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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 MAVEN spacecraft launched in November 2013, arrived at Mars in September 2014, and completed commissioning and began its one-Earth-year primary science mission in November 2014. The orbiter’s science objectives are to explore the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere, to determine the structure of the upper atmosphere and ionosphere and the processes controlling it, to determine the escape rates from the upper atmosphere to space at the present epoch, and to measure properties that allow us to extrapolate these escape rates into the past to determine the total loss of atmospheric gas to space through time. These results will allow us to determine the importance of loss to space in changing the Mars climate and atmosphere through time, thereby providing important boundary conditions on the history of the habitability of Mars. The MAVEN spacecraft contains eight science instruments (with nine sensors) that measure the energy and particle input from the Sun into the Mars upper atmosphere, the response of the upper atmosphere to that input, and the resulting escape of gas to space. In addition, it contains an Electra relay that will allow it to relay commands and data between spacecraft on the surface and Earth.

The contribution of solar-wind ions exchanging electrons with helium and hydrogen near the Sun is shown to be only about 40 per cent of the 1/4-keV X-ray flux observed in the Galactic plane; this supports the existence of a local ‘hot bubble’ filled with X-ray-emitting gas, accounting for the rest of the flux. Dual source for local X-rays Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-keV X-rays, coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, have led to a picture of a local cavity, or hot bubble, filled with X-ray-emitting hot gas. Suggestions that the emission could instead be produced within the Solar System by charge exchange between heavy solar-wind ions and neutral hydrogen and helium have raised questions over this model. Massimiliano Galeazzi et al . report observations showing that solar wind charge exchange contributes about 40% of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports models that include a million-degree hot bubble extending about a hundred parsecs from the Sun. The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays 1 , 2 , coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas 3 , led to a picture of a ‘local cavity’ filled with X-ray-emitting hot gas, dubbed the local hot bubble 4 , 5 , 6 . This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space 7 , 8 , 9 , 10 , 11 , potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk 12 , 13 , 14 , 15 . Here we report observations showing that the total solar-wind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.

Magnetic reconnection in collisional plasmas has been widely studied in solar and laboratory plasma disciplines, but in situ measurements in space plasmas have rarely been utilized to explore this reconnection regime. Here we study collisional effects on magnetic reconnection in the Martian ionosphere by analyzing in situ data obtained by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. A case study and statistical results demonstrate that current sheets are commonly observed in both collisionless and collisional regions in the Martian ionosphere. Meanwhile, reconnection ion jets become hardly detectable in collisional current sheets, suggesting suppression of reconnection outflows by ion‐neutral friction effects. MAVEN observations allow us to access multiple regimes of magnetic reconnection, thereby providing valuable opportunities for cross‐disciplinary studies of collisional reconnection.

Two of the primary goals of the MAVEN mission are to determine how the rate of escape of Martian atmospheric gas to space at the current epoch depends upon solar influences and planetary parameters and to estimate the total mass of atmosphere lost to space over the history of the planet. Along with MAVEN’s suite of nine science instruments, a collection of complementary models of the neutral and plasma environments of Mars’ upper atmosphere and near-space environment are an indispensable part of the MAVEN toolkit, for three primary reasons. First, escaping neutrals will not be directly measured by MAVEN and so neutral escape rates must be derived, via models, from in situ measurements of plasma temperatures and neutral and plasma densities and by remote measurements of the extended exosphere. Second, although escaping ions will be directly measured, all MAVEN measurements are limited in spatial coverage, so global models are needed for intelligent interpolation over spherical surfaces to calculate global escape rates. Third, MAVEN measurements will lead to multidimensional parameterizations of global escape rates for a range of solar and planetary parameters, but further global models informed by MAVEN data will be required to extend these parameterizations to the more extreme conditions that likely prevailed in the early solar system, which is essential for determining total integrated atmospheric loss. We describe these modeling tools and the strategies for using them in concert with MAVEN measurements to greater constrain the history of atmospheric loss on Mars.

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.