KeywordsAsteroidal dust Cometary dust Dust debris Exo-zodiacal clouds Gegenschein IDPs Meteors Zodiacal cloud
The zodiacal light is a faint veil of light covering the Earth’s sky. Brighter towards the Sun and the ecliptic, it originates in solar light scattered by a cloud of dust particles, mostly of cometary and asteroidal origin. The increase of brightness, toward the Sun and the near-ecliptic region, indicates an increase in the space density of the interplanetary dust cloud, which has a lenticular shape around the Sun; a slight brightness enhancement toward the antisolar region (gegenschein) corresponds to a backscattering effect. Thermal emission from the zodiacal particles forms, away from the galactic plane, the most prominent component in the near-infrared sky.
Although some hints are suspected in ancient texts, Childrey (1661) possibly gave the first description of the zodiacal light. Cassini (1683) provided a detailed interpretation, with the zodiacal light originating in a flattened cloud that scatters better the light when the line of sight gets closer to the Sun or its equator. Dortous de Mairan (1733) suggested that the cloud expands further than the terrestrial orbit and Brorsen (1854) described a slight brightness enhancement around the antisolar direction, later called gegenschein by Humboldt. Precise measurements have only taken place in the twentieth century; the question of the stability of the zodiacal cloud on a scale of more than a few centuries or millennia is thus still open.
Dust Properties, Orbits, and Sources
The brightness and thermal emission vary along the observational line of sight; inversion techniques are thus required to derive local values of, for example, brightness, polarization, albedo, spatial density, and temperature. Such approaches have established that the tiny dust particles are not at all spherical and that the interplanetary dust cloud is heterogeneous, with the local polarization and albedo varying with solar distance, while they do not depend upon the dust spatial density nor the temperature. Numerical simulations of the polarimetric and thermal properties suggest that, for solar distances of about 1 AU near the ecliptic, the dust particles consist of a roughly equally divided mixture of silicates and more absorbing organic molecules (such as HCN polymers), which progressively suffer sublimation while they get closer to the Sun. These dust particles may be either compact or, for at least 20 % of them, made up of fluffy aggregates.
Such results may be interpreted, taking into account the dynamical properties of the particles that build up the zodiacal cloud. The size distribution of the dust particles (as deduced from near-Earth cumulative fluxes measurements) follows approximately a power law with an index of −3 for particles below 20 μm, with a value of about −4.4 for larger sizes. The orbits of such dust particles are not stable. They may suffer some collisions, fragmentation, weathering, and sublimation; besides, they are not only subjected to solar gravity, but also to nongravitational forces, for example, the Poynting-Robertson drag that causes their orbits to spiral in toward the Sun and the solar radiation pressure that expels the smallest particles (with sizes approximately below 1 μm) out of the solar system. Therefore, assuming that the zodiacal cloud is (or at least has been for a while) stable, continuous sources of new particles are required.
The question of the origin of the dust particles that are replenishing the interplanetary dust cloud has been extensively discussed all over the past years. Before the 1980s, the main source was assumed to be the dust released by active cometary nuclei, as suggested by the orbital properties of meteor streams. In 1983, with the detection of asteroidal bands and cometary trails by IRAS, it has been suggested that the main source could be dust released by asteroidal collisions; by the way, evidence was found for dust particles (of possibly asteroidal origin) trapped in a circumsolar ring in resonant lock with the Earth. While other minor sources of dust have been detected (dust from the Jupiter and Saturn systems, interstellar dust), the main source of interplanetary dust (at least in the inner solar system) seems actually, from two independent approaches, to be cometary. The above-mentioned modeling of the zodiacal light and thermal emission indicates, taking into account the cometary dust morphology deduced from Giotto and Stardust data, that the presence of aggregates in the interplanetary dust cloud is a clue to a contribution from cometary dust in the 50–100 % range in the inner solar system; besides, the derived space density is inversely proportional to the solar distance, in agreement with what is expected for dust particles ejected by comets under Poynting-Robertson drag in their formation region. Also, a recent model based on IRAS observations, on the orbital properties and lifetimes of comets and asteroids, and on the dynamical evolution of dust after ejection indicates that about 90 % of the observed mid-infrared zodiacal thermal emission is produced by particles ejected from Jupiter family comets.
Astrobiology and Exo-zodiacal Dust Clouds
There is evidence for a major contribution of particles of cometary origin in the inner solar system, as established from their morphology, composition, and region of formation. Today, such particles are likely to be the source of meteors and micrometeorites on Earth. At the Late Heavy Bombardment (LHB) epoch, while the spatial density of dust in the interplanetary dust cloud was orders of magnitude greater, the survival of significant amounts of organics during the atmospheric entry of dust particles has probably been favored by the structure of the particles of cometary origin. Computing the meteoritic ablation while taking into account the deceleration of the particle by collisions with the atmospheric molecules implies (all parameters staying the same) that irregularly shaped particles and fluffy aggregates can bring up to ∼π3 times more organics in volume without being ablated to the Earth’s surface than compact spherical particles. During the LHB epoch, interplanetary dust particles ejected from cometary nuclei might thus have significantly contributed to the enrichment of early Earth (and telluric planets) in carbonaceous compounds, necessary for the origin of life.
Finally, far away from our solar system, the existence of exozodiacal dust clouds, or debris disks within a few astronomical units of the star, has been discussed as a potential source of noise in the detection of exoplanets, and specifically of exo-Earths inside the star’s habitable zone. A good understanding of the properties of our zodiacal cloud is mandatory to define relevant models of exo-zodiacal clouds. It is most likely that their dust particles are also irregular and that their properties change with distance to the star. However, while a significant part of the dust may, as well as in our solar system, originate in small bodies, questions about, for example, the contribution of dust streams from hot Jupiters and the asymmetric geometries of planetary systems with highly eccentric planets, need to be carefully considered.
Studies of zodiacal light and interplanetary dust flourished at the beginning of the space age with concerns about the risks of impacts with spacecraft; they are actually limited by the low spatial density of the zodiacal dust (of about 10−17 kg m−3 in the vicinity of the Earth orbit). About five decades later, the key questions are related to the characteristics of exo-zodiacal clouds and to the importance of zodiacal dust for the delivery of organics to the young Earth and terrestrial planets (not to mention exo-Earths). Meanwhile, new measurements of the zodiacal light (e.g., with the JAXA Akatsuki spacecraft during the cruising phase to Venus) should find out whether the inclination of the plane of symmetry varies or not with the solar distance and better explore its properties further away from the ecliptic plane.
References and Further Reading
- Brorsen T (1854) Über eine neue Erscheinung am Zodiacallichtes. Geogr Witterungskunde 8:156–160Google Scholar
- Cassini JD (1683) Découverte de la lumière qui paroist dans le zodiaque. Acad roy des sci, ParisGoogle Scholar
- Childrey J (1661) Britannia Baconica or the natural rarities of England, Scotland and Whales (London)Google Scholar
- Dortous de Mairan JJ (1733) Traité physique et historique de l’aurore boréale. Académie royale des sciences, ParisGoogle Scholar