Biomarker on Callisto

Ozone (O3), regarded as a biomarker, has been observed on the icy satellites Ganymede [1a], Dione and Rhea [1b]. Presence of O3 on the icy surfaces of Ganymede, Rhea and Dione are due to energetic processing of oxygen-bearing molecules. Laboratory experiments had shown efficient synthesis of O3 in oxygen-bearing molecules such as COx, SOx and NOx. Most of the experiments used InfraRed (IR) spectroscopy to detect O3 [2a]. However, unambiguous O3 detection in planetary objects using IR signatures is difficult due to the presence of silicates. Therefore, the Hartley band of O3, 220 – 310 nm, was used to find O3 presence on icy surfaces [2b]. Apart from the three satellites of the outer Solar System, there may be other satellites that might harbour O3. UltraViolet (UV) spectrum of Callisto recorded by the HST was reported to show spectral signatures of SO2 [3]. Based on this observation, the irradiation experiments simulating SO2 ices on Callisto revealed the coexistence of SO2 and O3. The spectral signatures in the UV were found to extend from 220 – 310 nm with a broad peak 255 – 285 nm, clear indication of O3 embedded in the SO2 ice matrix. Here we will present the detailed analysis that suggests the presence of O3 on Callisto.   References: [1] Noll et al. [a] (1996) Science, 273, 341. & [b] (1997) Nature, 388, 45.[2] Sivaraman et al. [a] (2007)  ApJ, 669, 1414. & [b] (2014) Chem Phy Lett, 603, 33.[3] Noll et al. (1997) GRL, 24, 1139.