Dirty snowball

Mission Deep Impact In July 2005, NASA steered a projectile, about 370 kg in weight, at the comet 9F/Tempel (dimensions 4x4x14km), in order to obtain more exact information on its structure and composition. The impact was visible from Earth the Rosetta spacecraft discussed above also sent pictures to Earth. The dust/ice ratio determined after the impact is very probably greater than unity, so that comets are probably icy dustballs rather than (as had previously been surmised) dirty snowballs . The density of the cometary nucleus, which seems to consist of porous material, is roughly equal to that of ice. The impact set free around 19 GJ of... 

Comets Dirty snowballs with short and long orbits originating from the Kuiper Belt and Oort Cloud, respectively... 

Comet A dirty snowball object in a solar system in orbit around a local star. 

The activity that characterizes a comet is driven by solar heating. As the comet approaches the Sun, jets of dust and gas erupt from active areas (Fig. 12.1), as they periodically rotate into the sunlight. The nucleus becomes surrounded by a spherical coma formed by the emitted gas and dust. Emitted gas becomes ionized due to interaction with solar ultraviolet radiation, and the ions are swept outward by the solar wind to form the comet s ion tail. A separate dust tail commonly has a different orientation, reflecting variations in the velocities of solid particles and ions. A popular term describing comets is dirty snowballs , although that description probably understates the proportion of rock and dust relative to ices. 

By far the most important compound of oxygen and hydrogen is water, HzO. Water is essential to life as we know it and possibly to life anywhere in the universe. A lot of it arrived on Earth from outer space in the form of comets, which are like huge dirty snowballs. When the Earth was young, comets collided with it frequently, and each one brought huge amounts of water to the planet. Another source of water was the rocks from which the young Earth initially formed. The water locked up inside these solids is released when they melt in the depths of the Earth. The emission from volcanoes—even today—contains vast quantities of water. 

While many asteroids may have previously been comet-like, even active comets are somewhat asteroid like. The traditional concept of comets as dirty snowballs has been modified by some authors to consider them to be more like frosty rocks, because they contain more rock than ice. The dust gas production rate from the spectacularly active comet Hale-Bopp exceeded its gas production rate by a factor of 5 (Jewitt and Matthews, 1999). In considering the water contents of comets, it is interesting to consider that some comets may have ice abundances similar to the bound water content of hydrated silicate-rich asteroids. At least in some cases, comets and asteroids might have similar capacities for carrying water to other solar system bodies. 

Comet nuclei range from homogeneous dirty snowballs to rubble piles that are held together by self-gravitation and dirty-ice/icy-dirt glue" [81-85]. The building blocks include ... 

Probably the best general description of a comet ever made was by the American astronomer and comet authority Fred Whipple (1906-2004). Whipple called comets "dirty snowballs." That term is apt because cometary nuclei consist primarily of ices of water, ammonia, carbon dioxide, and methane mixed with dust particles. A comet that is distant from the Sun is essentially invisible because its nucleus is so small and dark. As the comet approaches the Sun, however, solar radiation vaporizes some of the ices that make up the nucleus. The gases thus released, along with some of the dust in the nucleus, form the familiar and spectacular features of a comet. 

Hydrates may also be involved in the outgassing from cornets, which are often described as dirty snowballs, as they warm during their close approach to the sun. 

Figure 29 Top SANS data and form factor fits of a PNIPMAM-PNIPAM(50/50) copolymer microgel in D2O at different temperatures two data sets are shifted verticaiiy. Bottom schematic drawing of the internal nanophase-separated dirty snowball morphology of the PNIPMAM-PNIPAM(50/50) copoiymer microgei in D2O at the transition temperature. Note that the fuzziness of the microgel surface is not illustrated.

In order to interpret the scattering data, we developed a dirty snowball model, that is, we proposed that the copolymer microgel consists of two different regions phases with a higher density consisting of (mainly) PNIPAM which is in the collapsed state at 38 ° C (VPTTpNipAM = 34 °C ) and a matrix with a lower density consisting of (mainly) PNIPMAM which is still in the swollen state at 38 °C. 

Nucleus composed of rock, dust, ices, frozen gases often described as dirty snowballs, however the ices are covered by dark dust and rocks. The nuclei are irregularly shaped and the diameters range from several 100 m to some dozens of km. Up to now only the nuclei of the comets Halley (Fig. 5.3), Borrelly, Wild and Tempel 1 have been investigated by satellite missions and photographed directly. 

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