BY ROBERT L. MARCIALIS
UNIVERSITY OF ARIZONA
LUNAR & PLANETARY LABORATORY
1629 E. UNIVERSITY BLVD.
TUCSON, AZ 85721
http://www.lpl.arizona.edu/~umpire
INTRODUCTION
Planetary Science is the branch of knowledge benefiting most as a result of the space age. In less than half a century, the field has been revolutionized by the influx of new images, observations, and other forms of data. Most planets and their larger satellites have been transformed from telescopic objects to individual worlds. Each has revealed a distinctive geological pedigree. New twists on well-known evolutionary processes are seen, and totally unanticipated phenomena revealed every time a planet is visited by a spacecraft. What was “planetary astronomy” in one generation has become “comparative planetology.”
Technological developments such as adaptive optics, large-mirrored telescopes, and infrared array detectors have re-vitalized our observatories. These new tools enable significant, but parallel, progress to be made from the ground. Computers allow numerical experiments to test theory, and run simulations where solution of the analytic equations is impossible.
As a result of our newfound knowledge and tool kits, increasingly detailed studies of the beginnings of our solar system are being undertaken. Planets, albeit giant, Jupiter-class ones, are being discovered around other stars, extending the reach of planetary science beyond its grasp. Astronomical observation is still is a major player in the study of the solar system. The Kuiper Belt, a region of our solar system only suspected a decade ago, now has over 410 catalogued members; it took 95 years to discover that many asteroids. Distant Pluto is the only planet not yet visited by a spacecraft. It likely will remain a “telescopic” object for years to come, with the recent cancellation of the Pluto-Kuiper Express mission.
The status of our knowledge of the planetary system is summarized in the following pages. The treatment of each body is brief, and up-to-date at the turn of the millennium. However, rapid progress continues. In the time it took to prepare this article, there were discovered sixteen new satellites, 390 asteroids, and what may be ancient shorelines on Mars.
FORMATION
Our solar system began when a large cloud of gas and dust began to collapse. Comprised of a solar mixture of elements (Table 1), the mix is mostly hydrogen and helium, with a few percent sprinkling of compounds containing carbon, oxygen, nitrogen, and even lesser quantities of silicon and metals. The preponderance of hydrogen in this mixture means that most of the C, O, and N present react to form ices: mainly CH4, H2O, and NH3, with some CO and N2. To conserve angular momentum, motions in the cloud became accentuated as collapse continued, much as an ice skater spins faster as her arms are drawn to her body. The cloud fragmented into several hundred sub-clouds, each destined to form a separate stellar system.
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As the sub-cloud continued its collapse, compression of the gas component caused heating. The gas component tends to obey the hydrostatic equation, while small solid particles undergo Keplerian orbits around the barycenter. This difference in kinematics causes a drag force on the particles as they orbit the protosun. This drag force has several effects: elliptical orbits are circularized, inclined orbits damp down toward the equatorial plane, and particles tend to spiral inward. Since drag on a particle is proportional to its cross-sectional area and mass, different size and mass particles are affected to varying degrees. The result is low-velocity collisions. A non-negligible percentage of the particles stick together, beginning the inexorable process of protoplanetary accretion.
The cloud has become disk-shaped. Temperature and density gradients increase toward the equatorial plane and radially inward to the center of the disk. At a given position in the disk, solid particles will vaporize if the local temperature is warm enough. Therefore, particles residing in the inner regions of the disk are mainly refractory materials: high-temperature species like metals, oxides, and silicate minerals. Farther out, in the cooler regions, ices dominate solid particles, as ices are hundreds to thousands of times more abundant than metals or silicates. The biggest of the accreting “iceballs” (5 to 10 earth masses, ME) are large enough that their gravity can gobble up vast quantities of the surrounding gas (mainly hydrogen and helium). They grow more massive still, allowing even more gas to be captured (i.e., runaway accretion).
This scenario explains several features of our solar system. Most objects orbit in a single plane about the Sun called the ecliptic. The source of the compositional gradient seen in the present-day solar system is also explained. In the regions where water and other ices exist as solids we find the Jovian, or Jupiter-like planets. Hundreds of times more massive than the terrestrial “crumbs,” their composition largely reflects the original gaseous component of the nebula. Farther out, collision timescales were longer and building materials much more rare. The outermost planets did not grow to the runaway accretion stage, and are depleted in H and He relative to Jupiter and Saturn.
Eventually, the center of the nebula reaches temperature and density conditions sufficient for the initiation of thermonuclear fusion. The protosun ignites and begins to produce its own internal energy. Heating and radiation pressure from the young Sun caused residual gas in the nebula to dissipate (T Tauri phase). Except for a sweeping up and redistribution of the leftover crumbs, the formation of our planetary system is essentially complete.
MERCURY
Looking at the surface of Mercury, the innermost planet, one sees a heavily cratered surface interspersed with maria, impact basins subsequently flooded with basaltic lava. Mercury’s appearance is that of a slightly larger version of our Moon. But appearances are deceiving. To account for its density of 5.44 gm cm–3, the planet must have a huge iron-nickel core, nearly ¾ of the planet’s radius and 42% of its volume. The Moon’s core is only 2% or 3% of its radius; Earth’s is 45%.
Mercury orbits the Sun at 0.38 AU, in a decidedly elliptical path (e = 0.206). At aphelion it is 63% farther from the Sun than at perihelion. Virtually airless, Mercury’s surface has the most extreme conditions of any terrestrial body. Thermal infrared measurements from Earth-based telescopes show daytime temperatures can peak at 7000 °K; at night the minimum plunges to 90 °K. But a centimeter deep into the dusty soil the diurnal temperature pulse is “only” from 450 °K to 210 °K. So molten lakes of lead or zinc, common to old science fiction stories, are not plausible. Nonetheless, the surface environment is extremely hostile to most materials. Robotic exploration of the surface is not likely in the near future.
Mercury is devilishly difficult to observe from Earth—it is never more than 27° from the Sun. Therefore it must be observed either at high airmass (just after sunset or before sunrise), or during daylight hours when Earth’s atmosphere is very turbulent. In 1965 a radar beam was bounced off Mercury’s surface. The Doppler spread in the return demonstrated that the planet rotates in 58.6 days, not 88 days as expected. Mercury exhibits a 3:2 spin-orbit resonance wherein it rotates exactly three times on its axis for every two revolutions about the Sun. It is believed that the spin-orbit coupling is a result of tidal interaction with the Sun removing angular momentum, slowing its originally higher spin rate.
A “day” on Mercury is rather remarkable. The time between noons is 176 Earth days. However due to Mercury’s elliptical orbit, the Sun will rise in the east, stop, reverse its direction through the sky for a while, then resume its westward march. The apparent size of the Sun changes by about 62% during the course of 88 days. And, at perihelion, the Sun is directly overhead at one of only two points on the equator—180° apart—called “thermal” poles.
Recent observations have revealed “radar bright” regions near the eternally shadowy poles. Several candidate minerals are reflective at radio wavelengths. It is intriguing that subsurface water ice is a leading candidate. It may take a lander to determine the truth.
Mercury lies very deep in the Sun’s gravitational well. Mariner 10 is the only spacecraft to have flown past the planet. In addition to craters and impact basins, the images show a global network of “compression ridges,” best explained by cooling (shrinking) of the planet by some 15 km in radius. Yet Mariner’s magnetometer also detected a weak magnetic field—evidence the interior is still molten. Jumbled, “chaotic” terrain antipodal to the Caloris impact basin is likely due to the convergence of seismic energy from the impact event half a world away. The region was kicked several km skyward, then came crashing down.
VENUS
Venus’ radius and mass are slightly less than Earth’s at 0.95 RE and 0.82 ME, respectively. Presumably, our planet’s closest neighbor formed in the same general region of the solar nebula, and interior composition and structure of both planets are similar. Venus orbits the Sun at a mean distance of 0.72 AU in the most circular of all planetary orbits. It has no moons. Long supposed to be Earth’s “twin,” it is difficult to see any family resemblance in Venus upon closer inspection.
Venus is eternally and completely covered by thick clouds, which circulate around the planet in about 4 Earth days. The planet’s true rotation period was determined in 1962, when radar was bounced off the surface. Doppler broadening gave a value: 243 Earth days—retrograde! Too lazy for an internal dynamo to generate an appreciable magnetic field, the solar wind slams into the upper atmosphere, slowly but inexorably eroding it.
The atmosphere was discovered in 1761, but it wasn’t until 1932 when the major constituent, CO2, was identified via spectroscopy. In 1972 the composition of the clouds was determined: sulfuric acid! It may rain H2SO4, but it never reaches the surface. Application of Wien’s Law to far-infrared and radio data showed the lower atmosphere to be ~750 °K. The Soviet Venera 7 probe landed on the surface in 1970, confirming the high temperature and reporting a surface pressure of 90 bars. The greenhouse effect is responsible for the high temperature. The atmosphere is largely transparent to solar radiation (predominantly ~500 nm), which can penetrate to heat the surface. Energy is re-radiated as heat (black body peak ~4000 nm). However, at these longer wavelengths, the CO2 atmosphere is opaque, and acts as a very efficient thermal blanket. Equator to pole, daytime or night, the surface would have a uniform temperature (±2 °K) save for the effects of topography.
And it’s a dry heat. Most of Earth is covered with km of water. Venus’ surface is dry, the atmosphere only 30 parts per million water. Did Venus originate with an earth-like water inventory and lose it, or was the planet born dry? This question is key to understanding the planet’s evolution.
Four of six Soviet landers have returned surface panoramas. Angular, platey rocks, gravel, and fine soils in varying states of erosion are revealed. Chemical analyses at all but one the landing sites showed basaltic composition, comparable to terrestrial oceanic crust.
Although preceded by both Soviet and American radar mapping missions, the true oracle in our understanding of Venus’ surface was NASA’s 1991 Magellan mission. 98% of the planet was mapped at a resolution of 100–200 meters per pixel. From the travel time of the radar, accurate altimetry was derived. The histogram of surface radii is narrow and bell-shaped, very different from the bimodal histogram for Earth. (Our planet shows peaks at oceanic and continental radii.) This argues against earth-like global plate tectonics on Venus.
Magellan images revealed regions of local tectonism (crustal folding and stretching), lava channels, and aeolian features. Weathering processes on Venus seem to be extremely slow. Accurate impact crater counts were also made. The older a planetary surface, the more impact craters it will accumulate. The Apollo landing sites allowed absolute ages to be assigned to certain crater densities on the Moon; these can be extrapolated to Venus and other solar system bodies. The results for Venus are interesting, if puzzling.
Earth-style plate tectonics results in continual recycling of the crust at subduction zones, and regeneration at mid-oceanic ridges. It seems that Venus underwent a global resurfacing event about 500 million years ago, with craters accumulating ever since. Was Venus tectonically active for over 90% of its history, becoming inert only relatively recently? Unlikely: Earth still has plenty of tectonic activity. Or is resurfacing on Venus a periodic process? Very un-Earthly! Understanding the disparate styles of tectonism on Earth and Venus is crucial to comprehend the differences between these two worlds.
EARTH
The third planet from the Sun shows several traits making it unique among terrestrial (rocky) planets. Approaching Earth from space, we would first notice the presence of a large moon. An active magnetic field (~0.6 G) shields our planet from solar and galactic charged particles. Abundant water, in all three phases, coats the planet. Were aliens to drop 10 probes at random locations, about ⅓ would travel through water clouds and ¾ of them would splash down into ocean. A paucity of impact craters (~300 total) implies a very young surface. The atmosphere, 78% N2, 21% O2, 0.9% Ar, and 0.05-2% H2O is a mix far removed from chemical equilibrium with the surface materials; it betrays the existence of photosynthetic life forms generating vast amounts of free oxygen. A layer of ozone (O3) in the upper stratosphere screens the planet below from a large percentage of the solar ultraviolet (shortward of 310 nm). Advanced life is no doubt the most unique feature of our planet.
Relatively rapid rotation (currently 23h56m04s.0989) allows diurnal temperatures to be moderated and supplies the Coriolis force giving weather systems their characteristic size scale. The spin axis is inclined 23½° to the orbit normal, causing seasons. Our Moon tends to stabilize this obliquity (i.e., axial tilt), thereby moderating climate on a time scale of hundreds of million years. The time characteristic for continental drift is also about this size, as is the time characteristic for evolution. Plate tectonics (a.k.a. continental drift) is constantly renewing and rearranging the surface. It is known that evolutionary “explosions” have occurred coincident with the formation of new oceans, such as the Tethys Sea (now Indian Ocean) and the Atlantic. Life originated (perhaps several times over) within the first few hundred million years of our planet’s history, judging by the age of the oldest known fossils (3.5–3.9 Gyr). It is increasingly clear that evolution has been influenced, even shepherded, by geological processes.
Heat from radioactive decay led to early melting of the Earth. Dense materials (metals, and elements that readily dissolve in them) sunk to the center, and less dense materials floated in a process called chemical differentiation. The interior structure we “see” by seismic monitoring became established. At the center is a solid Fe-Ni inner core, surrounded by a Fe‑Ni liquid outer core (phase change due largely to decreased pressure). The next layer is the mantle, its base at just over 50% the Earth’s radius. It is composed of dense, molten rock, and transports heat to the surface by convection. (We recently have learned there is significant topography at the core-mantle boundary.) Only in the outer several hundred kilometers does the mantle cool from fluid, to plastic, to the brittle (solid) top layer called the crust. The crust begins ~5 km below the seafloor, ~30 km below the continents. Ocean floor material is mainly basalt rock (common to terrestrial planets), while the continents generally are composed of a less-dense rock type called granite.
Earth’s surface is divided into ~10 nearly rigid tectonic plates, which move relative to each other at rates of a few cm yr–1. Plates spread apart at mid-oceanic ridges. Trenches and island arcs occur at subduction zones (Japan, the Andes), where plates converge and one slips beneath the other. Horizontal slippage occurs at transform faults, such as California’s San Andreas. Plumes carry hot, low-viscosity material from depth, and volcanic island chains (Hawaii) form where these hot spots puncture a plate. The hypothesis provides a unifying framework for solid-Earth science in terms of thermal convection. Underlying dynamics of the process—the—“how” and “why” are just beginning to be understood. Only recently have computers become capable of simultaneous numerical models of both physical and chemical aspects. The mystery of why Earth has had vigorous plate tectonics throughout its history, but Venus has not, is far from solved. Until models can reproduce the difference, we cannot truly say we understand either planet.
MOON
Our Moon likely was produced near the end of Earth’s accretion phase 4.6 billion yr ago, when a Mars-sized body crashed into Earth, knocking large chunks of the mantle into orbit. This “Giant Impact Hypothesis” explains many major characteristics of the system. It accounts for Earth’s obliquity and rapid spin (initially 12–16 hr), the Moon’s diminutive core, apparent high-temperature chemistry, low density (3.3 g cm–3 vs. 5.53 g cm–3 for Earth), desiccated state, and the spot-on oxygen isotope match with Earth found in the samples returned by Apollo.
The chunks rapidly coalesced, and heating by radioactive decay soon melted the early Moon. A magma ocean several hundred km deep formed. Cooling ensued, and a 1000‑km lithosphere of largely anorthositic rock formed. The surface suffered a period of intense heavy bombardment as Earth and Moon swept up their remaining building blocks. Large impact basins, several hundred km across, were created on both worlds. Intense cratering ended about 3 Gyr ago. Darker, basaltic lava later erupted in many places, preferentially filling the low-lying thinner-crusted and heavily fractured basins.
This scenario of lunar evolution was pieced together from superposition relationships—an oft-used geological principle whereby “newer stuff tends to cover older stuff.” One of the foremost results from analyzing the 400+ kg of samples returned by six Apollo and three Soviet Luna probes was to apply radioisotopic dating to fix absolute ages to each step in the process. For lack of anything better, this time scale is extrapolated, with additional assumptions, to estimate ages for other planets and satellites in the solar system.
Small bodies have large surface area-to-volume ratios, and active lunar geology soon ended. The only substantive geologic events in the last 2.5–3.0 Gyr have been occasional impacts. Luna’s gravity (1/6× Earth; mass = 0.0123 ME) is too feeble to retain an atmosphere. Surface temperatures swing between 370 °K and 100 °K. Impact gardening and intense thermal cycling comminute the “soil” (to misuse a term) to the consistency of talcum powder. Craggy peaks described in old science fiction novels are in reality mantled and muted.
The lunar far side is deficient in the dark, basalt-filled maria so common to the near side. It is largely covered with impact craters. Far side crust is about twice as thick near side crust. There is a 2-km offset toward Earth in the center of mass from the center of figure, due to the maria. Tidal forces tug on the “bulge” to keep this face oriented earthward.
Seismometers left by the Apollo astronauts have been used to learn about the Moon’s interior. A topmost layer (regolith) typically is 3–30 m deep, consists of rubble created by impacts. While the seismometers functioned, they recorded the Moon being whacked by a number of things: grenade-sized explosives, lunar module upper stages, and spent rocket boosters. It rang like a bell, indicating a much less dissipative interior than Earth—further evidence for a largely solid interior. Passive monitoring showed moonquakes originate in two zones: the surface and ~1000 km depth. Surface events are ascribed to impacts; deeper ones hint at a transition zone from solid to partially molten. No core could be detected, although the magnetometer on NASA’s 1998 Lunar Prospector mission detected a small metallic core, 1-2% of the Moon’s total mass.
Another interesting result came from Lunar Prospector’s Neutron Spectrometer. Dips in the epithermal neutron energy spectra occurred at both poles. One interpretation of this is 10 to 300 million metric tons of water ice buried approximately 50 cm in permanently shadowed craters (water is an excellent moderator of neutrons).
Corner cube reflectors at the Apollo landing sites are used to conduct laser-ranging experiments. The Moon currently is receding from the Earth at about 6 cm yr–1.
MARS
Roughly half the diameter of Earth, diminutive Mars is nevertheless a world of “big geology.” Its southern hemisphere is ancient, heavily cratered uplands reminiscent of the Moon, complete with large impact basins. The northern half of the planet is relatively younger, less-cratered plains. Straddling the boundary is the Tharsis bulge, a construct of four major and several minor shield volcanoes, roughly half the area of the United States. The sheer size of Tharsis indicates Mars has a very thick, rigid crust to support the load, an estimated at 100–200 km thick. Olympus Mons is the solar system’s largest volcano, 27 km high, and the size of Arizona. Though inactive, the paucity of impact craters betrays a young age, perhaps ~1 Gyr.
Downslope of Tharsis is Valles Marineris, a 5000-km-long canyon, in places hundreds of km across and 6 km deep. This fracture is evidence Mars’ crust is thinner than the Moon’s. Its eastern end branches out into a chaos of hundreds of outflow channels. There is global evidence for a past era when abundant water flowed on Mars. Dry riverbeds are seen in the ancient south. Classic groundwater sapping patterns, fretted terrain, valley networks abound—and water is the only liquid in sufficient cosmic abundance to be the culprit. A stated goal of NASA’s Mars program is to locate sequestered water.
Today Mars’ atmosphere is thin and dry: 6–9 mbar (600–900 Pa), comprised of 95.3% CO2, 2.7% N2, 1.6% Ar, and only ~10 precipitable microns of water. Since this is below water’s triple point, liquid water cannot exist on the surface for long. Mars has polar caps, though. A substantial fraction of the atmospheric CO2 condenses at the winter pole. The result is a semiannual fluctuation in the surface pressure, with seasonal sublimation winds predominantly away from the summer pole. Mars’ orbit around the Sun is eccentric (e = 0.0934) resulting in asymmetric seasons. Presently perihelion is at the end of southern spring: southern summers are hotter than northern. Global dust storms tend to initiate around perihelion. Typical atmospheric optical depth due to dust loading is 0.3–0.6, but can reach 5 during dust storms.
The thin atmosphere is a poor thermal blanket. At the Mars Pathfinder landing site in Ares Valles, diurnal temperatures cycled between 197 °K and 263 °K. Mars lacks an ozone layer to shield the surface from solar ultraviolet radiation. Chemical reactions between the atmosphere and surface rocks in the presence of UV produce a suite of superoxides and peroxides that become adsorbed onto the surface. These chemicals are very destructive to organic compounds. In effect, the surface of Mars is self-sterilizing.
Mars Global Surveyor orbiter has discovered stripes of magnetized surface rock—bands of magnetic material aligned in one direction alternating with magnetic material of the opposite polarity. This is somewhat reminiscent of the patterns seen at mid-ocean rift zones on Earth. On our planet, the alternating stripes testify to plate tectonics. Earth’s reversing magnetic field imprints in upwelling magma as it cools below the temperature at which a magnetic field is preserved. Movement of crust away from a spreading center creates a “recording” of the magnetic field versus time (or age). The simplest conclusion: Mars too might have had a strong internal magnetic field and experienced limited tectonic activity when the oldest southern terrain formed about 4 Gyr ago.
The size of Mars’ core is constrained by the planetary mean density (3.933 g cm–3) and dimensionless moment of inertia (0.365). At one extreme, a 1300-km radius metallic iron core fits the data. At the other, a 2000-km radius FeS core also works. Petrochemical evidence from known Martian meteorites (the SNCs) shows the upper mantle is depleted in siderophile (“iron-loving”) and chalcophile (“sulfur-loving”) elements. This implies the latter model is preferred. In addition, FeS is less electrically conductive than pure Fe, explaining the lack of a strong internal magnetic field in Mars today.
Mars has two small satellites, Phobos and Deimos. The party line is that these bodies are captured asteroids. However, recent near-IR spectra show their surface materials are not matched by any of the common asteroid spectra.
ASTEROIDS
In the late 1700s, astronomers were puzzled by the vast, seemingly empty region of the solar system between Mars and Venus, from 1.52 to 5.20 AU. They began a search for the “missing planet” in this region. On the first night of the 19th century—01 January 1801—Giuseppe Piazzi discovered the “missing planet.” It was named Ceres, after the Roman goddess of the harvest. The mystery had been solved. Or had it? By 1807, three more asteroids had been discovered in similar orbits, what we now call the “main belt.” (The term “asteroid” means “star-like,” in reference to their telescopic appearance.) Official nomenclature for asteroids is a number (sequential, in order of discovery), followed by a proper name (typically chosen by the discoverer). Hence, 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, etc. Today there are about 10,000 numbered asteroids. Sampling of the main belt is likely complete down to ~20 km diameter. It is estimated some 100,000 observable asteroids are yet to be discovered. Were all asteroids combined, they would comprise a single body less than 1500 km in diameter—about 2/3 the Moon’s diameter. 1 Ceres is the largest asteroid, with a diameter of 960 km.
Main-belt asteroids never accreted as a single body. The region is riddled with locations where a body’s orbital period around the Sun is an exact ratio of Jupiter’s orbital period, e.g., 2:1, 3:1, 8:3, etc. Any asteroid wandering into this region is removed in short order. These resonance regions, named Kirkwood Gaps after their discoverer, are very apparent now that the statistical sample of known asteroid orbits has grown so large. This gravitational sculpting by Jupiter is responsible for replenishing the population of “near-Earth asteroids (NEAs).” The primary source of meteorites, NEAs are continually swept up by the terrestrial planets. Apollo asteroids are those whose orbits cross Earth’s; the Amor and Aten families comprise asteroids with perihelia less than Mars’ or Venus’ orbit, respectively. Asteroid “families” are named after the first-known member in a given class.
A third class of asteroids is the Trojans. These bodies (currently 850 are known) orbit at Jupiter’s distance from the Sun, 5.2 AU. Found roughly 60° ahead and 60° behind Jupiter along its orbit, they surround two gravitationally stable regions predicted theoretically by J.L. Lagrange. (Incidentally, Mars has two confirmed “Trojans,” and two more are suspected. Searches for Earth-Trojans have been to date unsuccessful.)
While the asteroids never accreted as a single planet, it is also plain that in the past there existed several bodies large enough to have undergone planetary differentiation. Spectra show various compositional types, or families, of asteroids. Some are metallic, others show silicate mineral composition, while some have spectral absorptions due to a few percent organic (carbon bearing) and/or hydrated minerals. The M-class (metallic) asteroids presumably are samples of the cores of these shattered, larger planetoids. 4 Vesta is of the rare basaltic achondrite class, and probably represents a piece of the surface of an evolved body whose interior had melted and flooded its surface with lava flows. Most asteroids, however, are chemically undifferentiated. They represent relatively primordial relics from the time when the solar system formed.
Understanding asteroid compositions is complicated by several factors. Composition of the solid component of the protoplanetary nebula varied with temperature, i.e., radial distance from the Sun, and with time. In general, inner asteroids are composed of higher-temperature minerals than those further out. Somewhere within the present-day belt was the “ice line,” or the region where temperature dropped enough for water to condense. Water of hydration is seen in spectra of many outer-belt asteroids.
Asteroids are the most energetically accessible bodies to the Earth, since their gravitational wells are tiny. They represent a vast, untapped mineral resource for future generations, as human exploration of the solar system becomes reality. We have only begun detailed examination of this class of objects. The first asteroid orbiter, the NEAR spacecraft, continues its examination of 433 Eros. Outer planet probes typically are targeted to fly by asteroids as they pass through the belt. It is clear that asteroids are important to our future, and to our past. Impact of a 10-km-sized asteroid into Mexico’s Yucatán 67 Myr ago had significant effects on terrestrial evolution by killing off the dinosaurs.
JUPITER
Giant Jupiter is more massive (318 ME) and bigger (11 RE) than all the other planets combined. It is the first of four “jovian” gas worlds, planets with no solid surfaces and compositions that are to first order similar to the Sun. By mass, the atmosphere is 78% H2 and 19% He. The remaining percent is mostly “hydrated” molecules of CH4 (0.2%) and NH3 (0.5%). Trace amounts of other species, many produced through UV photochemistry, give color to its clouds. Jupiter began as a “seed” of rock and ice, perhaps 12 Earth masses, onto which accreted a sizeable amount of its local solar nebula.
Many of the planet’s characteristics are understood as consequences of this “runaway accretion.” Drawing in gas from far away caused Jupiter to spin rapidly due to conservation of angular momentum. The jovian day is a mere 9h55m30s, short enough that the planet’s polar diameter is 6.4% less than its equatorial span. Accretion caused much heating. The core is estimated to be about 30,000 °K, although the cloud tops are a brisk 150 °K. Today, Jupiter emits about twice as much energy as it receives from the Sun. The source of this energy is a slow, continued contraction of about 10 cm yr–1. Jupiter is a bona fide planet and not a “failed star.” It would require 13 times its mass for the core to burn deuterium, and 75 times its mass for plain hydrogen fusion to initiate.
Rapid rotation and heating from below causes Jupiter’s cloud structure to be very different from Earth’s. Clouds are organized into bands parallel to the equator, called belts and zones. Convection causes rising, hot air parcels (zones) to cool, condensing out dissolved species when saturation vapor pressure is reached. Sinking parcels heat, vaporizing larger (less backscattering, therefore darker) ice crystals. Jupiter rotates differentially, generally faster at the equator than at the poles. Weather systems remain confined to their original latitudes, blowing at up to 150 m s–1. The Great Red Spot is the most prominent of these generally oval features. About 3 times the size of Earth, it has persisted since at least 1665. Theoretical calculations give it a minimum age of 50,000 yr.
The atmosphere has many cloud levels. Topmost is ammonia (NH3), then ammonium hydrosulfide (NH3SH), with a thick H2O cloud deck at about 10 bars pressure. A probe released by the Galileo spacecraft unfortunately did not survive much deeper than this. Dozens of deeper layers are predicted but obscured by higher layers. Deep convection of the atmosphere makes the planet self-sterilizing—any organic chemicals produced by UV photochemistry are destroyed when re-equilibrated at the bottom of a convection cell.
Interior models show the atmosphere gives way to a layer of liquid molecular hydrogen. Pressure increases with depth until the electrons and nuclei become dissociated. The molecular layer transitions into liquid hydrogen metal, at the base of which presumably lies the original “seed” core of rock. Metallic hydrogen is an excellent conductor of electricity and, combined with the planet’s rapid rotation and convection, gives rise to a magnetic field of 400,000 nT (10 times that of the Earth). Charged particles trapped in this field form intense radiation belts. This is the source of Jupiter’s decimeter radiation. All six spacecraft to Jupiter were built of radiation-hard materials. A simple flyby gives a dosage similar to 109 chest x-rays. The aging Galileo spacecraft, in orbit since 1995, only briefly dips close to the inner satellites to minimize radiation damage. A human’s body temperature would be raised about 1 °C at Io’s distance from Jupiter due to radiation.
Jupiter has 27 satellites—ten of which were discovered as this article goes to press. Many of the outer satellites are in irregular or retrograde orbits, evidence (along with light curves and spectra) they may be captured asteroids. The four innermost satellites also are small, odd-shaped, rocky bodies. Deep in Jupiter’s gravity well, micrometeorite impacts and charged particle impacts spatter from their surfaces a steady supply of submicron particles (about the size of smoke particles) that comprise Jupiter’s ring.
The four largest satellites, Io, Europa, Ganymede, and Callisto qualify as planets in their own right (the latter two are larger than Mercury). Co-discovered by Marius and Galileo (the man) in 1610, they were pivotal evidence against Ptolemy’s Earth-centered worldview. These worlds probably accreted in the jovian subnebula, in some ways making Jupiter a smaller version of the solar system. As such, they exemplify many principles of comparative planetology, such as compositional gradients (more ice) with increasing distance from Jupiter. Outer Callisto is the most heavily cratered surface in the solar system, attesting to great age and little internal geologic activity. The inner three Galileans are in a resonance, one consequence of which is varying degrees of internal heating due to tides. Surface age correlates with distance from Jupiter, consistent with more tidal energy being deposited in the inner satellites. Totally desiccated by geological activity, innermost Io is the most volcanically active body in the solar system, complete with hot lava flows and sulfur dioxide geysers and snow. Europa and Ganymede have veneers of dirty water ice, overlying liquid oceans and rocky cores. Europa’s surface is very young (few craters), while Ganymede has regions older than Europa but younger than Callisto. Lately Europa’s ocean has become a fashionable place to consider in terms as an abode of life (joining Mars and Saturn’s satellite Titan). The next mission to Jupiter likely will be a Europa orbiter.
SATURN
Save for its prominent ring system, Saturn might appear as just a slightly smaller Jupiter. Appearances are deceiving. Although 85% Jupiter’s diameter (9.5 DE), it has only one-third of Jupiter’s mass (95.1 ME). Saturn’s density is 0.69 g cm–3: find a large enough bathtub and it would float. To understand the differences between Jupiter and Saturn, consider their relative positions in the solar system.
Saturn orbits the Sun at an average distance of 9.54 AU, nearly twice Jupiter’s 5.20 AU. During accretion there was less “seed” material per unit volume, and the local nebular gas density was less. There were fewer building blocks, and a higher ice-to-rock ratio than closer to the Sun. To first order solar in composition, Saturn did not accrete sufficient mass to retain the full hydrogen inventory in its region of the nebula. “Energy of assembly” was far less than for Jupiter, resulting in less internal heat. Central pressure is also much less. Like Jupiter, Saturn emits more energy than it receives from the Sun, but it is not sufficiently massive for slow contraction to be the source. Rather, at Saturn’s reduced interior pressure, helium is less soluable in hydrogen. Helium droplets “rain” onto the core, releasing gravitational potential energy as heat. Otherwise, the interior structure of the two planets is rather similar. A much deeper transition region from molecular H2 to metallic H, and slower spin (10h39m25s) result in a magnetic field only 5% that of Jupiter.
Less internal heat, and only ¼ the incident sunlight has atmospheric consequences. The cloud structure is essentially similar, but the colder atmosphere means each cloud deck occurs lower on Saturn. Above the topmost ammonia clouds, methane photochemical haze forms. These two effects conspire to mute Saturn’s cloud features relative to Jupiter’s, and give it a higher albedo.
Saturn’s spin axis is inclined 26½° to the orbit normal, causing marked seasons (Jupiter’s obliquity is a paltry 1/3°). Large equatorial storms tend to occur at 30-year intervals, Saturn’s period around the Sun. Equatorial winds typically exceed 500 m s–1.
To the dynamicist, Saturn’s rings are every bit as beautiful as they are to the casual observer. Girdling the planet’s equator, they span 274,000 km but are a mere 100 m thick. Each Saturnian equinox the rings are edge-on to the Sun and apparently “disappear” for a few days. Ring spectroscopy reveals 100 °K water ice. Radar reflectivity and models of the scattering phase function yields a particle size distribution from house-sized down to dust. The most probable size is ~10 cm in diameter, although this varies with distance from Saturn. Gaps exist at several locations in the rings. The most prominent, the Cassini Division, lies at the 2:1 orbit resonance of the satellite Mimas. When Voyager arrived at Saturn in late 1979, the rings were resolved into literally thousands of ringlets and gaps. Other features, such as radial spokes, non-planar warps, and braids show there is much more to ring dynamics than solely orbit-orbit resonances with satellites. Ring science is a field enriched by theories of spiral density waves, bending waves, shepherd satellites, and imbedded moons. Erosion by micrometeorite impacts and radial “spreading” due to collisions imply the rings are only a few hundred million years old—far younger than Saturn. Current thinking is they are ephemeral structures, perhaps produced when a small (few km) satellite was shattered by a comet impact.
Saturn has 24 known moons. In general, they are small bodies, with albedos in excess of 90%. Densities, where known, are 0.90 g cm–3, consistent with the water ice composition of the rings. One satellite, Iapetus, has an unknown dark surface material on its leading hemisphere. In general, the satellites are heavily cratered, ancient surfaces, with few signs of internal geologic activity (except Enceladus).
Then there is Titan. At 5,150 km diameter (larger than Mercury), the satellite has a dense atmosphere, 85% N2, 3% CH4, and up to 12% Ar, a spectrally inert gas. Titan’s surface pressure is about 1.6 times Earth’s. The nitrogen likely was produced by UV photolysis of NH3 over the eons. Titan’s surface is eternally shrouded in a thick smog of organic compounds. Unlike the self-sterilizing environments of jovian planet atmospheres, however, photochemically produced organics can precipitate to the surface and accumulate. Oceans of primeval soup hypothesized two decades ago are unlikely based on recent radar observations, but Titan remains arguably the most profitable place in the solar system to search for life (excluding Earth). Its meteorology may be dominated by a “hydrologic” cycle where CH4 takes the place of H2O in forming clouds and precipitation. The Cassini spacecraft will enter orbit around Saturn in 2004, dropping the Huygens descent probe into Titan’s atmosphere. Its two-hour descent to the surface will open our eyes to this mysterious world.
URANUS
Of the four jovian planets, Uranus is least understood. The spin axis is tilted nearly into the orbital plane. The poles actually receive more solar energy than the equator, averaged over Uranus’ 84.01-yr orbit. When Voyager 2 encountered Uranus in 1985, the southern hemisphere had been basking in the Sun for decades. Considerable image enhancement revealed few distinct clouds, although atmospheric circulation is symmetric about the spin axis. Things have changed in the intervening decades. Recently, clouds appear increasingly frequently, consistent with visual reports made at the last equinox. This “zebra” changes its stripes with season. The temperature above the cloud tops (53 °K) is uniform planet-wide to within a few degrees, since the time constant for thermal equilibration is longer than the uranian year. Odd for a jovian, Uranus has almost no internal heat source.
At 14.54 ME, Uranus was unable to retain nearly as much hydrogen and helium as were the more massive jovians. It is smaller (4 DE) and denser (1.19 gm cm–3) than Saturn. Above an Earth-sized rocky core lies a deep layer of high-pressure water, likely containing dissolved NH3, CH4, H2, He, and various salts. Its upper boundary is at the H2O critical point, 218 bars and 273 °K. Sandwiched between this layer and the visible atmosphere is a region of molecular H2. The planet lacks sufficient mass to reach the required internal pressure for metallic hydrogen.
Uranus has a strange magnetic field, tilted 60° to the spin axis, and offset from the planet’s center by 0.3 radius. Its source is in the briny water layer. While the offset is understood, the tilt is not. It shows a 17.24 hr periodicity, betraying the planet’s spin rate. This, and the observed oblateness (0.024), places strong constraints on interior structure.
Methane and ammonia are enriched in the atmosphere relative to Jupiter or Saturn, although the composition is still mostly hydrogen and 15% helium. Cloud decks are similar to Saturn’s, but deeper due to a cooler temperature profile. The topmost cloud layer is methane (CH4) ice crystals. Methane has an absorption band at 725 nm, giving the mostly whitish clouds a slightly blue tinge compared to Saturn.
On 10 March 1977 Uranus occulted a star. By monitoring the star’s brightness as the atmosphere refracts its light, one can derive the ratio (T/µ) for Uranus, where T is temperature and µ the mean molecular weight. However, the stellar flux also showed symmetric, sharp dips before and after planetary occultation. The cause is a series of narrow rings between 1.60–1.90 planetary radii. There are a total of 11 rings, 2–100 km in width. Ring particles are rather different from Saturns’: dark as coal (4% albedo) and larger (~1 meter). Thought to be mainly water ice, the blackening probably results from UV photolysis of a methane component in the ice. (Saturn’s distance from the Sun is too warm for methane to condense and the rings stay bright.) The outermost (epsilon) ring is elliptical and varies in radial width. Voyager imaged two small (<25 km) “shepherd” satellites that serve to limit radial spreading of this ring. Similar moonlets may confine the other rings, but they have not been seen. Post-encounter Voyager images (in forward-scattered light) reveals a dust-sized component due to collision grinding. As for Saturn, it is difficult to believe the rings are as old as the planet.
Uranus has 20 known moons. The five largest are tidally locked to Uranus, as our Moon is to Earth. They are darker, denser, and smaller than Saturn’s major satellites. Surprisingly they show indications of varying degrees of past internal activity, despite the colder environment. The secret probably lies in their composition: a mix of water, methane, and ammonia ices has a much lower melting temperature than water alone.
Tiny Miranda—only 480 km in diameter—seems to have the most tortured history of all the satellites. Strange, square- to oval-shaped features (coronae) contrast with the surrounding heavily cratered but muted terrain. Coronae are relatively dark and riddled with concentric, sub-parallel grooves. How did such a tiny world evolve?
An early hypothesis posited that after initial accretion and differentiation, Miranda was blasted apart in a collision. It then re-accreted piecemeal. However, “joints” between coronæ and surrounding regions are geologically inconsistent with this scenario. An alternate theory is that Miranda underwent “chaotic de-spinning.” Normally, a satellite gradually de-spins at a certain rate, eventually pointing its long axis toward its primary. Eons ago, Miranda orbit overlapped several other satellite resonance regions. In such an instance, the de-spinning rate is augmented by a factor of e–2, where e is the orbital eccentricity, a small number. Rapid de-spinning deposited all of Miranda’s rotational energy as internal heat in short order. This energy was enough to melt the planet and fuel geologic activity.
NEPTUNE
Except for a “saner” axial tilt of 28.8°, the bulk properties of Neptune seem rather similar to Uranus. The interiors are thought to be analogous. Slightly smaller in diameter, and slightly more massive, Neptune’s density is 1.66 gm cm–3. It orbits the Sun at 30.06 AU, half again as far as its neighbor does, every 164.79 years. At Neptune’s distance, the Sun has only 1/900 the intensity as seen from Earth. Neptune’s magnetic field is weaker than Uranus.’ It is also offset from the planet’s center and inclined some 50° to the rotation axis. The spin period is 16.05 hr. Unlike Uranus, Neptune has a substantial internal heat source. Consequently, cloud top temperature is relatively balmy at 57 °K, some 4 °K warmer than its neighbor. Both planets have average albedos near 35%. While vertical cloud structure is quite similar, horizontal differences are substantial.
1989 Voyager 2 observations recorded abundant cloud features. There is significant variation in both latitude and rotation rate of the clouds, from 450 m s–1 westward at the equator to 300 m s–1 prograde at latitude 70° south. The range of wind speeds is wider than on any other planet, and a substantial fraction of the sound speed. How such wind velocities can be maintained in the cold, low energy environment is a mystery. Cyclonic features such as the Great Dark Spot (GDS) were seen to persist in Voyager images for over 8 months, but have long since disappeared. The GDS’ overall lifetime could have been as long as 10 years. A similar region appeared for a while in Neptune’s northern hemisphere, but it too has dissipated. Ephemeral cloud features have been monitored from Earth via disk-integrated photometry since about 1978. Recently, adaptive optics permits direct terrestrial observations of Neptunian clouds at resolution similar to Voyager far-encounter images, although the entire planet is only about 2 seconds of arc in apparent diameter. The changing face of the planet is a meteorologist’s dream.
The search for Neptunian rings using stellar occultations was ambiguous. Symmetric events on both sides of the planet would have confirmed the presence of a ring, but this was not the case. Voyager resolved the dilemma. Neptune does indeed have a ring system, situated from 1.7–2.5 planetary radii. The optical depth of the six known rings ranges from 0.0045 down to 0.00008 or less. (Saturn’s main rings have optical depths from 0.4–2.5.) Neptunian ring material is much more tenuous. The particles are dark (4% albedo) as for Uranus. The outermost “Adams” ring clumps into “arcs” instead of being evenly distributed about the planet in longitude. How these arcs (named Courage, Liberté, Egalité 1, Egalité 2, and Fraternité) are constrained from spreading is unknown.
Neptune’s satellite system can be divided into two regions. The inner region comprises six small satellites, ranging from 58 to 208 km in diameter, with the exception of outermost Proteus. Proteus is somewhat larger, 418 km mean diameter, comparable to Uranus’ Miranda. These inner satellites orbit in regular, prograde orbits of low inclination and eccentricity. The outer region contains two very irregular satellites, Triton and Nereid. Triton orbits in retrograde fashion, with inclination 157.3° to Neptune’s pole. Nereid’s sense is prograde, but its inclination and eccentricity are 27.6° and 0.75, respectively. Triton is thought to be a captured body. The process whereby this large, Pluto-sized moon was captured probably disrupted any initial, regular satellite system. Nereid is a survivor of this time, but has been scattered in the process.
Triton is a bizarre world. Its surface temperature is a mere 38 °K, the coldest planetary-sized body. The thin atmosphere, 16 µbar of mainly N2, is in vapor equilibrium with surface frosts of N2, CH4, and CO. Few craters on the tan- to cream-colored surface attest to a young age (<50–100 Myr). Voyager caught several geyser-like eruptions shooting dark particles to an altitude of 8 km, where prevailing winds turn the flows horizontal. Other presumably inactive vents have associated dark deposits. The surface shows fractures, a thin polar cap, and enigmatic areas called “cantaloupe terrain.” Once considered Pluto’s twin, there is little doubt Triton and Pluto are distinct, unique worlds.
PLUTO
Pluto is the only planet in our solar system not yet visited by a spacecraft. Its status as the only “telescopic” planet in our Sun’s family is unique—and daunting—to planetary scientists trying to uncover its secrets.
Clyde Tombaugh discovered Pluto in 1930. Despite a flurry of efforts next few years, Pluto’s faintness and sub-arc second diameter thwarted most investigations. For the next quarter century, about all that could be done was to refine the determination of its strange orbit. With a period of 248.8 yr about the Sun, Pluto’s orbit is more eccentric (e = 0.25) and more inclined (i = 17.2°) than that of any other planet. At perihelion, which occurred in 1996, it is only 60% as far from the Sun as at aphelion—coming closer to the Sun than does Neptune. Yet, the planets cannot collide for two reasons. First, the relative inclinations mean their orbital paths do not intersect. Second, Pluto is in a 2:3 orbit-orbit resonance with Neptune. When Pluto is at perihelion, Neptune is on the other side of the Sun.
In 1955, photoelectric measurements of Pluto revealed a periodicity of 6.38 days—the length of Pluto’s day. Subsequent photometry reveals two trends to the light curve. First, its amplitude has increased from about 10% to a current value of 30%. Second, the average brightness has faded, even after accounting for the inverse square law. These lines of evidence tell us Pluto’s poles are brighter than the equatorial region, and that the planet must have a large obliquity. Further, the sub-solar point has been moving equatorward for most of the time since the planet’s discovery. Decades of photometry have been interpreted to derive albedo maps of Pluto’s surface. These are comparable in detail with what the Hubble Space Telescope has been able to reveal.
Little was known regarding Pluto’s size or composition until recently. In 1976, the spectral signature of methane was discovered. This implied a bright albedo, and therefore a small radius. In 1978, James Christy discovered Pluto’s satellite Charon. Orbiting Pluto with the same 6.38-day period as Pluto’s spin, Charon was the key to unlocking Pluto’s secrets. Application of Kepler’s 3rd Law and the estimated separation between the two yielded a mass determination for the system, only about 0.002 ME. Charon orbits retrograde, and Pluto is the third planet of our solar system that spins backwards.
More importantly, Charon’s orbital plane above Pluto’s equator was seen edge-on in 1988. This produced a series of occultations and eclipses of and by the satellite, each half-orbit, from 1985 to 1992. Timing these “mutual events” gave the radii of both bodies—approximately 1153 km for Pluto and 640 km for Charon—the sum is about the radius of our Moon. With Charon hidden behind the planet, Pluto’s spectrum could be observed uncontaminated by its moon. This spectrum, when subtracted from a combined spectrum of the pair, yielded the spectrum of Charon. Pluto’s spectrum showed methane frost; Charon’s revealed nothing but water ice. Independent measurements have shown that methane abundance on Pluto varies with longitude, the brighter regions being more enriched than the darker. Charon shows little water variation with longitude. When Charon passed between Pluto and Earth, it (and its shadow) selectively covered different portions of its primary. While the data set is complicated, interpretation has allowed refined albedo maps for one hemisphere of Pluto to be extracted.
The surface temperature of Pluto is currently being debated. Two results have been published: ~40 °K and ~55 °K. The first value is Triton-like; the latter is more consistent with Pluto’s lower albedo (0.44-0.61). In either case, it’s pretty cold. There are misconceptions about how dark it would be for an observer on Pluto. Despite the planet’s remote distance, the Sun would appear to have the brightness of ~70 full Moons on Earth. This, combined with the high albedo, implies it would be easy to navigate on the surface.
On 9 June 1985, Pluto occulted a star. Rather than blinking out as in a knife-edge experiment, the light gradually dimmed due to refraction by an atmosphere. Too dense to be methane alone, N2 and CO were suspected. Both have since been identified on Pluto’s surface, with nitrogen comprising about 97% of the ground material. A “kink” or “knee” in the light curve, initially interpreted as a haze layer, is now thought to be due to a temperature gradient in the lower troposphere. Pluto’s atmospheric pressure is only a few microbars, and may vary by orders of magnitude depending on distance from the Sun.
The Hubble Space Telescope has been used to refine estimates of Charon’s orbital radius, a, from 19,405 to 19,662 km (approximately 1.5 DE). Since all distances derived from the mutual events are cast in terms of a, this is an important result. The mass ratio of satellite/planet has been constrained as 0.122 ± 0.005 by watching both bodies orbit around their mutual barycenter. The densities thereby derived, 1.8–2.0 g cm–3 for Pluto and 1.6–1.8 g cm–3 for Charon, tell us about internal composition: roughly a 50:50 mix of rock and ice. For the first time, realistic interior models can be made.
The future is both bright and dim. New large earth-based telescopes equipped with adaptive optics will allow many new observations to be made in unprecedented detail, surpassing even Hubble’s capabilities. On the other hand, recently a “stop work” order was issued on the Pluto-Kuiper Express spacecraft. Unless this mission is launched by 2004, it will lose its Jupiter gravity assist, and the trip to Pluto will take years longer. We will have to wait the better part of a Jupiter’s year for the geometry to repeat itself. Until the task is taken seriously, Pluto will remain the only planet unvisited by a spacecraft.
COMETS, THE KUIPER BELT, AND THE OORT CLOUD
Aristotle thought comets were a phenomenon of the Earth’s atmosphere. The word “comet” means “hairy.” It has taken the better part of two millennia to understand the true nature of these objects, and most of that progress has come in the 20th century. Note that comets are named after their discoverer(s).
The head (coma) is a roughly spherical region surrounding the tiny nucleus. It can be up to several Earth diameters across; it is a very tenuous cloud of gas with entrained dust particles. The tail originates in the coma, and typically extends for 10-100 million km (up to 1 AU). Comets have two distinct tails: one of dust, and one of partially ionized gas. The dust tail points more or less away from the Sun, as it is swept back by radiation pressure. Dynamics of the ion tail are dominated by the direction of the instantaneous interplanetary magnetic field.
A typical comet nucleus is an irregularly shaped chunk of dirty snow, between 1 and 20 km in diameter. It orbits the Sun in a very elliptical orbit, and is active only for a short amount of time near perihelion. Primarily made of frozen water, it contains small amounts of other ices such as carbon dioxide, ammonia, methane, and perhaps nitrogen. If comet ice were only water, then activity would cease beyond 5 AU. Therefore, other more volatile ices must be present. Although icy, comets are very dark (albedo 2-4%, about that of charcoal). A few percent carbon-bearing material and silicate minerals effectively reduce multiple scattering. The surface is mantled by this residuum of dark material, which is left behind when the volatile ices sublime due to solar heating. Only about 10% of an active comet’s surface vents material at a given time.
The European Giotto and Soviet Vega spacecraft were dispatched to Comet Halley in 1986. Images returned from these probes are our only close-up views of a comet to date. NASA’s Stardust mission currently is en route to Comet Wild 2. Encounter is in January of 2004, when the probe will image and do real-time chemical analysis of the coma. Particles in the coma will be collected by an aerogel coating on the spacecraft panels, and returned to Earth for analysis in 2006.
Examining the orbits of the approximately 700 known comets reveals a lot about their source regions. Short-period comets are those with orbital periods of about a century. Their orbits are overwhelmingly prograde, and generally lie within about ±30° of the ecliptic plane. Their origins are in the Kuiper Belt, a recently discovered icy version of the asteroid belt. It spans the region from Neptune out to about 50 AU and is disk-shaped.
Long-period comets, those with orbital periods more than about two centuries, have distinctly different orbital characteristics. These much larger orbits are 50-50 prograde and retrograde, and their orientations are random in inclination. They suggest a spherical source region between 20,000 and 100,000 AU. This region is named the Oort Cloud, after the Dutch astronomer who first suggested it. Estimates put its population at about 2 trillion objects. Passing stars and galactic tides occasionally will alter the orbits of a few of these bodies by just a few cm sec–1, and the long fall sunward begins.
Both source regions must continually supply fresh comets to the inner solar system, as a comet’s lifetime close to the fire is limited. A typical comet sublimes about one meter of material per orbit. Eventually comets (or their spent husks) crash into the Sun, a planet, or are ejected from the solar system entirely by Jupiter. In 1994, Comet Shoemaker-Levy 9 smashed into Jupiter. This spectacular event was the first time a cometary impact into a planet has been seen.
Interestingly, comets did not form in either the Kuiper Belt or the Oort Cloud—the dynamical timescale for accretion in situ is too long that far from the Sun. We believe the Kuiper Belt originated as leftover “crumbs” in the Uranus-Neptune region of the forming solar system. Planetesimals not accreted by these nascent planets were scattered into the Kuiper Belt. The Oort Cloud formed in a similar manner. However, the much more massive young Jupiter was able to hurl leftovers from its formation region with much more authority, despite orbiting closer to the Sun than its more diminutive brothers.
The classic picture is that Kuiper Belt and Oort Cloud are distinct regions. However, recent work shows diffusion of objects may have resulted in a smooth, continuous transition from one region to the other. The study of dynamics and interactions between these regions is very much a state-of-the-art topic.
REFERENCES
The University of Arizona Space Science Series is a most thorough compilation of information about the solar system. These volumes, and the references cited in them, represent the state of our knowledge as of their publication dates. Progress since publication of these volumes is well-documented in the journal Icarus, and the popular magazines Sky and Telescope and Astronomy, and Scientific American.
Vilas, F.; Chapman, C.R.; Matthews, M.S.; Eds. Mercury; University of Arizona Press: Tucson, Arizona, 1988; 794 pp.
Bougher, S.W.; Hunten, D.M.; Phillips, R.J.; Eds. Venus II—Geology, Atmosphere, and Solar Wind Environment; University of Arizona Press: Tucson, Arizona, 1997; 1376 pp.
Kieffer, H.H.; Jakosky, B.M.; Snyder, C.W.; Matthews, M.S., Eds. Mars; University of Arizona Press: Tucson, Arizona, 1988; 794 pp.
Binzel, R.P.; Gehrels, T.; Matthews, M.S.; Eds. Asteroids II; University of Arizona Press: Tucson, Arizona, 1989; 1258 pp.
Gehrels, T.; Ed. Jupiter; University of Arizona Press: Tucson, Arizona, 1976; 1254 pp.
Burns, J.A.; Matthews, M.S.; Eds. Satellites; University of Arizona Press: Tucson, Arizona, 1986; 1021 pp.
Gehrels, T.; Matthews, M.S.; Eds. Saturn; University of Arizona Press: Tucson, Arizona, 1984; 968 pp.
Bergstralh, J.T.; Miner, E.D.; Matthews, M.S.; Eds. Uranus; University of Arizona Press: Tucson, Arizona, 1991; 1076 pp.
Cruikshank, D.P.; Ed. Neptune and Triton; University of Arizona Press: Tucson, Arizona, 1991; 1076 pp.
Stern S.A.; Tholen, D.J.; Eds. Pluto and Charon; University of Arizona Press: Tucson, Arizona, 1997; 756 pp.
Atreya, S.K.; Pollack, J.B.; Matthews, M.S.; Eds. Origin and Evolution of Planetary and Satellite Atmospheres; University of Arizona Press: Tucson, Arizona, 1989; 881 pp.
Greenberg, R.; Brahic, A.; Eds. Planetary Rings; University of Arizona Press: Tucson, Arizona, 1984; 784 pp.
POTENTIAL GLOSSARY WORDS
solar mixture
hydrostatic equation
Keplerian orbits
barycenter
drag
accretion
runaway accretion
refractory
ecliptic
maria
Astronomical Unit (AU)
aphelion
perihelion
planetesimal
airmass
spin-orbit resonance
solar nebula
Wien's Law
greenhouse effect
plate tectonics
subduction
mid-oceanic ridge
chemical differentiation
lithosphere
superposition
seismometer
regolith
shield volcano
sublimation
siderophile
chalcophile