Statistics – Computation
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p23b1365g&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P23B-1365
Statistics
Computation
1009 Geochemical Modeling (3610, 8410), 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 6280 Saturnian Satellites
Scientific paper
The Ion and Neutral Mass Spectrometer onboard the Cassini spacecraft detected hydrogen cyanide (HCN) in plumes on Enceladus [1]. We explored computationally the thermodynamic stability and kinetic reactivity of HCN. Thermochemical equilibrium calculations show that HCN is unstable in liquid water, and should be overwhelmingly hydrolyzed to formic acid (HCOOH) and ammonia (NH3) under all conditions relevant to Enceladus. Even in an aqueous solution with an unreasonably high NH3 activity of 10, the HCOOH/HCN activity ratio is predicted to be greater than 107 at 0°C. This ratio decreases with increasing NH3 activity and with increasing temperature. Even at high temperatures (e.g., 200°C), it is at least 3-4 orders-of-magnitude above unity. It is notable that neither formic acid nor ammonia have been detected in any plumes [1]. We thus infer that the detected HCN is unlikely to have equilibrated with an aqueous solution below Enceladus' surface. Chemical kinetic considerations indicate that the hydrolysis lifetime of HCN has a complex dependence on pH, and decreases with increasing temperature. We find that, even at the pH value most conducive to the survival of HCN (~5), HCN can persist for only about 5 Ma in liquid water at 0°C. Thus, the HCN in the plumes could not have come from a long-lasting ocean or sea, and probably came from a recent aqueous melt or some icy phase. A liquid source for the HCN would be more restrictive than an ice source. For example, an aqueous solution must have a pH between 4-6 for HCN to be preserved in it for at least ~1 Ma at 0°C. However, this pH restriction would be lifted if the HCN source were a very young pool (i.e., centuries-millennia old). On the other hand, it is expected that HCN could survive indefinitely in an icy matrix. The proposed HCN sources may be less favorable abodes for life than a long-lived aqueous reservoir. Reference: [1] Waite J.H. et al. (2008) Eur. Planet. Sci. Congr. 3, EPSC2008-A-00593.
Glein Christopher R.
Shock Everett L.
Zolotov Mikhail Yu.
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