COMETARY ORIGIN OF THE ZODIACAL CLOUD AND CARBONACEOUS MICROMETEORITES. IMPLICATIONS FOR HOT DEBRIS DISKS

Abstract

The zodiacal cloud is a thick circumsolar disk of small debris particles produced by asteroid collisions and comets. Their relative contribution and how particles of different sizes dynamically evolve to produce the observed phenomena of light scattering, thermal emission, and meteoroid impacts are unknown. Until now, zodiacal cloud models have been phenomenological in nature, composed of ad hoc components with properties not understood from basic physical processes. Here, we present a zodiacal cloud model based on the orbital properties and lifetimes of comets and asteroids, and on the dynamical evolution of dust after ejection. The model is quantitatively constrained by Infrared Astronomical Satellite ( IRAS ) observations of thermal emission, but also qualitatively consistent with other zodiacal cloud observations, with meteor observations, with spacecraft impact experiments, and with properties of recovered micrometeorites (MMs). We find that particles produced by Jupiter-family comets (JFCs) are scattered by Jupiter before they are able to orbitally decouple from the planet and drift down to 1 AU. Therefore, the inclination distribution of JFC particles is broader than that of their source comets and leads to good fits to the broad latitudinal distribution of fluxes observed by IRAS . We find that 85%–95% of the observed mid-infrared emission is produced by particles from JFCs and <10% by dust from long-period comets. The JFC particles that contribute to the observed cross section area of the zodiacal cloud are typically D ≈ 100 μm in diameter. Asteroidal dust is found to be present at <10%. We suggest that spontaneous disruptions of JFCs, rather than the usual cometary activity driven by sublimating volatiles, is the main mechanism that liberates cometary particles into the zodiacal cloud. The ejected mm to cm-sized particles, which may constitute the basic grain size in comets, are disrupted on ≲10,000 yr to produce the 10–1000 μm grains that dominate the thermal emission and mass influx. Breakup products with D > 100 μm undergo a further collisional cascade with smaller fragments being progressively more affected by Poynting–Robertson (PR) drag. Upon reaching D < 100 μm, the particles typically drift down to <1 AU without suffering further disruptions. The resulting Earth-impact speed and direction of JFC particles is a strong function of particle size. While 300 μm to 1 mm sporadic meteoroids are still on eccentric JFC-like orbits and impact from antihelion/helion directions, which is consistent with the aperture radar observations, the 10–300 μm particles have their orbits circularized by PR drag, impact at low speeds, and are not detected by radar. Our results imply that JFC particles represent ∼85% of the total mass influx at Earth. Since their atmospheric entry speeds are typically low (≈14.5 km s −1 mean for D = 100–200 μm with ≈12 km s −1 being the most common case), many JFC grains should survive frictional heating and land on Earth's surface. This explains why most MMs collected in antarctic ice have primitive carbonaceous composition. The present mass of the inner zodiacal cloud at <5 AU is estimated to be 1–2 × 10 19 g, mainly in D = 100–200 μm particles. The inner zodiacal cloud should have been >10 4 times brighter during the Late Heavy Bombardment (LHB) epoch ≈3.8 Gyr ago, when the outer planets scattered numerous comets into the inner solar system. The bright debris disks with a large 24 μm excess observed around mature stars may be an indication of massive cometary populations existing in those systems. We estimate that at least ∼10 22 , ∼2 × 10 21 , and ∼2 × 10 20 g of primitive dark dust material could have been accreted during LHB by the Earth, Mars, and Moon, respectively.