The Zodiacal Emission Spectrum as Determined byCOBEand Its Implications

Abstract

We combine observations from the DIRBE and FIRAS instruments on the COBE satellite to derive an annually averaged spectrum of the zodiacal cloud in the 10-1000 lm wavelength region. The spectrum exhibits a break at $150 lm that indicates a sharp break in the dust size distribution at a radius of about 30 lm. The spectrum can be fitted with a single blackbody with a À2 emissivity law beyond 150 lm and a temperature of 240 K. We also used a more realistic characterization of the cloud to fit the spectrum, including a distribution of dust temperatures representing different dust compositions and distances from the Sun, as well as a realistic representation of the spatial distribution of the dust. We show that amorphous carbon and silicate dust with respective temperatures of 280 and 274 K at 1 AU, and size distributions with a break at grain radii of 14 and 32 lm, can provide a good fit to the average zodiacal dust spectrum. The total mass of the zodiacal cloud is 2-11 Eg (Eg ¼ 10 18 g), depending on the grain composition.

Microwave thermal emission from the zodiacal dust cloud predicted with contemporary meteoroid models

“…They found that the bands keep high Q abs ∼ 1 up to λ ∼ 3 mm, implying that their constituent particles are macroscopic. The absorption efficiency of the smooth cloud decays in the microwaves, however, not exactly as sharply as the simple approximation of Fixsen & Dwek (2002) suggests. The inverse problem solution sometimes led Planck Collaboration XIV (2014) to negative absorption efficiencies of the smooth cloud or circumsolar dust ring and Earth-trailing blob.…”

“…Both the smooth cloud and the dust bands extend only up to the heliocentric distance of 5.2 AU, since Kelsall et al (1998) Kelsall et al (1998), gray area on the plot, and for the Planck HFI wavelengths by Planck Collaboration XIV (2014). Fixsen & Dwek (2002) have also used the data from the COBE FIRAS instrument to determine the annually averaged spectrum of the zodiacal cloud plotted here with a dotted line. the outer boundaries of the corresponding asteroid families.…”

“…Planck Collaboration XIV (2014) used the cross-section area density of the Kelsall model and found that the absorption efficiencies for its components approximately describe the thermal emission from interplanetary dust at the wavelengths of Planck's High Frequency Instrument (HFI). Fixsen & Dwek (2002) used the FIRAS (Far-Infrared Absolute Spectrometer) data from COBE and found that the annually averaged spectrum of the zodiacal cloud can be fitted with a single blackbody at a temperature of 240 K with an absorption efficiency that is flat at wavelengths shorter than 150 μm and Q abs ∝ λ −2 beyond 150 μm. The absorption efficiencies of dust from all three bands coincide in Kelsall et al (1998) .…”

“…A negative absorption efficiency found for the smooth cloud by Planck Collaboration XIV (2014) at λ = 2.1 mm is replaced with the result of interpolation of the positive efficiencies found at λ = 1.4 and 3 mm, whereas negative efficiencies for the circumsolar ring at λ = 0.85, 1.4, and 2.1 mm are simply nullified. The absorption efficiency is extrapolated beyond the Planck HFI wavelengths (λ > 3 mm, i.e., in the WMAP range) using the approximation of Fixsen & Dwek (2002) for the smooth cloud Q abs ∝ λ −2 , similar to that of Maris et al (2006) who assessed the level of contamination of the Planck data by the zodiacal microwave emission based on the Kelsall model, and using flat Q abs = 1, 0.5, 1, and 0.1 for the asteroid dust bands 1, 2, 3, and the circumsolar ring with the trailing blob, respectively.…”

“…IMEM has only 6 × 10 19 g mass and 6 × 10 −6 AU 2 cross-section area of meteoroids within 5.2 AU from the Sun. Fixsen & Dwek (2002) assumed a certain size distribution of interplanetary meteoroids and used the optical properties for silicate, graphite, and amorphous carbonaceous particles in order . Thermal emission spectra of the zodiacal cloud predicted by IMEM for the apex of Earth's motion around the Sun and the ecliptic pole, assuming that the dust particles are composed of amorphous carbon (top), silicate (middle), and iron (bottom) material.…”