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Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH₄ and N₂). Up to now, it has been assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (~1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH₄ and N₂) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (~8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.

The Lunar Environment heliospheric X-ray Imager (LEXI) is a wide field-of-view soft Xray telescope developed to study solar wind-magnetosphere coupling. LEXI is part of the Blue Ghost 1 mission comprised of 10 payloads to be deployed on the lunar surface. LEXI monitors the dayside magnetopause position and shape as a function of time by observing soft X-rays (0.1–2 keV) emitted from solar wind charge-exchange between exospheric neutrals and high charge-state solar wind plasma in the dayside magnetosheath. Measurements of the shape and position of the magnetopause are used to test temporal models of mesoand macro-scale magnetic reconnection. To image the boundary, LEXI employs lobster-eye optics to focus X-rays to a microchannel plate detector with a 9.1◦ × 9.1◦ field of view.

The Mars thermosphere-ionosphere-exosphere (TIE) system constitutes the atmospheric reservoir (i.e. available cold and hot planetary neutral and thermal ion species) that regulates present day escape processes from the planet. The characterization of this TIE system, including its spatial and temporal (e.g., solar cycle, seasonal, diurnal, episodic) variability is needed to determine present day escape rates. Without knowledge of the physics and chemistry creating this TIE region and driving its variations, it is not possible to constrain either the short term or long term histories of atmosphere escape from Mars. MAVEN (Mars Atmosphere and Volatile Evolution Mission) will make both in-situ and remote measurements of the state variables of the Martian TIE system. A full characterization of the thermosphere (∼100–250 km) and ionosphere (∼100–400 km) structure (and its variability) will be conducted with the collection of spacecraft in-situ measurements that systematically span most local times and latitudes, over a regular sampling of Mars seasons, and throughout the bottom half of the solar cycle. Such sampling will far surpass that available from existing spacecraft and ground-based datasets. In addition, remote measurements will provide a systematic mapping of the composition and structure of Mars neutral upper atmosphere and coronae (e.g. H, C, N, O), as well as probe lower altitudes. Such a detailed characterization is a necessary first step toward answering MAVEN’s three main science questions (see Jakosky et al. 2014 , this issue). This information will be used to determine present day escape rates from Mars, and provide an estimate of integrated loss to space throughout Mars history.

The ion and electron sensor (IES) is part of the Rosetta Plasma Consortium (RPC). The IES consists of two electrostatic plasma analyzers, one each for ions and electrons, which share a common entrance aperture. Each analyzer covers an energy/charge range from 1 eV/e to 22 keV/e with a resolution of 4%. Electrostatic deflection is used at the entrance aperture to achieve a field of view of 90°× 360° (2.8π sr). Angular resolution is 5°× 22.5° for electrons and 5°× 45° for ions with the sector containing the solar wind being further segmented to 5°× 5°. The three-dimensional plasma distributions obtained by IES will be used to investigate the interaction of the solar wind with asteroids Steins and Lutetia and the coma and nucleus of comet 67P/Churyumov–Gerasimenko (CG). In addition, photoelectron spectra obtained at these bodies will help determine their composition.

The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan's upper atmosphere and its interaction with Saturn's magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon's surface to form hydrocarbon-nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn's ring system and icy moons and on the identification of positive ions and neutral species in Saturn's inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of approximately 12 planetary radii and about the genesis and evolution of the rings. The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N2 and CH4; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of not less than 10 exp 4/cu cm will be detected (compared with not less than 10 exp 5/cu cm in the open source neutral mode). For ions the detection threshold is on the order of 10 exp -2/cu cm at Titan relative velocity (6 km/sec). The INMS instrument has a mass range of 1-99 Daltons and a mass resolution M/Delta-M of 100 at 10 percent of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C6H6. The INMS instrument was built by a team of engineers and scientists working at NASA's Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution.

Auroral X‐ray emissions from Jupiter with a total power of about 1 GW have been observed by the Einstein Observatory, Roentgen satellite, Chandra X‐ray Observatory, and XMM‐Newton. Previous theoretical studies have shown that precipitating energetic sulfur and oxygen ions can produce the observed X‐rays. This study presents the results of a hybrid Monte Carlo (MC) model for sulfur and oxygen ion precipitation at high latitudes, looks at differences with the continuous slow‐down model, and compares the results to synthetic spectra fitted to observations. We concentrate on the effects of altitude on the observed spectrum. The opacity of the atmosphere to the outgoing X‐ray photons is found to be important for incident ion energies greater than about 1.2 MeV per nucleon for both sulfur and oxygen. Model spectra are calculated for intensities with and without any opacity effects. These synthetic spectra were compared with the results shown by Hui et al. (2010) which fit Chandra X‐ray Observatory observations for the north and south Jovian auroral emissions. Quenching of long‐lived excited states of the oxygen ions is found to be important. Opacity considerably diminishes the outgoing X‐ray intensity calculated, particularly when the viewing geometry is not favorable.

We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of ∼700 cm−3 below ∼1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through “fast” ion‐neutral chemistry and the latter through “slow” electron dissociative recombination. The strong correlation between the observed night‐to‐day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time‐dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long‐lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short‐lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short‐lived ions observed on Titan's nightside.

Observations by instruments onboard the Cassini spacecraft of Saturn's icy satellite Enceladus have shown that a plume containing water vapor and ice grains is present in the southern hemisphere. Energy distributions of electrons in this plume were measured by the electron sensor part of the Cassini plasma spectrometer (CAPS – ELS). A significant suprathermal electron population was detected. The nature of the electron population is important for understanding the composition and chemistry of the plume plasma because the electron‐ion recombination rate depends on the energy distribution and because ionizing collisions by energetic electrons creates new plasma. We present the results of a two‐stream electron transport model for plume electrons that includes neutral densities that agree with Cassini Ion and Neutral Mass Spectrometer (INMS) data. Electron production within the plume due to photoionization by solar radiation and by electron impact ionization was included, as were energy losses due to electron‐neutral collisions. Model cases were considered that both included and did not include electron inputs from outside the plume. Comparisons are made of model fluxes with measured fluxes by CAPS – ELS on October 9, 2008. The model‐data comparisons indicate that photoelectrons (10 eV–70 eV energies) locally produced within the plume can explain the data. The possible role of electron‐grain collisions was also explored and it was determined that nanograin densities in excess of 106 cm−3 would be needed to affect the electron distribution. Key Points Suprathermal electrons strongly interact with neutral gas in the Enceladus plume Photoelectrons produced by solar radiation explain Cassini plume electron data Interaction between electrons and nanograins is probably insignificant

We report on Cassini Ion and Neutral Mass Spectrometer (INMS) observations above Titan's exobase at altitudes of 2225 km to 3034 km. We observe significant densities of CH5+, HCNH+ and C2H5+that require ion‐molecule reactions to be produced in the quantities observed. The measured composition and ion velocity (about 0.8–1.5 km/s) suggest that the observed ions must have been created deep inside Titan's ionosphere (below the exobase) and then transported to the detection altitude. Plasma motion from below Titan's exobase to large distances can be driven by a combination of thermal pressure and magnetic forces. The observed outward flows may link the main ionosphere with the more distant wake and provide a source of hydrocarbon ions in the Saturnian system. Key Points The Cassini INMS observes ions from below Titan's exobase near 3000 km altitude The composition is ionospheric, namely CH3+, CH4+, CH5+, HCNH+, and C2H5+ The ions are observed to have a speed of 0.8‐1.5 km/s relative to Titan

The Cassini radio science facility provided 13 occultation electron density profiles of Titan during the period of 2006 and 2009. This paper presents the results of all of these occultation observations. It shows that ten of the observed electron density profiles are similar, but three are significantly different. The number of observations is relatively small for meaningful statistical conclusions, but it is shown, using the corresponding measured electron spectra, that the three anomalous profiles in the ionospheric peak regions are likely to be the result of unusually intense electron precipitation events. Key Points The structure and density of the Titan ionosphere display sporadic changes Changes are most likely related to enhanced electron precipitation