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Abstract. The efficient sputtering and decomposition of Europa's regolith by energetic charged particles produces an atmosphere representative of its surface composition. In addition to 02 and H2 from the decomposition of ice, we show that molecules representative of organics and salts will be present in ionic form at levels detectable using an ion mass spectrometer on an orbiting spacecraft. Such an instrument can also measure isotope ratios to determine surface age.

sub-Earth point (i.e., the X ray limb is the bright visible limb). Except during theShoemaker-Levy 9 impacts, the total X ray emissions are not strongly correlated with System III central meridian longitude. However, a composite map of the emissions shows that they are generally located in regions of low surface magnetic field strength at low latitudes, and in regions of large horizontal surface magnetic field strength gradients at high latitudes.

The origin of Titan's atmospheric methane is a key issue for understanding the origin of the Saturnian satellite system. It has been proposed that serpentinization reactions in Titan's interior could lead to the formation of the observed methane. Meanwhile, alternative scenarios suggest that methane was incorporated in Titan's planetesimals before its formation. Here, we point out that serpentinization reactions in Titan's interior are not able to reproduce the deuterium over hydrogen (D/H) ratio observed at present in methane in its atmosphere, and would require a maximum D/H ratio in Titan's water ice 30% lower than the value likely acquired by the satellite during its formation, based on Cassini observations at Enceladus.Alternatively, production of methane in Titan's interior via radiolytic reactions with water can be envisaged but the associated production rates remain uncertain. On the other hand, a mechanism that easily explains the presence of large amounts of methane trapped in Titan in a way consistent with its measured atmospheric D/H ratio is its direct capture in the satellite's planetesimals at the time of their formation in the solar nebula.In this case, the mass of methane trapped in Titan's interior can be up to ∼1,300times the current mass of atmospheric methane.

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We describe a formation scenario of Enceladus constrained by the deuterium-to-hydrogen ratio (D/H) in the gas plumes as measured by the Cassini Ion and Neutral Mass Spectrometer. We propose that, similarly to Titan, Enceladus formed from icy planetesimals that were partly devolatilized during their migration within the Kronian subnebula. In our scenario, at least primordial Ar, CO, and N 2 were devolatilized from planetesimals during their drift within the subnebula, due to the increasing temperature and pressure conditions of the gas phase. The origin of methane is still uncertain since it might have been either trapped in the planetesimals of Enceladus during their formation in the solar nebula or produced via serpentinization reactions in the satellite's interior. If the methane of Enceladus originates from the solar nebula, then its D/H ratio should range between ∼4.7 × 10 −5 and 1.5 × 10 −4. Moreover, Xe/H 2 O and Kr/H 2 O ratios are predicted to be equal to ∼7 × 10 −7 and 7 × 10 −6 , respectively, in the satellite's interior. On the other hand, if the methane of Enceladus results from serpentinization reactions, then its D/H ratio should range between ∼2.1 × 10 −4 and 4.5 × 10 −4. In this case, Kr/H 2 O should not exceed ∼10 −10 and Xe/H 2 O should range between ∼1 × 10 −7 and 7 × 10 −7 in the satellite's interior. Future spacecraft missions, such as Titan Saturn System Mission, will have the capability to provide new insight into the origin of Enceladus by testing these observational predictions.

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