B. A. Magee

Cassini Finds an Oxygen–Carbon Dioxide Atmosphere at Saturn’s Icy Moon Rhea

Extraterrestrial Atmosphere The detection of oxygen in the atmospheres of Jupiters icy moons, Europa and Ganymede, and the presence of this gas as the main constituent of the atmosphere that surrounds Saturns rings, has suggested the possibility of oxygen atmospheres around the icy moons that orbit inside Saturns magnetosphere. Using the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft, Teolis et al. (p. 1813, published online 25 November; see the Perspective by Cruikshank) report the detection of a very tenuous oxygen and carbon dioxide atmosphere around Saturns icy moon Rhea. As with other icy satellites, this atmosphere is maintained through the dissociation of surface molecules and ejection into the atmosphere as a result of Saturns magnetospheric radiation. Rhea’s atmosphere is maintained by chemical decomposition of surface water ice under irradiation from Saturn’s magnetosphere. The flyby measurements of the Cassini spacecraft at Saturn’s moon Rhea reveal a tenuous oxygen (O2)–carbon dioxide (CO2) atmosphere. The atmosphere appears to be sustained by chemical decomposition of the surface water ice under irradiation from Saturn’s magnetospheric plasma. This in situ detection of an oxidizing atmosphere is consistent with remote observations of other icy bodies, such as Jupiter’s moons Europa and Ganymede, and suggestive of a reservoir of radiolytic O2 locked within Rhea’s ice. The presence of CO2 suggests radiolysis reactions between surface oxidants and organics or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism.

Titan's thermospheric response to various plasma environments

[1] TheCassini‐HuygensmissionhasbeenobservingTitansinceOctober2004,resultingin over 70 targeted flybys. Titan’s thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn’s magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn’s plasma sheet. Finally the data analysis is supplemented with Navier‐Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar‐driven diurnal variation in temperatures in the upper atmosphere. Citation: Westlake, J. H., J. M. Bell, J. H. Waite Jr., R. E. Johnson, J. G. Luhmann, K. E. Mandt, B. A. Magee, and A. M. Rymer (2011), Titan’s thermospheric response to various plasma environments, J. Geophys. Res., 116, A03318,

Titan's ionospheric composition and structure: Photochemical modeling of Cassini INMS data

Titans upper atmosphere produces an ionosphere at high altitudes from photoionization and electron impact that exhibits complex chemical processes in which hydrocarbons and nitrogen-containing molecules are produced through ion-molecule reactions. The structure and composition of Titans ionosphere has been extensively investigated by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. We present a detailed study using linear correlation analysis, 1-D photochemical modeling, and empirical modeling of Titans dayside ionosphere constrained by Cassini measurements. The 1-D photochemical model is found to reproduce the primary photoionization products of N2 and CH4. The major ions, CH5+, C2H5+, and HCNH+ are studied extensively to determine the primary processes controlling their production and loss. To further investigate the chemistry of Titans ionosphere we present an empirical model of the ion densities that calculates the ion densities using the production and loss rates derived from the INMS data. We find that the chemistry included in our model sufficiently reproduces the hydrocarbon species as observed by the INMS. However, we find that the chemistry from previous models appears insufficient to accurately reproduce the nitrogen-containing organic compound abundances observed by the INMS. The major ion, HCNH+, is found to be overproduced in both the empirical and 1-D photochemical models. We analyze the processes producing and consuming HCNH+ in order to determine the cause of this discrepancy. We find that a significant chemical loss process is needed. We suggest that the loss process must be with one of the major components, namely C2H2, C2H4, or H2.

THE 12C/13C RATIO ON TITAN FROM CASSINI INMS MEASUREMENTS AND IMPLICATIONS FOR THE EVOLUTION OF METHANE

We have re-evaluated the Cassini Ion Neutral Mass Spectrometer (INMS) 12 C/ 13 C ratios in the upper atmosphere of Titan based on new calibration sensitivities and an improved model for the NH3 background in the 13 CH4 mass channel. The INMS measurements extrapolated to the surface give a 12 C/ 13 Ci n CH4 of 88.5 ± 1.4. We compare the results to a revised ratio of 91.1 ± 1.4 provided by the Huygens Gas Chromatograph Mass Spectrometer and 86.5 ± 7.9 provided by the Cassini Infrared Spectrometer and determine implications of the revised ratios for the evolution of methane in Titan’s atmosphere. Because the measured 12 C/ 13 C is within the probable range of primordial values, we can only determine an upper boundary for the length of time since methane began outgassing from the interior, assuming that outgassing of methane (e.g., cryovolcanic activity) has been continuous ever since. We find that three factors play a crucial role in this timescale: (1) the escape rate of methane, (2) the difference between the current and initial ratios and the rate of methane, and (3) production or resupply due to cryovolcanic activity. We estimate an upper limit for the outgassing timescale of 470 Myr. This duration can be extended to 940 Myr if production rates are large enough to counteract the fractionation due to escape and photochemistry. There is no lower limit to the timescale because the current ratios are within the range of possible primordial values.

An empirical approach to modeling ion production rates in Titan's ionosphere I: Ion production rates on the dayside and globally

Titans ionosphere is created when solar photons, energetic magnetospheric electrons or ions, and cosmic rays ionize the neutral atmosphere. Electron densities generated by current theoretical models are much larger than densities measured by instruments on board the Cassini orbiter. This model density overabundance must result either from overproduction or from insufficient loss of ions. This is the first of two papers that examines ion production rates in Titans ionosphere, for the dayside and nightside ionosphere, respectively. The first (current) paper focuses on dayside ion production rates which are computed using solar ionization sources (photoionization and electron impact ionization by photoelectrons) between 1000 and 1400 km. In addition to theoretical ion production rates, empirical ion production rates are derived from CH4, CH3+, and CH4+ densities measured by the INMS (Ion Neutral Mass Spectrometer) for many Titan passes. The modeled and empirical production rate profiles from measured densities of N2+ and CH4+ are found to be in good agreement (to within 20%) for solar zenith angles between 15 and 90°. This suggests that the overabundance of electrons in theoretical models of Titans dayside ionosphere is not due to overproduction but to insufficient ion losses.

High-Time Resolution In-situ Investigation of Major Cometary Volatiles around 67P/C-G at 3.1 - 2.3 AU Measured with ROSINA-RTOF

Comets considered to be pristine objects contain key information about the early formation of the solar system. Their volatile components can provide clues about the origin and evolution of gases and ices in the comets. Measurements with ROSINA/RTOF at 67P/Churyumov-Gerasimenko have now allowed, for the first time, a direct in situ high-time resolution measurement of the most abundant cometary molecules originating directly from a comets nucleus over a long time-period, much longer than any previous measurements at a close distance to a comet between 3.1 and 2.3 au. We determine the local densities of H 2 O, CO 2 , and CO, and investigate their variabilities.

FORMATION CONDITIONS OF ENCELADUS AND ORIGIN OF ITS METHANE RESERVOIR

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 N2 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/H2O and Kr/H2O 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/H2O should not exceed ∼10 −10 and Xe/H2O 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.

The source of heavy organics and aerosols in Titan's atmosphere

Ion-neutral chemistry in Titans upper atmosphere (∼ 1000 km altitude) is an unex- pectedly prodigious source of hydrocarbon-nitrile compounds. We report observations from the Cassini Ion Neutral Mass Spectrometer (INMS; Waite et al. 2004) and Cassini Plasma Spec- trometer (CAPS; Young et al. 2004) that allow us to follow the formation of the organic material from the initial ionization and dissociation of nitrogen and methane driven by several free en- ergy sources (extreme ultraviolet radiation and energetic ions and electrons) to the formation of negative ions with masses exceeding 10,000 amu.