Evaporating Grains in p/ Halley's Coma

Details Evaporating Grains in p/ Halley's Coma Evaporating Grains in p/ Halley's Coma

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987a%26a...187..801w&link_type=abstract

Astronomy and Astrophysics, Vol.187, NO. 1+2/NOV, P. 801, 1987

Astronomy and Astrophysics

Astrophysics

26

Scientific paper

Evidence is presented for the hypothesis that the organic grains streaming from a cometary nucleus lose substantial volatile constituents within 105 s. To develop the hypothesis, we study grain properties appropriate to both the comet-probe encounters at 0.8-0.9 AU and to comas observed at 4-5 AU. While submicron-sized grains of ice contaminated by 1 % or more absorptive graphite can explain the strong OH coma far from the Sun, as in comet Bowell at 4.6 AU, the dynamical lifetime of the grain coma implies mm-sized and therefore cooler grains, with weaker H2O or OH attachment than ice at about 10 kcal/mole. Grains in comet Halley, if composed of similar organic material must also contain fractions of more strongly bound components. In this study we adopt kerogen as a prototype organic complex having a range of bond energies. Temperatures and evaporation rates of kerogen grains are calculated using Arrhenius relations and Mie-Guttler radiative scattering theory with complex refractive index as measured for two cases: organic material of biological origin and UV-processed methane ice ("tholin"). A time-scale of 104 s at 0.9 AU corresponds to submicron grains with rather strongly-bound organics of ˜40 kcal/mole, identified with bridges of ether, ketone, sulphide etc., bonds. The steeply increasing strength of H and 0 inside 1 AU (∝ r-4), unlike OH, corresponds to increasing breakdown following rupture of the chemical bridges. The 10 μm IR feature can be explained by grains of an organo-siliceous material or organic mantle on a siliceous core, a few μm in size and about 20 kcal/mole bond strength.

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