Comets albedo

Some energy is reflected from the body, controlled by a property called the Albedo. Objects with larger albedos reflect more of the incoming light whereas lower albedos absorb more light. Clearly, albedo is a function of wavelength and this must be known for the surface composition of the comet. 

For all comets visited by spacecraft, the measured albedo is <5%, making comet nuclei among the darkest objects in the solar system. This coloration is due to black crusts that cover most of the nuclei. The materials now comprising the crusts were presumably spewed out of the interior at speeds below the escape velocity, so they can be considered lag deposits. The parts of surfaces covered by such deposits are inert, and active areas releasing gas and dust constitute only a small percentage of a comet s total surface area. 

The size of a cometary nucleus cannot be measured directly, since even in the largest telescopes it remains an unresolved point of light. Photometric brightness measurements of comets still far away from the Sun before a radiating halo has formed, together with a phase law and a plausible value for the albedo, yield diameters of the order of 1-20 km (Roemer ). Periodic comets are, on the average, smaller than new ones, since they lose about 0.1 % of their masses per revolution. 

Figure 7 High-resolution image of comet Borrelly with topographic and albedo regions mapped (source Soderblom et al, 2002).

Figure 1. Dependence of bolometrie albedo on phase angle. The data are from (0) [15], (1) [16], (2-7) [8], (8) [13]. The solid line represents the least-square fit to the data for comets C/1975 VI (West) and C/1980 Y1 (Bradfield) and is interpolated to eonneet smoothly to the backscattering curve from [13] normalized at a = 30°. The data from [13] are normalized to the data for comet Bradfield at a = 127°.

The disappearance of the silicate feature near heliocentric distances of 2 AU correlates v/ell with the expected stability of water clathrate grains at this distance (Sekanina 1973). However, the albedo of the grains did not change significantly at this distance in Comet Uest, indicating that water ice nay act as a glue that holds the grains in clumps until they are well removed from the nucleus. 

Spacecraft missions like GIOTTO (see Fig. 5.3) (comet Halley) and Deep Space 1 (comet Borrelley) have shown that cometary nuclei are very dark (albedo less than 0.04). 

As we have already pointed out, comets may have been very important for the Earth and terrestrial planets because they deposited during collisions considerable amounts of water on the surfaces of these planets. On March 18, 1988 dark features on nine consecutive photographs were observed on Venus. Since film defects and other interferences (e.g. from an artificial Earth satellite or interplanetary object) can be ruled out, it is highly probable that this event was an impact of a small cometary like object that took place on the upper haze layer of the dense Venusian atmosphere. Because such an object consists mainly of water, evaporation of H2SO4 particles occurred which decreased the albedo at the point of entrance and therefore a dark feature appeared (Kolovos, Varvoglis and Pylarinou, 1991 [189]). 

Davies et al., 1997 [96] investigated spectra of Comet Hale-Bopp (C/1995 Ol) covering the range 1.4-2.5 pm that were recorded when the comet was 7 AU from the Sun and they found broad absorption features at 1.5 and 2.05 pm. Much of the light from the comet is scattered from the coma, some of the absorption features can be matched by an intimate mixture of water ice and a low-albedo material such as carbon on the nucleus. Furthermore, the absence of the 1.65 pm absorption feature of crystalline ice suggests that the cometary ice was probably in an amorphous state at the time of these observations. 

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