Nature and Source of Organic Matter in the Shoemaker–Levy 9 Jovian Impact Blemishes

The 0.3–1.0 μm optical constants of the aerosols in the dark SL-9 Jupiter impact blemishes are nearly identical to those of the organic residue in the Murchison carbonaceous chondrite. Porous poly-HCN is also a reasonable match. The mass of organics needed to produce a typical blemish is 0.8–8 × 10 13 g, depending on the aerosol porosity. The 10 μm emission feature seen during the splash-back phase for the impact of fragment R, however, requires ∼1–5 × 10 12 g of astronomical silicate. The organics from the blemishes cannot be present at this point, or else its greater abundance would dominate the thermal flux, hiding or greatly modifying the 10 μm silicate feature. Furthermore, this mass of silicates, but not the mass of organics in the blemishes, is consistent with the HST reflectances of the impact plumes if the aerosols have radii ∼0.1 μm. In addition to the blemishes, a large expanding ring was observed for the G and K impacts between 3 and 4 μm but was not detected outside that range. Titan tholin has a strong absorption edge near 3 μm which decays by a factor of 8 by 4 μm. Poly-HCN and Murchison organic residue have similar but less extreme behavior. If the radiation from the ring is thermal emission instead of reflected sunlight, this feature is best explained by a very thin cloud of hot Titan tholin-like aerosols at the few microbar level. The rings require ∼10 10 g of material. The mass of organics required to produce the blemishes is comparable to the organic abundance expected in the ∼10 14 -g fragments, assuming cometary composition. However, the high temperature of the fireball would likely destroy most, if not all, of the organic matter contained within the fragments. Because of the low abundance of HCN seen at the impact sites, shock synthesis of organics from the jovian atmosphere is an improbable production mechanism. Quench synthesis of organics from cometary dissociation products, though, may satisfy the large mass requirements. The expanding ring requires a low enough mass of tholin that the latter is reasonably the result of shock synthesis, possibly during the splash-back phase.