Biemann1 calls into question our preliminary interpretations of our experimental results2. A comparison of laboratory and flight measurements should settle the uncertainties he raises. In addition to evaluating instrumental characteristics such as the 'piston effect', a technique we used for injecting oven-gas content for gas chromatography–mass spectrometry (GC–MS), such studies should enable a wide range of possible compositions for Titan's aerosols to be investigated. Our aerosol collection and pyrolysis (ACP) measurements in Titan's atmosphere can then be revisited.
Biemann doubts that we were detecting NH3 and HCN after the pyrolysis steps at 600 °C, as well as questioning our deductions. If the detection is valid, our inference is clear. The data obtained from temperature sensors in flight undoubtedly show that the oven heaters did work nominally, so any volatile material collected by ACP along with the haze particles must have been vaporized during the 250 °C heating step, and — as indicated by the reading of the oven pressure sensors — evacuated from the ACP–GC–MS. Consequently, only the refractory part of the aerosols could have remained in the oven afterwards. The detection of NH3 and HCN in the products formed during the pyrolysis of this unknown, but refractory, material indicates that it must comprise a mixture of carbon-, hydrogen- and nitrogen-containing species.
Biemann suggests that the laboratory analogues of Titan's aerosols were only crudely characterized and not even partially separated for structural analysis; however, this is not correct. The laboratory tholins were recovered and studied using different techniques in order to determine their chemical composition and their cracking patterns, which are the parameters required for interpreting observational data3,4,5,6,7. He also misinterprets our comments on the specific structure of the aerosols. Our supplementary Fig. 6 showing the probable structure of Titan's aerosols (ref. 2) is intended to illustrate a representative example of what could be part of the structure of the complex organic material, and not what it actually is. This structure was constructed from the nature of the identified pyrolysates: nitrile groups to produce HCN, and amino or imino groups to produce NH3. This type of illustration has previously been used8, based on experimental work9.
An important puzzle that will be addressed by laboratory simulations is why there are no fragments of high relative molecular mass in the spectra of the pyrolysate formed at 600 °C, which may be explained by their destruction during transport to the detector. We must, of course, accept the possibility that we could have been misled in our preliminary interpretations by a remarkable set of coincidences, which is why we are now completing a rigorous set of laboratory calibrations before reaching a final judgement on the results of the experiment.
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The Astronomy and Astrophysics Review (2009)