Zodiacal Dust - an overview | ScienceDirect Topics

The Zodiacal Cloud: Dynamics

The Zodiacal Cloud is a thick circumsolar disk made of dust particles produced by asteroid collisions and cometary activity. The fraction of cometary vs. asteroidal dust in the ZC changes according to each ZC model. That is, it depends on the parameterization of dust production in both periodic and non-periodic comets. In the past, JFCs were thought to release much less dust than actually detected. Asteroidal dust is found to be present at < 10%, while cometary dust is predicted to be about 90%, mainly by particles from JFCs (Nesvorný et al., 2010). While this model predicts more dust from comets than from asteroids in the ZC, other models predict just the opposite. Probably the two fractions have a similar order of magnitude.

The main force driving dust dynamics in the ZC is solar radiation pressure. The dust orbital velocity (v) implies an aberration angle of the photons impacting the dust particle of the order of v/c, i.e. drag (Poynting-Robertson drag) on the dust which is forced to slowly spiral down toward the Sun. This mechanism accumulates ZC dust toward the Sun, and explains the increasing ZC dust space density in closer proximity to the Sun. Such an increasing space density implies increasing collisions, which fragment the particles into smaller ones. In addition, close to the Sun (F corona), dust minerals start to sublimate, further decreasing the dust size. The dust orbit size depends on the β parameter (if β ≥ 1, the orbit cannot be bound), thus on the dust size. It follows that the ZC has two main components: (1) particles larger than one micron, slowly spiraling toward the Sun, and (2) smaller dust particles, named β-meteoroids and built-up close to the Sun by collisions and sublimation. The latter escape from the Sun and eventually exit the Solar System, along hyperbolic orbits. Since the radial velocity of the latter is orders of magnitudes larger than the velocity of the former, the space density of the β-meteoroids is negligible with respect to the bound ZC space density.

The ZC dust size distribution has been directly measured by space probes and in the Earth's upper atmosphere. Given the complexity of ZC dynamics, the link between ZC dust size distribution and its sources heavily depends on models. Inputs of these models are: (1) the flux data measured in situ by space probes; and (2) the photometric data of the zodiacal light observed from ground and by orbiting IR satellites, i.e. the light scattered by the ZC. The light scattering by dust heavily depends on its structure, size distribution and chemistry, which again heavily depend on the sources input of ZC dust. Collection in the upper Earth atmosphere of sub-mm ZC dust implies strong bias, e.g. compact asteroidal sub-mm dust mostly melts as it enters the atmosphere, because its low cross-section-to-mass ratio implies a slow deceleration, thus high velocities in the lower and denser atmosphere. Aggregate IDPs sample the ZC cometary population very well but poorly sample the asteroidal one. The opposite is true for meteorites: these are big enough to survive a complete melting, i.e. providing a good complete sampling of asteroids; cometary meteoroids (larger than 1 cm and up to kilograms in mass) are not sufficiently decelerated and are completely destroyed in the atmosphere before reaching the ground (Rietmeijer, 2000). Most meteor streams (e.g. the Perseids and Leonids) are associated with comets, and have never yielded any meteorites that could directly sample the parent comet. They all melt in the upper atmosphere and only silicate spheres in the size range of tens of microns, possibly originating from Perseids and Leonids, are collected in the stratosphere as IDPs.

2.3 Zodiacal Light

The wedge-shaped appearance of the zodiacal light (see Figure 29.1) demonstrates its concentration in the ecliptic plane. For an observer on the Earth, the zodiacal light extends along the ecliptic plane all the way around the sky to the antisolar direction, although at strongly reduced intensities. In the direction opposite to the Sun, this light forms a hazy area of a few degrees in dimension known as the gegenschein, or counterglow. If seen from the outside, the zodiacal dust cloud would have a flattened, lenticular shape that extends along the ecliptic plane about seven times farther from the Sun than perpendicular to the ecliptic plane.

The brightness of the zodiacal light is the result of light scattered by a huge number of particles in the direction of observation. The observed zodiacal brightness is a mean value, averaged over all sizes, compositions, and structures of particles along the line of sight. Zodiacal light brightness can be traced clearly into the solar corona. However, most of this dust is foreground dust close to the observer because of a favorable scattering function. Nevertheless, the vicinity of the Sun is of considerable interest for zodiacal light measurements because it is expected that close to the Sun the temperature of the dust rises, and the dust particles start to sublimate, first the more volatile components and closer to the Sun, even the refractory ones. Inside about four solar radii distance, dust should completely sublimate. Some observers indeed found a sharp edge of a dust-free zone at four solar radii, while others did not see such a sharp edge. Perhaps the inner edge of the zodiacal cloud changes with time.

The large-scale distribution of the zodiacal dust cloud is obtained from zodiacal light measurements on board interplanetary spacecraft spanning a distance ranging from 0.3 to approximately 3 AU from the Sun. Even though the intensity decreases over this distance by a factor of 150, the spatial density of dust needs to decrease by only a factor of 15. The radial dependence of the number density is slightly steeper than an inverse distance dependence. A slight inclination of about 3° of the symmetry plane of zodiacal light with respect to the ecliptic plane has been determined from zodiacal light measurements.

At visible wavelengths, the spectrum of the zodiacal light closely follows the spectrum of the Sun. A slight reddening (i.e. the ratio of red to blue intensities is larger for zodiacal light than for the Sun) indicates that the majority of particles are larger than the mean visible wavelength of 0.54 μm. In fact, most of the zodiacal light is scattered by 10- to 100-μm-sized particles. Therefore, the dust seen as zodiacal light is only a subset of the interplanetary dust cloud. Submicrometer- and micrometer-sized particles, as well as millimeter-sized and bigger particles, do not contribute much to the zodiacal light at optical wavelengths but they exist.

Above about 1 μm in wavelength, the intensities in the solar spectrum rapidly decrease. The zodiacal light spectrum follows this decrease up to about 5 μm, while at longer wavelengths the thermal emission of the dust particles prevails. Because of the low albedo (fraction of incident sunlight reflected and scattered in all directions is smaller than 10%) of IDPs, most visible radiation (>90%) is absorbed and emitted at infrared wavelengths. The maximum of the thermal infrared emission from the zodiacal dust cloud lies between 10 and 20 μm. From the thermal emission observed by the IRAS and Cosmic Background Explorer (COBE) satellites, an average dust temperature at 1 AU distance from the Sun between 0 and 20 °C has been derived. Some spatial structure has been observed at thermal infrared wavelengths. Asteroid bands mark several asteroid families as significant sources of solar system dust just as comet trails identify dust emitted from individual comets.

Optical and infrared observations of other extraterrestrial dusty phenomena have also provided important insights into the zodiacal dust complex. Cometary and asteroidal dust is considered to be an important source of the zodiacal cloud. The study of circumplanetary dust and rings has stimulated much research on the dynamics of dust clouds. Interstellar dust is believed to be the ultimate source of all refractory material in the solar system. Circumstellar dust clouds like the one around β Pictoris are “zodiacal clouds” of their own right, the study of which may eventually give information on extrasolar planetary systems (see Infrared Views of the Solar System from Space; Planetary Rings; Extra-Solar Planets).

Space Dust

Another glow due to cosmic dust is the Gegenschein, seen as a faint patch exactly opposite to the Sun in the sky. It is extremely elusive, and is visible only under near-ideal conditions. The best opportunities occur when the anti-Sun position is well away from the Milky Way, from February to April and from September to November. Generally it is oval in shape, measuring about 10° by 22°, so that its maximum diameter is 40 times that of the full moon.

The Zodiacal Band is a very dim, parallel-sided band of radiance which may extend to either side of the Gegenschein, or prolonged from the apex of the Zodiacal Light Cone to join the Zodiacal Light with the Gegenschein. It also is due to sunlight being reflected from interplanetary particles near the main plane of the Solar System.

2.3.1.2 Dust measurement methods

What methods are used for probing the dust in the solar system? A first category is ground-based heritage instruments: classical astronomical observations are made of the zodiacal cloud, comets and active asteroids, mostly in the optical part of the spectrum using ground-based telescopes. Millimeter-sized or tens of micrometer-sized dust particles that enter the atmosphere become meteors that can be measured by optical (photo, video, fireball networks) and radar instruments. Two types of radars exist: the specular backscattering meteor radars (e.g., AMOR, CMOR) which measure the ionized-atmosphere tail of a meteor, and the high power large aperture (HPLA) radars like Arecibo (until 2020) which measure head echoes. This is the portion of the atmosphere that ionizes in front of the meteor; it has a shorter time span and moves along with the meteor.

Smaller interplanetary dust particles of only one to a few micrometers in size that slowly move downwards through the atmosphere have been collected in the stratosphere using aeroplanes. These planes have collectors with an oily substance, and the particles can be analyzed in the laboratory. Also analyzed in the laboratory, but on the large end of the scale-bar, are the meteorites that are found everywhere on Earth, but especially in the Arctic or Antarctic and in deserts where they are easy to identify and collect.

Space-based telescopes observe dust in the infrared. For instance, the Spitzer space telescope measured several cometary trails, and IRAS and COBE satellite data were used for mapping the zodiacal cloud (Rowan-Robinson et al., 2013). Cameras onboard spacecraft take detailed images of dust coming off from comets (e.g., Rosetta—OSIRIS). Sample return missions bring back samples of dust from cometary flyby's (e.g., Stardust) or asteroid rendez-vous (Hyabusa, OSIRIS-Rex), and from interplanetary space (e.g., Stardust InterStellar Preliminary Examination).

The workhorse of space-based dust instruments are the impact ionization instruments, because of their high sensitivity (also to small dust particles: “nanodust” down to 10⁻²¹ kg) and their reliability (multi-coincidence detectors). Examples are the Ulysses Dust Analyzer, Galileo Dust Detection System, and Cassini Cosmic Dust Analyzer. Time-of-Flight mass spectrometers are a derivative of these types of instruments, and provide the elemental composition of the particles (e.g., Cassini CDA, Stardust CIDA). The working principle of these instruments is the following: upon impact of a dust particle on the target of such instruments, the particle and some of the target material vaporizes and ionizes. The rise-time of the signal largely depends on the impact velocity of the particle, and the total charge released after impact depends on the impact velocity as well as on particle mass. The pointing direction of the instrument confines the directionality of the particle. The composition of the material is measured through the flight times of the electrons and/or ions after impact. Multiple channels are triggered upon a dust impact, which helps to determine whether an impact is a noise event or not.

PVDF and piezo detectors are the still regularly used single-incidence detectors. Their advantages are lower spacecraft resources and larger detector surfaces, but noise characterization can be more of a challenge. These instruments are generally sensitive to larger particle impacts (10⁻¹⁵–10⁻¹² kg) and basically measure the impact momentum. Successful examples of PVDF-type of detectors are the New Horizons student dust counter, the Cassini High Rate Detector and the Stardust Dust Flux Monitor Instrument (DFMI). Examples of piezo detectors are Rosetta GIADA and the BepiColombo Mercury Dust Monitor (MDM). Pressurized-cell types of detectors (also called ‘beer-can' type of detectors) like onboard Pioneer 10 and 11, and the early microphones, had specific problems, and are generally not further used. An in-depth overview of instruments and dust science is given in Grün et al. (2019).

The last category of instruments can be called ‘serendipity instruments’: after the Voyager 2 spacecraft had detected impacts of dust on the spacecraft body using its plasma wave instrument during its Saturn encounter, many more missions have followed where plasma wave instrument data were analyzed for dust impacts, e.g., WIND, Cassini, STEREO, and most recently, Parker Solar Probe. Particle impacts are measured through the (transient) potential difference that occurs between spacecraft surface charge and antenna, via the impact plasma cloud that is created after particle impact on the spacecraft surface (or on the antenna). This can be detected as a spike in the plasma wave detector data (e.g., Malaspina et al., 2014). A second type of ‘serendipity measurements” are done by high-precision interferometry missions, like LISA Pathfinder, where the positions of two free floating blocks are monitored to the nanometer level with an interferometric system (Thorpe et al., 2016). A biproduct of these measurements can be dust detection.

VI.A Limitations of Space-Based Observations

The minimum at 3 μm is at the crossover point between reflected and emitted zodiacal radiation. At shorter wavelengths the emission is dominated by reflected sunlight. At wavelengths longer than 3 μm, the main emission is thermal emission from the dust. The minimum at 400 μm is due to the fall off of the warmer emission from the zodiacal dust and extrasolar dust, termed cirrus, and the short wavelength falloff of the cosmic microwave background, which has a much colder temperature. These background minima are very useful regions for the observation of faint sources such as distant galaxies.

Although there have been few space infrared missions, the missions carried out to date have proved quite successful. Due to the advantages of space infrared observations, there are also several missions in the planning stage. We will review a few of the major missions here.

2 The Zodiacal Dust Cloud and Its Sources

The zodiacal light is caused by the scattering of sunlight off of small particles near the Earth's orbit viewed when the geometry is optimal. Comets were long thought to be the origin of the zodiacal cloud. However, early estimates of dust production by short-period comets fell far short of that needed to maintain the cloud in steady state against losses from particles spiraling into the Sun. This mechanism, where the absorption and reemission of solar radiation continually decreases particle velocity, is called Poynting–Robertson drag.

Dust production from collisions is continuous down to sizes at which they are finally removed from the production region by radiation forces. When the fragments are around 1 μm in size they are immediately ejected from the solar system along hyperbolic orbits. These are known as β-meteoroids. For larger particles the solar radiation field and solar wind act as a friction to their orbital motion (Poynting–Robertson drag) and they will slowly spiral past the orbit of the Earth into the Sun. It is thought that the dust ultimately vaporizes and is incorporated into the Sun or recondenses into small particles that are then lost to the solar system as β-meteoroids.

Initially, the dust bands were thought to be associated with the principal Hirayama asteroid families because of the proximity of their apparent latitudes with the orbital inclinations of those groups. These families are thought to have arisen from the catastrophic disruption of asteroids 100–250 km in diameter more than a billion years ago. If the asteroid belt as a whole was grinding down and generating dust, then it would follow that the most dust would be generated in the regions of greatest asteroid concentration—the largest asteroid families. Assuming this dust to be the main source of the zodiacal cloud, the cloud itself would be something expected to change slowly over much of the age of the solar system. An alternative hypothesis proposed that the dust bands arose from more recent collisions of smaller asteroids and that the zodiacal cloud was highly variable over time. David Nesvorny and colleagues identified the sources of the two most prominent pairs of dust bands as the Karin and Veritas families and determined that the collisions forming these families occurred within the past 10 million years. This demonstrates that the zodiacal cloud, once assumed to be in relative steady state, may vary substantially over time as dust production in a given family slowly declines as more and more of its mass is ground up and removed by radiation forces and a new random collision creates family of debris that generates more dust. A faint inner pair of bands was originally associated with the very ancient Themis family, the largest asteroid family, which was formed by the catastrophic disruption of a 240 km diameter asteroid billions of years ago. Nesvorny and colleagues subsequently identified the origin of these bands as the Beagle cluster within the Themis family, arising from the disruption of a ∼20-km-diameter asteroid less than 10 million years ago. Thus, the asteroidal contribution to the zodiacal dust complex appears to be dominated by recent catastrophic disruptions of small asteroids.

8 Summary

Comets are a diverse population of icy, sublimating bodies that display large-scale phenomena. The central body, the nucleus, has typical dimensions of 1–10 km. The bulk composition is mostly H₂O ice and dust, and the composition of comets is remarkable uniform. The physical processes involved—sublimation of ices in the interior, the flow of gases away from the nucleus, the dissociation and ionization of molecules, and the interaction with the solar wind—continue to provide challenges for scientists. Comets are important to our understanding of other solar system phenomena such as meteors and the zodiacal light. Many problems in comet physics can be solved only by sending spacecraft to the immediate vicinity for close-up imaging and in situ measurements. The past few years have seen several space missions to comets and an extraordinary increase in our knowledge of comets and their diversity. The interiors of comets are not well understood, but results from the Deep Impact and EPOXI missions provide an important first step. These missions showed that comet Tempel 1's nucleus is porous and that at least the outer layers have very low tensile strength. Ultimately, samples of cometary material must be returned to the Earth for analysis in the laboratory. This has begun with the return of dust particle samples from the Stardust mission in 2006. Although the Rosetta mission to comet Churyumov–Gerasimenko is expected to greatly expand our knowledge of comets, with the main spacecraft spending an extended time period near the comet and the lander spacecraft landing on and anchoring itself to the nucleus, the return of icy materials to the Earth for analysis is probably well in the future. Comet Tempel 1 has been suggested as a good candidate for a sample return mission.

1 INTRODUCTION

Perhaps the place to begin is the realization that almost all stars evidently have “astrospheres”, all too transparent to be seen but undoubtedly as complex as our own heliosphere. The stellar wind is the creator of the astrosphere, just as the solar wind sweeps out the cavity in interstellar space that we call the heliosphere. Thus the origin of the heliosphere and the astrosphere traces back to the hydrodynamics of the million degree solar and stellar coronas. The solar corona appears to be created by the dissipation of mechanical and magnetic energy in the tenuous gas above the dense photosphere. It is that dissipation, evidently in the form of the microflaring in the magnetically “quiet” regions of the Sun, that creates the heliosphere. The staggering complexity of the convective and magnetic machinations on all scales down into the unresolved microstructure of the solar activity gives some idea of the mystery of the stellar corona and astrosphere. Indeed, the mystery does not stop with the microflaring, for we are in the dark as to the origin of the fibril magnetic fields that seem to drive the system from below the visible surface. With the variety of stellar types and circumstances that may be presumed to create stellar winds and astrospheres, the inquiry into the heliosphere and the extrapolation to other stars is bewildering.

The first primitive model of the heliosphere was sketched some 45 years ago, and the subject has come a long way since that time with the advent of the space age. We begin, then, by noting that the heliosphere evidently has been in place since the formation of the Sun and Earth some 4.6 × 10⁹ years ago. Unknown to classical astronomy, the heliosphere remained “silent” until the advance of technology and science first began to uncover its effects. Only in the last half century have we appreciated its existence. Then once we ventured into space the “silent” heliosphere became noisy indeed. There are a number of terrestrial effects, but in the early years they were more puzzling than informative. Some effects are obvious, e.g. the aurora, while others e.g. geomagnetic fluctuations, cosmic ray variations, etc. are detected only by scientific instruments. It was the geomagnetic storm that a century ago first suggested bursts of “solar corpuscular radiation” from the Sun, consisting mainly of protons and an equal number of electrons to provide electrical neutrality. Otherwise space was regarded as a hard vacuum capable of supporting unlimited electric potential differences, at the same time that the zodiacal light was interpreted as sunlight scattered from about 500 free electrons/cm³ at the distance of Earth (1 AU). Then about half a century ago Biermann’s ([1], [2]) studies of the anti-solar acceleration of comet tails led to his fundamental pronouncement of the perpetual universal emission of solar corpuscular radiation. The velocity of the solar corpuscular radiation had long been estimated at 10³ km/sec, from the time delay of a couple of days between the flaring on the Sun and the impact of the corpuscular radiation against the outer boundary of the geomagnetic field. Biermann inferred from the measured anti-solar acceleration of gaseous comet tails that the number density of the solar corpuscular radiation at the orbit of Earth is in excess of 103 electrons and ions per cm 3, later revised downward to perhaps as little as 500/cm a based on resonant charge exchange with the cometary atoms. This density seemed to be confirmed by the comparable interplanetary electron density inferred from the intensity of the zodiacal light, considered at that time to be Thomson scattering of sunlight by free electrons. So the solar corpuscular radiation was powerful stuff. Its impact against the geomagnetic dipole field was calculated to confine the field to a distance of about five Earth’s radii on the sunward side.

It should be noted here that the origin of the solar corpuscular radiation at the Sun was a mystery at that time, with vague ideas about acceleration in or around the magnetic fields of active regions, sunspots, and flares. Thus the origin was made even more mysterious by Biermann’s basic point that the Sun emitted corpuscular radiation in all directions at all times, regardless of the presence or absence of magnetic active regions.

Now by 1956 John Simpson ([25], [13]) had succeeded in determining the energy spectrum of the variation of the cosmic ray intensity with the varying level of activity of the Sun. The variations were first detected by Scott Forbush, using ion chambers, which are sensitive to the muons produced in the atmosphere by cosmic ray protons with energies of 10-20 Gev and up. Simpson invented the cosmic ray neutron monitor which responds to the nucleonic component in the atmosphere, thereby registering the effect of cosmic ray protons down to about 1 Gev, where the time variations are much larger. Using five neutron monitors distributed from the geomagnetic equator to Chicago (at 55° geomagnetic latitude) he exploited the geomagnetic field of Earth as a magnetic spectrometer. He showed that the variations had an energy spectrum that could not be a consequence of an electrostatic potential difference in space, which would be presumed to decrease the energy of each particle by the same amount. Instead, the variations, apart from the bursts of solar cosmic rays from the occasional large flare, showed simply a removal of particles that increased with declining cosmic ray particle energy. He noted that the variations suggested time varying magnetic fields in space.

The great cosmic ray flare of 23 February 1956 showed direct passage of the solar cosmic rays from their origin on the Sun to Earth, arriving promptly at Earth from the direction of the Sun ([12]). Thereafter the solar cosmic ray intensity was observed to decline slowly as if escaping by diffusing through a magnetic barrier beginning at about the orbit of Mars and extending outward to the orbit of Jupiter. The simplest model suggested by the observations was a radial magnetic field extending from the Sun out to the orbit of Mars, with a disordered nonradial magnetic field beyond.

Collectively this indicated a dynamical state of the solar corpuscular radiation and magnetic field in interplanetary space. The challenge, then, was to understand how the corpuscular radiation and interplanetary magnetic field were created by the Sun.

2.4 Cosmic dust particles

The lowest-mass objects of the Solar System are cosmic dust particles. Cosmic dust can be divided in two categories: interstellar dust (ISD) – visible as the dust blocking the light of the stars of the milky way – and interplanetary dust particles (IDPs) – visible as the zodiacal light, which is sunlight scattered by interplanetary dust particles. Interstellar dust particles reside in diffuse or in dense clouds and are the basic building blocks of what eventually become stars, planets and, later on, life. They play a crucial role in astrochemistry for cloud thermodynamics. Characterizing ISD is important for astronomical observations: it is the medium we look through to observe the universe and the dust physical properties are needed for interpreting observations of faraway protoplanetary disks, for example. Classical astronomical observations of ISD over long kiloparsec (kpc) scales are used to reveal ISD composition and size distribution using measurements of wavelength-dependent extinction and polarization of starlight, emission by the dust in the infrared and observations of chemical abundances in the gas (assuming the missing elements, in comparison with the abundances of a reference like the Sun, are locked in the dust). Using this ensemble of observations, models are built for ISD size distribution and composition.

In 1993, a new type of observation became available, providing ground truth information on ISD: for the first time, interstellar dust had been detected in situ in the Solar System with a dust detector onboard the spacecraft Ulysses. This is possible thanks to the relative motion of the Solar System and the Local Interstellar Cloud. Ulysses flew out of the ecliptic plane, and its orbit, being almost perpendicular to the inflow direction of ISD, has facilitated distinguishing interstellar from interplanetary dust. Ulysses has detected between about 500 and 900 particles over 16 years and opened the era of in situ ISD research in the Solar System. More observations followed (Galileo, Helios, Cassini) and in 2016, the Cassini Cosmic Dust Analyzer (CDA) measured the composition of 36 ISD particles, whereas the Stardust mission brought back some samples of ISD in its sample return capsule (2006, with analysis in 2014). A comprehensive review on interstellar dust in the Solar System (incl. relevant references) is given in Sterken et al. (2019).

The zodiacal dust has been explored with in situ detectors for more than half a century! In addition to ordinary IDPs, various types of dust “between” the planets have been examined, such as dust coming from active moons, stream particles, planetary rings, cometary dust and dust clouds around airless bodies.

Enceladus is an example of such an active moon with a subsurface ocean, where water ice particles escape via vents into space. Cassini CDA (a time-of-flight mass spectrometer) measured the composition of the dust particles in Enceladus' plumes and in Saturn's E-ring, illustrating that subsurface ocean compositions can be probed without the need for landing on such a moon. Also, Io has volcanoes whose tiny particles are accelerated in the Jovian magnetic field and become very fast nanometer-sized “stream particles.” Their composition was also measured by Cassini CDA. Dust impacts on airless bodies cause ejecta and as such, airless moons are surrounded by an ejecta cloud. Measurements of these ejecta can be used to compositionally map the surfaces of these moons without a landing (Postberg et al., 2011).

Thus, the importance of in situ cosmic dust measurements goes far beyond “just” measuring dust. Furthermore, interplanetary dust, which mostly stems from comet activity and asteroid collisions, provides us with insights into the history of our Solar System; these particles are also a means toward understanding geochemical conditions on subsurfaces of active moons and toward probing the surface composition of airless bodies. Also, physical processes of the dust and dust as charged probes to investigate the Interplanetary Magnetic Field (IMF) or planetary magnetospheres are subjects of study. A comprehensive review of interplanetary dust is given in Grün et al. (2019) and Koschny et al. (2019).

16.3 An extrasolar perspective of the Kuiper belt

In fact, the level of thermal emission from dust in the Kuiper belt is not well known. Its emission has not been detected since it is masked by that of the zodiacal cloud which is much closer to the Earth, although the far-IR all-sky surveys like IRAS, COBE, and Planck do provide upper limits on the level of emission that can be present (e.g., Backman et al., 1995). Direct detection of dust grains in the outer solar system gives an indication of the number of dust grains present (Piquette et al., 2019), but this must be combined with dynamical models for dust production and evolution to understand the whole dust structure (Vitense et al., 2014). In doing so, the best estimate is that Kuiper belt emission peaks at ∼70 μm at a flux 1% that of the Sun (Vitense et al., 2012). Even for bright stars their far-IR fluxes cannot be predicted with such accuracy, which combined with calibration uncertainties in photometric measurements, means that ≫10% excesses are required for a confident detection.

If it were possible to resolve the emission from Kuiper belt dust, the structures that would be observed at long wavelengths would resemble the structures known to be present in the distribution of Kuiper belt objects (e.g., Lawler, 2014); that is, there would be a prominent ring at ∼40 AU from the classical Kuiper belt (see Fig. 16.2 left), with some nonaxisymmetric structure caused by the resonant Kuiper belt objects (see Fig. 16.2 middle), and with emission extending out to larger radii from the scattered disk (which is present but hardly noticeable in Fig. 16.2 left). Given the low density of the dust distribution, the smallest dust (as traced by the shorter far-IR wavelengths) would migrate in by Poynting-Robertson drag and so fill in the 40 AU hole. However, very little dust would make it past Jupiter or even Saturn, meaning that this tenuous dust distribution would have drops in density associated with these planets (Liou and Zook, 1999; Moro-Martín and Malhotra, 2002).

There are, however, indications that the Kuiper belt was more massive in the past by several orders of magnitude. At such an epoch there would have been correspondingly more dust, resulting in detectable levels of emission (see Fig. 16.2 right, Booth et al., 2009). Thus the known debris disks could be analogues to the primordial Kuiper belt, and thus representative of systems that either never will, or have yet to undergo, large-scale instabilities. Note though that the exact timing of the depletion in the solar system remains uncertain (e.g., Gomes et al., 2005; Morbidelli et al., 2018).