Feasibility studies of laser desorption to study surfaces of Jupiter's icy moons

Abstract

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/cm 2 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/cm 2 , 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.