We report the first efforts to characterize a laser desorption and thermal emission spectroscopic technique that could be used to detect and analyze the abundances of organic and inorganic compounds in the surfaces of Jupiter's icy moons. Based on the literature, an infrared laser near 3.1 μm at a moderate fluence 120 mJ/cm2 may desorb the compounds in the ice surface into the gas phase through an efficient explosive phase-transition process. The desorbed compounds, which are much warmer than the ice, can be analyzed by monitoring their IR thermal emission spectra against the colder icy surface by use of an IR spectrometer. However, there appears to have some discrepancies in the literature on the exact threshold fluence for desorption. We have conducted experiments to determine the threshold fluence for the laser desorption of ice by use of a sensitive photoacoustic spectroscopy technique. Our results suggest that the threshold fluence near 3.1 μm, at the peak of optical absorption of ice, is close to 120 mJ/cm2, as compared to some other higher values being reported. In addition, our data shows a delay time of about 24 μs or longer for an explosive removal of a layer of the ice surface after the irradiation of a laser pulse. Implications for mission and instrument design are discussed.
Spherical detonations of C2H2/O2/N2 mixtures in an open flow system (initially at 1 atmosphere) and planar detonations of C2H2/O2 and H2/O2/C2H2 mixtures in an enclosed tube are successfully initiated by use of an ArF laser at 193 nm. The required critical energy for the initiation of spherical detonations is found to be relatively low: approximately 12 +/- 2 mJ for a 40% C2H2 in C2H2/O2 mixtures. This small critical energy may be attributed to a relatively strong absorption of C2H2 at 193 nm, and possible enhancement by the photodissociation products of C2H and H. The initiation appears to be accomplished without overdriving the mixtures through a blast wave. The critical energy, delay time, detonation velocity and pressures are measured as functions of stoichiometric mixture ratio, initial pressure and incident laser energy, for both spherical and planar detonations.