not. The measured central position of the band at 25 and 60/xm is close to the expected ¡3 = -11.6 deg derived from DIRBE data, but the width of the band in the high resolution scans is significantly less than the 3.5 deg measured by DIRBE. The amplitudes of the Gaussians are close to the numbers derived from DIRBE [19]. The repeated 25/xm observation, observed one year later, outlines a rather different profile with higher amplitude and larger width. It is not clear if we see real physical changes on a timescale of one year, or if the differences are introduced by the slightly different observing strategy and data reduction of the two scans. The 25/60/mi colour temperature of the band is 208 K, placing the —10° asteroidal band at a heliocentric distance of about 1.8 AU.

3.2. Cometary trails

Crossing the orbits of a few periodic comets (Tempel 2, Encke, Kopff, Tempel 1, Gunn, Schwassmann-Wachmann 1, Churyumov-Gerasimenko, and Pons-Winnecke) IRAS detected long narrow streams of dust particles [24], These dust 'trails' typically extend 10° behind and 1° ahead of the comet, and they should not be confused with the visually more prominent 'tails' formed by sub-micrometer-sized particles blown away by the solar radiation pressure. Rather they correspond to the anti-tails observed in the visible for particular geometric arrangements of the Sun, Earth and comet, and apparently extending towards the Sun (a spectracular unpublished picture of comet Encke and its trail was taken at mid-infrared wavelengths with the ISOCAM camera by Reach). The trails consist of mm-sized dust particles ejected from the comet during active times extending over many years [25]. Theoretical investigations of dust ejection from comets predict that the common dust production of 85 short-period and 101 long-period comets is of the order of only 3 x 104 g s_1 [26]. This production rate is by far too low to maintain the zodiacal cloud. Recent infrared observations indicate, however, that due to the very low albedo of the cometary dust, the mass input rate may be higher than suspected [27], In addition, the dust injection rate is not necessarily constant in time, and the contribution from active comets in the past is not known. A higher dust supply rate, in the form of comet showers at 37 and 50 million years ago, is indicated by measurements of the 3He content in interplanetary dust particles from oceanic core samples [28].

A key parameter in the calculations of the dust supply rate from comets is the timescale on which the ejected grains are mixed with the general interplanetary medium. Davies

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