Cometary impacts

A new reservoir of comets may have formed at around 5 AU in a local orbit around Jupiter or at least perturbed by its gravitational attraction. A comet close to Jupiter would simply have been captured, delivering its chemical payload to the ever-increasing gas giant. Some comets would merely have been deflected towards the inner terrestrial planets, delivering a similar payload of water and processed molecules. Cometary impacts such as the spectacular collision of the comet Shoemaker-Levy 9 with Jupiter would have been common in the early formation phase of the solar system but with a much greater collision rate. Calculations of the expected collision rate between the Earth and potential small comets deflected from the snow line may have been sufficient to provide the Earth with its entire... 

Exogenous delivery of organics Cometary impact and the origin of the oceans loading the full cometary molecular inventory onto the prebiotic Earth... 

Such a measurement can tell us about the chemical evolution of oxygen, such as whether the isotopes differentiated via a thermal cycle in which lighter leO fractionates from the heavier lsO, much as Vostok ice-core oxygen ratios reveal the Earth s prehistoric climate. From this fixed point of the Sun s oxygen ratios, we can then trace the history of water in other planetary bodies since their birth in the solar nebulae through the subsequent cometary bombardment [13]. In NASA s search for water on the Moon, important for the establishment of a future Moon base, such isotopic ratios will determine whether the water is a vast mother lode or just a recent cometary impact residue. 

Pierazzo, E. and Chyba, C.R, 1999. Amino acid survival in large cometary impacts. Meteoritics Planet. Sci., 34, 909-18. 

Acetylene has been observed in the atmospheres of Jupiter and Titan [33, 34] and more recently has been identified in significant abundance in comet Hyakutake [35]. Following the discovery of acetylene in Hyakutake, photochemical experiments have demonstrated [36] that this molecule is a likely precursor of C2, a widely observed component of comets. Acetylene itself may therefore be a ubiquitous constituent of comets. It has been proposed [37] that polymerization of acetylene in cometary impact on planetary atmospheres may be responsible for the formation of polycyclic aromatic hydrocarbons (PAHs) which may in turn be responsible for the colors of the atmospheres of Jupiter and Titan. Shock-induced polymerization of acetylene has been observed in the gas phase [38], and static high-pressure experiments have demonstrated polymerization of orthorhombic solid acetylene above 3 to... 

The flyer plate velocity range employed in these simulations, from 8 to 25 km/s, is comparable to the speeds expected for cometary impact into planetary atmospheres [42]. It is clear from the simulations that significant shock-induced pol5nnerization can occur in cometary acetylene under these conditions. Ring structures characteristic of polycyclic aromatic hydrocarbons (PAHs) were not found in the simulations, but unsamrated oligomer precursors would presumably react to form these more complex structures over a much longer time interval than could be followed in the shock impact simulations. 

These anthracene simulations indicate that PAHs of moderate size may survive and even undergo polymerization reactions under shock impact conditions to be expected in cometary impacts on planetary atmospheres. Preliminary simulations of shock-induced chemistry in naphthalene [51] suggest a similar reaction threshold for the smallest PAH as well. 

Koeberl C (1994) Tektite origin by hypervelocity asteroidal or cometary impact Target rocks, source craters, and mechanisms. In Dressier BO, Grieve RAF, Sharpton VL (eds) Large meteorite impacts and planetary evolution. Geological Society of America, Special Paper 293. Boulder, CO, pp 133-151... 

Collisions between comets and planets have occurred often during the evolution of the solar system. Cometary impacts on early Earth may have deposited a substantial amount of water to the Earth. The masses of lost and retained water after the impacts of comets and asteroids on oceans of various depths were studied by Svetsov, 2009 [330], The bombardment of an atmosphereless planet by fast asteroids can wipe out the most part of an ocean. Because of their mass loss during perihelion passage, it is difficult to predict cometary orbits with high precision. Therefore an impact of a comet on a planet cannot be predicted precisely. 

During the impact of comet Shoemaker Levy on Jupiter (1994), a 22 GHz water MASER emission line was observed. This line can be used as a diagnostic for the existence of water on exoplanets. An observational program (called ITASEL = Italian Search for Extraterrestrial Life) started 1999 using the 32 m dish of the Medic-ina radiotelescope and from 32 candidates four emissions can be suspected. If such observations can be definitely confirmed it will prove the importance of cometary impacts for the deposit of water on exoplanets (Cosmovici et al., 2006 [80]). 

Maser detection in the interstellar medium was described by Elitzur, 1987 [118]. Water maser emission at 22 GHz may be a hint for cometary impacts that occur in extrasolar planetary systems. Such radiosignatures were proposed by Cos-movici et al., 2001 [79]. During the impact of comet Shoemaker-Levy 1994 on Jupiter observations made with the 32-m dish Medicina radiotelescope (Bolgna, Italy) revealed the delivery of water and for the first time also a water maser in the solar system. After the comet s nucleus exploded in the upper part of Jupiter s atmosphere, this maser was produced. 

Cosmovid, C.B., Pogrebenko, S., Montebugnoli, S., Maccaferri, G. Radio searching for extrasolar cometary impacts at 22 Ghz (water MASER emission). In Catastrophic Events and Mass Extinctions Impacts and Beyond, p. 3002 (2001)... 

---