The IRAS dust band contribution to the interplanetary dust complex - Evidence seen at 60 and 100 microns

We examine the beta and gamma dust band pairs in images constructed from the 60 and 100 micron wavelength observations in the 30 x 30 arcmin IRAS Zodiacal Observation History File in an attempt to asses the fraction of solar system dust associated with the bands. The alpha band pair is not separable from the beta pair in this data set. The gamma pair's surface brightness at 100 microns is too low to allow it to be confidently distinguished from background cirrus, and is much lower than suggested by earlier analyses. Analysis of the cleaner 60-micron data implies that 0.1-0.5 percent of the zodiacal dust complex derives from the gamma bands, while 0.2-1.0 percent of the dust is associated with the beta pair. Gravitational focusing near the Earth of dust spiraling sunward under Poynting-Robertson orbital decay could result in a proportion of beta band dust as high as 15 percent in collected micrometeorites. The alpha bands are not distinguished from the beta bands, which suggests that the two pairs have similar heliocentric distances as well as inclinations.