4336
The Journal of Physical Chemistty, Vol. 87, No. 21, 1983
therefore very dark. It only becomes highly reflective when it is broken up into small separated particles. So the high reflectivity is actually evidence for snow. It may be possible to use the spectral reflectivity to estimate the sizes of these small particles, as well as their composition (Wiscombe; Warren J. Atmos. Sci. 1980,37, 2712). Consolmagno: Polarization studies can also reveal the small particle size. Since these are airless bodies, subjected to constant micrometeorite bombardment, we expect the surfaces to be broken up into small, snowlike fragments. These bodies also have a very strong radar reflectivity, which is still not well understood but which may be evidence for a complex surface structure full of voids and fractures.
T. Takahashi: May I ask why the ice coverage on planets is scattered in appearance? Consolmagno: We must remember that these are bodies made up primarily of ice; your question could be turned around and asked, why is the rocky coverage of the ice scattered? The dark material may be micrometeorites which have contaminated the surface, or may be a thin layer of rocky material from the primordial ice-rock mixture that made up these bodies which was left behind after some process (sublimation or ion sputtering) has stripped away the ice. We see craters with icy floors that appear to have “punctured” this layer of rocky material. Icy regions on Ganymede appear to be younger, resurfaced regions or associated with extensional “cracks”. Some transport of icy material to polar regions does occur, but since there is no atmosphere and the temperatures are quite low, it is not a very efficient process.
Extraterrestrial Ice. A Review (J. Klinger) R. Smoluchowski: There may be plenty of water on Mars. High temperatures occur on Mars every few 1OOOOO years. Enceladus surface temperature is approximately 75 K. Thus there is a need for considerable heat to cause its crystallization near 150 K. The heat evolved below 150 K is very small. Klinger: In fact we need plenty of heat to heat up Enceladus to the transition temperature, but I think that the model of tidal heating as it was proposed by Yoder (ref 31) will work much easier when phase transitions take place. Further it has been shown by Ghormley (ref 33) that amorphous ice will release some heat even before the phase transition. What may help too is resurfacing. J . C. Comiso: What additional ice observational data would you require to gain a better understanding of the origin of the solar system? Klinger: I think that the identification of different ice phases (for example by infrared reflection spectra) may give us some ideas the thermal history of the material. A . H. Delsemme: You mentioned that the OH band was observed in comets by radio astronomers. This is true. However, it was discovered in its 3050-A ultraviolet band in the spectrum of comet Cummingham (1941) by Swings and co-workers, and repeatedly observed in many comets in the ultraviolet. Klinger: OK!
Ice in Comets (A. H. Delsemme) J . Klinger: May the fact that Kohoutek was essentially outgassing COz be related to the finding that after perihelion the production of OH, in a wide range of heliocentric distances, showed no variation? Delsemme: Yes indeed; on its leg before perihelion, the surface temperature of the nucleus was controlled by C02 sublimation; apparently, COz became depleted near perihelion, but took a long time to restore a new steady state involving only water ice; this explains why the sublimation of water ice remained for a while a t a constant rate, the rise in temperature due to C02depleting being compensated by the larger and larger heliocentric distance of the comet.
Energetic Charged Particle Erosion of Ices in the Solar System (R. E. Johnson) J. M . Warman: You seemed to be attaching some significance to the fact that several molecules were being detached from the
surface per incident 1.5-MeV He+ ion. However, the total number of molecules undergoing excitation and dissociation will in fact be in the region of lo4 to lo6 per ion. Detachment therefore corresponds to a very small fraction of excitation events. A likely process of molecular detachment would seem to be molecular dissociation of surface molecules followed by escape of high velocity (“hot”)radical fragments. Would you be able tp detect such radical products and if so what is their ratio to detached water molecules? Johnson: It is true that an enormous number of bonds are broken as the electronic excited states relax. In fact decomposition apparently occurs fairly efficiently. The g value is of the order of 0.6 for a-particles. We see only a fraction of the O2 and Dz, those produced and ejected in the surface region. When I discussed the modification of the surface layer of a comet irradiated in the Oort cloud I was referring to the total effect you mentioned. I used the word large, I meant the number ejected from the surface was large compared to sputtering produced by direct collisions. Radical fragments are probably lost, but our estimates indicate they are not the dominant ejected species. We can detect these radicals by using water having isotopes of oxygen. Certainly if the events at the surface were produced by individual excitation events radicals may be dominant products of dissociative electronic relaxation as,for example, in desorption. However, the nonlinear dependences of yield on (dE/dn) suggests that the relaxation events might be producing “hot” overlapping regions which produces the observed molecular ejection.
R. Smoluchowski: You indicated that Saturnian rings are eroded photoelectrically rather than by particulate radiation. How did you estimate the electronic and proton flux in proximity of the rings? Is this based on Morfill, Grein et al. data? Johnson: The fluxes were obtained by modeling the diffusive loss in the vicinity of the main ring system. It is based on a recent paper by Cheng, Lanzerotti, and Pirronello. J. M. Greenberg: What is the depth of penetration of a 1.5-MeV He+ ion? If it is deep enough you are here dealing with bulk (well inside surface) phenomena differentiate. Johnson: A 1.5-MeV He+ ion will penetrate to micron depths. However, we believe the energy deposited only within a few 100 A at most would affect the single particle surface erosion. If the erosion continues or if the surface is subsequently heated the effects a t greater depth will eventually contribute.
R. Wolff: The energy of ejected H 2 0 from your experiment is generally
