroids by means of infrared observations.

In 1968, a radar echo was successfully obtained from the small asteroid Icarus during its close approach to the Earth. Another radar contact was made in 1980.

Since 1976, several new Earth crossing asteroids, i.e., those which cross the orbit of the Earth, far from the inner boundary of the asteroid belt, have been discovered. Among them was Ra-Shalom, discovered in 1979.

Most asteroids have been found to very dark, perhaps a symptom of carbon content, and chemically bound water has been detected on some of the darker surfaces.

photo of a small boy with the allenede meteorite

Sample from the stars: the Allende, Mexico meteorite and a puzzled Earthman. (Photograph courtesy of Brian Mason, National Museum of Natural History, Smithsonian Institution.)

The mass of the large asteroid Vesta has been determined, and it has been found that the surface composition of Vesta resembles that of basalt lava.

In 1979, a minor planet (Chiron) was discovered between the orbits of Saturn and Uranus, far outside the known limits of the asteroid belt.

Laboratory study of meteorites reveals that they typically have spent only 10 million to 100 million years exposed to the space environment, a finding that suggests that relatively recent collisions have occurred in the asteroid belt.

Some meteorites are composed of lava, indicating that early melting and volcanic eruptions occurred on their parent asteroids.

Daughter products of extinct, primordial , radioactive isotopes, such as iodine-129, plutonium-244, and aluminum-26, have been found in meteorites. These isotopes, when still present in the parent objects of the meteorites, may have served as important heat sources.

Amino acids have been found in meteorites (see Appendix section on Exobiology).

White inclusions of minerals formed at high temperatures have been found in the Allende meteorite. These may be samples of the first material to solidify in the original cloud from which the solar system was born.

Elemental anomalies found in the white Allende inclusions indicate that some matter may have been introduced into the solar system cloud from another star, presumably a supernova.

The formation ages of meteorites date back 4.6 billion years, providing a firm estimate for the age of the solar system.

Gases from the solar wind have been found to be trapped and preserved in meteorites.

The rates at which iron meteorites cooled from the molten state within their parent objects have been determined; they range from 1 to 20 degrees centigrade per million years. This cooling is too rapid to have occurred within the iron core of a large planet.

Comets

In 1970, the existence of huge hydrogen clouds around the comets Tago-Sato-Kosaka (1969 IX) and Bennett (1970 II) was discovered with the OAO-2 satellite. The clouds, also observed by OGO-5, are detectable only in ultraviolet light. The existence of such clouds had been predicted on the basis of the icy conglomerate or "dirty iceball" model of comets, and thus the discovery helped to confirm the theory.

The predicted presence within the cometary hydrogen cloud of high velocity material, with speeds of about 20 kilometers per second (12 miles per second), was found to be consistent with the intensity distribution of the hydrogen clouds mapped by OAO-2 and subsequent spacecraft. It was recently confirmed by spectroscopic observations from the Copernicus satellite.

The hydrogen clouds provide direct evidence for a rate of mass flow from the nucleus of a comet that is adequate to account for the so-called "nongravitational force" that disturbs the motions of comets.

Radio telescopes have observed the hydroxyl (OH) radical in several comets and have made apparent de tections of the substances CH₃CN, HCN, and CH in Comet Kohoutek (1973 XII).

The first definite detections of carbon and oxygen atoms in a comet were made by ultraviolet spectroscopy from two sounding rockets launched to study Comet Kohoutek in 1974.

Infrared measurements made from the ground have revealed the presence of silicate dust grains in several comets.

Radio observations of Comet Kohoutek and Comet West (1975n) revealed the presence of transient microwave emission apparently produced by the icy-grain halo, a structural feature of comets that had been predicted to exist.

Observations of Comets Seargent (1978m) and Bradfield (19791) with the IUE satellite revealed the presence of atomic sulfur and CS in the coma and showed that these species are produced from an extremely short lived (less than about 100 seconds) parent molecule, perhaps CS₂. The forbidden neutral oxygen line at 2972 A was identified; it is thought to come from photodissociation of water and thus provides a means of determining the spatial distribution of water vapor in the coma.

Using IUE, the water vapor production rate of Comet Bradfield was determined over a range of heliocentric distance from 0.7 to 1.5 astronomical units, the first time this has been measured for a comet. The variation was inconsistent with the idea that the vaporization might depend only on the input of heat from the Sun.

Interplanetary Dust

Pegasus 1, in 1965, and other space craft showed that interplanetary dust particles are about 10,000 times less abundant than had been indicated by early space experiments.

Pioneer 10 and 11, in 1973 and 1974, found that Mars sweeps up the interplanetary dust near its orbit.

Pioneer observations also showed that the zodiacal light is produced by dust that orbits the Sun at great distances from the Earth.

The first definite samples of interplanetary dust have been collected by high-altitude research aircraft since 1978.

A tiny bead (chondrule) in a meteorite displays a jewel-like arrangement of microscopic crystals and glass. (Photograph courtesy of Laurel Wilkening, University of Arizona.)